Viewpoint https://academic.oup.com/bioscience February 2020 / Vol. 70 No. 2 • BioScience 115 Planning for Change: Conservation-Related Impacts of Climate Overshoot CHRISTA M. ANDERSON, CHRISTOPHER L. WEBER, CHRISTO FABRICIUS, LOUISE GLEW, JEFF J. OPPERMAN, PABLO PACHECO, LINWOOD H. PENDLETON, DAVID THAU, SONJA J. VERMEULEN, AND M. REBECCA SHAW T he Intergovernmental Panel on  Climate Change’s (IPCC) special report on global warming of 1.5 degrees Celsius (°C) makes clear that most sce- narios (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 declin- ing to the end-of-century 1.5°C goal (IPCC 2018). An overshoot is also pos- sible for 2°C scenarios, given the lack of ambition in existing mitigation commit- ments. 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 conserva- tion may be large. Efforts to avoid an overshoot must be increased through more ambitious mitigation commit- ments and a greater focus on peak warming rather than end-of-century outcomes. Simultaneously, conserva- tion 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. Overshooting global warming targets 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 conserva- tion-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 fol- lowed by a period of cooling back to the temperature target—received sig- nificant attention in the report’s miti- gation pathways, the potential impacts of overshoot are mentioned only once in the widely read summary for poli- cymakers. That is, although most miti- gation 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 con- cern 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 consid- eration of the impacts of a 1.5°C over- shoot, 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 incorpo- rates an overshoot risk is also needed. We review in the present article what is known about the impacts of a tem- perature overshoot for conservation, showing that both climate and con- servation 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 main- taining average warming below 1.5°C for the entire century. Among the high-overshoot scenarios, the aver- age peak warming is 1.68°C (range, 1.6°C–1.85°C), and the average over- shoot 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 magni- tude, duration, and rate of temperature change, both increasing and decreasing (Ricke et al. 2017, IPCC 2018). Periods of overshoot that are higher in magni- tude 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 conserva- tion-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 Downloaded from https://academic.oup.com/bioscience/article/70/2/115/5670753 by guest on 09 July 2022