Overestimate of committed warming

Abstract

Palaeoclimate variations are an essential component in constraining future projections of climate change as a function of increasing anthropogenic greenhouse gases. The Earth System Sensitivity (ESS) describes the multi-millennial response of Earth (in terms of global mean temperature) to a doubling of CO₂ concentrations. A recent study used a correlation of inferred temperatures and radiative forcing from greenhouse gases over the past 800,000 years to estimate the ESS from present day CO₂ is about 9°C, and to imply a long-term commitment of 3–7°C even if greenhouse gas levels remain at present-day concentrations. However, we demonstrate that the methodology of ref. does not reliably estimate the ESS in the presence of orbital forcing of ice age cycles and therefore conclude that the inferred present-day committed warming is considerably overestimated. There is a Reply to this Comment by Snyder, C. W. Nature 547, http://dx.doi.org/10.1038/nature22804 (2017).

Figure 1

a) Two examples of synthetic temperature and carbon dioxide cycles over 1 million years driven by an 80 kyr cycle in ice sheet forcing using a simple 3-component model for temperature, CO₂ and glacial ice (see Supplementary Information). b) The ratio of the scaled regression (the linear regression coefficient in K W⁻¹ m² multiplied by 3.7 W m⁻²) to the model ESS over a range of plausible parameter space.