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Anthropogenic emissions of greenhouse gases and ozone-depleting substances are expected to continue to affect concentrations of ozone in the stratosphere through the 21st century. While a range of estimates for when stratospheric ozone is expected to return to unperturbed levels is available in the literature, quantification of the spread in results is sparse. Here we present the first probabilistic study of latitudinally resolved years of return of stratospheric ozone to 1960 levels. Results from our 180-member ensemble, simulated with a newly developed simple climate model, suggest that the spread in return years of ozone is largest around 40°N/S and in the southern high latitudes and decreases with increasing greenhouse gas emissions. The spread in projections of ozone is larger for higher greenhouse gas scenarios and is larger in the polar regions than in the midlatitudes, while the spread in ozone radiative forcing is smallest in the polar regions.

[1] Episodic explosive volcanic eruptions are a natural part of the climate system but are often omitted from atmosphere-ocean general circulation model (AOGCM) preindustrial spin-up and control experiments. This omission imposes a negative bias on ocean heat uptake in simulations of the historical period. In models of a range of complexity, we find that global-mean sea level rise due to thermal expansion during the last ∼ 150 years is consequently underestimated by 5–30 mm, which is a substantial proportion of the model mean of 50 mm in Coupled Model Intercomparison Project Phase 3 AOGCMs with anthropogenic forcing only, and is therefore important in accounting for 20th century sea level rise. We test and recommend a procedure for removing the bias.

Precipitation changes are a key driver of climate change impacts. On average, global precipitation is expected to increase with warming. However, model projections show that precipitation does not scale linearly with surface air temperature. Instead, global hydrological sensitivity, the relative change of global-mean precipitation per degree of global warming, seems to vary across different scenarios and even with time. Based on output from 20 coupled Atmosphere-Ocean-General-Circulation-Models for up to 7 different scenarios, we discuss to what extent these variations can be explained by changes in the tropospheric energy budget. Our analysis supports earlier findings that long-and shortwave absorbers initially decrease global-mean precipitation. Including these absorbers into a multivariate scaling approach allows to closely reproduce the simulated global-mean precipitation changes. We find a sensitivity of global-mean precipitation to tropospheric greenhouse gas forcing of -0.42 +/- 0.23%/(W/m(2)) ( uncertainty given as one std of inter-model variability) and to black carbon emissions of -0.07 +/- 0.02%/(Mt/yr). In combination with these two predictors the dominant longer-term effect of surface air temperatures on precipitation is estimated to be 2.2 +/- 0.52%/K - much lower than the 6.5%/K that may be expected from the Clausius-Clapeyron relationship. .

[1] We present a comprehensive evaluation of uncertainties in the Global Warming Potential (GWP) and Global Temperature Change Potential (GTP) of CH4, using a simple climate model calibrated to AOGCMs and coupled climate-carbon cycle models assessed in the IPCC Fourth Assessment Report (AR4). In addition, we estimate uncertainties in these metrics probabilistically by using a method that does not rely on AOGCMs but instead builds on historical constraints and uncertainty estimates of current radiative forcings. While our mean and median GWPs and GTPs estimates are consistent with previous studies, our analysis suggests that uncertainty ranges for GWPs are almost twice as large as estimated in the AR4. Relative uncertainties for GTPs are larger than for GWPs, nearly twice as high for a time horizon of 100 years. Given this uncertainty, our results imply the possibility for substantial future adjustments in best-estimate values of GWPs and in particular GTPs.