Global climate change

Most of the last 100,000 years or longer has been characterized by large, abrupt, regional-to-global climate changes. Agriculture and industry have developed during anomalously stable climatic conditions. New, high-resolution analyses of sediment cores using multiproxy and physically based transfer functions allow increasingly confident interpretation of these past changes as having been caused by "band jumps" between modes of operation of the climate system. Recurrence of such band jumps is possible and might be affected by human activities.     

Assessing agricultural risks of climate change in the 21st century in a global gridded crop model intercomparison

Here we present the results from an intercomparison of multiple global gridded crop models (GGCMs) within the framework of the Agricultural Model Intercomparison and Improvement Project and the Inter-Sectoral Impacts Model Intercomparison Project. Results indicate strong negative effects of climate change, especially at higher levels of warming and at low latitudes; models that include explicit nitrogen stress project more severe impacts. Across seven GGCMs, five global climate models, and four representative concentration pathways, model agreement on direction of yield changes is found in many major agricultural regions at both low and high latitudes; however, reducing uncertainty in sign of response in mid-latitude regions remains a challenge. Uncertainties related to the representation of carbon dioxide, nitrogen, and high temperature effects demonstrated here show that further research is urgently needed to better understand effects of climate change on agricultural production and to devise targeted adaptation strategies.The magnitude, rate, and pattern of climate change impacts on agricultural productivity have been studied for approximately two decades. To evaluate these impacts, researchers use biophysical process-based models (e.g., refs. 1–5), agro-ecosystem models (e.g., ref. 6), and statistical analyses of historical data (e.g., refs. 7 and 8). Although these and other methods have been widely used to forecast potential impacts of climate change on future agricultural productivity, the protocols used in previous assessments have varied to such an extent that they constrain cross-study syntheses and limit the ability to devise relevant adaptation options (9, 10). In this project we have brought together seven global gridded crop models (GGCMs) for a coordinated set of simulations of global crop yields under evolving climate conditions.This GGCM intercomparison was coordinated by the Agricultural Model Intercomparison and Improvement Project (AgMIP; 11) as part of the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP; 12). In order to facilitate analyses across models and sectors, all global models are driven with consistent bias-corrected climate forcings derived from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive (13). The objectives are to (i) establish the range of uncertainties of climate change impacts on crop productivity worldwide, (ii) determine key differences in current approaches used by crop modeling groups in global analyses, and (iii) propose improvements in GGCMs and in the methodologies for future intercomparisons to produce more reliable assessments.We examine the basic patterns of response to climate across crops, latitudes, time periods, regional temperatures, and atmospheric carbon dioxide concentrations [CO₂]. In anticipation of the wider scientific community using these model outputs and the expanded application of GGCMs, we introduce these models and present guidelines for their practical application. Related studies in this special issue focus on crop water demand and the freshwater supply for irrigation (14), the application of the crop model results as part of wider intersectoral analyses (15), and the integration of crop-climate impact assessments with agro-economic models (16).     

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.     

Multisectoral climate impact hotspots in a warming world

The impacts of global climate change on different aspects of humanity’s diverse life-support systems are complex and often difficult to predict. To facilitate policy decisions on mitigation and adaptation strategies, it is necessary to understand, quantify, and synthesize these climate-change impacts, taking into account their uncertainties. Crucial to these decisions is an understanding of how impacts in different sectors overlap, as overlapping impacts increase exposure, lead to interactions of impacts, and are likely to raise adaptation pressure. As a first step we develop herein a framework to study coinciding impacts and identify regional exposure hotspots. This framework can then be used as a starting point for regional case studies on vulnerability and multifaceted adaptation strategies. We consider impacts related to water, agriculture, ecosystems, and malaria at different levels of global warming. Multisectoral overlap starts to be seen robustly at a mean global warming of 3 °C above the 1980–2010 mean, with 11% of the world population subject to severe impacts in at least two of the four impact sectors at 4 °C. Despite these general conclusions, we find that uncertainty arising from the impact models is considerable, and larger than that from the climate models. In a low probability-high impact worst-case assessment, almost the whole inhabited world is at risk for multisectoral pressures. Hence, there is a pressing need for an increased research effort to develop a more comprehensive understanding of impacts, as well as for the development of policy measures under existing uncertainty.     

Global food security under climate change

This article reviews the potential impacts of climate change on food security. It is found that of the four main elements of food security, i.e., availability, stability, utilization, and access, only the first is routinely addressed in simulation studies. To this end, published results indicate that the impacts of climate change are significant, however, with a wide projected range (between 5 million and 170 million additional people at risk of hunger by 2080) strongly depending on assumed socio-economic development. The likely impacts of climate change on the other important dimensions of food security are discussed qualitatively, indicating the potential for further negative impacts beyond those currently assessed with models. Finally, strengths and weaknesses of current assessment studies are discussed, suggesting improvements and proposing avenues for new analyses.     

Global climate change, war, and population decline in recent human history

Although scientists have warned of possible social perils resulting from climate change, the impacts of long-term climate change on social unrest and population collapse have not been quantitatively investigated. In this study, high-resolution paleo-climatic data have been used to explore at a macroscale the effects of climate change on the outbreak of war and population decline in the preindustrial era. We show that long-term fluctuations of war frequency and population changes followed the cycles of temperature change. Further analyses show that cooling impeded agricultural production, which brought about a series of serious social problems, including price inflation, then successively war outbreak, famine, and population decline successively. The findings suggest that worldwide and synchronistic war-peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change. The findings also imply that social mechanisms that might mitigate the impact of climate change were not significantly effective during the study period. Climate change may thus have played a more important role and imposed a wider ranging effect on human civilization than has so far been suggested. Findings of this research may lend an additional dimension to the classic concepts of Malthusianism and Darwinism.     

Global inequities and political borders challenge nature conservation under climate change

Underlying sociopolitical factors have emerged as important determinants of wildlife population trends and the effectiveness of conservation action. Despite mounting research into the impacts of climate change on nature, there has been little consideration of the human context in which these impacts occur, particularly at the global scale. We investigate this in two ways. First, by modeling the climatic niches of terrestrial mammals and birds globally, we show that projected species loss under climate change is greatest in countries with weaker governance and lower Gross Domestic Product, with loss of mammal species projected to be greater in countries with lower CO₂ emissions. Therefore, climate change impacts on species may be disproportionately significant in countries with lower capacity for effective conservation and lower greenhouse gas emissions, raising important questions of international justice. Second, we consider the redistribution of species in the context of political boundaries since the global importance of transboundary conservation under climate change is poorly understood. Under a high-emissions scenario, we find that 35% of mammals and 29% of birds are projected to have over half of their 2070 climatic niche in countries in which they are not currently found. We map these transboundary range shifts globally, identifying borders across which international coordination might most benefit conservation and where physical border barriers, such as walls and fences, may be an overlooked obstacle to climate adaptation. Our work highlights the importance of sociopolitical context and the utility of a supranational perspective for 21st century nature conservation.

Earth’s biodiversity is set to face major disruption under climate change, with substantial implications for natural ecosystems and human societies that depend on them (1–4). However, the fate of biodiversity depends not only on the severity and distribution of climate impacts, but also on the human context in which they occur (5). For example, socioeconomic factors such as governance, corruption, and conflict frequency are important predictors of wildlife population trends and the effectiveness of conservation efforts (6–9). Political borders, too, have important conservation implications where they fragment policy and legislation across species ranges (10) or where they present physical barriers to movement (11–14). Here, we use ensemble niche modeling to investigate climate-induced biodiversity change in the context of these two key human considerations: socioeconomic factors of relevance for biodiversity conservation and the political borders that circumscribe and delineate their influence.     

Multimodel assessment of water scarcity under climate change

Water scarcity severely impairs food security and economic prosperity in many countries today. Expected future population changes will, in many countries as well as globally, increase the pressure on available water resources. On the supply side, renewable water resources will be affected by projected changes in precipitation patterns, temperature, and other climate variables. Here we use a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways) to synthesize the current knowledge about climate change impacts on water resources. We show that climate change is likely to exacerbate regional and global water scarcity considerably. In particular, the ensemble average projects that a global warming of 2 °C above present (approximately 2.7 °C above preindustrial) will confront an additional approximate 15% of the global population with a severe decrease in water resources and will increase the number of people living under absolute water scarcity (<500 m³ per capita per year) by another 40% (according to some models, more than 100%) compared with the effect of population growth alone. For some indicators of moderate impacts, the steepest increase is seen between the present day and 2 °C, whereas indicators of very severe impacts increase unabated beyond 2 °C. At the same time, the study highlights large uncertainties associated with these estimates, with both global climate models and GHMs contributing to the spread. GHM uncertainty is particularly dominant in many regions affected by declining water resources, suggesting a high potential for improved water resource projections through hydrological model development.     

Global and local implications of biotechnology and climate change for future food supplies

The development of improved technology for agricultural production and its diffusion to farmers is a process requiring investment and time. A large number of studies of this process have been undertaken. The findings of these studies have been incorporated into a quantitative policy model projecting supplies of commodities (in terms of area and crop yields), equilibrium prices, and international trade volumes to the year 2020. These projections show that a "global food crisis," as would be manifested in high commodity prices, is unlikely to occur. The same projections show, however, that in many countries, "local food crisis," as manifested in low agricultural incomes and associated low food consumption in the presence of low food prices, will occur. Simulations show that delays in the diffusion of modern biotechnology research capabilities to developing countries will exacerbate local food crises. Similarly, global climate change will also exacerbate these crises, accentuating the importance of bringing strengthened research capabilities to developing countries.     

The causality analysis of climate change and large-scale human crisis

Recent studies have shown strong temporal correlations between past climate changes and societal crises. However, the specific causal mechanisms underlying this relation have not been addressed. We explored quantitative responses of 14 fine-grained agro-ecological, socioeconomic, and demographic variables to climate fluctuations from A.D. 1500-1800 in Europe. Results show that cooling from A.D. 1560-1660 caused successive agro-ecological, socioeconomic, and demographic catastrophes, leading to the General Crisis of the Seventeenth Century. We identified a set of causal linkages between climate change and human crisis. Using temperature data and climate-driven economic variables, we simulated the alternation of defined "golden" and "dark" ages in Europe and the Northern Hemisphere during the past millennium. Our findings indicate that climate change was the ultimate cause, and climate-driven economic downturn was the direct cause, of large-scale human crises in preindustrial Europe and the Northern Hemisphere.     

Global climate change and mammalian species diversity in U.S. national parks

National parks and bioreserves are key conservation tools used to protect species and their habitats within the confines of fixed political boundaries. This inflexibility may be their "Achilles' heel" as conservation tools in the face of emerging global-scale environmental problems such as climate change. Global climate change, brought about by rising levels of greenhouse gases, threatens to alter the geographic distribution of many habitats and their component species. With these changes comes great uncertainty about the future ability of parks and protected areas to meet their conservation mandates. We report here on an analysis aimed at assessing the extent of mammalian species turnover that may be experienced in eight selected U.S. national parks if climate change causes mammalian species within the continental U.S. to relocate to new geographic locations. Due to species losses of up to 20% and drastic influxes of new species, national parks are not likely to meet their mandate of protecting current biodiversity within park boundaries. This approach represents a conservative prognosis. As species assemblages change, new interactions between species may lead to less predictable indirect effects of climate change, increasing the toll beyond that found in this study.     

气候变化与鼠疫流行的耦合分析

目的研究气候变化与人间鼠疫流行之间的关系。方法选择内蒙古和福建省分别作为我国北方和南方鼠疫疫源地和历史鼠疫病区的典型省,采用对比分析和相关分析的方法,研究气候变化(降水变化以旱涝指数表示,气温变化以冷暖指数表示)与人间鼠疫发病强度之间的关系。结果在北方地区,人间鼠疫发病强度与前一年和当年的旱涝指数及冷暖指数呈显著负相关;而在南方地区,人间鼠疫的发病强度仅与当年秋季的冷暖指数有显著负相关性。结论气候变化与鼠疫流行之间的关系有明显的区域差异。降水丰富及气候温暖利于北方鼠疫的流行与传播;降水变化对南方鼠疫的流行没有明显的影响,秋冬季的气温与南方鼠疫的流行强度有一定的正相关关系。     

Carbon residence time dominates uncertainty in terrestrial vegetation responses to future climate and atmospheric CO2

Future climate change and increasing atmospheric CO₂ are expected to cause major changes in vegetation structure and function over large fractions of the global land surface. Seven global vegetation models are used to analyze possible responses to future climate simulated by a range of general circulation models run under all four representative concentration pathway scenarios of changing concentrations of greenhouse gases. All 110 simulations predict an increase in global vegetation carbon to 2100, but with substantial variation between vegetation models. For example, at 4 °C of global land surface warming (510–758 ppm of CO₂), vegetation carbon increases by 52–477 Pg C (224 Pg C mean), mainly due to CO₂ fertilization of photosynthesis. Simulations agree on large regional increases across much of the boreal forest, western Amazonia, central Africa, western China, and southeast Asia, with reductions across southwestern North America, central South America, southern Mediterranean areas, southwestern Africa, and southwestern Australia. Four vegetation models display discontinuities across 4 °C of warming, indicating global thresholds in the balance of positive and negative influences on productivity and biomass. In contrast to previous global vegetation model studies, we emphasize the importance of uncertainties in projected changes in carbon residence times. We find, when all seven models are considered for one representative concentration pathway × general circulation model combination, such uncertainties explain 30% more variation in modeled vegetation carbon change than responses of net primary productivity alone, increasing to 151% for non-HYBRID4 models. A change in research priorities away from production and toward structural dynamics and demographic processes is recommended.     

Secure human attachment can promote support for climate change mitigation

Attachment theory is an ethological approach to the development of durable, affective ties between humans. We propose that secure attachment is crucial for understanding climate change mitigation, because the latter is inherently a communal phenomenon resulting from joint action and requiring collective behavioral change. Here, we show that priming attachment security increases acceptance (Study 1: n = 173) and perceived responsibility toward anthropogenic climate change (Study 2: n = 209) via increased empathy for others. Next, we demonstrate that priming attachment security, compared to a standard National Geographic video about climate change, increases monetary donations to a proenvironmental group in politically moderate and conservative individuals (Study 3: n = 196). Finally, through a preregistered field study conducted in the United Arab Emirates (Study 4: n = 143,558 food transactions), we show that, compared to a message related to carbon emissions, an attachment security–based message is associated with a reduction in food waste. Taken together, our work suggests that an avenue to promote climate change mitigation could be grounded in core ethological mechanisms associated with secure attachment.

The negative effects of climate change are looming, demanding forceful and immediate action from multiple stakeholders, including households and individuals. Despite the pressing need for climate change mitigation, many people still deny the reality or seriousness of climate change (1). Furthermore, even when people do acknowledge climate change, they often do not change their behavior in substantive ways to reduce carbon emissions (2). Several reasons have been proposed to explain the limited household action to mitigate climate change (3), and among alternative accounts, the hypothesis of motivated reasoning is gaining momentum. The motivated reasoning hypothesis proposes that limited action on climate change may function to safeguard core identity motives, ideologies, and worldviews, including perceived conflicts between mitigating climate change and maintaining economic prosperity (4). This perspective is increasingly prominent in the literature, particularly with research suggesting that individuals construe scientific evidence about climate change in ways that are self-serving (5). Therefore, given that attitudes and action about climate change are, at least partially, influenced by motivational processes, the question remains of how to motivate individuals toward more proenvironmental pathways.We propose that climate change mitigation may be promoted when psychological structures related to human interconnection are developed and active. This is based on the premise that the willingness to mitigate climate change involves accepting human (co)accountability, caring for others (present and future generations), and a readiness to act (together) as a mitigation response.Here, we specifically examine the role of attachment orientation in the willingness to mitigate climate change, in line with increasing calls to invest in demand side solutions to address global warming (6). We focus on attachment because it relates to the primal form of emotional bonding between humans (7). From an evolutionary perspective, attachment is conceptualized as an innate behavioral system aimed at safeguarding against potential threats by assuring proximity to caring and supportive others (8). The motivation to seek proximity to protective others is functionally different from more general affiliation needs (9); it aims to establish a secure base, which is defined as a sense that protective others are available and responsive in case of threat. This concept of protection from threat is pertinent to climate change because global warming poses an existential threat to humankind, which is within the realm of stimuli that could be expected to activate the attachment system.Researchers have identified two primary attachment orientations^* (10–12): a secure attachment orientation, which is used to describe people who have experienced a sense of safe haven, protection, and comfort from close others in times of distress, and an insecure attachment orientation, which is associated with experiences of being rejected or ignored by close others in times of need or threat (13).This distinction between attachment orientations is significant because the attachment system is linked to other behavioral systems (14), namely the caregiving system. The caregiving system is thought to have evolved to provide protection and support to others, and is inherently altruistic in nature (15). These behavioral systems are linked in a way that, when people feel comforted and safe in threatening situations (securely attached), the activation of caregiving is facilitated, enabling them to focus on the distress of others (16). By comparison, insecurely attached individuals tend to remain focused on their own distress and are less likely to engage in altruistic behaviors. Only when relief from threat is achieved, and a sense of safety restored, can individuals shift resources to other behavioral systems such as caregiving (17). Thus, attachment security does not activate the caregiving system directly but rather offers a solid psychological foundation for altruism (18). Previous research has shown the positive impact of secure attachment in multiple instances of caregiving, including volunteering and helping behavior (18, 19). Essentially, secure attachment, as a basic psychological need related to safety and protection, anchors the progression toward higher-level psychological processes (19) and offers a strong, theoretical foundation to understand prosocial behavior. In prior research, mitigating climate change has been defined as a form of prosocial behavior, which places individual self-interest behind the collective welfare (20).The interplay between the attachment and caregiving systems has several important implications for research about climate change. First, attachment theory sheds a different light on prosocial behavior, examined through an evolutionary perspective. Research based on evolutionary frameworks suggests that prosocial behavior occurs primarily to protect reputation and build reciprocity as a means to guarantee return benefits, particularly from in-group members (21, 22). However, the distinctiveness of the attachment system lies in the ethology of the bonding process, which goes beyond the notion of transactional social ties. Seminal evidence from humans and nonhuman primates (23, 24) has shown that the attachment system is not rooted in reward reinforcement from the caregiver (such as food) but rather is motivated by a need for protective bonding. Secure attachment has been linked to volunteering and helping behavior (which are features of the caregiving system) beyond the boundaries of close in-group members—for example, toward strangers and unrelated individuals (14–18).Second, secure attachment may be the psychological infrastructure on which several factors previously associated with caring for the environment are built. There is evidence showing the role of altruistic social orientation, empathy, and universality values in proenvironmental behaviors (25–27). This previous research, however, leaves unaddressed what promotes these factors. We propose that attachment security is a crucial, latent foundation because of its facilitating role in the activation of the caregiving system, which offers a broader conceptual perspective to previous findings.Third, the literature on adult attachment theory provides a validated, experimental apparatus for the design of interventions (18) to increase the manifestation of prosocial behavior. A core assumption in attachment theory is that attachment orientations are relatively stable over time (12) but prone to temporary variations (e.g., such as a parent’s death, a job loss, or a new intimate relationship) and transient fluctuations, including experimental manipulations (14). There is consistent causal evidence showing the effect of priming attachment security in increasing levels of empathy, trust, and helping behavior (11, 15, 18). Therefore, such validated, experimental manipulations also allow one to experimentally test the effect of attachment security on beliefs and action toward anthropogenic climate change.Finally, attachment theory is not culturally bound, unlike other perspectives on climate change (28). A central feature of attachment theory is the universality of its premise (7), with minor cultural variations (29). Fundamentally, attachment security is a psychological feature that can be nurtured in all humans. Given its link to a generalized concern for others’ welfare (14), attachment security could help the conservation of global public goods (20) that require protective action based on common concerns that affect all of humankind—of which climate change is a paradigmatic example.     

受艾滋病影响儿童心理健康干预方法探索

目的探索能够有效促进受艾滋病影响儿童心理健康的干预策略。方法选取河南省某县受艾滋病影响的10～15岁儿童45名,进行为期10个月的综合心理支持,干预前后进行问卷调查测量干预效果。结果干预前有焦虑不安等负性心理体验的儿童占44.4%(20/45),干预后下降到20.0%(9/45);症状自评量表SCL-90测量的强迫症状、人际关系敏感、抑郁、焦虑以及偏执等各个因子,干预之后均显著改善。结论综合心理支持对于促进受艾滋病影响儿童的心理健康具有积极意义。     

Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO2 and modulated by climate and plant functional types

We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance (g_s), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO₂ (C_a), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C_a was a dominant environmental driver of iWUE, the effects of increasing C_a were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases in A_net dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in A_net, whereas reductions in g_s occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C_a and other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in g_s as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

How terrestrial plants respond to more frequent, and often prolonged, environmental stressors will have profound impacts on, and feedbacks to, the Earth-climate system at regional to continental scales (1, 2). Central to these feedbacks are plant stomata, microscopic pores on the leaves of plants that act as a control valve over the fluxes of carbon dioxide (C_a) into the leaf during photosynthesis and water vapor (H₂O) out of the leaf during transpiration. Importantly, changes in stomatal aperture do not affect the fluxes of C_a and H₂O equally, as the sum of resistances for the diffusion of C_a from the atmosphere to mesophyll cells where Rubisco is located are much greater than those for H₂O from the surface of leaf mesophyll cells to the atmosphere (3). Indeed, as stomatal aperture changes, so does water use efficiency (WUE), or the ratio of C_a uptake to H₂O released from the leaf to canopy scale (4). Consequently, understanding the environmental factors driving changes in leaf physiology is of paramount concern in the context of climate change as small changes in tree WUE can have major effects on the carbon and hydrologic cycles over large geographical areas (1, 5).Approaches using tree ring carbon isotopes (6–9), eddy-flux measurements (4, 9, 10), atmospheric carbon isotope composition analysis (11), and Earth system modeling techniques (9–13) have shown trends of recently increasing WUE. These increases can occur by stimulation of leaf photosynthetic rates (A_net) (14, 15), reduced stomatal conductance to water (g_s) (14, 15), or some combination of the two. A fundamental physiological response found in numerous C_a enrichment experiments is that WUE of many plants is improved as a result of increasing C_a stimulating photosynthesis and causing partial stomatal closure (14, 16). However, environmental factors distinct from C_a, such as vapor pressure deficit (VPD), precipitation, and temperature, have independent effects on A_net and g_s and, therefore, may modulate the response of WUE to rising C_a, especially across functionally distinct plant groups with differences in wood anatomy (9) and leaf morphology (17). Despite this, few studies have thoroughly examined the effects of multiple environmental factors over controls of WUE, and even fewer have considered the underlying component parts, A_net and g_s (9, 13, 17–20). This has, in part, been due to the complexity of partitioning H₂O gas fluxes at ecosystem scales (21), in addition to the difficulty in attributing changes in isotopically derived intrinsic water use efficiency (iWUE) (the ratio of A_net to stomatal conductance to water, g_s) to A_net or g_s without the accompaniment of physiological measurements (18, 19).A promising technique couples carbon isotopically derived estimates of iWUE with oxygen isotope leaf water enrichment above source water (∆¹⁸O_lw; derived from tree ring δ¹⁸O and source water δ¹⁸O assumed to be ∼δ¹⁸O_{precipitation}) to provide a qualitative attribution of changes in iWUE to underlying A_net and g_s (9, 20, 22–29). As ∆¹⁸O_lw is inversely related to g_s (26–30), if increases in iWUE were due to increases in A_net, then Δ¹⁸O_lw should be constant or decrease with iWUE. However, if increases in iWUE were due to decreases in g_s, or a combination of a decrease in g_s and an increase in A_net, Δ¹⁸O_lw would increase with iWUE (9, 25, 30). As such, there currently exists a wealth of previously untapped long-term records of tree physiological responses to environmental change within numerous dendrochronological studies from around the world, providing a historical view of how iWUE has changed globally over the last century. In this analysis, we 1) synthesize published data from tree ring carbon and oxygen isotope chronologies to examine global trends in tree ring–derived iWUE, 2) identify those environmental factors, and their interactions, that best explain multidecadal to centurial trends in iWUE, and 3) investigate the potential underlying changes in A_net and g_s through an analysis of coupled tree ring–derived iWUE and ∆¹⁸O_lw over time (9, 20, 22, 23, 25).Using 113 unique tree ring carbon and oxygen isotope chronologies comprising 36 different species across 84 sites globally (Fig. 1A and SI Appendix, Table S1), we show tree-level iWUE increased, on average, by ∼40% (0.35% y⁻¹, iWUE = 0.23*y − 369.16) over the last century (1901–2015) (Fig. 1B). We statistically identified a breakpoint in the combined iWUE chronology at 1963, after which iWUE increased linearly at a rate of 0.39 ± 0.01 µmol CO₂⋅mol⁻¹ H₂O⋅y⁻¹ (1.67% y⁻¹), or ∼3.9 times faster than the previous 63 y (F = 207.14, P < 0.0001) (Fig. 1B and SI Appendix, Fig. S1). This change in the rate at which tree iWUE increased after 1963 coincided with a statistical breakpoint in C_a at 1969 where CO₂ began increasing 4.1 times faster than between 1901 and 1969 (SI Appendix, Fig. S2) (31), and is similar to the trend in iWUE from a recent global synthesis by Adams et al. (32) who also showed a similar breakpoint in the 1960s. When considering individual chronologies, the increases in iWUE were widespread, with 93% (105 of 113 chronologies) of those examined having positive trends over 1901–2015 and 84% (95 of 113) of those examined having positive trends over 1963–2015 (SI Appendix, Table S1). These data for trees during the Anthropocene spanning 10 biomes in six continents that represent a spectrum of leaf and wood types reinforce reports of increasing iWUE in the United States (9), Europe (6, 8), and tropical forests (7), in addition to a recent compilation of global iWUE trends (32).Open in a separate windowFig. 1.Tree locations from which chronologies of iWUE and ∆¹⁸O_lw were developed (A) and group mean-centered iWUE by species within site over the period 1901–2015 (B). The color of data points in A and B correspond to the biome from which trees were growing, while the size of the circle in A corresponds to the number of trees used in the development of each carbon and oxygen isotope-derived chronology, respectively. The vertical dashed line in B occurs at the year 1963 where the rate of change in iWUE increases. The solid lines in B denote the average trend in iWUE for the period before (1901–1963) and after (1963–2015) the identified breakpoint. Data for iWUE trends and the number of trees per chronology are listed in SI Appendix, Table S1.Globally, differences in vegetation physiognomy may have large impacts on iWUE, and despite much work examining plant physiological processes within biomes, there have been few large-scale comparisons of historical tree iWUE across biomes at a global scale (6, 8, 9, 12, 32). In this study, we found the rate of increase of iWUE from 1963 to 2015 differed among the 10 biomes represented (F = 7.21, P < 0.001) and ranged between 0.101 ± 0.07 µmol CO₂⋅mol⁻¹ H₂O⋅y⁻¹ for trees growing in deserts and xeric shrublands to 0.611 ± 0.07 µmol CO₂⋅mol⁻¹ H₂O⋅y⁻¹ for Mediterranean forests, woodlands, and scrub (SI Appendix, Fig. S3A and Table S2). Like deserts and xeric shrublands, the rate of iWUE increase since 1963 for trees growing in the tundra and in temperate grasslands, savannas, and shrublands was low, with a mean across the three biomes of 0.131 µmol CO₂⋅mol⁻¹ H₂O⋅y⁻¹. All other biomes exhibit mean rates of iWUE increase after 1963 greater than 0.409 µmol CO₂⋅mol⁻¹ H₂O⋅y⁻¹ (SI Appendix, Table S2). Furthermore, mean iWUE over 1963–2015 ranged from a low of 59.9 µmol CO₂⋅mol⁻¹ H₂O in tropical and subtropical moist broadleaf forests to a high of 91.5 µmol CO₂⋅mol⁻¹ H₂O in temperate conifer forests (SI Appendix, Fig. S3A).When considering changes in iWUE since 1963 with respect to wood anatomy, the rate of iWUE increase was ∼21% higher for conifers relative to diffuse porous trees (P = 0.022), but we found no difference in the rate of iWUE increase between conifer trees and ring porous trees (P = 0.61) or between diffuse porous trees and ring porous trees (P = 0.27) (SI Appendix, Fig. S4A). Our results are similar to climate-corrected iWUE trends from tree rings presented by Frank et al. (6) in European forests, but are in contrast to work by Saurer et al. (33) and Wang et al. (34) using tree rings and flux tower measurements, respectively, who showed broadleaf deciduous trees (i.e., ring porous and diffuse porous) in the Northern hemisphere having greater rates of increase in iWUE than conifer trees over the last century. It is important to note, however, that minor discrepancies among absolute iWUE values and their trends over time in these studies when compared to ours may have resulted from the formulation (i.e., our inclusion of a photorespiratory term) (35) and methodology differences (i.e., tree ring, flux tower) (36). We found no differences in the rate of iWUE increase since 1963 among trees with different leaf functional types (F = 1.05, P = 0.37) (SI Appendix, Fig. S4C). Additionally, we found mean iWUE during 1963–2015 was different among all wood types (F = 782.96, P < 0.001), being the highest in conifer species and lowest in ring porous species (SI Appendix, Fig. S4B), consistent with recent findings from 12 tree species at eight forested sites in the United States (9). Of important note, the patterns in mean iWUE for each wood type fall along a gradient of hydraulic conductivity and safety trade-offs (37). Conifer trees, which have relatively low hydraulic conductivity, but are more resistant to drought induced cavitation and embolism, had the highest mean iWUE, in contrast to ring porous trees, which have higher hydraulic conductivity, but are more vulnerable to hydraulic failure, which had lower mean iWUE (38–40). Mean iWUE was also different among trees with different leaf types, with needleleaf evergreen trees being the highest, followed by needleleaf deciduous trees, broadleaf evergreen trees, and finally broadleaf deciduous trees (SI Appendix, Fig. S4D).Changes in climate and C_a have strong effects on vegetation function from the leaf (14, 15) to global scale (13, 41, 42), yet how environmental change has influenced iWUE across large spatial scales over the last century is not fully resolved. Guerrieri et al. (9) and Frank et al. (6) recently showed increasing C_a and climate change have led to increased iWUE in tree species across the United States and Europe, but whether this response is conserved across biomes experiencing a much larger range in climate is still unknown. To address this knowledge gap, we used linear mixed effects (LME) models to examine the importance of environmental factors in driving tree iWUE. Across all species and study locations from 1963–2015, LME model results explained as much as ∼89% of the variance in iWUE and indicated a strong, positive relationship with C_a and growing season vapor pressure deficit (VPD_grw) and a negative relationship with growing season precipitation (PPT_grw), although a large proportion of the total variance was attributed to differences among sites (Table 1 and SI Appendix, Table S3). Remaining unaccounted variance may include nonclimatic factors, such as nitrogen availability or acidic air pollution, which are known to have important influences over iWUE (19, 43, 44), but these were not included in the studies from which we extracted isotopic data. When extending this analysis to changes in iWUE over the last 115 y (1901–2015), LME model main effects were remarkably consistent with data from 1963–2015, with the exception of growing season temperature (TMP_grw) having a marginally positive effect on iWUE across the 115-y chronology (SI Appendix, Table S4), which suggests TMP_grw is becoming more important in recent years.Table 1.LME model results and parameters for the best model (lowest AICc) examining the drivers of tree ring–derived iWUE for the period 1963–2015