Small increase in CO2 can have a major impact on global climate

Most climate models consider only short-term processes such as cloud and sea-ice formation when assessing Earth's sensitivity to greenhouse-gas forcing. Mounting evidence indicates that the response could be stronger if boundary conditions change drastically.

In the Early Pliocene, three to five million years ago, global temperatures were about 3–4° C warmer than today in the low latitudes, and up to 10° C warmer nearer the poles1, 2. Climate simulations and reconstructions of this relatively recent period (geologically speaking) may help constrain realistic magnitudes of future warming3. Under commonly assumed greenhouse forcing scenarios, atmospheric carbon dioxide concentrations of 500–600 ppmv — roughly twice the preindustrial level — would be required to produce the climate of the Pliocene. Writing in Nature Geoscience, Pagani et al. and Lunt et al. suggest that much lower carbon dioxide concentrations in the atmosphere governed the Early Pliocene warm period, with potentially dire implications for the long-term future of the planet.