Temperature increase of 21st century mitigation scenarios

Abstract

Estimates of 21st Century global-mean surface temperature increase have generally been based on scenarios that do not include climate policies. Newly developed multigas mitigation scenarios, based on a wide range of modeling approaches and socioeconomic assumptions, now allow the assessment of possible impacts of climate policies on projected warming ranges. This article assesses the atmospheric CO 2 concentrations, radiative forcing, and temperature increase for these new scenarios using two reducedcomplexity climate models. These scenarios result in temperature increase of 0.5-4.4°C over 1990 levels or 0.3-3.4°C less than the no-policy cases. The range results from differences in the assumed stringency of climate policy and uncertainty in our understanding of the climate system. Notably, an average minimum warming of Ϸ1.4°C (with a full range of 0.5-2.8°C) remains for even the most stringent stabilization scenarios analyzed here. This value is substantially above previously estimated committed warming based on climate system inertia alone. The results show that, although ambitious mitigation efforts can significantly reduce global warming, adaptation measures will be needed in addition to mitigation to reduce the impact of the residual warming.climate ͉ climate policy ͉ stabilization ͉ integrated assessment ͉ scenario A key indicator for climate change is the expected globalmean surface temperature increase. Future global temperature changes will be determined primarily by future emissions of greenhouse gases, ozone, and aerosol precursors and the response of the Earth system to those emissions. Any calculation of the potential range of future climate change requires consideration of both a plausible range of emissions scenarios and uncertainties in Earth system response, preferably by using results from multiple scenarios and models. The present analysis aims to map out the potential benefits of climate mitigation actions in terms of how much temperature increase can be avoided as a function of abatement effort. By including scenarios that are among the most stringent in the current literature, the analysis also provides quantitative insight into how much warming is likely to remain as a result of inertia within the energy system as well as the climate system. Such information is of critical importance in the climate policies that are currently being formulated.The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) (1) recently projected that by 2100, global mean surface temperature would increase by 1.1-6.4°C over the 1990 level using the range of illustrative baseline (nonmitigation) emissions scenarios from six energyeconomic modeling teams that had been developed in the IPCC Special Report on Emissions scenarios (SRES) (2) (the low end of the range results from the so-called B1 scenario; the upper range from the A1FI scenario). This uncertainty range originates both from the range in emissions scenarios and from the limited understanding of the climate system. Earlier, ...

Near-term climate mitigation by short-lived forcers

“…This is an inconsistent comparison, however, because actions to reduce CO 2 emissions will also result in reductions in methane and pollutant emissions because of many common sources of these emissions (18,25).…”

Differences between carbon budget estimates unravelled

“…Earlier studies have indeed shown that this is the case 7,8,18,19 , at least for peak warming and monotonously increasing cumulative carbon emissions. If a set carbon budget limit is exceeded, CO 2 needs to be removed actively from the atmosphere afterwards [20][21][22] to bring emissions back to within the budget.…”

Broader perspectives for comparing different greenhouse gases

“…If one considers pathways to climate stabilization, such as recently given in the reference concentration pathways (RCPs) [30,31], then it is clear that continuing updates of the GWP could have quite significant implications in the future. Along the RCP3 pathway, which is aimed at stabilizing global warming at 2 • C, the methane GWP would increase by about 33 per cent as the concentration of methane decreased to about 65 per cent of present-day levels, and CO 2 stabilized at levels about 10 per cent higher than its present-day levels.…”

“…Emission scenarios consistent with achieving climate stabilization have been developed in some detail [30,31], but changes in the emissions of different greenhouse gases that keep to the same total GWP-weighted emissions do not keep to the same RF over time. For example, if technological development led to faster reduction in methane emissions and there was a GWP-based compensating increase in CO 2 emissions, then the result would be inconsistent with climate stabilization.…”