Predictions of extinction risks due to climate change vary widely depending on a study’s assumptions, geographic region, and species focus. In a paper published by Mark Urban of the University of Connecticut, more than 130 of the existing studies on extinction were included in a meta-analysis to estimate the global mean rate of extinction we can expect due to anthropogenic (human-caused) climate change. Urban also determined which factor(s) contribute the most uncertainty to extinction risks.

Urban found that global extinction risks will increase as temperatures rise, putting up to 16 percent of the existing species at risk of extinction if the global climate were to rise 4.3 degrees Celsius above pre-industrial levels. He also found that projections of future temperature increases were the most influential factor driving global extinction risk.

Prior to this study, the existing predictions of extinction risks varied widely, ranging from zero percent to over half of species being at risk of going extinct due to climate change.

To provide what he hopes is more reliable information, Urban focused exclusively on multi-species studies to avoid biases that may exist in studies that focus on a single species. The overall proportion of global species threatened was estimated using a meta-analysis that took into account the variation among and within the studies that were considered, with each study weighted by sample size. Estimates of the variation in extinction risk were made for several predictors, including future global temperature increases, taxonomic groups of the species being studied, geographic regions, modeling techniques, and species dispersal assumptions.

The studies included in Urban’s analysis used a variety of techniques for estimating extinction risk, such as species-area relationships, correlations between current population distributions and predictions of future suitable habitats, demography, and expert opinions. However, most of the models ignored some of the factors considered essential to determining future extinction risk, such as species interactions, dispersal differences, and evolutionary forces.

In his analysis, Urban found that the amount of climate change was the most consistent predictor of extinction risk in these studies. At the current international target of two degrees Celsius of global warming, extinction risk for species worldwide will increase from 2.8 percent to 5.2 percent. If the Earth warms by three degrees Celsius, the worldwide extinction risk will jump to 8.5 percent of all species. If no serious policy intervention is implemented and the current emissions trajectory continues, the Earth’s climate will warm a total of 4.3 degrees Celsius; at that point, up to 16 percent of all global species will be at risk of extinction. In other words, business-as-usual means that one in six species will be at risk of extinction before the end of the century.

In terms of regional variation in extinction risk, species in North America and Europe had the lowest extinction risks, at just five and six percent respectively. By contrast, South America had a 23 percent extinction risk, while Australia and New Zealand had a 14 percent extinction risk. These three areas have diverse collections of species with limited habitat ranges, which may explain their higher risk. However, it is also possible that some lesser-studied regions (like Asia) have higher extinction risks—there isn’t enough data available on them at the moment. Refocusing research on less-closely examined regions would help to flesh out the literature.

Urban found that many studies used different assumptions, which influenced predictions of extinction risk.

For example, some studies assumed that 100 percent habitat loss was required for species extinction, whereas other studies used thresholds of 95 percent or 80 percent. Urban found that an 80 percent threshold predicted a 15 percent extinction risk, while requiring a 100 percent threshold predicted just a five percent extinction risk.

In his analysis, Urban also considered whether the studies included assumptions of species dispersal into novel habitats—some studies assumed no dispersal, while others assumed different dispersal options: into contiguous habitats, into habitats dependent on species’ ability, or even universal dispersal. He found that these assumptions significantly affected predictions of extinction risks. For example, assumption of universal dispersal placed extinction risk at 10 percent, while the assumption of no dispersal increased extinction risk to 12 percent.

The studies included in Urban’s analysis used different techniques to predict future extinctions, such as correlations between current distributions and climate or more sophisticated models. He found that modeling techniques had only a marginal effect on expected extinction risk, with the largest risks coming from models that included species-area relationships and expert opinions.

In his conclusions, Urban writes that extinction-risk-based analysis likely only captures a small subset of those species that are influenced by climate change—it doesn’t account for species that would face substantial changes in abundance and distribution. Very few studies specifically model other factors that could increase biological realism in extinction analyses, such as evolutionary adaptation, landscape dispersal barriers, and population trait distribution.

Urban has clearly identified a number of ways to improve future research in this area. But his most pronounced conclusion is that extinction risks from climate change are likely to increase with each degree rise in global temperature. Risking the extinction of up to 16 percent of all species on Earth would involve radical changes to the planet.

Science, 2015. DOI: 10.1126/science.aaa4984 (About DOIs).