Gregory Munday

Abstract:

Overshooting global warming targets risks irreversible Earth system changes. However, uncertainties remain in how critical Earth system components such as permafrost and the Atlantic Meridional Overturning Circulation (AMOC) respond to and interact under warming. By using three climate models of varying complexity to assess the permafrost carbon response under idealized climate mitigation and overshoot scenarios, we here show that permafrost loses 11–21 PgC of carbon per 100 degree-years of warming exposure (cumulative warming over 100 years) during temperature overshoot in a robust linear relationship. This relationship also holds true under relative Northern Hemisphere cooling from a temporary AMOC slowdown during temperature overshoot. This relative cooling partially offsets climate change impacts on permafrost, while in itself causing adverse impacts on climate and society. These results underscore the importance of including both destabilizing and stabilizing Earth system feedbacks when assessing overshoot impacts, critical for informing carbon budgets, net-zero planning, and climate change reversibility.

Abstract:

The sequence of compounding winter storms of 2013/14 in the United Kingdom (UK) caused a range of significant impacts across the country, totalling an economic cost of approximately £1.3 billion. Heavy rain, totalling 545mm over the season, caused widespread flooding, and coastal impacts were exacerbated by high spring tides and strong winds. The Somerset Levels particularly felt the impact of the flooding, accounting for 30% of the total UK area of flooded agricultural land. This event is a case study for the Horizon-Europe COMPASS project (Compound events attribution to climate change: towards an operational service), where the main goal is to produce a flexible and harmonised methodological framework for such compound extremes, with a focus on impact attribution.Using the flood model SFINCS (Super-Fast Inundation of Coasts), developed by Deltares, we model the total flood extent for a small region of the Somerset levels for the season. We drive this model with both factual and counterfactual (“natural”, with anthropogenic warming removed) simulations of the winter precipitation, using the HadGEM3-A large-ensemble attribution runs. Initial results demonstrate the magnitude of the observed flood extent to be 1.21 times more likely due to climate change, based on return periods. We also found that a flood event under “natural” forcing but with the same return period as the factual event would be slightly less severe in its extent, 113.40km² compared to 114.02km².We aim to extend this initial work further both by improving the representation of flood defences, and including the sea level rise component. Attribution of the flood extent to sea level rise can be investigated by removing post-industrial sea level rise from the model boundary conditions to create a counterfactual; this opens another avenue for exploring the compound nature of the event. The impact of the proportion of sea level rise specifically attributable to anthropogenic influence will also be explored.

Abstract:

Global warming leads to widespread permafrost thaw and subsequent emissions of carbon dioxide and methane, driving the permafrost carbon–climate feedback (PCF) which amplifies climate change. Many current-generation climate models omit this feedback, limiting projections of long-term temperature outcomes and associated Earth system tipping risks. Here, we investigate how this feedback affects the risk and timing of crossing tipping points in the Earth system by integrating permafrost carbon emissions into temperature projections for idealized stabilization and overshoot scenarios. In a conceptual network model of interacting climate tipping elements—including the Greenland and West Antarctic ice sheets, the Atlantic Meridional Overturning Circulation, and the Amazon rainforest, we find that PCF leads to additional warming that increases the probability of exceeding one or more tipping elements by up to 50% and can substantially accelerate the timing of tipping events by hundreds of years. The effect of PCF on tipping risk and acceleration is particularly evident in overshoot scenarios with lower peak temperatures below 2–3 ∘C or peak cumulative emissions of below 2000 PgC. Considering PCF, therefore, narrows the critical temperature space for overshoot pathways. Our results highlight that failing to account for PCF underestimates both climate risks and the urgency of mitigation, increasing the likelihood of tipping events across the Earth system.

Abstract:

The sequence of compounding winter storms of 2013/14 in the United Kingdom (UK) caused a range of significant impacts across the country, totalling an economic cost of approximately £1.3 billion (Environment Agency, 2016). Heavy rain, totalling 545mm over the season, caused widespread flooding, and coastal impacts were exacerbated by high spring tides and strong winds. The Somerset Levels particularly felt the impact of the flooding, accounting for 30% of the total UK area of flooded agricultural land. This event is a case study for the COMPASS project, where the main goal is to produce a flexible and harmonised methodological framework for such compound extremes with a focus on impact attribution.Using the flood model SFINCS (Super-Fast Inundation of Coasts), developed by Deltares, we model the total flood extent for a small region of the Somerset levels for the season. We drive this model with both factual and counterfactual (“natural”, with anthropogenic warming removed) simulations of the winter precipitation, using the HadGEM3-A large-ensemble attribution runs (Ciavarella et al., 2018). We find the likelihood of the magnitude of the observed flood extent to be 1.21 times more likely due to climate change, based on return periods. We also find that a flood event under “natural” forcing but with the same return period as the factual event would be slightly less severe in its extent, 113.40km² compared to 114.02km².Although these results are not statistically significant, this agrees with generally inconclusive results from other studies on the 2013/14 UK winter storms, such as that of Schaller et al. (2016) who found a small, non-significant increase due to climate change in the number of properties impacted by flooding in the Thames river catchment. Potential modelling improvements to refine results for Somerset include increasing resolution and adding flood defences to better represent the coastal inundation. Investigation of attribution of the flood extent to post-industrial sea level rise also opens another avenue for exploring the compound nature of the event.The Horizon-Europe COMPASS project (Compound events attribution to climate change: towards an operational service) is exploring climate and impact attribution of different complex extreme events, and scoping an operational attribution service. It aims to develop transferable attribution methods for operational attribution of compound extremes to support climate change evidence and policy.