Global fish production and climate change

Current global fisheries production of ≈160 million tons is rising as a result of increases in aquaculture production. A number of climate-related threats to both capture fisheries and aquaculture are identified, but we have low confidence in predictions of future fisheries production because of uncertainty over future global aquatic net primary production and the transfer of this production through the food chain to human consumption. Recent changes in the distribution and productivity of a number of fish species can be ascribed with high confidence to regional climate variability, such as the El Niño–Southern Oscillation. Future production may increase in some high-latitude regions because of warming and decreased ice cover, but the dynamics in low-latitude regions are governed by different processes, and production may decline as a result of reduced vertical mixing of the water column and, hence, reduced recycling of nutrients. There are strong interactions between the effects of fishing and the effects of climate because fishing reduces the age, size, and geographic diversity of populations and the biodiversity of marine ecosystems, making both more sensitive to additional stresses such as climate change. Inland fisheries are additionally threatened by changes in precipitation and water management. The frequency and intensity of extreme climate events is likely to have a major impact on future fisheries production in both inland and marine systems. Reducing fishing mortality in the majority of fisheries, which are currently fully exploited or overexploited, is the principal feasible means of reducing the impacts of climate change.     

Long-term natural variability and 20th century climate change

Global mean temperature at the Earth''s surface responds both to externally imposed forcings, such as those arising from anthropogenic greenhouse gases, as well as to natural modes of variability internal to the climate system. Variability associated with these latter processes, generally referred to as natural long-term climate variability, arises primarily from changes in oceanic circulation. Here we present a technique that objectively identifies the component of inter-decadal global mean surface temperature attributable to natural long-term climate variability. Removal of that hidden variability from the actual observed global mean surface temperature record delineates the externally forced climate signal, which is monotonic, accelerating warming during the 20th century.     

Long-term resistance to simulated climate change in an infertile grassland

Climate shifts over this century are widely expected to alter the structure and functioning of temperate plant communities. However, long-term climate experiments in natural vegetation are rare and largely confined to systems with the capacity for rapid compositional change. In unproductive, grazed grassland at Buxton in northern England (U.K.), one of the longest running experimental manipulations of temperature and rainfall reveals vegetation highly resistant to climate shifts maintained over 13 yr. Here we document this resistance in the form of: (i) constancy in the relative abundance of growth forms and maintained dominance by long-lived, slow-growing grasses, sedges, and small forbs; (ii) immediate but minor shifts in the abundance of several species that have remained stable over the course of the experiment; (iii) no change in productivity in response to climate treatments with the exception of reduction from chronic summer drought; and (iv) only minor species losses in response to drought and winter heating. Overall, compositional changes induced by 13-yr exposure to climate regime change were less than short-term fluctuations in species abundances driven by interannual climate fluctuations. The lack of progressive compositional change, coupled with the long-term historical persistence of unproductive grasslands in northern England, suggests the community at Buxton possesses a stabilizing capacity that leads to long-term persistence of dominant species. Unproductive ecosystems provide a refuge for many threatened plants and animals and perform a diversity of ecosystem services. Our results support the view that changing land use and overexploitation rather than climate change per se constitute the primary threats to these fragile ecosystems.     

Catalogue of abrupt shifts in Intergovernmental Panel on Climate Change climate models

Abrupt transitions of regional climate in response to the gradual rise in atmospheric greenhouse gas concentrations are notoriously difficult to foresee. However, such events could be particularly challenging in view of the capacity required for society and ecosystems to adapt to them. We present, to our knowledge, the first systematic screening of the massive climate model ensemble informing the recent Intergovernmental Panel on Climate Change report, and reveal evidence of 37 forced regional abrupt changes in the ocean, sea ice, snow cover, permafrost, and terrestrial biosphere that arise after a certain global temperature increase. Eighteen out of 37 events occur for global warming levels of less than 2°, a threshold sometimes presented as a safe limit. Although most models predict one or more such events, any specific occurrence typically appears in only a few models. We find no compelling evidence for a general relation between the overall number of abrupt shifts and the level of global warming. However, we do note that abrupt changes in ocean circulation occur more often for moderate warming (less than 2°), whereas over land they occur more often for warming larger than 2°. Using a basic proportion test, however, we find that the number of abrupt shifts identified in Representative Concentration Pathway (RCP) 8.5 scenarios is significantly larger than in other scenarios of lower radiative forcing. This suggests the potential for a gradual trend of destabilization of the climate with respect to such shifts, due to increasing global mean temperature change.The gradual rise in greenhouse gas concentrations is projected to drive a mostly smooth increase in global temperature (1). However, the Earth system is suspected to have a range of “tipping elements” with the characteristic that their gradual change will be punctuated by critical transitions on regional scales (2, 3). That is, for relatively small changes in atmospheric concentrations of greenhouse gases, parts of the Earth system exhibit major changes. The recent fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) presents a catalog of possible abrupt or irreversible changes (table 12.4 in ref. 4). This catalog builds on a previous literature review (2) of components believed to have the potential for an acceleration of change as fossil fuel burning changes atmospheric composition and thus radiative forcing.The expert elicitation (2) motivated discussion of a multitude of environmental threats to the planet in which it was critically argued that atmospheric carbon dioxide concentration should not cross 350 ppm (5), trying to determine what constitutes safe levels of global warming. This threshold was suggested in ref. 5 to minimize the risk due to massive sea ice change, sea level rise, or major changes to terrestrial ecosystems and crops. An alternative purely temperature-based threshold is that from the Copenhagen accord, setting an upper limit of 2° (6). However, major uncertainty exists in knowledge of climate sensitivity (7), which makes it difficult to relate this warming level to a precise CO₂ concentration. However, despite this and the growing interest in the societal effects of such transitions, there has been no systematic study of the potential for abrupt shifts in state-of-the-art Earth System Models.To explore what may be deduced from the current generation of climate models in this context, we analyze the simulations produced by Coupled Model Intercomparison Project 5 (CMIP5) (8) that were used to inform the IPCC. CMIP5 provides a compilation of coordinated climate model experiments. Each of 37 analyzed models includes representations of the oceans, atmosphere, land surface, and cryosphere. The climate models have been forced with future changes in atmospheric gas concentrations, depicted in four Representative Concentration Pathways (RCPs) (9), starting in year 2006. Of these, we analyze RCP2.6, RCP4.5, and RCP8.5 to explore a range of changes in radiative forcing, reaching levels of 2.6 W⋅m⁻², 4.5 W⋅m⁻², and 8.5 W⋅m⁻², respectively, by year 2100 (including all available simulations that go beyond 2100). We also analyze historical simulations, capturing changes from preindustrial conditions in year 1850 to the present, and preindustrial control simulations.To assess future risks of abrupt, potentially irreversible, changes in important climate phenomena, we first need to define what we mean by “abrupt.” This term clearly refers to time scale and is usually defined as when changes observed are faster than the time scale of the external forcing. Here we choose a methodology consisting of three stages. Firstly, we systematically screen the CMIP5 multimodel ensemble of simulations for evidence of abrupt changes using search criteria (Methods) to make a first filtering of regions of potentially relevant abrupt events from this dataset (stage 1). These criteria are motivated by the definition of the assessment report, AR5 (4): “A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.” Other definitions have emphasized the timescales of the change, e.g., 30 y (10), and rapidity in comparison with the forcing (11), which also meet our search criteria. Global maps of quantities with potential to change abruptly are expressed as (i) the mean difference between end and beginning of a simulation, (ii) the SD of the detrended time series, and (iii) the maximum absolute change within 10 y. These maps are made for all scenario runs and compared with values for the preindustrial control runs. When at least two indicators suggest locations of major change, we construct time series for area averages of at least 0.5 × 10⁶ km² (roughly 10 by 10 degrees) and visually inspect these for abrupt shifts standing out from the internal variability (stage 2). Subsequently, we check whether the selected cases can indeed be considered examples of abrupt change applying formal classification criteria (Methods) such as the criterion that the change should be larger than 4 times the SD of the preindustrial simulation, in combination with additional statistical tests (stage 3).We find a broad range of transitions passing our classification criteria (Fig. 1, SI Appendix, Table S1), which can be grouped into four categories (Fig. 2). They include abrupt shifts in sea ice and ocean circulation patterns as well as abrupt shifts in vegetation and the terrestrial cryosphere. Fig. 2 shows a selected example for each category. All other time series are displayed in Fig. 3. Information on the regions where the shifts occur and the results of the statistical tests used for classification are displayed in SI Appendix, Tables S2 and S3, respectively. A list of the climate models and their acronyms is provided in SI Appendix, Table S1.Open in a separate windowFig. 1.Geographical location of the abrupt climate change occurrences. All 30 model cases listed in

Growing impact of wildfire on western US water supply

Streamflow often increases after fire, but the persistence of this effect and its importance to present and future regional water resources are unclear. This paper addresses these knowledge gaps for the western United States (WUS), where annual forest fire area increased by more than 1,100% during 1984 to 2020. Among 72 forested basins across the WUS that burned between 1984 and 2019, the multibasin mean streamflow was significantly elevated by 0.19 SDs (P < 0.01) for an average of 6 water years postfire, compared to the range of results expected from climate alone. Significance is assessed by comparing prefire and postfire streamflow responses to climate and also to streamflow among 107 control basins that experienced little to no wildfire during the study period. The streamflow response scales with fire extent: among the 29 basins where >20% of forest area burned in a year, streamflow over the first 6 water years postfire increased by a multibasin average of 0.38 SDs, or 30%. Postfire streamflow increases were significant in all four seasons. Historical fire–climate relationships combined with climate model projections suggest that 2021 to 2050 will see repeated years when climate is more fire-conducive than in 2020, the year currently holding the modern record for WUS forest area burned. These findings center on relatively small, minimally managed basins, but our results suggest that burned areas will grow enough over the next 3 decades to enhance streamflow at regional scales. Wildfire is an emerging driver of runoff change that will increasingly alter climate impacts on water supplies and runoff-related risks.

Recent declines in soil moisture, streamflow, and reservoir storage signal the precariousness of water supplies in the western United States (WUS) and the urgency of managing associated risks (1, 2). Declining WUS water supplies are qualitatively consistent with modeled trends due to anthropogenic climate change (3, 4), but projections are uncertain due to not only climate but also the complexity of vegetation responses to climate change and associated disturbances such as wildfire (5–9). In addition to transpiration and interception, which directly divert moisture from runoff, vegetation also affects hydrology by shaping soil depth and structure and by modulating turbulent energy fluxes that alter snowpack and evaporation (10). In addition to direct effects on vegetation, wildfires can further affect streamflow by promoting water repellency and soil erosion (11–13). Given that the headwater areas of major WUS rivers are generally forested, altered forest cover or ecosystem water demand could potentially affect water resources at regional scales.In recent decades, the annual forest area burned in the WUS has risen rapidly, in step with climate trends toward warming and drying (14–20). In general, forest disturbances such as wildfire are known to temporarily enhance streamflow (21–23), although cases of postdisturbance streamflow declines, especially in arid areas, have also been documented (21, 24, 25). The likelihood that rapid increases in regional forest fire activity will continue (26, 27) suggests that wildfire may increasingly impact water resources in the water-limited WUS (6). Yet, the duration and seasonality of postdisturbance increases in runoff are unknown, raising the question of whether increased forest fire activity will meaningfully affect water availability in the WUS.Here we use stream gauge records from 179 river basins in the WUS to assess the strength, duration, and seasonality of postfire changes in streamflow and whether increasing forest fire activity is likely to have a detectable effect on regional streamflow.     

Association between climate variability and malaria epidemics in the East African highlands

The causes of the recent reemergence of Plasmodium falciparum epidemic malaria in the East African highlands are controversial. Regional climate changes have been invoked as a major factor; however, assessing the impact of climate in malaria resurgence is difficult due to high spatial and temporal climate variability and the lack of long-term data series on malaria cases from different sites. Climate variability, defined as short-term fluctuations around the mean climate state, may be epidemiologically more relevant than mean temperature change, but its effects on malaria epidemics have not been rigorously examined. Here we used nonlinear mixed-regression model to investigate the association between autoregression (number of malaria outpatients during the previous time period), seasonality and climate variability, and the number of monthly malaria outpatients of the past 10-20 years in seven highland sites in East Africa. The model explained 65-81% of the variance in the number of monthly malaria outpatients. Nonlinear and synergistic effects of temperature and rainfall on the number of malaria outpatients were found in all seven sites. The net variance in the number of monthly malaria outpatients caused by autoregression and seasonality varied among sites and ranged from 18 to 63% (mean=38.6%), whereas 12-63% (mean=36.1%) of variance is attributed to climate variability. Our results suggest that there was a high spatial variation in the sensitivity of malaria outpatient number to climate fluctuations in the highlands, and that climate variability played an important role in initiating malaria epidemics in the East African highlands.     

Dynamical malaria models reveal how immunity buffers effect of climate variability

Assessing the influence of climate on the incidence of Plasmodium falciparum malaria worldwide and how it might impact local malaria dynamics is complex and extrapolation to other settings or future times is controversial. This is especially true in the light of the particularities of the short- and long-term immune responses to infection. In sites of epidemic malaria transmission, it is widely accepted that climate plays an important role in driving malaria outbreaks. However, little is known about the role of climate in endemic settings where clinical immunity develops early in life. To disentangle these differences among high- and low-transmission settings we applied a dynamical model to two unique adjacent cohorts of mesoendemic seasonal and holoendemic perennial malaria transmission in Senegal followed for two decades, recording daily P. falciparum cases. As both cohorts are subject to similar meteorological conditions, we were able to analyze the relevance of different immunological mechanisms compared with climatic forcing in malaria transmission. Transmission was first modeled by using similarly unique datasets of entomological inoculation rate. A stochastic nonlinear human–mosquito model that includes rainfall and temperature covariates, drug treatment periods, and population variability is capable of simulating the complete dynamics of reported malaria cases for both villages. We found that under moderate transmission intensity climate is crucial; however, under high endemicity the development of clinical immunity buffers any effect of climate. Our models open the possibility of forecasting malaria from climate in endemic regions but only after accounting for the interaction between climate and immunity.Climate plays a key role in driving the seasonal outbreaks of malaria in areas of low or unstable malaria transmission (1–4). Recent studies have shown the possibility of forecasting malaria outbreaks on the basis of climate information and disease features in these low-transmission settings (3, 5). For instance, in highland malaria the role of warming temperatures is vividly debated (4, 6–8) and in desert-epidemic fringes early studies reported predictions of a widespread increase in malaria transmission (9–12). Recent malaria models also predict a global net increase of the population at risk (13); however, others suggest a shift in spatial distribution rather than a large net increase in total malaria incidence worldwide (14, 15). In epidemic fringes, variation in the incidence of disease is largely determined by the seasonal variation of the mosquito population’s occurrence and density, which are essentially modulated by local rainfall [e.g., if water limited (3, 16)] or temperature [e.g., if altitude limited (2, 4, 8)]. This is not the case in holoendemic transmission settings, where incidence of disease is determined not only by external forces, but also by the development of clinical and antiparasite immunity. Under intense transmission, clinical immunity develops during childhood after many infections (17, 18), whereby the individual can tolerate nonnegligible parasite densities without showing symptoms. Subsequently, antiparasite immunity, which enables control of parasite density, develops much more slowly (19), leading to a state of premunition, whereby individuals harbor chronic, potentially subpatent infections (20). Continued exposure to the parasite is seemingly required to maintain such premunition (21). Complete protection from further infections is rarely, if ever, achieved. In such high-transmission regions, the relationship between local climate and disease is difficult to disentangle.In this study, two unique long-term cohort datasets from villages separated by 5 km but with markedly different malaria transmission intensity (Fig. 1, Upper) enable us to showcase the relative roles of internal and external factors in malaria epidemiology, assess the potential degree of predictability emanating from climatic variability, and generate estimates of key parameters in determining malaria population dynamics. To this end, we use a recently developed inference methodology for nonlinear stochastic dynamical systems, successfully applied to epidemic dynamics (3, 16) but never applied to endemic settings. A general coupled mosquito–human compartment model that includes possible key mechanisms common to both villages serves our aim of disentangling differences related to immunity, infectivity, superinfection, and asymptomatic infections as well as to measure the relevance of local climate for each village.Open in a separate windowFig. 1.(Upper) P. falciparum malaria incidence for Dielmo (red) and Ndiop (green). Vertical dotted black lines separate the four different drug regimes (from left to right: Quinine, Chloroquine, Fansidar, and ACT). Incidence units are episodes per person per month. (Lower) Average annual cycles computed as the average month by month for the whole time series of P. falciparum monthly incidence for Dielmo (red) and Ndiop (green), local rainfall (blue), and temperature (orange). Shaded regions correspond to the SD.     

Late Pleistocene climate change and the global expansion of anatomically modern humans

The extent to which past climate change has dictated the pattern and timing of the out-of-Africa expansion by anatomically modern humans is currently unclear [Stewart JR, Stringer CB (2012) Science 335:1317–1321]. In particular, the incompleteness of the fossil record makes it difficult to quantify the effect of climate. Here, we take a different approach to this problem; rather than relying on the appearance of fossils or archaeological evidence to determine arrival times in different parts of the world, we use patterns of genetic variation in modern human populations to determine the plausibility of past demographic parameters. We develop a spatially explicit model of the expansion of anatomically modern humans and use climate reconstructions over the past 120 ky based on the Hadley Centre global climate model HadCM3 to quantify the possible effects of climate on human demography. The combinations of demographic parameters compatible with the current genetic makeup of worldwide populations indicate a clear effect of climate on past population densities. Our estimates of this effect, based on population genetics, capture the observed relationship between current climate and population density in modern hunter–gatherers worldwide, providing supporting evidence for the realism of our approach. Furthermore, although we did not use any archaeological and anthropological data to inform the model, the arrival times in different continents predicted by our model are also broadly consistent with the fossil and archaeological records. Our framework provides the most accurate spatiotemporal reconstruction of human demographic history available at present and will allow for a greater integration of genetic and archaeological evidence.     

Timing outweighs magnitude of rainfall in shaping population dynamics of a small mammal species in steppe grassland

Climate change–induced shifts in species phenology differ widely across trophic levels, which may lead to consumer–resource mismatches with cascading population and ecosystem consequences. Here, we examined the effects of different rainfall patterns (i.e., timing and amount) on the phenological asynchrony of population of a generalist herbivore and their food sources in semiarid steppe grassland in Inner Mongolia. We conducted a 10-y (2010 to 2019) rainfall manipulation experiment in 12 0.48-ha field enclosures and found that moderate rainfall increases during the early rather than late growing season advanced the timing of peak reproduction and drove marked increases in population size through increasing the biomass of preferred plant species. By contrast, greatly increased rainfall produced no further increases in vole population growth due to the potential negative effect of the flooding of burrows. The increases in vole population size were more coupled with increased reproduction of overwintered voles and increased body mass of young-of-year than with better survival. Our results provide experimental evidence for the fitness consequences of phenological mismatches at the population level and highlight the importance of rainfall timing on the population dynamics of small herbivores in the steppe grassland environment.

The Earth is facing a great challenge from accelerated climate change. The global surface air temperature has increased by about 1° during the past century and is projected to exceed 1.5 to 2 °C by the end of the 21st century (1). Climate change has caused profound impacts on the Earth’s ecosystems, such as local extinctions (2), range shifts (3), and population fluctuations (4, 5) of many species. Many organisms have advanced the timing of phenological events in response to climate warming, such as earlier leaf-out in plants, earlier emergence of insects, or accelerated egg hatching dates for birds (6). For consumers, phenological events are timed to match peak food resources for breeding; however, the direction of consumer’s phenological response to climate change may differ from the response of species occupying lower trophic levels, leading to asynchrony between resources and consumers (7, 8). With respect to climate change, numerous studies have focused on the impact of temperature and its role in driving phenological asynchrony (9–11) since this is especially critical for species population dynamics and ecosystem functioning. However, relatively little is known about how rainfall mediates asynchrony between resources and consumers and its potential demographic consequences, especially in arid environments.Shifts in rainfall patterns have been greatly affected by climate warming (12) and play a key role in regulating vertebrate population dynamics (13), the species composition of communities, and ecosystem functions and services (14). Both the timing and the amount of rainfall are recognized as distinct but major components that synergistically influence the timing of vegetation phenology, e.g., the timing of plant germination and seed ripening (15, 16). However, it remains unclear whether changes in the timing or the amount of rainfall play the more dominant role in the processes of phenological asynchrony between interacting species despite their distinct effects on aboveground annual net primary productivity (17). It is therefore important to disentangle the independent effects of rainfall timing and amount if we are to predict responses of species’ populations and ecosystems to global climate change.Among small herbivores, rainfall is well recognized to induce a bottom-up increase in abundance via increasing food availability, as observed in Phyllotis darwini and Octodon degus in South America (18, 19), Pseudomys hermannsburgensis and Mus domesticus in Australia (20, 21), Spermophilus dauricus (22) and Cricetulus barabensis (23) in East Asia, Dipodomys merriami in North America (24), and Mastomys natalensis in Africa (25). However, these observations are all based on the correlation between rodent abundance and precipitation; the mechanism underlying the bottom-up effects of precipitation on rodents through plant productivity is often assumed but has been rarely investigated by manipulative experiments. While valuable in their own right, most previous studies have been unable to elucidate fully the role of rainfall as a potential proximate cue in regulating phenology. In natural environments, many biotic factors (e.g., predation and interspecific competition) and abiotic factors (e.g., flooding of burrows) may interact to influence how phenological processes can affect population dynamics. To understand the effects of rainfall on the role of phenological asynchrony in the population dynamics of target species, including the effects of rainfall amount and timing, it is therefore necessary to exclude or control for confounding factors. Conducting more tightly controlled manipulative experiments is a requirement when assessing the fitness consequences of phenological asynchrony (6, 7), although it is very challenging for small rodents owing to the need for large field enclosures that prevent immigration/emigration of individuals and impacts by predators.We conducted a 10-y, large-scale, manipulative experiment to examine the bottom-up effects of changes in rainfall regime (including timing and amount; SI Appendix, Fig. S1) on the phenological asynchrony between plants and herbivores, demographic parameters, and population dynamics of Brandt’s voles Lasiopodomys brandtii. In our study region in Inner Mongolia, an increase in annual rainfall, especially during the early growing season, can markedly enhance annual net primary productivity (26), with more rain increasing the biomass of rye grass Leymus chinensis (27, 28), a major and favored food source for Brandt’s voles (29). Additional rainfall in the early growing season can provide a match between the peak food resources and peak food requirements of young voles. Therefore, we hypothesized that rainfall would change the population density of voles by mediating the timing and peak amount of preferred foods and that rainfall timing (in the early growing season) would be of vital importance in triggering population increases, or outbreaks, of voles in arid steppe grassland.     

Ice cores record significant 1940s Antarctic warmth related to tropical climate variability

Although the 20th Century warming of global climate is well known, climate change in the high-latitude Southern Hemisphere (SH), especially in the first half of the century, remains poorly documented. We present a composite of water stable isotope data from high-resolution ice cores from the West Antarctic Ice Sheet. This record, representative of West Antarctic surface temperature, shows extreme positive anomalies in the 1936–45 decade that are significant in the context of the background 20th Century warming trend. We interpret these anomalies—previously undocumented in the high-latitude SH—as indicative of strong teleconnections in part driven by the major 1939–42 El Niño. These anomalies are coherent with tropical sea-surface temperature, mean SH air temperature, and North Pacific sea-level pressure, underscoring the sensitivity of West Antarctica's climate, and potentially its ice sheet, to large-scale changes in the global climate.     

Perception of climate change

“Climate dice,” describing the chance of unusually warm or cool seasons, have become more and more “loaded” in the past 30 y, coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (3σ) warmer than the climatology of the 1951–1980 base period. This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small. We discuss practical implications of this substantial, growing, climate change.     

Discriminating between climate observations in terms of their ability to improve an ensemble of climate predictions

In view of the cost and complexity of climate-observing systems, it is a matter of concern to know which measurements, by satellite or in situ, can best improve the accuracy and precision of long-term ensembles of climate projections. We follow a statistical procedure to evaluate the relative capabilities of a wide variety of observable data types for improving the accuracy and precision of an ensemble of Intergovernmental Panel on Climate Change (IPCC) models. Thirty-two data types are evaluated for their potential for improving a 50-y surface air temperature trend prediction with data from earlier periods, with an emphasis on 20 y. Data types can be ordered in terms of their ability to increase the precision of a forecast. Results show that important conclusions can follow from this ordering. The small size of the IPCC model ensemble (20 members) creates uncertainties in these conclusions, which need to be substantiated with the larger ensembles expected in the future. But the larger issue of whether the methodology can provide useful answers is demonstrated.     

Linking global climate and temperature variability to widespread amphibian declines putatively caused by disease

The role of global climate change in the decline of biodiversity and the emergence of infectious diseases remains controversial, and the effect of climatic variability, in particular, has largely been ignored. For instance, it was recently revealed that the proposed link between climate change and widespread amphibian declines, putatively caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), was tenuous because it was based on a temporally confounded correlation. Here we provide temporally unconfounded evidence that global El Niño climatic events drive widespread amphibian losses in genus Atelopus via increased regional temperature variability, which can reduce amphibian defenses against pathogens. Of 26 climate variables tested, only factors associated with temperature variability could account for the spatiotemporal patterns of declines thought to be associated with Bd. Climatic predictors of declines became significant only after controlling for a pattern consistent with epidemic spread (by temporally detrending the data). This presumed spread accounted for 59% of the temporal variation in amphibian losses, whereas El Niño accounted for 59% of the remaining variation. Hence, we could account for 83% of the variation in declines with these two variables alone. Given that global climate change seems to increase temperature variability, extreme climatic events, and the strength of Central Pacific El Niño episodes, climate change might exacerbate worldwide enigmatic declines of amphibians, presumably by increasing susceptibility to disease. These results suggest that changes to temperature variability associated with climate change might be as significant to biodiversity losses and disease emergence as changes to mean temperature.     

Bedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth's elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth's instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous 'pulse' of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly.     

Rapid reductions and millennial-scale variability in Nordic Seas sea ice cover during abrupt glacial climate changes

Constraining the past sea ice variability in the Nordic Seas is critical for a comprehensive understanding of the abrupt Dansgaard-Oeschger (D-O) climate changes during the last glacial. Here we present unprecedentedly detailed sea ice proxy evidence from two Norwegian Sea sediment cores and an East Greenland ice core to resolve and constrain sea ice variations during four D-O events between 32 and 41 ka. Our independent sea ice records consistently reveal a millennial-scale variability and threshold response between an extensive seasonal sea ice cover in the Nordic Seas during cold stadials and reduced seasonal sea ice conditions during warmer interstadials. They document substantial and rapid sea ice reductions that may have happened within 250 y or less, concomitant with reinvigoration of deep convection in the Nordic Seas and the abrupt warming transitions in Greenland. Our empirical evidence thus underpins the cardinal role of rapid sea ice decline and related feedbacks to trigger abrupt and large-amplitude climate change of the glacial D-O events.

Sea ice is a critical component of the global climate system as it affects Earth’s albedo, phytoplankton productivity, ocean-atmosphere heat and gas exchange, and ocean circulation (1). Rapid sea ice retreat, as observed in the modern Arctic Ocean, exerts important climate feedbacks that may lead to an accelerated climate warming at northern high latitudes (2). While many climate models have difficulties in reproducing the currently observed Arctic sea ice decline (3), the rates of ongoing atmospheric warming in some Arctic regions are already comparable with those of prominent abrupt climate changes that occurred during the last glacial period (4). The latter are referred to as Dansgaard–Oeschger (D-O) climate events and known from Greenland ice core records as abrupt shifts between cold Greenland stadials (GS) and warmer Greenland interstadials (GI) occurring repeatedly ∼10–110 ka (5, 6). The millennial-scale glacial climate variability was a global phenomenon with different characteristics in the northern and southern hemispheres, but the most striking feature of the D-O events is an extremely abrupt climate transition that includes an atmospheric warming of 5–16.5 °C over the Greenland ice sheet happening in just a few decades (7). Analogous to the modern and future sea ice retreat and resulting warming in the Arctic, the abrupt D-O climate transitions are widely believed to have been amplified by rapid sea ice retreat in the Nordic Seas (8–15).Today, the Nordic Seas are largely ice-free, and warm Atlantic surface waters flow into the Norwegian Sea as far north as Svalbard at ∼80°N (Fig. 1), where the Arctic sea ice cover is being eroded, in particular in the Barents Sea. The warm Atlantic surface waters release heat to the atmosphere as it flows northward, which is accompanied by convective intermediate and deep-water formation between Norway and Greenland, feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) (16). A portion of the Atlantic waters continues flowing into the stratified Arctic Ocean as subsurface waters (17). While the pattern of ocean circulation during GI was fairly comparable to that today, proxy data indicate that the glacial Nordic Seas exhibited a stable surface stratification during GS, similar to the modern Arctic Ocean (13, 18). The AMOC and associated northward surface heat transport into the Nordic Seas were weakened during GS, with most extreme weakening related to Heinrich events signified by massive iceberg discharges to the North Atlantic (19, 20). Intermediate and deep waters in the stadial Nordic Seas were 2–4 °C warmer as compared with GI or modern conditions, resulting from a stable halocline and reduced open-ocean convection (21, 22). Contemporaneously, an extended sea ice cover reaching at least as far south as the Greenland–Scotland Ridge at ∼60°N insulated the high-latitude atmosphere from the deep oceanic heat reservoir (23, 24). Model simulations support a subsurface warming scenario under extended sea ice during GS (22, 25, 26) and suggest that a rapid removal of the sea ice cover might have caused the abrupt and high-amplitude D-O climate warming (11, 12, 14, 15).Open in a separate windowFig. 1.Core sites and regional context of the study area. Yellow diamonds mark the core sites investigated in this study. The map shows the core-top P_BIP₂₅ distribution (42, 43, 63), illustrating the great potential of the biomarker approach for sea ice reconstruction. Orange, yellow, and green dots mark core-top sites north, east, and south of Greenland, respectively, data of which are investigated in this study. Small black dots indicate locations of published core-top data. Purple lines mark the modern sea ice extent during September (dashed) and March (solid), averaged between A.D. 1981 and 2010 (https://nsidc.org/; ref. 64). The thin blue line shows the P_BIP₂₅ = 0.2 isoline, representing best the modern winter/spring sea ice extent. Red arrows illustrate the warm and saline North Atlantic Current (NAC). The map was produced with Ocean Data View software (65).Although there is some evidence of millennial-scale sea ice fluctuations during the last glacial, the few available sea ice proxy records (23, 24, 27–31) are mostly restricted to the southern Norwegian Sea and the Arctic Ocean, often have a limited temporal resolution, and partly reflect opposing trends regarding stadial–interstadial sea ice changes depending on the proxies used. Here we present high-resolution sea ice biomarker records from two key sites that form a North–South transect within the Atlantic inflow region in the Norwegian Sea and are thus ideally suited to record spatiotemporal shifts in sea ice cover in both the entrance and the interior of the ocean basin, oceanic fronts, and Atlantic water inflow during the last glacial (Fig. 1). Furthermore, we combine these marine sea ice proxy records with an independent sea ice record based on bromine-enrichment (Br_enr) values from an East Greenland ice core, which significantly enhances the spatial coverage, the robustness of results, and temporal constraint of the sea ice reconstruction. We focus on five representative glacial D-O cycles between 32 and 41 ka, which comprise long- and short-lasting GI as well as several GS, one of which includes Heinrich Event 4. The application of the cryptotephra-based chronological constraints provides a level of robustness as to the timing, duration, and nature of the events unfolding during abrupt climate changes. Our study provides robust empirical evidence that resolves rapid and widespread sea ice retreat in the Nordic Seas and its role in initiating and amplifying the abrupt climate change of the glacial D-O events.     

Short communication: impact of climate variability on the incidence of dengue in Mexico

We evaluated the impact of weather variables and climatic indicators associated with the incidence of dengue in two municipalities of the state of Veracruz, Mexico, from 1995 to 2003. A retrospective ecological study was conducted, using time‐series analysis in which we compiled the weekly reported cases of dengue and the weather and climatic parameters: temperature, rainfall and sea‐surface temperature (SST), the latter as an El Niño Southern Oscillation indicator. We statistically evaluated the data with autogressive models. The models’ predictive abilities were evaluated using data collected from 1995 to 2002 and were validated with those observed for 2003. Each degree Centigrade increase in SST was followed by an increase in the number of dengue cases: 46% in San Andrés Tuxtla (P = 0.001) 16 weeks later and 42% in Veracruz 20 weeks later (P = 0.002). Increases in weekly minimum temperature and rainfall were also significant factors in the increase in the reported cases of dengue. We recommend future studies using the same method, involving larger populations with different geographic location, climate and weather. We also recommend strengthening environmental, health and entomological surveillance systems to improve preparedness and emergency responses.     

North American tree migration paced by climate in the West,lagging in the East

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.

Effective planning for the redistribution of habitats from climate change will depend on understanding demographic rates that control population spread at continental scales. Mobile species are moving, some migrating poleward (1, 2) and/or upward in elevation (3, 4). Species redistribution is also predicted for sessile, long-lived trees that provide the resource and structural foundation for global forest biodiversity (5–7), but their movement is harder to study. Contemporary range shifts are recognized primarily where contractions have followed extensive die-backs (8) or where local changes occur along compact climate gradients in steep terrain (9, 10). Whether migration capacity can pace habitat shifts of hundreds of kilometers on decade time scales depends on seed production and juvenile recruitment (Fig. 1A), which have not been fitted to data in ways that can be incorporated in models to anticipate biogeographic change (11–13). For example, do the regions of rapid warming coincide with locations where species can produce abundant seed (Fig. 1B)? If so, does seed production translate to juvenile recruitment? Here, we combine continent-wide fecundity estimates from the Masting Inference and Forecasting (MASTIF) network (13) with tree inventories to identify North American hotspots for recruitment and find that species are well-positioned to track warming in the West and North, but not in parts of the East.Open in a separate windowFig. 1.Transitions, hypothesized effects on spread, and sites. (A) Population spread from trees (BA) to new recruits is controlled by fecundity (seed mass per BA) followed by recruitment (recruits per seed mass). (B) The CTH that warming has stimulated fecundity ahead of the center of adult distributions, which reflect climate changes of recent decades. Arrows indicate how centroids from trees to fecundity to recruitment could be displaced poleward with warming climate. (C) The RSH that cold-sensitive fecundity is optimal where minimum temperatures are warmer than for adult trees and, thus, may slow northward migration. The two hypotheses are not mutually exclusive. B and C refer to the probability densities of the different life stages. (D) MASTIF sites are summarized in SI Appendix, Table S2.2 by eco-regions: mixed forest (greens), montane (blues), grass/shrub/desert (browns), and taiga (blue-green).Suitable habitats for many species are projected to shift hundreds of kilometers in a matter of decades (14, 15). While climate effects on tree mortality are increasingly apparent (16–19), advances into new habitats are not (20–23). For example, natural populations of Pinus taeda may be sustained only if the Northeast can be occupied as habitats are lost in the South (Fig. 2). Current estimates of tree migration inferred from geographic comparisons of juvenile and adult trees have been inconclusive (2, 7, 21, 24, 25). Ambiguous results are to be expected if fecundity and juvenile success do not respond to change in the same ways (20, 26–29). Moreover, seedling abundances (7, 30) do not provide estimates of recruitment rates because seedlings may reside in seedling banks for decades, or they may turn over annually (31–33). Another method based on geographic shifts in population centers calculated from tree inventories (3, 34) does not separate the effects of mortality from recruitment, i.e., the balance of losses in some regions against gains in others. The example in Fig. 2 is consistent with an emerging consensus that suitable habitats are moving fast (2, 14, 15), even if population frontiers are not, highlighting the need for methods that can identify recruitment limitation on population spread. Management for forest products and conservation goals under transient conditions can benefit from an understanding of recruitment limitation that comes from seed supply, as opposed to seedling survival (35).Open in a separate windowFig. 2.Suitable habitats redistribute with decade-scale climate change for P. taeda (BA units m² /ha). (Suitability is not a prediction of abundance, but rather, it is defined for climate and habitat variables included in a model, to be modified by management and disturbance [e.g., fire]. By providing habitat suitability in units of BA, it can be related it to the observation scale for the data.) Predictive distributions for suitability under current (A) and change expected from mid-21^st-century climate scenario Representative Concentration Pathway 4.5 (B) showing habitat declines in the Southwest and East. Specific climate changes important for this example include net increases in aridity in the southeast (especially summer) and western frontier and warming to the North. Occupation of improving habitats depends on fecundity in northern parts of the range and how it is responding. Obtained with Generalized Joint Attribute Modeling (see Materials and Methods for more information).We hypothesized two ways in which fecundity and recruitment could slow or accelerate population spread. Contemporary forests were established under climates that prevailed decades to centuries ago. These climate changes combine with habitat variables to affect seeds, seedlings, and adults in different ways (36, 37). The “climate-tracking hypothesis” (CTH) proposes that, after decades of warming and changing moisture availability (Fig. 3 A and B), seed production for many species has shifted toward the northern frontiers of the range, thus primed for poleward spread. “Fecundity,” the transition from tree basal area (BA) to seed density on the landscape (Fig. 1A), is taken on a mass basis (kg/m² BA) as a more accurate index of reproductive effort than seed number (38, 39). “Recruitment,” the transition from seed density to recruit density (recruits per kg seed), may have also shifted poleward, amplifying the impact of poleward shifts in fecundity on the capacity for poleward spread (Fig. 1B). Under CTH, the centers for adult abundance, fecundity, and recruitment are ordered from south to north in Fig. 1B as might be expected if each life-history stage leads the previous stage in a poleward migration.Open in a separate windowFig. 3.Climate change and tracking. (A) Mean annual temperatures since 1990 have increased rapidly in the Southwest and much of the North. (Zero-change contour line is in red.) (B) Moisture deficit index (monthly potential evapotranspiration minus P summed over 12 mo) has increased in much of the West. (Climate sources are listed in SI Appendix.) (C) Fecundity (kg seed per BA summed over species) is high in the Southeast. (D) Recruits per kg seed (square-root transformed) is highest in the Northeast. (E and F) Geographic displacement of 81 species show transitions in Fig. 1A, as arrows from centroids for adult BA to fecundity (E) and from fecundity to recruitment (F). Blue arrows point north; red arrows point south. Consistent with the RSH (Fig. 1B), most species centered in the East and Northwest have fecundity centroids south of adult distributions (red arrows in E). Consistent with the CTH, species of the interior West have fecundity centroids northwest of adults (blue arrows). Recruitment is shifted north of fecundity for most species (blue arrows in F). SI Appendix, Fig. S2 shows that uncertainty in vectors is low.The “reproductive-sensitivity hypothesis” (RSH) proposes that recruitment may limit population growth in cold parts of the range (Fig. 1C), where fecundity and/or seedling survival is already low. Cold-sensitive reproduction in plants includes late frost that can disrupt flowering, pollination, and/or seed development, suggesting that poleward population frontiers tend to be seed-limited (40–44). While climate warming could reduce the negative impacts of low temperatures, especially at northern frontiers, these regions still experience the lowest temperatures. The view of cold-sensitive fecundity as a continuing rate-limiting step, i.e., that has not responded to warming in Fig. 1C, is intended to contrast with the case where warming has alleviated temperature limitation in Fig. 1B. Lags can result if cold-sensitive recruitment naturally limits growth at high-latitude/high-altitude population frontiers (Fig. 1C). In this case, reproductive sensitivity may delay the pace of migration to an extent that depends on fecundity, recruitment, or both at poleward frontiers. The arrows in Fig. 1C depict a case where optimal fecundity is equator-ward of optimal growth and recruitment. The precise location of recruitment relative to fecundity in Fig. 1C will depend on all of the direct and indirect effects of climate, including through seed and seedling predators and disturbances like fire. Fig. 1C depicts one of many hypothetical examples to show that climate variables might have opposing effects on fecundity and recruitment.Both CTH and RSH can apply to both temperature and moisture; the latter is here quantified as cumulative moisture deficit between potential evapotranspiration and precipitation, D=∑m=112(PETm−Pm) for month m, derived from the widely used Standardized Precipitation Evapotranspiration Index (45). Whereas latitude dominates temperature gradients and longitude is important for moisture in the East, gradients are complicated by steep terrain in the West, with temperature tending to decline and moisture increase with elevation.We quantified the transitions that control population spread, from adult trees (BA) to fecundity (seeds per BA) to recruitment (recruits per kg seed) (Fig. 1A–C). Fecundity observations are needed to establish the link between trees and recruits in the migration process. They must be available at the tree scale across the continent because seed production depends on tree species and size, local habitat, and climate for all of the dominant species and size classes (13, 46). These estimates are not sufficient in themselves, because migration depends on seed production per area, not per tree. The per-area estimates come from individual seed production and dispersal from trees on inventory plots that monitor all trees that occupy a fixed sample area. Fecundity estimates were obtained in the MASTIF project (13) from 211,000 (211K) individual trees and 2.5 million (2.5M) tree-years from 81 species. We used a model that accommodates individual tree size, species, and environment and the codependence between trees and over time (Fig. 1C). In other words, it allows valid inference on fecundity, the quasisynchronous, quasiperiodic seed production typical of many species (47). The fitted model was then used to generate a predictive distribution of fecundity for each of 7.6M trees on 170K forest inventory plots across the United States and Canada. Because trees are modeled together, we obtain fecundity estimates per plot and, thus, per area. BA (m² /ha) of adult trees and new recruits into the smallest diameter class allowed us to determine fecundity as kg seed per m² BA and recruitment per kg seed, i.e., each of the transitions in Fig. 1A.Recruitment rates, rather than juvenile abundances, come from the transitions from seedlings to sapling stages. The lag between seed production and recruitment does not allow for comparisons on an annual basis; again, residence times in a seedling bank can span decades. Instead, we focus on geographic variation in mean rates of fecundity and recruitment.We summarized the geographic distributions for each transition as 1) the mean transition rates across all species and 2) the geographic centroids (central tendency) for each species as weighted-average locations, where weights are the demographic transitions (BA to fecundity, fecundity to recruitment, and BA to recruitment). We analyzed central tendency, or centroids (e.g., refs. 3 and 34) because range limits cannot be accurately identified on the basis of small inventory plots (21). If fecundity is not limiting poleward spread (CTH of Fig. 1B), then fecundity centroids are expected to be displaced poleward from the adult population. If reproductive sensitivity dominates population spread (RSH of Fig. 1C), then fecundity and/or recruitment centroids will be displaced equator-ward from adult BA. The same comparisons between fecundity and recruitment determine the contribution of recruitment to spread.     

Network-based forecasting of climate phenomena

Network theory, as emerging from complex systems science, can provide critical predictive power for mitigating the global warming crisis and other societal challenges. Here we discuss the main differences of this approach to classical numerical modeling and highlight several cases where the network approach substantially improved the prediction of high-impact phenomena: 1) El Niño events, 2) droughts in the central Amazon, 3) extreme rainfall in the eastern Central Andes, 4) the Indian summer monsoon, and 5) extreme stratospheric polar vortex states that influence the occurrence of wintertime cold spells in northern Eurasia. In this perspective, we argue that network-based approaches can gainfully complement numerical modeling.