TODAY’S STUDY: THE YEAR CLIMATE CHANGE WILL ARRIVE

The projected timing of climate departure from recent variability

Camilo Mora, et. al., 10 October 2013 (Nature)

Abstract

Ecological and societal disruptions by modern climate change are critically determined by the time frame over which climates shift beyond historical analogues. Here we present a new index of the year when the projected mean climate of a given location moves to a state continuously outside the bounds of historical variability under alternative greenhouse gas emissions scenarios. Using 1860 to 2005 as the historical period, this index has a global mean of 2069 (±18 years s.d.) for near-surface air temperature under an emissions stabilization scenario and 2047 (±14 years s.d.) under a ‘business-as-usual’ scenario. Unprecedented climates will occur earliest in the tropics and among low-income countries, highlighting the vulnerability of global biodiversity and the limited governmental capacity to respond to the impacts of climate change. Our findings shed light on the urgency of mitigating greenhouse gas emissions if climates potentially harmful to biodiversity and society are to be prevented.

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Introduction

Climate is a primary driver of biological processes, operating from individuals to ecosystems, and affects several aspects of human life. Therefore, climates without modern precedents could cause large and potentially serious impacts on ecological and social systems1, 2, 3, 4, 5. For instance, species whose persistence is shaped by the climate can respond by shifting their geographical ranges4, 5, 6, 7, remaining in place and adapting5, 8, or becoming extinct8, 9, 10, 11. Shifts in species distributions and abundances can increase the risk of extinction12, alter community structure3 and disrupt ecological interactions and the functioning of ecosystems. Changing climates could also affect the following: human welfare, through changes in the supply of food13 and water14, 15; human health16, through wider spread of infectious vector-borne diseases17, 18, through heat stress19 and through mental illness20; the economy, through changes in goods and services21, 22; and national security as a result of population shifts, heightened competition for natural resources, violent conflict and geopolitical instability23. Although most ecological and social systems have the ability to adapt to a changing climate, the magnitude of disruption in both ecosystems and societies will be strongly determined by the time frames in which the climate will reach unprecedented states1,2. Although several studies have documented the areas on Earth where unprecedented climates are likely to occur in response to ongoing greenhouse gas emissions24, 25, our understanding of climate change still lacks a precise indication of the time at which the climate of a given location will shift wholly outside the range of historical precedents.

To provide an indication of the projected timing of climate departure under alternative greenhouse gas emissions scenarios, we have developed an index that determines the year when the values of a given climatic variable exceed the bounds of historical variability for a particular location (Fig. 1a). We emphasize that although our index commonly identifies future dates, this does not imply that climate change is not already occurring. In fact, our index projects when ongoing climate change signals the start of a radically different climate. For this analysis we used the projections of 39 Earth System Models developed for the Coupled Model Intercomparison Project phase 5 (CMIP5). The bounds of climate variability were quantified as the minimum and maximum values yielded by the Earth System Models with the CMIP5 ‘historical’ experiment, which for all models included the period from 1860 to 2005. This experiment included observed changes in atmospheric composition (reflecting both anthropogenic and natural sources) and was designed to model the climate’s recent past and allow the validation of model outputs against available climate observations26. The year at which a climate variable moves out of the historical bounds was estimated independently with data from the Representative Concentration Pathways 4.5 (RCP45) and 8.5 (RCP85), which included the period from 2006 to 2100. These pathways or scenarios represent contrasting mitigation efforts between a concerted rapid CO2 mitigation and a ‘business-as-usual’ scenario (CO2 concentrations could increase to 538 and 936 p.p.m. by 2100, according to RCP45 and RCP85, respectively27, 28). A more aggressive mitigation scenario (RCP 2.6) was not analysed, because it was not consistently used among models, and the implicit mitigation effort is considered currently unfeasible29.

We analysed five climate variables for the atmosphere and two for the oceans (Extended Data Table 1). However, we report results with mean annual near-surface air temperature as our indicator of the climate, unless otherwise specified. Simulated and actual measurements of temperature for the period 1986–2005 were remarkably similar (Extended Data Fig. 1). In comparison with any individual model’s results, the multi-model average best fitted the actual data (Extended Data Fig. 1). We therefore describe our results on the basis of multi-model averages. We show standard deviations to report the spatial variability in our results. Multi-model uncertainty (that is, the variability between models in the predicted years) was measured as the standard error of the mean, which for near-surface air temperature had a global value of 4.2 years for RCP45 and 2.7 years for RCP85 (Extended Data Fig. 2a). Multi-model uncertainty for all variables is shown inExtended Data Fig. 2.

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Sensitivity of the Index

Three factors could affect the result of our index: first, the number of years used as the historical reference period (a longer historical reference period could yield broader bounds of climate variability); second, the number of consecutive years out of historical bounds in order to declare the timing of climate departure (for example, one single year out of historical bounds will probably occur sooner than several consecutive years); and third, the extent to which the historical reference period has been affected by anthropogenic greenhouse gases (the use of a period that already contains anthropogenic effects would yield broader bounds of climate variability).

To address the first two concerns we calculated the year when the climate exceeded the bounds of historical variability, using different historical reference periods and varying numbers of consecutive years out of the historical bounds. We found that the year in which the climate exceeded the bounds of historical variability changed minimally when using historical time bins ranging from 20 to 140 years (Fig. 1b). Despite the fact that our analysis was constrained by the 140 years of available model data, the observed relationship between historical time bins (X) and the year exceeding bounds of historical climate variability (Y) showed a strong correlation for both RCPs (Y = 1.2773 ln X + 2063.2 for RCP45; Y = 1.0865 ln X + 2042.2 for RCP85 (dashed line in Fig. 1b); r2 = 0.99 and P < 0.001 in both cases). Under both RCPs, extrapolating these equations from a 140-year time bin to a 1,000-year time bin increased the estimated year exceeding the bounds of historical climate variability by only about 2 years. In contrast, the year at which the climate exceeded the bounds of historical variability was sensitive to the number of consecutive years out of historical bounds considered (Fig. 1c). As illustrated for the location in Fig. 1a, the climate will experience three consecutive years out of historical bounds by 2012, 11 consecutive years by 2023, and all consecutive years after 2036. To ensure robustness in our results we used the minimum and maximum values in the entire time series of the historical experiment from 39 Earth System Models available, suggesting that our results included the broadest historical climate bounds possible, given available data. Similarly, we used the first year after which all values would continuously exceed the bounds of historical climate variability.

To address the third concern we compared our results from the historical experiment with those obtained from an additional CMIP5 experiment, ‘historicalNat’, which simulated the same time span as the historical experiment but with only natural forcing (for example volcanoes and solar variability), while excluding anthropogenic greenhouse gas emissions26. The results of this analysis (Fig. 1d) indicated that the climate surpasses the bounds of historical variability about 18.5 years earlier under RCP45, and 11.5 years earlier under RCP85 when using historical simulations that excluded anthropogenic greenhouse-gas forcing (historicalNat experiment) compared with those that included it (historical experiment). We did not use the historicalNat experiment in our main results because it was available for only 17 out of 39 Earth System Models in CMIP5 (Extended Data Table 1) and this would have sacrificed the robustness obtained by using all available models. However, the sensitivity test above provides a quantitative measure of the likely adjustment in the projected timing of climate departure if historic periods without human effects were used to quantify past climate variability, and suggests that the reported values using the historical experiment are highly conservative.

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The timing of climate departure

We found that the year at which the climate exceeds the bounds of historical variability depended on the modelled scenario. Under RCP45, the projected near-surface air temperature of the average location on Earth will move beyond historical variability by 2069 (±18 years s.d., 56 years in the future; solid blue line in Fig. 2d) and two decades earlier under RCP85 (that is, 2047 (±14 years s.d.), 34 years in the future; solid red line in Fig. 2d). These results are sobering indicators of the pace of climate change if one considers that the timing of climate departure will occur sooner if ‘pristine’ climate conditions (that is, the historicalNat experiment) are used to set historical climate bounds: 37.5 years in the future under RCP45, or 22.5 years under RCP85. For the ocean, the historical bounds of sea surface temperature will be surpassed on average by 2072 (±17 years s.d.) under RCP45 and by 2051 (±16 years s.d.) under RCP85 (Extended Data Fig. 3). When the index is calculated using monthly values (see Methods), all consecutive months will be out of the monthly historical bounds later in the century (Fig. 2b, dotted lines in Fig. 2d; see also Extended Data Fig. 3b, d). The estimated year when the climate exceeds historical variability is delayed when using monthly instead of annual averages, because one anomalous year is not necessarily caused by all months being extreme; thus, an anomalous year on average is likely to occur earlier than the year for which all months fall outside the monthly climate bounds. It is remarkable, however, that after 2050 most tropical regions will have every subsequent month outside of their historical range of variability (Fig. 2b). Although this is later than the yearly averages, we stress that this is an extreme situation, in which every month will be an extreme climatic record.

We used mean annual near-surface air temperature as the main proxy for the climate. However, other climate variables can change earlier or later than temperature in response to greenhouse gas emissions. To assess this effect, we analysed additional climate variables under both emissions scenarios and provided an overall climate assessment by estimating the year at which the first climate variable exceeded its historical bounds of variability. These variables included evaporation, transpiration, sensible heat flux and precipitation for the Earth’s atmosphere, and surface pH for the ocean. For the atmosphere, the projected timing of climate departure did not change when considering other climate variables along with air temperature (Extended Data Fig. 4). This occurred because air temperature will experience the earliest and most sustained changes outside historical climate bounds in comparison with other atmospheric variables (that is, other atmospheric variables will continuously surpass their historical variability later than temperature; Extended Data Fig. 4). However, the projected timing of the ocean’s climate departure was pushed forward to this decade when pH was considered alongside sea surface temperature. Global mean ocean pH moved outside its historical variability by 2008 (±3 years s.d.), regardless of the emissions scenario analysed (Extended Data Fig. 4). This result, which is consistent with recent studies30, is explained by the fact that ocean pH has a narrow range of historical variability and that a considerable fraction of anthropogenic CO2 emissions has been absorbed by the ocean30, 31.

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Timing and absolute changes in climate

Absolute changes in the climate are often the means of detecting or assessing climate change and are expected to be considerably larger at higher latitudes (Fig. 2c; see also ref. 25). Measures of absolute changes in the climate have also dominated the dialogue on climate change (for example, avoiding 2 °C warming is a broadly recognized goal among scientists, policy makers and the public, because such change is forecast to generate deleterious consequences for society and the environment25, 32). However, we found poor spatial correlation (Fig. 2e) between the absolute change in the climate expected by the year 2100 (Fig. 2c) and the year at which the climate would surpass historical precedents (Fig. 2a); this pattern was consistent among other climate variables (Extended Data Fig. 4). This result suggests that some aspects of climate change, which may be detrimental to biodiversity, are poorly accounted for by metrics of absolute changes in the climate; and implies that global biodiversity could face a climate change ‘double jeopardy’ from either large absolute changes or the early arrival of unprecedented climates.

We also found that the tropics will experience the earliest emergence of historically unprecedented climates (Fig. 2a, b). This probably occurs because the relatively small natural climate variability in this region of the world generates narrow climate bounds that can be easily surpassed by relatively small climate changes. However, small but fast changes in the climate could induce considerable biological responses in the tropics, because species there are probably adapted to narrow climate bounds5, 33, 34, 35. This is a prime explanation for the decline in the range sizes of species towards lower latitudes (Rapoport’s rule): having narrower tolerances, tropical species are largely restricted to the tropics; in comparison, the broader physiological tolerances of temperate species allow them to survive across a broader latitudinal span33. Furthermore, empirical and theoretical studies in corals5, 36, 37, terrestrial ectotherms34 and plants and insects35 show that tropical species live in areas with climates near their physiological tolerances and are therefore vulnerable to relatively small climate changes.

The earliest emergence of unprecedented climates in the tropics and the limited tolerance of tropical species to climate change are troublesome results, because most of the world’s biodiversity is concentrated in the tropics (Extended Data Fig. 5; see also ref. 33). We found that, on average, the projected timing of climate departure in marine and terrestrial biodiversity hotspots (sensu ref.38, the top 10% most species-rich areas on Earth where a given taxon is found) will occur one decade earlier than the global average under either emissions scenario (Fig. 3). Coral hotspots will experience the earliest arrival of unprecedented climates: 2050 under RCP45 (about 23 years earlier than the global average), or 2034 under RCP85 (about 17 years earlier than the global average) (Fig. 3). With the exception of marine birds, whose hotspots are located at high latitudes (Extended Data Fig. 5d; see also ref. 39), unprecedented climates will occur at the latest by 2063 (RCP45) or 2042 (RCP85) in the hotspots of all other taxa considered (Fig. 3). Overall these results suggest that the overarching effect of climate change on biodiversity may occur not only as a result of the largest absolute changes in climate at high latitudes but also perhaps more seriously from small but prompt changes in the tropics. In short, the tropics will be highly vulnerable to climate change for at least three reasons: first, the earliest emergence of unprecedented climates will be there; second, tropical species are more vulnerable to small climate changes; and third, this region holds most of the Earth’s species.

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Discussion

The biological responses expected from the rapid emergence of historically unprecedented climates are likely to be idiosyncratic40 and will depend on attributes such as species adaptive capacity, current genetic diversity, ability to migrate, current availability of habitats, disruption of ecological interactions, and ecological releases40, 41, 42, 43, 44. Although the extent of these responses in the future has been a topic of debate45, 46, considerable changes in community structure3 and extinction10 have been shown to have coincided with the emergence of unprecedented climates in the past. In addition, recent short-term extreme climatic events have been associated with die-offs in terrestrial47, 48, 49 and marine37 ecosystems, highlighting the potentially serious consequences of reaching historically unprecedented climates. Unfortunately, key conservation strategies such as protected areas, which may ameliorate the extent of several anthropogenic stressors, are unlikely to provide refuge from the expected effects of climate change, because protected areas within biodiversity hotspots will experience unprecedented climates at the same time as non-protected hotspot areas (Fig. 4a; see also ref. 50). The expansion and/or effectiveness of protected areas and other conservation strategies could be further impaired by limited governmental capacity, because the earliest emergence of unprecedented climates will occur among hotspots predominantly located in low-income countries

The emergence of unprecedented climates could also induce responses in human societies1, 2, 13,14, 15, 16, 19, 20, 21, 22, and the resulting adjustments could be considerable because according to RCP45 roughly 1 billion people (about 5 billion people under RCP85) currently live in areas where climate will exceed historical bounds of variability by 2050 (Fig. 5a). The fact that the earliest climate departures occur in low-income countries (Fig. 5b) further highlights an obvious disparity between those who benefit economically from the processes leading to climate change and those who will have to pay for most of the environmental and social costs. This suggests that any progress to decrease the rate of ongoing climate change will require a bigger commitment from developed countries to decrease their emissions but will also require more extensive funding of social and conservation programmes in developing countries to minimize the impacts of climate change. Our results on the projected timing of climate departure from recent variability shed light on the urgency of mitigating greenhouse gas emissions if widespread changes in global biodiversity and human societies are to be prevented…

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