Professor Myles Allen CBE FRS

Abstract:

Committed warming describes how much future warming can be expected from historical emissions due to inertia in the climate system. It is usually defined in terms of the level of warming above the present for an abrupt halt of emissions. Owing to socioeconomic constraints, this situation is unlikely, so we focus on the committed warming from present-day fossil fuel assets. Here we show that if carbon-intensive infrastructure is phased out at the end of its design lifetime from the end of 2018, there is a 64% chance that peak global mean temperature rise remains below 1.5 °C. Delaying mitigation until 2030 considerably reduces the likelihood that 1.5 °C would be attainable even if the rate of fossil fuel retirement was accelerated. Although the challenges laid out by the Paris Agreement are daunting, we indicate 1.5 °C remains possible and is attainable with ambitious and immediate emission reduction across all sectors.

Abstract:

Anthropogenic climate change is likely to increase the frequency of extreme weather events in future. Previous studies have robustly shown how and where climate change has already changed the risks of weather extremes. However, developing countries have been somewhat underrepresented in these studies, despite high vulnerability and limited capacities to adapt. How additional global warming would affect the future risks of extreme rainfall events in Bangladesh needs to be addressed to limit adverse impacts. Our study focuses on understanding and quantifying the relative risks of seasonal extreme rainfall events in Bangladesh under the Paris Agreement temperature goals of 1.5 °C and 2 °C warming above pre-industrial levels. In particular, we investigate the influence of anthropogenic aerosols on these risks given their likely future reduction and resulting amplification of global warming. Using large ensemble regional climate model simulations from weather@home under different forcing scenarios, we compare the risks of rainfall events under pre-industrial (natural), current (actual), 1.5 °C, and 2.0 °C warmer and greenhouse gas only (anthropogenic aerosols removed) conditions. We find that the risk of a 1 in 100 year rainfall event has already increased significantly compared with pre-industrial levels across parts of Bangladesh, with additional increases likely for 1.5 and 2.0 degree warming (of up to 5.5 times higher, with an uncertainty range of 3.5 to 7.8 times). Impacts were observed during both the pre-monsoon and monsoon periods, but were spatially variable across the country in terms of the level of impact. Results also show that reduction in anthropogenic aerosols plays an important role in determining the overall future climate change impacts; by exacerbating the effects of GHG induced global warming and thereby increasing the rainfall intensity. We highlight that the net aerosol effect varies from region to region within Bangladesh, which leads to different outcomes of aerosol reduction on extreme rainfall statistics, and must therefore be considered in future risk assessments. Whilst there is a substantial reduction in the impacts resulting from 1.5 °C compared with 2 °C warming, the difference is spatially and temporally variable, specifically with respect to seasonal extreme rainfall events.

Abstract:

Targets agreed to in Paris in 2015 aim to limit global warming to "well below 2 °C and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels". Despite the far-reaching consequences of this multi-lateral climate change mitigation strategy, the implications for global river flows remain unclear. Here we estimate the impacts of 1.5ºC vs 2.0ºC mitigation scenarios on peak flows by using daily river flow data from a multi-model ensemble which follows the HAPPI Protocol (that is specifically designed to simulate these temperature targets). We find agreement between models with regard to changing risk of river flow extremes. Moreover, we find that the response at 2.0°C is not a uniform extension of the response at 1.5º, suggesting a non-linear global response of peak flows to the two mitigation levels. Yet committing to the 1.5ºC warming target, rather than 2ºC, is projected to lead to an increase in the frequency of occurrence of extreme flows in several large catchments. In the most affected areas, predominantly in South Asia, while region-specific features such as aerosol loads may determine precipitation patterns, we estimate that under our 1.5ºC scenario the historical 1-in-100-year flow occurs with a frequency of 1-in-25 years. At 2.0ºC similar increases are observed in several global regions. These shifts are also accompanied by changes in the duration of rainy seasons which influence the occurrence of high flows.