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 reduced-complexity climate models. These scenarios result in temperature increase of 0.5-4.4 degrees C over 1990 levels or 0.3-3.4 degrees 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 approximately 1.4 degrees C (with a full range of 0.5-2.8 degrees 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.

Fig. 1.

Emissions of equivalent CO₂ under the baseline scenarios (A) and mitigation scenarios (B), comparison of cumulative CO₂ emissions (C), and the net present value (NPV) of abatement costs (D). Emissions in A and B are expressed in CO₂-equivalent emissions for illustrative purposes. The numbers used to identify the scenarios refer to actual forcing target used within the models. D shows the approximate NPV of abatement costs (see SI Text) as a function of year-2100 radiative forcing as calculated by MAGICC. The colors indicate the different grouping (black, baseline; light blue, 4.5 W/m² stabilization scenarios from EMF-21; dark blue, scenarios with higher stabilization targets than EMF-21; pink, scenarios with targets in between 3.5 and 4 W/m²; and purple, scenarios with targets <3.5 W/m²).

Fig. 2.

Radiative forcing and temperature change in year 2100 (A and B), transient temperature change (C) and 2100 temperature increase as a function of cumulative emissions (D). Radiative forcing relative to a preindustrial state and temperature change relative to 1980–2000 are given for baseline (red) and mitigation (blue) scenarios (A and B). Central values are shown as symbols and uncertainty ranges as color bands. Uncertainty ranges in MAGICC originate from the 19 MAGICC runs emulating different AOGCMS (mean ± 1 σ across 19 MAGICC runs) with darker area showing the impact of climate sensitivity only (CS; CS range is 2.0–4.9°C), and the lighter shaded uncertainty ranges show the combined effect of climate sensitivity (CS) plus carbon cycle response (CC) uncertainties (i.e., CS + CC). The full Bern2.5CC model ranges (CS + CC) were obtained by combining different assumptions about the behavior of the CO₂ fertilization effect, the response of heterotrophic respiration to temperature, and the turnover time of the ocean, thus approaching an upper boundary of uncertainties in the carbon cycle (CC), and additionally accounting for the effect of varying climate sensitivity (CS) from 1.5 to 4.5°C. C includes the increase of global mean temperature over time for MAGICC (using the same color codes and symbols as Fig. 1). D shows the temperature increase (mean and CS + CC range) for both climate models as a function of cumulative CO₂ emissions from 2000 to 2100.