Planning for Change: Conservation-Related Impacts of Climate Overshoot

The Intergovernmental Panel on Climate Change's (IPCC) special report on global warming of 1.5 degrees Celsius (°C) makes clear that most scenarios (90%) that hold warming to 1.5°C by 2100 include an overshoot, or a period in which the temperature increase exceeds 1.5°C before declining to the end-of-century 1.5°C goal (IPCC 2018). An overshoot is also possible for 2°C scenarios, given the lack of ambition in existing mitigation commitments. Current conservation policy and planning does not adequately account for the high likelihood of a temperature overshoot in a 1.5°C scenario, but the impacts of an overshoot on conservation may be large. Efforts to avoid an overshoot must be increased through more ambitious mitigation commitments and a greater focus on peak warming rather than end-of-century outcomes. Simultaneously, conservation planning should account for such impacts by anticipating more dynamic systems that carry greater uncertainties and potentially irreversible changes that may persist even as temperatures peak and decline. The IPCC special report on global warming of 1.5°C (SR1.5) made a compelling case for limiting warming to 1.5°C, including by highlighting the key differences between impacts at 1.5°C and 2°C for several conservation-relevant outcomes (IPCC 2018). However, although the concept of a temperature overshoot—a period of warming above a temperature target (1.5°C in the case of SR1.5) that is followed by a period of cooling back to the temperature target—received significant attention in the report's mitigation pathways, the potential impacts of overshoot are mentioned only once in the widely read summary for policymakers. That is, although most mitigation pathways for 1.5°C include a period of overshoot, the impacts of 1.5°C are often focused on stabilized warming of 1.5°C, in part because of insufficient understanding of the impacts accrued during the overshoot. Overshoot has been a topic of concern in climate change literature for some time (Huntingford and Lowe 2007), including the acknowledgment that overshoot pathways to reaching a temperature goal can increase the risks of undesirable impacts relative to no-overshoot pathways (Schneider and Mastrandrea 2005). SR1.5 both highlights the need for further consideration of the impacts of a 1.5°C overshoot, including irreversible impacts, and cautions that there is still some uncertainty about the feasibility of decreasing temperatures after a peak (IPCC 2018). In light of this uncertainty, more ambitious mitigation commitments must be undertaken to reduce the likelihood of an overshoot and its impacts. In addition, a greater focus on peak warming rather than an endpoint temperature (in 2100) would improve the assessment of impacts (Rogelj et al. 2019). Given that an overshoot is likely even if climate commitments are greatly increased to meet a 1.5°C target, conservation planning that incorporates an overshoot risk is also needed. We review in the present article what is known about the impacts of a temperature overshoot for conservation, showing that both climate and conservation communities must urgently strive to better understand and plan for the ecological and social impacts of an overshoot, some of which may be irreversible. By undertaking flexible and adaptive management (McCarthy and Possingham 2007) that anticipates an overshoot, some impacts may be ameliorated. Most of the 90 scenarios analyzed in SR1.5 resulting in end-of-century warming of less than 1.5°C (n = 81) are identified as either a high or a low overshoot, with only nine maintaining average warming below 1.5°C for the entire century. Among the high-overshoot scenarios, the average peak warming is 1.68°C (range, 1.6°C–1.85°C), and the average overshoot duration is 50 years (range, 31–68 years), whereas the equivalents for the low-overshoot scenarios are 1.56°C (range, 1.51°C–1.6°C) and 30 years (range, 13–54 years; using the reduced complexity climate model MAGICC6). The expected impacts of an overshoot on the conservation of natural systems are related to the overshoot's magnitude, duration, and rate of temperature change, both increasing and decreasing (Ricke et al. 2017, IPCC 2018). Periods of overshoot that are higher in magnitude and have a faster rate of change may have greater impacts than those with a lower magnitude and slower change rate (which, in some cases, may have minimal or undetectable impacts; IPCC 2018). In general, the conservation-related impacts of an overshoot will be both direct, such as the loss of species habitat, and indirect, such as habitat conversion caused by human migration in response to the effects of an overshoot, as is explored below. Despite the significant remaining uncertainty regarding specific impacts of an overshoot on ecosystems, a reasonable starting point to explore the potential effects of an overshoot is a focus on ecological systems that are expected to face a transition in risk level within the range of global temperatures expected in an overshoot scenario. This requires comparing the potential shapes of overshoot mitigation scenarios with what is known about transitions in risk at different global temperature levels (Warszawski et al. 2013, IPCC 2018, Warren et al. 2018). Risk level transitions are estimated by expert judgment in ranges, so they necessarily include uncertainty and cannot indicate specific risks. However, this approach represents a useful first step in highlighting where further research on overshoot impacts is necessary. For example, figure 1 illustrates ­typical mitigation scenarios from SR1.5 next to expert-assessed risks for representative systems. Taking the system perhaps most closely linked to conservation, terrestrial ecosystems, risk shifts from moderate risk (yellow) to high risk (red) from 1.5°C to 1.8°C warming (Warszawski et al. 2013, IPCC 2018). Because the risks to terrestrial systems are expected to change in the overshoot temperature range, considerable potential exists for a difference in cumulative impacts between a scenario with no overshoot of 1.5°C (figure 1 line (a)) and a scenario of low (b) or high (c) overshoot of 1.5°C. The low 2°C scenario (d) might also be interpreted as a very long duration overshoot of 1.5°C, on the order of 120 years (IPCC 2018; MAGICC6 model). (An overshoot of such long duration includes a suite of additional challenges not discussed in the present article.) Similarly, the Arctic region shifts from high risk (red) to very high risk (purple) from 1.5°C to 2°C. Warm-water corals, in contrast, are under very high risk (purple) through the 1.5°C to 2°C range (figure 1). Additional systems and risks ranging from soil erosion and wildfire damage to effects on kelp forests and sandy beaches have been assessed using the same method in recent IPCC reports on land and on the oceans and cryosphere (IPCC 2019a, 2019b). Of course, understanding whether these ecosystem risks will translate to impacts and how conservation management should address these impacts requires further consideration, including whether the impacts are reversible or not. 1.5°C overshoot scenarios compared next to risk levels from the IPCC burning embers diagram for terrestrial ecosystems (IPCC 2018; figure SPM.2). This comparison illustrates that overshoot scenarios (lines (b) and (c)) may coincide with changes in risk level for some systems. For example, terrestrial ecosystems shift from moderate risk (yellow) into the high (red) risk zone from 1.5°C to 1.8°C and the arctic region shifts from high risk (red) to very high risk (purple) from 1.5°C to 2°C. Warm-water corals, in contrast, are under very high risk (purple) through the 1.5°C to 2°C range. Temperature profiles are shown for (a) no overshoot, (b) 1.5°C low overshoot, (c) 1.5°C high overshoot, (d) low 2°C, and (e) high 2°C scenarios (MAGICC6 model). 1.5°C overshoot scenarios compared next to risk levels from the IPCC burning embers diagram for terrestrial ecosystems (IPCC 2018; figure SPM.2). This comparison illustrates that overshoot scenarios (lines (b) and (c)) may coincide with changes in risk level for some systems. For example, terrestrial ecosystems shift from moderate risk (yellow) into the high (red) risk zone from 1.5°C to 1.8°C and the arctic region shifts from high risk (red) to very high risk (purple) from 1.5°C to 2°C. Warm-water corals, in contrast, are under very high risk (purple) through the 1.5°C to 2°C range. Temperature profiles are shown for (a) no overshoot, (b) 1.5°C low overshoot, (c) 1.5°C high overshoot, (d) low 2°C, and (e) high 2°C scenarios (MAGICC6 model). SR1.5 documented thoroughly that impacts are expected to be more severe at 2°C of warming than at 1.5°C (IPCC 2018), and although fewer studies have assessed specific warming differences at smaller increments, it is likely that some impacts will gradually worsen as warming occurs. For example, at 1.5°C of warming, 6% of insects, 8% of plants, and 4% of vertebrates are projected to lose more than half of their range, whereas, at 2°C, the impacts double to 18% of insects, 16% of plants, and 8% of vertebrates (Warren et al. 2018). These percentages apply just to a warming of 1.5°C, not taking account of the likely overshoot involved in meeting a 1.5°C target. Impacts like these may be reversible if entire species and ecosystems aren’t lost and if appropriate conservation measures are undertaken. However, even when the impacts are reversible, the pattern of change may not be the same for the warming and cooling period of an overshoot. For example, species that migrate to more suitable climates during warming may need to migrate again during cooling (Warren et al. 2018), and migration back to their previous habitat may be affected by changes to migration pathways or characteristics of the original habitat arising from the overshoot (Williams and Blois 2018). Expected local variability in warming and in the pattern of the overshoot would also affect patterns of ecosystem change and migration. Some overshoot impacts may be irreversible, involving crossing thresholds that cause the loss of ecosystems (IPCC 2018), ecosystem transformation (Nolan et al. 2018), or regime shifts (Chevalier and Grenouillet 2018). In forest ecosystems, for example, impacts that become more frequent as temperature rises, such as forest fires and pest outbreaks (Anderegg et al. 2015), can stimulate ecosystem transformation and may cause greater forest mortality during an overshoot period than would be expected in a no-overshoot scenario. That is, even if temperatures decrease following an overshoot, forest fire or pest outbreaks may have already caused severe and lasting impacts on the conservation of forest ecosystems. Warm-water corals are perhaps the most frequently cited system that may face irreversible impacts (IPCC 2018). They are expected to be among the most affected by even limited warming (IPCC 2019b) and thus also by an overshoot, for those that survive under very high risk conditions (figure 1). Emerging evidence finds that natural selection against heat sensitive species (Hughes et al. 2019) may mitigate ecosystem-scale mortality. However, it is unclear whether corals would survive overshoot periods, and those that do may not be able to fully recover (IPCC 2018). Across many ecosystems, the magnitude of indirect impacts of an overshoot related to changes in land use, water, and agriculture may, at times, be greater than the direct impacts discussed above (Turner et al. 2010). For example, the land-use change required for carbon dioxide removal to decrease temperatures in the cooling period of an overshoot, particularly bioenergy with carbon capture and storage, are large—on the order of millions of hectares (IPCC 2018), which would have enormous impacts on the conservation of terrestrial and freshwater systems. In addition to these mitigation-based indirect impacts, the adaptation-based indirect impacts may also be considerable (Turner et al. 2010). For example, for some ecosystems, the most important impacts of an overshoot for conservation may be related not to direct ecosystem changes but to ecosystem impacts that occur through human responses to climate change, such as land-use change attributable to overshoot-induced human migration (Turner et al. 2010). Human migration to avoid drought, floods, and sea-level rise could affect ecosystems severely, with impacts ranging from land-use conversion for agriculture (Turner et al. 2010) to the anthropogenic spread of invasive biota (Shackleton et al. 2018). These impacts may be greater in an overshoot scenario than in a scenario of stabilized warming because migration may occur differentially both as the climate warms and again when it cools. An overshoot period may also increase the challenges associated with increased frequency or intensity of droughts and floods projected to occur because of climate change (Greve et al. 2018), with attendant indirect impacts on conservation. For example, in regions that experience higher flood risk during the overshoot period (Greve et al. 2018), management responses, such as the construction of infrastructure, including dams and levees, would have impacts on river and floodplain ecosystems that would persist long after the overshoot period. The impacts of a temperature overshoot on food systems have the potential to be severe as well and would therefore have indirect impacts on conservation. Failures in food production can lead to shifts to areas more suitable for crop growth, resulting in the conversion of natural habitats. Furthermore, the impacts of an overshoot could include decreased food security across the globe, with possible cascade effects on human nutrition, well-being, migration, and agricultural development (Schleussner et al. 2016). What is known about these impacts has infrequently been included in assessments of an overshoot. The best solution to these vexing problems, although it is exceedingly challenging, is to avoid an overshoot and its associated impacts by immediately and rapidly decreasing emissions and maintaining a mitigation scenario for a temperature change of below 1.5°C. Simultaneously however, broader work must be pursued to build climate change projections into mainstream conservation planning, including the need to address overshoot scenarios specifically. Such efforts would include the implementation of many of the strategies already developed for conservation management. Broadly, adaptive management through the overshoot period (McCarthy and Possingham 2007) will be needed, including targeting conservation efforts to areas of particular vulnerability (Dawson et al. 2011). Conservation planning that addresses reversible impacts will include approaches such as conservation corridors to allow for migration (Keeley et al. 2018). For impacts related to an overshoot's cooling phase, conservation management lessons may be drawn from existing knowledge of ecological restoration (Jackson and Hobbs 2009). In cases in which large ecosystem changes are unavoidable, deployment of conservation management can lessen impacts (Millar and Stephenson 2015). This set of strategies would support conservation that addresses spatial and temporal flexibility and shifting conditions on decadal time scales, including both warming and cooling, rather than managing for conditions of an eventual target temperature. Given the high likelihood of overshooting the 1.5°C goal, conservation planning must account for the direct conservation implications of an overshoot and consider the impacts of human responses to it. Christa M. Anderson (christa.anderson@wwf.org), Christopher L. Weber, Christo Fabricius, Louise Glew, Jeff J. Opperman, Pablo Pacheco, Linwood H. Pendleton, David Thau, and M. Rebecca Shaw are affiliated with the World Wildlife Fund, in Washington, DC. Sonja J. Vermeulen is affiliated with the CGIAR, in Montpellier, France. Christo Fabricius is affiliated with the Sustainability Research Unit of Nelson Mandela University, in George, South Africa. Linwood H. Pendleton is affiliated with Duke University, in Durham, North Carolina. Linwood H. Pendleton is affiliated with the Global Change Institute at the University of Queensland, in St Lucia, Australia. Linwood H. Pendleton is also affiliated with the University of Brest, in Plouzane, France.