Transient Climate Response – Declining Uncertainty as CO2 Forcing Dominates Climate Change into the Future

Australia: The Land Where Time Began

Transient Climate Response – Declining Uncertainty as CO₂ Forcing Dominates Climate Change into the Future

During the industrial era the largest portion of radiative forcing and change of surface temperature has been exerted by CO₂, though there have also been contributions from other anthropogenic influences (Myhre et al., 2013; Stocker et al., 2013). It has been difficult to derive climate sensitivity from historical observations as a result of large uncertainties in total forcing (Stocker et al., 2013; Aldrin et al., 2012; Andreae, Jones & Cox, 2005; Knutti & Hegerl, 2008; Otto et al., 2013; Roe & Amour, 2011). Between the 4^th and 5^th reports of the IPCC (IPCC; refs 1-8), anthropogenic forcing has increased, though there has been a decrease in its relative uncertainty. In this paper Myhre et al. have shown that this evolution towards lower uncertainty, which is based on data from the 2 reports, can be expected to continue into the future. The less uncertain CO₂ forcing is expected to become increasingly dominant, as it is easier to reduce air pollution than emissions of CO₂, and as a result of the long lifetime of CO₂. Myhre et al. have estimated, by the use of a statistical model, that in anthropogenic forcing of more than 40 % quoted in the latest IPCC report for 2011, the relative uncertainty will be almost halved by 2030, even if there is no better scientific understanding. If projected decreases in aerosols occur the uncertainty of absolute forcing will also decline for the first time. This stronger constraint on forcing, other factors being equal, will result in a significant reduction of uncertainty of observation-based estimates of the transient climate response, with a reduction of 50 % in its uncertainty rage expected by 2030.