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Human pressure on the environment is driving a global decline of biodiversity. Anticipating whether this trend can be reverted under future scenarios is key to supporting policy decisions. We used the InSiGHTS framework to model the impacts of land-use and climate change on future habitat availability for 2,827 terrestrial mammals at 15 arcmin resolution under five contrasting global scenarios based on combinations of representative concentration pathways and shared socio-economic pathways between 2015 and 2050. Mammal habitat declined globally by 5%–16% depending on the scenario. Africa (with declines up to 25%) and South America were the most affected regions. African insectivores, primates, Australian carnivorous marsupials and marsupial moles, and South American opossums declined the most. Tackling this loss would require a mix of actions across scales, including a global shift toward sustainability, addressing land-use change in sub-Saharan Africa, and helping endemic species track climate change in South America.

Although it is widely accepted that future climatic change—if unabated—is likely to have major impacts on biodiversity, few studies have attempted to quantify the number of species whose populations have already been impacted by climate change. Using a systematic review of published literature, we identified mammals and birds for which there is evidence that they have already been impacted by climate change. We modelled the relationships between observed responses and intrinsic (for example, body mass) and spatial traits (for example, temperature seasonality within the geographic range). Using this model, we estimated that 47% of terrestrial non-volant threatened mammals (out of 873 species) and 23.4% of threatened birds (out of 1,272 species) may have already been negatively impacted by climate change in at least part of their distribution. Our results suggest that populations of large numbers of threatened species are likely to be already affected by climate change, and that conservation managers, planners and policy makers must take this into account in efforts to safeguard the future of biodiversity.

The effects of climate change on biodiversity are increasingly well documented, and many methods have been developed to assess species' vulnerability to climatic changes, both ongoing and projected in the coming decades. To minimize global biodiversity losses, conservationists need to identify those species that are likely to be most vulnerable to the impacts of climate change. In this Review, we summarize different currencies used for assessing species' climate change vulnerability. We describe three main approaches used to derive these currencies (correlative, mechanistic and trait-based), and their associated data requirements, spatial and temporal scales of application and modelling methods. We identify strengths and weaknesses of the approaches and highlight the sources of uncertainty inherent in each method that limit projection reliability. Finally, we provide guidance for conservation practitioners in selecting the most appropriate approach(es) for their planning needs and highlight priority areas for further assessments.

Assessing species' vulnerability to climate change is a prerequisite for developing effective strategies to conserve them. The last three decades have seen exponential growth in the number of studies evaluating how, how much, why, when, and where species will be impacted by climate change. We provide an overview of the rapidly developing field of climate change vulnerability assessment (CCVA) and describe key concepts, terms, steps and considerations. We stress the importance of identifying the full range of pressures, impacts and their associated mechanisms that species face and using this as a basis for selecting the appropriate assessment approaches for quantifying vulnerability. We outline four CCVA assessment approaches, namely trait‐based, correlative, mechanistic and combined approaches and discuss their use. Since any assessment can deliver unreliable or even misleading results when incorrect data and parameters are applied, we discuss finding, selecting, and applying input data and provide examples of open‐access resources. Because rare, small‐range, and declining‐range species are often of particular conservation concern while also posing significant challenges for CCVA, we describe alternative ways to assess them. We also describe how CCVAs can be used to inform IUCN Red List assessments of extinction risk. Finally, we suggest future directions in this field and propose areas where research efforts may be particularly valuable. This article is categorized under: Climate, Ecology, and Conservation > Extinction Risk Assessing species' vulnerability to climate change is becoming a prerequisite for conservation planning, but choosing approaches, methods and data can be challenging. Key to informing such choices is consideration of the full range of climate change pressures and their likely mechanisms of impact on individuals, subpopulations and species. Navigate a sound path through do's and don'ts, and explore resources and future perspectives in this exciting field.

In the Supplementary Information PDF published with this Letter, under the heading ‘Keywords used to select the papers for the literature review’, operator (*) use in the search strings is inconsistent and at times incorrect. In addition, the full list of references shortlisted from the Web of Science search criteria used in this study was not provided. The amended PDF is available as Supplementary Information to this Correction; those references not cited in the main paper and Methods have been included: refs 59–124 relate to mammals and refs 125–190 to birds.