On p. 109 is a scale of impacts and temperature rise:
The pre-2010 estimates on the impact on life had been: if +2 C, then 9%-31% of species will go extinct; if +4 C, then 40%-70% of species will go extinct. Urban's 2015 paper corrects this estimate. The new finding is based on a data synthesis (a meta-analysis and model-averaging approach) of climate-and-extinction studies published over the past five years, with some clean-up (like getting rid of outliers). The Science editorial sums it up:
Older studies show a wide variation of estimated losses, which is no big mystery, since estimates vary with temperature rise as well as with region. South America will be hit hardest, Europe will be spared the worst, and how hard Asia is going to be hit needs further research. (That South America may lose a quarter of its species makes sense due to its huge diversity in the Amazon Basin.) One factor that apparently does not create extra variability is taxonomic group. That's new. Conventional wisdom says that amphibians will suffer disproportionately, but the meta-analysis doesn't seem to bear this out. The upshot now (2015) is this:Biologists worry that the rapid rates of warming projected for the planet will doom many species to extinction. Species could face extinction with climate change if climatically suitable habitat disappears or is made inaccessible by geographic barriers or species' inability to disperse. Previous studies have provided region- or taxon-specific estimates of biodiversity loss with climate change that range from 0% to 54%, making it difficult to assess the seriousness of this problem. ... Urban provides a synthetic and sobering estimate of climate change–induced biodiversity loss by applying a model-averaging approach to 131 of these studies. The result is a projection that up to one-sixth of all species may go extinct if we follow “business as usual” trajectories of carbon emissions.
Four points are noteworthy:Overall, 7.9% of species are predicted to become extinct from climate change; … Results were robust to model type, weighting scheme, statistical method, potential publication bias, and missing studies … The factor that best explained variation in extinction risk was the level of future climate change. The future global extinction risk from climate change is predicted not only to increase but to accelerate as global temperatures rise … Global extinction risks increase from 2.8% at present to 5.2% at the international policy target of a 2°C post-industrial rise, which most experts believe is no longer achievable. If the Earth warms to 3°C, the extinction risk rises to 8.5%. If we follow our current, business-as-usual trajectory [4.3°C rise], climate change threatens one in six species (16%). Results were robust to alternative data transformations and were bracketed by models with liberal and conservative extinction thresholds. (Emphasis mine. Condensed and with annotations deleted; cf. Mark Urban, loc. cit., 571).
- The 2 C target seems no longer achievable. (As source for this consensus, Urban cites S. Fuss et al., "Betting on negative emissions." Nature Clim. Change 4, 850–853 (2014).)
- The 4 C trajectory is the now the likeliest track. On this track, one in six species will go extinct.
- This is bad but better than the 2009 Mann/Kump estimate of 40%-70% losses on this track.
- The conclusion of the negative impact of climate change on life remains unchanged.
In either of these cases, Mann/Kump would be right on target.