Best expert evidence proves that climate change causes extinction and exacerbates every other impact.

Climate change is an existential threat to humans and global biodiversity, as was reinforced in the sixth assessment report (1) of the Intergovernmental Panel on Climate Change (IPCC), as well as Conferences of the Parties of the IPCC (Glasgow, UK; November 2021) and the UN Convention on Biological Diversity (Montreal, Canada; December 2022). But these gatherings also implicitly highlight severe shortcomings in how conservation scientists and policymakers are addressing the climate change threat.

Research in climate change ecology and conservation is still dominated by attempts to understand how species are vulnerable to the altered abiotic conditions driven by climate change alone (e.g., rising temperature) (2). This threat is undoubtedly important. But we argue that other facets of climate disruption could have vastly greater impacts—in particular, the ways in which climate-altered human societies, demographics, and economies interact with natural ecosystems and species. Unfortunately, these human dimensions of climate change ecology have received scant attention. Broad-scale assessment of the scale of biodiversity responses to climate change (3) are therefore underestimating the problem—potentially on a massive scale—when only incorporating direct, abiotic impacts.

Climate change is often considered to be one of the threats facing biodiversity, distinct from other threats and not considered—at least currently—to have as strong an impact as habitat loss and overexploitation (4, 5). But rather than being a separate factor, climate change magnifies these other threats and is now exacerbating all of the pathways by which humans directly cause species endangerment (Fig. 1). Policymakers and planners still do not recognize this combined impact when considering how to abate climate disruption and biodiversity loss. Indeed, it is these synergies, along with the pace of change, that make the current extinction crisis unique in Earth’s history. Although global climate change of about 6 °C caused Earth's biggest mass extinction event to date (at the end of the Permian), this phenomenon likely occurred an order of magnitude more slowly than what we are inducing today—and in a world without significant habitat loss, degradation, and other biodiversity threats from the actions of a single species.

Appreciating the Complexity

Although conservation efforts have been key to slowing recent losses of biodiversity, scientists and decision makers need to stop viewing climate change as just another separate threat to Earth’s biosphere. Climate change magnifies the intensity of the other drivers of biodiversity decline, such as habitat loss, overexploitation, and invasive species. Regional drying is making formerly remote areas of tropical rainforests accessible, thereby increasing hunting, fishing, and logging pressure (6). Warming can expand the cultivation zone of cold-intolerant crops upwards in elevation, threatening montane areas with deforestation and land conversion (7). Climate-facilitated expansion of oil palm (Elaeis guineensis) cultivation in Southeast Asia, for example, would threaten native species in these hyperdiverse systems much more than either habitat loss or climate change alone (7). Rising ocean temperatures can increase the spread of invasive species (8) and make environmental pollutants more toxic (9) (Fig. 1). Evidence increasingly shows that the destruction of coral reefs is particularly acute when warming waters act in combination with other stressors; plastic pollution, overfishing, and crown-of-thorns starfish (Acanthaster planci) outbreaks all interact synergistically with rising ocean temperatures to inhibit coral fertilization and larval recruitment (10). Loss of coral reefs can then cascade to harm at least two of the other highly diverse (and economically important) marine ecosystems, mangroves and seagrasses, through altered pH buffering, sediment capture, and abundance of shared animal species (11).

In turn, non-climatic biodiversity threats can intensify climate change impacts. Nutrient loading and turbidity from agricultural runoff and logging-induced erosion render corals more susceptible to mortality in the face of warmer temperatures and more-frequent storms (10). Fear of introducing potentially invasive species is slowing or preventing the widespread adoption of assisted colonization as a strategy to ameliorate climate change impacts (12); the bramble cay melomys (Melomys rubicola) is among the first documented extinctions from contemporary climate change—a loss that was preventable. Although ecologists have done a thorough job of addressing the direct impacts of changing abiotic conditions on organisms (e.g., 3), synergies between climate and other anthropogenic impacts have received much less attention.