Professor Myles Allen CBE FRS

Abstract:

Energy policy faces a triple challenge: increasing resilience and guaranteeing the security of supply of both fossil and non-fossil energy, minimising the impact on consumer energy prices, and retaining consistency with Paris Agreement climate goals. High prices and producer rents, however, also present an opportunity: to open a conversation about applying the principle of extended producer responsibility (EPR) to fossil fuels. We demonstrate that this could deconflict energy security and climate policy at an affordable cost by stopping fossil fuels from causing further global warming. Implementing EPR through a combination of geological CO₂ storage and nature-based solutions can deliver net zero at comparable or lower costs than conventional scenarios driven with a global carbon price and subject to constraints on CO₂ storage deployment. It would also mean that the principal beneficiary of high fossil fuel prices, the fossil fuel industry itself, plays its part in addressing the climate challenge while reducing the risk of asset stranding.

Abstract:

On 15 January 2022, the Hunga Tonga–Hunga Ha’apai (HTHH) eruption injected 146 MtH2O and 0.42 MtSO2 into the stratosphere. This large water vapour perturbation means that HTHH will probably increase the net radiative forcing, unusual for a large volcanic eruption, increasing the chance of the global surface temperature anomaly temporarily exceeding 1.5 °C over the coming decade. Here we estimate the radiative response to the HTHH eruption and derive the increased risk that the global mean surface temperature anomaly shortly exceeds 1.5 °C following the eruption. We show that HTHH has a tangible impact of the chance of imminent 1.5 °C exceedance (increasing the chance of at least one of the next 5 years exceeding 1.5 °C by 7%), but the level of climate policy ambition, particularly the mitigation of short-lived climate pollutants, dominates the 1.5 °C exceedance outlook over decadal timescales.

Abstract:

How confident are we that CO₂ emissions must reach net zero or below to halt CO₂-induced warming? The IPCC's sixth assessment report concluded that “limiting human-induced global warming to a specific level requires … reaching at least net zero CO₂ emissions.” This is much stronger language than the special report on the global warming of 1.5°C, which concluded that reaching net zero CO₂ emissions would be sufficient. Here we show that “approximately net zero” is better supported than “at least net zero.” We estimate the rate of adjustment to zero emissions (RAZE) parameter (−0.24 to +0.17%/yr), defined as the fractional change in CO₂-induced warming after CO₂ emissions cease. The RAZE determines the CO₂ emissions compatible with halting warming over multiple decades: in 1.5°C-consistent scenarios, CO₂ emissions consistent with halting anthropogenic warming are +2.2 GtCO₂/yr (5–95th percentile range spans −7.3 to +6.2 GtCO₂/yr), similar to the expected emissions from unmodelled Earth system feedbacks.

Abstract:

Anthropogenic climate change is likely to increase the risk (probability of occurrence of a hazard) of extreme weather events in the 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 extreme rainfall events in Bangladesh under the Paris Agreement temperature goals of 1.5 and 2.0°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; NAT), current (actual; ACT), 1.5 and 2.0°C warmer, and greenhouse gas (GHG)-only (with pre-industrial levels of anthropogenic aerosols) conditions. Both GHGs and anthropogenic aerosols have an impact on seasonal mean rainfall over this region. In general, higher global mean temperature levels lead to higher rainfall and higher aerosol concentrations to lower rainfall, however the relative importance of the two factors varies between the regions. For extreme rainfall events, we find that the risk of a 1 in 100 year rainfall episode has already increased significantly compared with pre- industrial levels across parts of Bangladesh, with additional increases likely for 1.5 and 2.0°C warming. Climate change impacts on the probabilities of extreme rainfall episodes are found during both pre-monsoon and monsoon seasons. Results show that reduction in anthropogenic aerosols will exacerbate the effects of GHG-induced warming and thereby increasing the rainfall intensity, which has otherwise attenuated the impacts. 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. While there is a substantial reduction in risk at 1.5°C warming when compared to 2°C warming, the difference is spatially and temporally variable too, specifically with respect to seasonal extreme rainfall events.