Differences in spontaneous mutation frequencies as a function of environmental stress in soil fungi at "Evolution Canyon," Israel

When various wild strains of Penicillium lanosum and Aspergillus niger were placed in the same mild laboratory environment, their frequencies of new spontaneous mutations were clearly related to whether they had been isolated from a region of high or low microclimatic stress. In the mild environment, the total frequencies of conidial color and morphological mutations in P. lanosum, summed over all relevant loci, ranged from 0.29% to 2.4% for six strains from the north-facing, less stressful "European" slope (ES/NFS) of "Evolution Canyon" I, compared with 6.5-11.6% for five strains from the south-facing "African" slope (AS/SFS), which is a much more stressful environment, being harsher, drier, more fluctuating in temperature, and receiving up to eight times more UV radiation than the opposite slope. The corresponding figures for A. niger were 0.42-1.50% for three strains from the ES/NFS and 2.3-4.9% for six strains from the AS/SFS. The more mutagenic environment of the AS/SFS than of the ES/NFS means that, in Evolution Canyon, the mutation frequency differences between the very stressful environment and the less stressful environment are probably even larger than the 4- and 6-fold differences found here in a mild laboratory environment. The evidence from these two filamentous fungi, which have no sexual cycle, is that there are inherited differences in spontaneous mutation rates according to the levels of stress in the environment, and this feature may well be adaptive. Evolution Canyon I is at Nahal Oren, Mount Carmel, Israel.     

Sympatric incipient speciation of spiny mice Acomys at “Evolution Canyon,” Israel

Does the paucity of empirical evidence of sympatric speciation in nature reflect reality, despite theoretical support? Or is it due to inappropriate searches in nature with overly restrictive assumptions and an incorrect null hypothesis? Spiny mice, Acomys, described here at Evolution Canyon (EC) incipiently and sympatrically speciate owing to microclimatic interslope divergence. The opposite slopes at EC vary dramatically, physically and biotically, representing the dry and hot south-facing slope savannoid-African continent [“African” slope (AS)], abutting with the north-facing slope forested south-European continent [“European” slope (ES)]. African-originated spiny mice, of the Acomys cahirinus complex, colonized Israel 30,000 y ago based on fossils. Genotypically, we showed significantly higher genetic diversity of mtDNA and amplified fragment length polymorphism of Acomys on the AS compared with the ES. This is also true regionally across Israel. In complete mtDNA, 25% of the haplotypes at EC were slope-biased. Phenotypically, the opposite slope’s populations also showed adaptive morphology, physiology, and behavior divergence paralleling regional populations across Israel. Preliminary tests indicate slope-specific mate choices. Colonization of Acomys at the EC first occurred on the AS and then moved to the ES. Strong slope-specific natural selection (both positive and negative) overrules low interslope gene flow. Both habitat slope selection and mate choices suggest ongoing incipient sympatric speciation. We conclude that Acomys at the EC is ecologically and genetically adaptively, incipiently, sympatrically speciating on the ES owing to adaptive microclimatic natural selection.Can mammals speciate sympatrically (i.e., the origin of a new species within a freely interbreeding population) without geographic isolation (1)? We have recently argued that blind subterranean mole rats, Spalax, possibly display incipient sympatric ecological speciation within the range of Spalax galili (2n = 52) in the eastern Upper Galilee in Israel (2). Sympatric speciation (SS) has been controversial since it was first proposed as a mode of speciation by Darwin (3). Skepticism about extensive SS in nature, except polyploidy in plants, has been recently expressed in a treatise on speciation (1). Clearly, natural selection in the Spalax case (2) overruled gene flow, as was the general case at Evolution Canyon (4). We concluded in the spalacid study that the constrained local gene flow, overruled by natural adaptive edaphic selection, permits incipient SS to occur. Moreover, we hypothesized that SS may indeed be abundant in nature, as visualized by Darwin (3), because abutting divergent ecologies are abundantly derived from geological, edaphic, and climatic divergences in nature (2). Here, we describe a similar case of incipient SS in the spiny mice Acomys at Evolution Canyon (EC), based on sharp local microclimatic divergence.The EC model reveals divergent interslope evolution in action involving biodiversity across life from bacteria to mammals, followed by divergent genomic, proteomic, and phenomic adaptive complexes (4–9). In the EC (details are given in Supporting Information) major adaptive complexes on the “African” slope (AS) are due to solar radiation, heat, and drought, whereas those on the “European” slope (ES) relate to shade or light stress and photosynthesis. Remarkably, interslope species divergence led to incipient SS in bacteria, fungi, plants (wild barley), insects (Drosophila and beetles), and mammals such as the spiny mice Acomys (7) to be described here in detail. Owing to this global pattern of incipient SS across life, the EC was dubbed the “Israeli Galapagos” (7). This local adaptive and speciational interslope microclimatic divergence also occurs in the other three “Evolution Canyons” studied in Israel in the Galilee, Golan, and Negev Mountains (8) as well as in other evolution canyons worldwide (9).Spiny mice of the genus Acomys are tropical murid rodents involving about 19 species ranging in Africa and southwest Asia in rocky habitats (10). Acomys cahirinus, belonging to the A. cahirinus– Acomys dimidiatus complex (11, 12), is referred to as A. cahirinus in this paper. Acomys is widely distributed across Israel and Sinai, ranging in both xeric and mesic environments comprising the Mediterranean, steppe, and desert climatic regimes and thus climatically is a wide-ranging species complex. However, it lives only in rocky habitats and is relatively stenotopic (i.e., restricted in niche preference). The three karyotypes of Acomys in Israel (11) seem to represent two speciation events, an old one, generating Acomys russatus, 2n = 66, ranging in the Sinai, Negev, and Judean deserts, and a new one involving the Acomys cahirinus complex, with 2n = 36 in xeric Sinai, and the derivative 2n = 38 in the mesic north, differing by a single Robertsonian change. The 2n = 38 occurs in Sinai and Israel, respectively (13), including the 2n = 38 karyotype at EC on both slopes based on 34 karyotypes analyzed (Fig. S1A). Allozyme analysis estimated the origin of speciation of A. russattus as 1,500,000 ± 50,000 y ago, and the divergence of the two new karyotypes’ speciation in A. cahirinus to 115,000 ± 40,000 y (14).The fossil record and zoogeographical evidence suggest that Acomys colonized the Near East from Africa in the Pliocene. However, A. cahirinus, based on fossil evidence (15), colonized mesic Israel only in the Upper Pleistocene some 30,000 y ago, conforming roughly to the allozyme data (14). Acomys carmeliensis, akin to A. cahirinus, was described by Georg Haas in the Natufian-Neolithic site of the Abu Usba cave at the upper ES slope of EC, lower Nahal Oren, Mount Carmel (15). The oldest fossil record of Acomys out of Africa, similar in morphology to those from the Abu Usba cave and to the recent A. cahirinus at EC, was found in the Levalloiso-Mousterian of the Kebara cave about 20 km south of EC (15). Here we provide evidence strongly suggesting that Acomys speciated incipiently and sympatrically at EC.     

Impact of geoengineering schemes on the global hydrological cycle

The rapidly rising CO(2) level in the atmosphere has led to proposals of climate stabilization by "geoengineering" schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earth's surface. In this article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO(2) forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4% K(-1) for solar forcing, but only 1.5% K(-1) for CO(2) forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.     

Impacts of climate change on the world's most exceptional ecoregions

The current rate of warming due to increases in greenhouse gas (GHG) emissions is very likely unprecedented over the last 10,000 y. Although the majority of countries have adopted the view that global warming must be limited to <2 °C, current GHG emission rates and nonagreement at Copenhagen in December 2009 increase the likelihood of this limit being exceeded by 2100. Extensive evidence has linked major changes in biological systems to 20th century warming. The "Global 200" comprises 238 ecoregions of exceptional biodiversity [Olson DM, Dinerstein E (2002) Ann Mo Bot Gard 89:199-224]. We assess the likelihood that, by 2070, these iconic ecoregions will regularly experience monthly climatic conditions that were extreme in 1961-1990. Using >600 realizations from climate model ensembles, we show that up to 86% of terrestrial and 83% of freshwater ecoregions will be exposed to average monthly temperature patterns >2 SDs (2σ) of the 1961-1990 baseline, including 82% of critically endangered ecoregions. The entire range of 89 ecoregions will experience extreme monthly temperatures with a local warming of <2 °C. Tropical and subtropical ecoregions, and mangroves, face extreme conditions earliest, some with <1 °C warming. In contrast, few ecoregions within Boreal Forests and Tundra biomes will experience such extremes this century. On average, precipitation regimes do not exceed 2σ of the baseline period, although considerable variability exists across the climate realizations. Further, the strength of the correlation between seasonal temperature and precipitation changes over numerous ecoregions. These results suggest many Global 200 ecoregions may be under substantial climatic stress by 2100.     

Drosophila flies in "Evolution Canyon" as a model for incipient sympatric speciation

The genetic basis of population divergence leading to adaptive radiation and speciation is a major unresolved problem of evolutionary biology. Molecular elucidation of "speciation genes" advanced recently, yet it remains without clear identification of the gene complexes participating in reproductive isolation between natural populations, particularly, in sympatry. Genetic divergence was discovered between Drosophila melanogaster populations inhabiting ecologically contrasting, opposite slopes in "Evolution Canyon" (EC), Mt. Carmel, Israel. Interslope migration of flies is easy and verified. Nevertheless, significant interslope D. melanogaster population divergence was established at EC involving habitat choice, mate choice, thermal and drought tolerances, adaptive genes, and mobile elements. Parallel patterns of stress tolerance, habitat choice, and mate choice were demonstrated in Drosophila simulans at EC, although on a smaller scale. However, some tests for interslope genetic differentiation in Drosophila, derived from the opposite EC slopes, gave somewhat controversial results. Here we present new empirical data on interslope genetic divergence of Drosophila at EC, and summarize previous supporting and controversial results. We suggest that Drosophila populations at EC represent a rare example, demonstrating how selection overrides migration, and propose an ad hoc ecological model of incipient sympatric divergence.     

Dynamical evidence for causality between galactic cosmic rays and interannual variation in global temperature

As early as 1959, it was hypothesized that an indirect link between solar activity and climate could be mediated by mechanisms controlling the flux of galactic cosmic rays (CR) [Ney ER (1959) Nature 183:451–452]. Although the connection between CR and climate remains controversial, a significant body of laboratory evidence has emerged at the European Organization for Nuclear Research [Duplissy J, et al. (2010) Atmos Chem Phys 10:1635–1647; Kirkby J, et al. (2011) Nature 476(7361):429–433] and elsewhere [Svensmark H, Pedersen JOP, Marsh ND, Enghoff MB, Uggerhøj UI (2007) Proc R Soc A 463:385–396; Enghoff MB, Pedersen JOP, Uggerhoj UI, Paling SM, Svensmark H (2011) Geophys Res Lett 38:{"type":"entrez-nucleotide","attrs":{"text":"L09805","term_id":"177519"}}L09805], demonstrating the theoretical mechanism of this link. In this article, we present an analysis based on convergent cross mapping, which uses observational time series data to directly examine the causal link between CR and year-to-year changes in global temperature. Despite a gross correlation, we find no measurable evidence of a causal effect linking CR to the overall 20th-century warming trend. However, on short interannual timescales, we find a significant, although modest, causal effect between CR and short-term, year-to-year variability in global temperature that is consistent with the presence of nonlinearities internal to the system. Thus, although CR do not contribute measurably to the 20th-century global warming trend, they do appear as a nontraditional forcing in the climate system on short interannual timescales.The basic principles behind a possible connection between galactic cosmic rays (CR) and global temperature (GT) are as follows: It has been known since the invention of the cloud chamber in 1911 by Charles Thomson Rees Wilson that ionizing radiation leads to atmospheric cloud nucleation. Although the prime source of ionizing radiation in the global troposphere is CR, the flux of CR reaching the troposphere depends on the solar wind. The solar wind is a stream of ionized gases that blows outward from the Sun, and its intensity varies strongly with the level of surface activity on the Sun. The Earth''s magnetic field shields the planet from much of the solar wind, deflecting that wind like water around the bow of a ship. When solar activity is great, the solar wind is strong, swiping away CR arriving at the top of the atmosphere. These CR are hypothesized to affect cloud formation and cloudiness, and therefore GT. The net radiative effect of cloudiness depends on the difference between incoming solar radiation and outgoing long-wave radiation. Increased cloudiness in the upper troposphere reduces outgoing long-wave radiation, thereby resulting in warming of the planet. Increased cloudiness in the lower troposphere causes less incoming radiation, and therefore cooling of the planet. Data suggest (6) that the amount of CR is positively correlated with the amount of low-level clouds but has no effect on middle- or high-level clouds. Although this is still an open question (7, 8), the reduction in flux in CR in times of high solar activity is hypothesized to result in less cloud nucleation and fewer cloud condensation nuclei, and consequently, reduced low-level cloud amounts. This, in turn, leads to a higher solar radiation flux at the Earth’s surface, and warmer temperatures. Conversely, a weaker solar wind results in cooler temperatures. The actual chemical processes and reactions involved in this problem are complex, but a growing body of experimental and theoretical work has uncovered a chemical pathway by which CR ionization may increase nucleation rates to levels appropriate for cloud condensation nuclei (2–5, 9–11 and references therein). This suggests a superficially simple network linking the Sun, CR, and global climate, with the interaction between the Sun and CR having a potential influence on the climate system. However reasonable this may be, as described in a 2006 review (12), “The suggested mechanisms are, however, too complex to evaluate meaningfully at present.”     

Atmospheric brown clouds: impacts on South Asian climate and hydrological cycle

South Asian emissions of fossil fuel SO(2) and black carbon increased approximately 6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled ocean-atmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.     

Potential for perceived failure of stratospheric aerosol injection deployment

As anthropogenic activities warm the Earth, the fundamental solution of reducing greenhouse gas emissions remains elusive. Given this mitigation gap, global warming may lead to intolerable climate changes as adaptive capacity is exceeded. Thus, there is emerging interest in solar radiation modification, which is the process of deliberately increasing Earth’s albedo to cool the planet. Stratospheric aerosol injection (SAI)—the theoretical deployment of particles in the stratosphere to enhance reflection of incoming solar radiation—is one strategy to slow, pause, or reverse global warming. If SAI is ever pursued, it will likely be for a specific aim, such as affording time to implement mitigation strategies, lessening extremes, or reducing the odds of reaching a biogeophysical tipping point. Using an ensemble climate model experiment that simulates the deployment of SAI in the context of an intermediate greenhouse gas trajectory, we quantified the probability that internal climate variability masks the effectiveness of SAI deployment on regional temperatures. We found that while global temperature was stabilized, substantial land areas continued to experience warming. For example, in the SAI scenario we explored, up to 55% of the global population experienced rising temperatures over the decade following SAI deployment and large areas exhibited high probability of extremely hot years. These conditions could cause SAI to be perceived as a failure. Countries with the largest economies experienced some of the largest probabilities of this perceived failure. The potential for perceived failure could therefore have major implications for policy decisions in the years immediately following SAI deployment.

Anthropogenic climate change, primarily driven by increasing concentrations of atmospheric greenhouse gasses, has caused Earth’s global mean temperature to reach its warmest level in at least the last 2,000 y (1). This global warming may exceed 1.5 °C above preindustrial temperatures later this decade, at least for a short period of time, and most years are likely to exceed the 1.5 °C threshold by 2040 across a range of emissions scenarios (1). By the middle of this century (2041–2060), warming in excess of 2.0 °C would be reached under intermediate, high, and very high emission scenarios (1), and current policies have the world on track to warm by roughly 3.0 °C by the end of the century (2). Moreover, emissions scenarios that target global temperature stabilization at either 1.5 or 2.0 °C require net-zero carbon emissions trajectories, which in practice will necessitate new and enormously scaled-up carbon dioxide removal technology (3).In parallel with global policy shortfalls, current levels of warming are driving substantial impacts on human and natural systems (4). For example, climate change is already leading to intensification of extreme events such as extreme heat, heavy rainfall, intense droughts, extreme wildfire weather, and marine heat waves (4). These and other climate changes are leading to a broad suite of impacts, such as migration of ecological niches (5), increases in global tree mortality (6), increases in financial losses from extremes (e.g., 7), and amplification of existing economic inequality (8) and social injustices (9). Furthermore, there is the possibility that biogeophysical tipping points may lead to new states in key Earth systems, such as irreversible Antarctic ice loss, tropical rainforest dieback, and slowing ocean circulations (10). These so-called tipping points are highly uncertain—in terms of whether, when, and how they may occur (1). Despite this uncertainty, there is paleoclimate evidence that tipping points have been crossed in the past, and emerging evidence suggests that they could be crossed as a result of anthropogenic change (11–13).To possibly grant humanity additional time to sufficiently reduce greenhouse gas emissions, lessen the existing negative impacts of climate change, and avoid transgression of irreversible tipping points, there is renewed interest in developing an international research agenda on solar radiation modification (SRM)—a speculative form of climate change response that has the potential to offset human-induced warming by reflecting a small amount of solar energy back to space before it enters and warms the planetary environment (14).There are numerous challenges for advancing SRM science and research. First, there are substantial ethical questions concerned with committing future generations to an uncertain technology and the potential burden of continuing climate intervention well into the future (15) or deciding when and how to ramp down SRM deployment (16–19). Second, there are important concerns related to how climate intervention may drive changes in essential Earth system processes (20, 21). Third, there are concerns that the negative consequences arising from SRM would disproportionately burden populations that are systematically already burdened by climate change impacts, global dispossession of resources, and wealth inequality (22, 23). Research investigating public opinion has found considerable heterogeneity in attitudes toward either research or use of climate intervention (24).In addition to these social challenges, there exist basic scientific questions about how to distinguish the climate effects of SRM from anomalies driven by internal variability of the Earth system (25, 26). This variability can lead to substantial short-term variation in socially relevant climate phenomena, such as the frequency of extreme hot and cold spells (27), the severity of drought (28), the path of the midlatitude storm tracks (29), changes in regional temperature and precipitation (30), the state of Arctic Sea ice (31), or the strength of tropical modes of variability such as the El Niño Southern Oscillation (32) or the Madden-Julian Oscillation (33). Research on the interaction between human-induced climate impacts, or “signals,” and internal climate variability, or “noise,” is a critical area of climate change science, not least for supporting policymakers and the public in navigating the expectations of climate change action against a backdrop of an internally varying climate system (34).Stratospheric aerosol injection (SAI) is the SRM strategy of releasing particles into the stratosphere to slow, pause, or reverse global warming (35). While climate simulations provide evidence that the long-term result of SAI could lead to stabilized global temperatures (17), the impacts of SAI may be regionally heterogeneous, with temperature and precipitation varying considerably (36–39). Moreover, internal climatic variability may mask the short-term perceived effectiveness of SAI; that is, it is possible that while SAI could successfully stabilize mean global temperatures, the perceived effectiveness on regional scales may be overwhelmed by local climatic variability over the short term. Psychologically, a climate change–related event connects to people’s perceptions most clearly when it is directly and locally relevant (40, 41). Moreover, people who are residents of a specific location may tacitly incorporate 10-y trends in their perception of changes in climate (42). Hence, local changes in climate—such as continued warming or the occurrence of extreme events—may cause climate interventions such as SAI to be perceived as a failure. Given the potential for SAI to abruptly cease and the likelihood of rapid climate change following such cessation (e.g., 19, 43), the perception of failure carries particular risks.If SRM is ever pursued, it will likely be for a specific social or geophysical aim (22). This may include halting an anticipated geophysical tipping point [such as accelerated Antarctic ice loss (44), permafrost melting, or forest die-off] or lessening the impacts of extremes such as deadly heat waves in large population centers (45). Yet, if climate variability were to mask the short-term perceived effectiveness of climate intervention, it could undermine coordinated, international policy action to address climate change broadly (46). Understanding the masking effects of climate variability on regional scales will thus be critical for interpreting the potential perceived success of any SRM strategy in the immediate years following deployment.To systematically distinguish the different possible outcomes associated with the masking effect of internal climate variability, we introduce a set of archetypal regional responses that could unfold under SAI. These archetypes are motivated by the fact that in the period prior to SAI deployment, a given region could be warming or not due to internal climate variability, even in the context of global-scale warming (47). Similarly, following deployment, that region could either experience warming or not, even if the global temperature is stabilized. Thus, we defined four archetypes of perceived success of climate intervention based on four categories of pre- and postdeployment experience: 1) Rebound Warming (i.e., no warming followed by warming); 2) Continued Warming (i.e., warming followed by more warming); 3) Stabilization (i.e., no warming either before or after deployment); and 4) Recovery (i.e., warming followed by no warming). The phenomena Rebound Warming and Continued Warming could both be locally perceived as a failure of SAI to deliver on its intended purpose; hence, throughout the rest of this work, the phrase “perceived failure” refers to the combination of these two archetypes.Past research into global SRM strategies employed climate or Earth system models to simulate how the natural system might respond to different intervention approaches (48). Here, we leveraged just one of them: the Assessing Responses and Impacts of SRM on the Earth system with Stratospheric Aerosol Injection (ARISE-SAI) ensemble carried out with the Community Earth System Model, version 2 (CESM2) (49). ARISE-SAI simulates a plausible deployment of SAI, designed to hold global mean temperature at 1.5 °C above preindustrial conditions in the context of the Shared Socioeconomic Pathway 2 (SSP2)-4.5 future emissions scenario (Fig. 1A) (49). Extending out to the year 2069, ARISE-SAI includes 10 ensemble members, each initiated from slightly different initial conditions to enable quantification of the irreducible uncertainty arising from internal climate variability (e.g., 50). The 1.5 °C threshold is relevant for global policy discourse in part because this is a global mean temperature increase that is considered both an important Earth system threshold as well as a key focus of global climate policy negotiations enshrined in the United Nations’ Paris Agreement (51). The fact that ARISE-SAI simulates SAI deployment that stabilizes global temperature at 1.5 °C while also representing the effect of internal variability via a substantial number of ensemble members makes ARISE-SAI a useful testbed for probing the possibility of perceived failure of climate intervention.Open in a separate windowFig. 1.Surface temperature trends. (A) Global mean surface temperature. Gray lines denote individual ensemble members, and the black line denotes the ensemble mean. (B and C) Ensemble-mean trends over years 2015 to 2034 under SSP2-4.5 (B) and 2035 to 2069 (C) with ARISE-SAI deployment. (D and E) Trends over the predeployment decade (D) and postdeployment decade (E) for ensemble member #9. (B–D) The percentage in the bottom of the maps denotes the percentage of land area that exhibited warming trends as defined in the text.     

Equatorial decline of reef corals during the last Pleistocene interglacial

The Last Interglacial (LIG; ca. 125,000 y ago) resulted from rapid global warming and reached global mean temperatures exceeding those of today. The LIG thus offers the opportunity to study how life may respond to future global warming. Using global occurrence databases and applying sampling-standardization, we compared reef coral diversity and distributions between the LIG and modern. Latitudinal diversity patterns are characterized by a tropical plateau today but were characterized by a pronounced equatorial trough during the LIG. This trough is governed by substantial range shifts away from the equator. Range shifts affected both leading and trailing edges of species range limits and were much more pronounced in the Northern Hemisphere than south of the equator. We argue that interglacial warming was responsible for the loss of equatorial diversity. Hemispheric differences in insolation during the LIG may explain the asymmetrical response. The equatorial retractions are surprisingly strong given that only small temperature changes have been reported in the LIG tropics. Our results suggest that the poleward range expansions of reef corals occurring with intensified global warming today may soon be followed by equatorial range retractions.     

From the Cover: Anthropogenic warming has increased drought risk in California

California is currently in the midst of a record-setting drought. The drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. The extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. We find that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. In addition, the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Climate model experiments with and without anthropogenic forcings reveal that human activities have increased the probability that dry precipitation years are also warm. Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼100% probability that any annual-scale dry period is also extremely warm. We therefore conclude that anthropogenic warming is increasing the probability of co-occurring warm–dry conditions like those that have created the acute human and ecosystem impacts associated with the “exceptional” 2012–2014 drought in California.The state of California is the largest contributor to the economic and agricultural activity of the United States, accounting for a greater share of population (12%) (1), gross domestic product (12%) (2), and cash farm receipts (11%) (3) than any other state. California also includes a diverse array of marine and terrestrial ecosystems that span a wide range of climatic tolerances and together encompass a global biodiversity “hotspot” (4). These human and natural systems face a complex web of competing demands for freshwater (5). The state’s agricultural sector accounts for 77% of California water use (5), and hydroelectric power provides more than 9% of the state’s electricity (6). Because the majority of California’s precipitation occurs far from its urban centers and primary agricultural zones, California maintains a vast and complex water management, storage, and distribution/conveyance infrastructure that has been the focus of nearly constant legislative, legal, and political battles (5). As a result, many riverine ecosystems depend on mandated “environmental flows” released by upstream dams, which become a point of contention during critically dry periods (5).California is currently in the midst of a multiyear drought (7). The event encompasses the lowest calendar-year and 12-mo precipitation on record (8), and almost every month between December 2011 and September 2014 exhibited multiple indicators of drought (Fig. S1). The proximal cause of the precipitation deficits was the recurring poleward deflection of the cool-season storm track by a region of persistently high atmospheric pressure, which steered Pacific storms away from California over consecutive seasons (8–11). Although the extremely persistent high pressure is at least a century-scale occurrence (8), anthropogenic global warming has very likely increased the probability of such conditions (8, 9).Despite insights into the causes and historical context of precipitation deficits (8–11), the influence of historical temperature changes on the probability of individual droughts has—until recently—received less attention (12–14). Although precipitation deficits are a prerequisite for the moisture deficits that constitute “drought” (by any definition) (15), elevated temperatures can greatly amplify evaporative demand, thereby increasing overall drought intensity and impact (16, 17). Temperature is especially important in California, where water storage and distribution systems are critically dependent on winter/spring snowpack, and excess demand is typically met by groundwater withdrawal (18–20). The impacts of runoff and soil moisture deficits associated with warm temperatures can be acute, including enhanced wildfire risk (21), land subsidence from excessive groundwater withdrawals (22), decreased hydropower production (23), and damage to habitat of vulnerable riparian species (24).Recent work suggests that the aggregate combination of extremely high temperatures and very low precipitation during the 2012–2014 event is the most severe in over a millennium (12). Given the known influence of temperature on drought, the fact that the 2012–2014 record drought severity has co-occurred with record statewide warmth (7) raises the question of whether long-term warming has altered the probability that precipitation deficits yield extreme drought in California.     

Strong relationship between elemental stoichiometry and metabolome in plants

Shifts in the elemental stoichiometry of organisms in response to their ontogeny and to changing environmental conditions should be related to metabolomic changes because elements operate mostly as parts of molecular compounds. Here we show this relationship in leaves of Erica multiflora throughout their seasonal development and in response to moderate experimental field conditions of drought and warming. The N/P ratio in leaves decreased in the metabolically active growing seasons, coinciding with an increase in the content of primary metabolites. These results support the growth-rate hypothesis that states that rapidly growing organisms present low N/P ratios because of the increase in allocation of P to RNA. The foliar N/K and P/K ratios were lower in summer and in the drought treatment, in accordance with the role of K in osmotic protection, and coincided with the increase of compounds related to the avoidance of water stress. These results provide strong evidence of the relationship between the changes in foliar C/N/P/K stoichiometry and the changes in the leaf's metabolome during plant growth and environmental stress. Thus these results represent a step in understanding the relationships between stoichiometry and an organism's lifestyle.     

Nonrandom mating in Drosophila melanogaster laboratory populations derived from closely adjacent ecologically contrasting slopes at "Evolution Canyon"

Ecological differentiation of natural populations of Drosophila melanogaster, Drosophila simulans, and another drosophilid, Zaprionus tuberculatus, in "Evolution Canyon," Mount Carmel, Israel, is well established. The fitness complex of D. melanogaster includes oviposition temperature preferences, tolerance to high temperature, drought stress and starvation, and different longevity patterns. This remarkable differentiation has evolved despite small interslope distances (only 100-400 m), within easy dispersal distance. The differences between populations are those expected from genetic adaptation to local microclimates. How such differentiation could evolve and be maintained despite the likelihood of genetic exchange between populations is a challenging question. We hypothesized that interslope microclimatic differences caused strong differential selection for stress tolerance, accompanied by behavioral differentiation (habitat choice and reduced migration rate), reinforced by sexual isolation. Here we report highly significant mate choice by flies from different slopes of the canyon, with preference for sexual partners originating from the same slope. No preferences were found when the sexual partners belonged to different isofemale lines from the same slope.     

Climate change in the Fertile Crescent and implications of the recent Syrian drought

Before the Syrian uprising that began in 2011, the greater Fertile Crescent experienced the most severe drought in the instrumental record. For Syria, a country marked by poor governance and unsustainable agricultural and environmental policies, the drought had a catalytic effect, contributing to political unrest. We show that the recent decrease in Syrian precipitation is a combination of natural variability and a long-term drying trend, and the unusual severity of the observed drought is here shown to be highly unlikely without this trend. Precipitation changes in Syria are linked to rising mean sea-level pressure in the Eastern Mediterranean, which also shows a long-term trend. There has been also a long-term warming trend in the Eastern Mediterranean, adding to the drawdown of soil moisture. No natural cause is apparent for these trends, whereas the observed drying and warming are consistent with model studies of the response to increases in greenhouse gases. Furthermore, model studies show an increasingly drier and hotter future mean climate for the Eastern Mediterranean. Analyses of observations and model simulations indicate that a drought of the severity and duration of the recent Syrian drought, which is implicated in the current conflict, has become more than twice as likely as a consequence of human interference in the climate system.Beginning in the winter of 2006/2007, Syria and the greater Fertile Crescent (FC), where agriculture and animal herding began some 12,000 years ago (1), experienced the worst 3-year drought in the instrumental record (2). The drought exacerbated existing water and agricultural insecurity and caused massive agricultural failures and livestock mortality. The most significant consequence was the migration of as many as 1.5 million people from rural farming areas to the peripheries of urban centers (3, 4). Characterizing risk as the product of vulnerability and hazard severity, we first analyze Syria’s vulnerability to drought and the social impacts of the recent drought leading to the onset of the Syrian civil war. We then use observations and climate models to assess how unusual the drought was within the observed record and the reasons it was so severe. We also show that climate models simulate a long-term drying trend for the region as a consequence of human-induced climate change. If correct, this has increased the severity and frequency of occurrence of extreme multiyear droughts such as the recent one. We also present evidence that the circulation anomalies associated with the recent drought are consistent with model projections of human-induced climate change and aridification in the region and are less consistent with patterns of natural variability.     

Observational evidence that cloud feedback amplifies global warming

Global warming drives changes in Earth’s cloud cover, which, in turn, may amplify or dampen climate change. This “cloud feedback” is the single most important cause of uncertainty in Equilibrium Climate Sensitivity (ECS)—the equilibrium global warming following a doubling of atmospheric carbon dioxide. Using data from Earth observations and climate model simulations, we here develop a statistical learning analysis of how clouds respond to changes in the environment. We show that global cloud feedback is dominated by the sensitivity of clouds to surface temperature and tropospheric stability. Considering changes in just these two factors, we are able to constrain global cloud feedback to 0.43 ± 0.35 W⋅m⁻²⋅K⁻¹ (90% confidence), implying a robustly amplifying effect of clouds on global warming and only a 0.5% chance of ECS below 2 K. We thus anticipate that our approach will enable tighter constraints on climate change projections, including its manifold socioeconomic and ecological impacts.

Clouds have long been recognized as the leading source of uncertainty in Earth’s climate response to anthropogenic forcing through their key role in modulating the global energy balance. While a combined assessment of all available lines of evidence—theory, modeling, and Earth observations—suggests that cloud feedback is likely positive, i.e., amplifies global warming (1–3), so far, a narrow constraint on this feedback has remained elusive. This is reflected in the broad 90% CI for cloud feedback (−0.09 to +0.99 W⋅m⁻²⋅K⁻¹) estimated in a recent assessment under the auspices of the World Climate Research Program [WCRP (3)], which relied both on a review of existing studies and expert judgment. Part of the challenge stems from the variety of physical processes contributing to the net cloud feedback, involving the interaction of clouds with both solar (shortwave [SW]) and terrestrial (longwave [LW]) radiative fluxes (4).Uncertainty in cloud feedback has persisted because each line of evidence comes with its limitations and challenges. Theory cannot provide precise projections. Global climate models (GCMs) are unable to explicitly represent small-scale cloud processes on their coarse spatial grids, resulting in large spread in their simulation of cloud feedback (4, 5). High-resolution models may better represent such cloud processes, but limitations in computational power prevent climate change experiments on global grids (6). Most of the available observational estimates of cloud feedback are restricted to particular regions and circulation regimes, such as tropical subsidence regions (7–11) or extratropical mixed-phase clouds (12, 13), and are uncertain, owing to the short satellite record of global cloud-radiative measurements and the numerous, covarying meteorological factors controlling clouds.     

Rapid responses of soil microorganisms improve plant fitness in novel environments

Global change is challenging plant and animal populations with novel environmental conditions, including increased atmospheric CO(2) concentrations, warmer temperatures, and altered precipitation regimes. In some cases, contemporary or "rapid" evolution can ameliorate the effects of global change. However, the direction and magnitude of evolutionary responses may be contingent upon interactions with other community members that also are experiencing novel environmental conditions. Here, we examine plant adaptation to drought stress in a multigeneration experiment that manipulated aboveground-belowground feedbacks between plants and soil microbial communities. Although drought stress reduced plant growth and accelerated plant phenologies, surprisingly, plant evolutionary responses to drought were relatively weak. In contrast, plant fitness in both drought and nondrought environments was linked strongly to the rapid responses of soil microbial community structure to moisture manipulations. Specifically, plants were most fit when their contemporary environmental conditions (wet vs. dry soil) matched the historical environmental conditions (wet vs. dry soil) of their associated microbial community. Together, our findings suggest that, when faced with environmental change, plants may not be limited to "adapt or migrate" strategies; instead, they also may benefit from association with interacting species, especially diverse soil microbial communities, that respond rapidly to environmental change.     

Carbon choices determine US cities committed to futures below sea level

Anthropogenic carbon emissions lock in long-term sea-level rise that greatly exceeds projections for this century, posing profound challenges for coastal development and cultural legacies. Analysis based on previously published relationships linking emissions to warming and warming to rise indicates that unabated carbon emissions up to the year 2100 would commit an eventual global sea-level rise of 4.3–9.9 m. Based on detailed topographic and population data, local high tide lines, and regional long-term sea-level commitment for different carbon emissions and ice sheet stability scenarios, we compute the current population living on endangered land at municipal, state, and national levels within the United States. For unabated climate change, we find that land that is home to more than 20 million people is implicated and is widely distributed among different states and coasts. The total area includes 1,185–1,825 municipalities where land that is home to more than half of the current population would be affected, among them at least 21 cities exceeding 100,000 residents. Under aggressive carbon cuts, more than half of these municipalities would avoid this commitment if the West Antarctic Ice Sheet remains stable. Similarly, more than half of the US population-weighted area under threat could be spared. We provide lists of implicated cities and state populations for different emissions scenarios and with and without a certain collapse of the West Antarctic Ice Sheet. Although past anthropogenic emissions already have caused sea-level commitment that will force coastal cities to adapt, future emissions will determine which areas we can continue to occupy or may have to abandon.Most studies on the projected impacts of anthropogenic climate change have focused on the 21st century (1). However, substantial research indicates that contemporary carbon emissions, even if stopped abruptly, will sustain or nearly sustain near-term temperature increases for millennia because of the long residence time of carbon dioxide in the atmosphere and inertia in the climate system, e.g., the slow exchange of heat between ocean and atmosphere (2–5). Earth system and carbon-cycle feedbacks such as the release of carbon from thawing permafrost or vegetation changes affecting terrestrial carbon storage or albedo may further extend and possibly amplify warming (6).Paleontological records indicate that global mean sea level is highly sensitive to temperature (7) and that ice sheets, the most important contributors to large-magnitude sea-level change, can respond to warming on century time scales (8), while models suggest ice sheets require millennia to approach equilibrium (9). Accordingly, sustained temperature increases from current emissions are expected to translate to long-term sea-level rise (SLR). Through modeling and with support from paleontological data, Levermann et al. (10) found a roughly linear global mean sea-level increase of 2.3 m per 1 °C warming within a time-envelope of the next 2,000 y.This relationship forecasts a profound challenge in light of warming likely to exceed 2 °C given the current path of emissions (11). Although relatively modest in comparison, projected SLR of up to 1.2 m this century has been estimated to threaten up to 4.6% of the global population and 9.3% of annual global gross domestic product with annual flooding by 2100 in the absence of adaptive measures (12). Higher long-term sea levels endanger a fifth of all United Nations Educational, Scientific and Cultural Organization world heritage sites (13). These global analyses depend on elevation data with multimeter rms vertical errors that consistently overestimate elevation and thus underestimate submergence risk (14). Here we explore the challenges posed under different scenarios by long-term SLR in the United States, where highly accurate elevation and population data permit robust exposure assessments (15, 16).Our analysis combines published relationships between cumulative carbon emissions and warming, together with two possible versions of the relationship between warming and sea level, to estimate global and regional sea-level commitments from different emissions totals. The first version, the “baseline” case, employs a minor modification of the warming–SLR relationship from Levermann et al. (10) The second version, the “triggered” case, makes a major adjustment to explore an important possibility suggested by recent research, by assuming that an inevitable collapse of the West Antarctic Ice Sheet (WAIS) already has been set in motion (17–19).For each case, we then use topographic, tidal, and census data to assess the contemporary populations living on implicated land nationwide, by state and by municipality. Although current populations will not experience full, long-term SLR, we use their exposure as a proxy for the challenge facing the more enduring built environment and the cultural and economic activity it embodies, given the strong spatial correlation between population and development. We focus most on cities, identifying and tabulating municipalities where committed sea levels would set land that is home to more than half (or other fractions) of the current population below the high tide line.By “committed” or “locked in” warming or sea level in a given year, we refer to the long-term effects of cumulative anthropogenic carbon emissions through that year: the sustained temperature increase or SLR that will ensue on a time scale of centuries to millennia in the absence of massive and prolonged future active carbon removal from the atmosphere. We call a city “committed” when sea-level commitments would affect land supporting more than half of its current population (or another percentage of the population, if specified). We assume zero future emissions when assessing commitments for a given year, with the exception of one analysis incorporating future emissions implied by current energy infrastructure. When we associate years with warming, sea level, and city commitments, we are referencing the 21st century years when the commitments are established through cumulative emissions, not the years farther in the future when the commitments are realized through sustained temperature increases and SLR.     

Tree diversity does not always improve resistance of forest ecosystems to drought

Climate models predict an increase in the intensity and frequency of drought episodes in the Northern Hemisphere. Among terrestrial ecosystems, forests will be profoundly impacted by drier climatic conditions, with drastic consequences for the functions and services they supply. Simultaneously, biodiversity is known to support a wide range of forest ecosystem functions and services. However, whether biodiversity also improves the resistance of these ecosystems to drought remains unclear. We compared soil drought exposure levels in a total of 160 forest stands within five major forest types across Europe along a gradient of tree species diversity. We assessed soil drought exposure in each forest stand by calculating the stand-level increase in carbon isotope composition of late wood from a wet to a dry year (Δδ¹³C_S). Δδ¹³C_S exhibited a negative linear relationship with tree species diversity in two forest types, suggesting that species interactions in these forests diminished the drought exposure of the ecosystem. However, the other three forest types were unaffected by tree species diversity. We conclude that higher diversity enhances resistance to drought events only in drought-prone environments. Managing forest ecosystems for high tree species diversity does not necessarily assure improved adaptability to the more severe and frequent drought events predicted for the future.Biodiversity plays an important role in ecosystem functioning by promoting a wide range of functions and services (1–3). This beneficial effect of biodiversity is determined by mechanistic processes directly under the influence of species interactions: complementarity among species for resource use through ecological niche partitioning and/or facilitation processes increase ecosystem performance because resources are better shared among neighboring species and are thus potentially more available (4). Previous studies have demonstrated that, apart from enhancing performance, diverse terrestrial ecosystems may also be more resilient and more resistant to biotic stresses such as insect pests or diseases (5, 6). However, it remains unclear whether higher biodiversity also leads to improved resistance of terrestrial ecosystems to the more frequent droughts expected in temperate regions (7). The rare case studies published thus far have shown contrasting results. Two reported that species in more diverse ecosystems could be more resistant to drought stress (8, 9), whereas another suggested that enhanced biodiversity could trigger higher exposure to drought (10). Improving our understanding of how species diversity influences the resistance of terrestrial ecosystems to a fluctuating climate is crucial.More frequent and intense droughts will greatly affect the carbon and water cycles of the terrestrial biosphere (11), in particular in forested ecosystems (12). Many societies around the world rely on forests for essential services such as wood production, hunting, or watershed protection. We therefore urgently need to improve our knowledge of the physiological response of these ecosystems to drier climatic conditions to propose new climate-smart management options. Forests, although influenced by local environmental conditions, play a major role in the global carbon and water balance as they release into and assimilate from the atmosphere huge amounts of CO₂ while losing water vapor through transpiration. Tree species are known to vary widely in the ecological strategies they use to deal with drought stress. It could therefore be expected that in highly diverse forests composed of tree species with contrasting functional traits, limited water resources could be better partitioned among the neighboring species as a result of complementarity and facilitation processes (4). Such forests should be more resistant to deal with drought stress because the trees should be able to maintain better access to diminishing water resources as the drought progresses. In contrast, if the interacting species in a diverse forest have similar functional traits (i.e., functional redundancy), ecological niche overlap (13) may lead to more stressful conditions during drought than in pure stands due to lower water availability for each species.Carbon isotope composition in C₃ plant tissues (δ¹³C) provides an integrated record of the ratio of intercellular to atmospheric CO₂ concentrations during the period when the carbon was fixed and thus reflects the balance between net CO₂ assimilation and stomatal conductance (14). Plants typically react toward drought stress by closing their stomata and reducing carbon assimilation rates. However, leaf stomatal conductance is affected to a greater extent than assimilation, causing a concomitant increase in δ¹³C (14, 15). Therefore, under soil drought conditions, δ¹³C from organic material has been widely accepted as an indicator of the intensity of drought exposure in plants (16, 17) (SI Text). If complementarity for water use is occurring among species, δ¹³C values should increase less between wet and dry soil conditions with increasing tree species diversity (i.e., a negative relationship). Inversely, if tree species occupy redundant ecological niches, δ¹³C values should either have a similar or higher increase between wet and dry conditions with increasing tree species diversity (i.e., a null or positive relationship).In a previous study, we analyzed the influence of drought on the relationship between tree species diversity and the increase in stand-level carbon isotope composition between a wet and dry year (Δδ¹³C_S) in boreal forests (10). Species diverse forests were shown to be more affected by drought stress than less diverse ones (i.e., a positive relationship between Δδ¹³C_S and tree species diversity). In the present study, we extend our research to five major forest types across Europe, which extends from northern hemiboreal forests to southern Mediterranean forests (Table S1). Our objective was to test whether the relationship between Δδ¹³C_S and tree species diversity would be consistent across a large range of climatic and edaphic conditions. At each of the five study sites, we selected a set of representative canopy trees (Table S2) in 21–42 forest stands varying in tree species diversity. For each site, we used a water balance modeling approach to select 1 y within the last 14 y with high drought stress and 1 reference y when no drought occurred (Figs. S1 and S2). We measured the δ¹³C in the tree rings of the selected canopy trees and calculated Δδ¹³C_S for each stand.     

Derivation of burn scar depths and estimation of carbon emissions with LIDAR in Indonesian peatlands

During the 1997/98 El Niño-induced drought peatland fires in Indonesia may have released 13–40% of the mean annual global carbon emissions from fossil fuels. One major unknown in current peatland emission estimations is how much peat is combusted by fire. Using a light detection and ranging data set acquired in Central Kalimantan, Borneo, in 2007, one year after the severe peatland fires of 2006, we determined an average burn scar depth of 0.33 ± 0.18 m. Based on this result and the burned area determined from satellite imagery, we estimate that within the 2.79 million hectare study area 49.15 ± 26.81 megatons of carbon were released during the 2006 El Niño episode. This represents 10–33% of all carbon emissions from transport for the European Community in the year 2006. These emissions, originating from a comparatively small area (approximately 13% of the Indonesian peatland area), underline the importance of peat fires in the context of green house gas emissions and global warming. In the past decade severe peat fires occurred during El Niño-induced droughts in 1997, 2002, 2004, 2006, and 2009. Currently, this important source of carbon emissions is not included in IPCC carbon accounting or in regional and global carbon emission models. Precise spatial measurements of peat combusted and potential avoided emissions in tropical peat swamp forests will also be required for future emission trading schemes in the framework of Reduced Emissions from Deforestation and Degradation in developing countries.     

Biological consequences of earlier snowmelt from desert dust deposition in alpine landscapes

Dust deposition to mountain snow cover, which has increased since the late 19^th century, accelerates the rate of snowmelt by increasing the solar radiation absorbed by the snowpack. Snowmelt occurs earlier, but is decoupled from seasonal warming. Climate warming advances the timing of snowmelt and early season phenological events (e.g., the onset of greening and flowering); however, earlier snowmelt without warmer temperatures may have a different effect on phenology. Here, we report the results of a set of snowmelt manipulations in which radiation-absorbing fabric and the addition and removal of dust from the surface of the snowpack advanced or delayed snowmelt in the alpine tundra. These changes in the timing of snowmelt were superimposed on a system where the timing of snowmelt varies with topography and has been affected by increased dust loading. At the community level, phenology exhibited a threshold response to the timing of snowmelt. Greening and flowering were delayed before seasonal warming, after which there was a linear relationship between the date of snowmelt and the timing of phenological events. Consequently, the effects of earlier snowmelt on phenology differed in relation to topography, which resulted in increasing synchronicity in phenology across the alpine landscape with increasingly earlier snowmelt. The consequences of earlier snowmelt from increased dust deposition differ from climate warming and include delayed phenology, leading to synchronized growth and flowering across the landscape and the opportunity for altered species interactions, landscape-scale gene flow via pollination, and nutrient cycling.     

Tropical nighttime warming as a dominant driver of variability in the terrestrial carbon sink

The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO₂-driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50–100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatures, which are projected to increase faster than global average temperatures, suggests that C stored in tropical forests may be vulnerable to future warming.Terrestrial ecosystems have been a substantial net sink of anthropogenic carbon (C) emissions since the 1960s (1–4), but the terrestrial C sink could switch to a C source in the 21st century, resulting in a positive C cycle-climate feedback that would accelerate global surface warming with potentially major consequences for the biosphere (5–7). The interannual variability of the terrestrial C sink can help constrain our understanding of C/climate feedbacks and identify regions and mechanisms of the terrestrial C cycle that are most sensitive to climate parameters, shedding light on the future of the sink and its possible transition to a source (8). Currently, several major drivers have been shown to be correlated with the interannual variability of the terrestrial C sink, including (i) tropical temperature, which is tightly coupled to interannual variability in the atmospheric growth rate (AGR) of CO₂ (8, 9); (ii) tropical drought stress, including major droughts in the Amazon (10–12), which has been suggested to underlie increasing sensitivity of the AGR to tropical temperature over the period from 1959–2010 (13); (iii) temperature and precipitation variability in semiarid regions (14, 15); and (iv) average minimum daily (hereafter “nighttime”) temperatures, which studies of several local field sites in the tropics have found play a major role in interannual productivity (16–18).Determining the mechanism underlying the interannual variability of the terrestrial C sink, including the relative roles of precipitation vs. temperature stress and their effects on gross primary productivity (GPP) vs. total respiration (both autotrophic and heterotrophic; R), is critical to predict the sink’s future and to improve Earth system models. Here, we quantify changes in the interannual variability of the terrestrial C sink over the past half-century and then statistically evaluate four hypotheses that the variability of the terrestrial sink is most strongly influenced by (i) tropical mean temperature, (ii) tropical precipitation, (iii) precipitation and temperature in semiarid regions, and (iv) nighttime tropical temperatures. We combine multiple simulations from an atmospheric mass balance of the land C sink [net ecosystem exchange (NEE)] from 1959 to 2010, remote sensing-modeled datasets of vegetation greenness and GPP from 1982 to 2010, and global gridded climate datasets to constrain globally the fundamental equation NEE = GPP − R and the relative sensitivities of each component to temperature and precipitation. We draw on a combination of model selection and partial correlation analysis to provide relative likelihood estimates of each driver and to account for covariation between predictor variables (e.g., tropical mean temperature vs. nighttime temperature).