Scientists call for research to rise to the challenge of HILL events

The higher global temperatures rise, the greater the likelihood of extreme weather events

Extreme weather events have become more frequent in recent years, and many of these can be attributed to climate change. The capabilities of weather forecasting and longer-term climate modelling have improved exponentially over recent decades, but even the best forecasting tools are challenged by “High Impact, Low Likelihood” (HILL) events.

HILL events go beyond traditional weather extremes, potentially taking the climate system into uncharted territories. For example, much of the UK’s climate is predicated on two large elements of the climate system: the North Atlantic jet stream, a core of strong winds five to seven miles above the Earth’s surface, and the Atlantic Meridional Overturning Circulation (AMOC), a system of ocean currents which transports warm water northwards in the Atlantic.

Changes in both of these systems linked to increasing warmth in the Arctic have been cited as potential HILL events. A change in either or both isn’t considered likely in the immediate future (low likelihood), but the impacts from such changes would be devastating (high impact). One example of an unusual weather event is extreme rainfall.

In July 2021, parts of London received up to a month’s rainfall in an hour, causing flash flooding and disrupting transport. This event – which went on to cause devastating flooding and loss of life in continental Europe – was created when an area of slow-moving low pressure became trapped in the meridian of the Jet Stream, creating what’s known as a “Quasi-stationary” storm.

Professor Lizzie Kendon, Science Fellow at the Met Office, published a paper with Dr Abdullah Kahraman, of Newcastle University, in 2021 which found that “quasi-stationary” storms that exceed specific thresholds in moisture and uplift, where they are pushed up by rising terrain (and so have the potential for high rainfall accumulations) could become 14 times more common across Europe as a whole by the end of the century.

Further analysis for the UK, using the latest UKCP Local (2.2km) projections indicate that the number of days with rainfall exceeding 30mm/h (for at least one 5km grid box across London) could – using the most pessimistic scenario of action to tackle climate change increase 2.5 times by the 2070s compared to 1990s under the RCP8.5 scenario. These results suggest a big increase in the frequency of flash-flood producing rainfall events.

HILL events were a key theme of the Met Office Climate Science Conference in May 2021, and the Met Office also hosted a session on HILL events at the COP26 Science Pavilion.

Met Office Science Strategic Head Dr Richard Wood said: “Both the climate science and policy communities would benefit from devoting more attention to HILL events because they are a major component of climate change risk. There are some key questions that need answering as we move towards implementing a desired pathway for the global climate and living with the consequences. Although it isn’t certain where the thresholds are for these HILL events, we can be sure that higher levels of global warming will make them more likely.”

Prof Richard Betts MBE is the Head of Climate Impacts Research in the Met Office Hadley Centre and a Professor at the University of Exeter. Prof Betts, who led the team which prepared the Technical Report for the UK’s 3rd Climate Change Risk Assessment (CCRA3), is calling for a monitoring, attribution and prediction system that can provide early warning of HILLs.

Professor Betts said: “With rising global temperatures, we are edging closer to the thresholds for more and more HILL events. Greater research into these events will help scientists advise policy makers on their thresholds and impacts.”

Risk registers could form an essential part of the future planning of all key sectors of the economy, such as finance and energy, to mitigate against HILL events. The better adapted we are, the more lives can be protected. There will be financial benefits too. For instance, by assessing potential flood risk as a result of climate change, science can help developers avoid building on flood plains, save money for the insurance industry, and protect peoples’ lives and livelihoods.

It will never be possible to predict extreme events with a hundred per cent accuracy. What’s important is that we do all we can to assess how factors such as carbon-dioxide levels, melting ice sheets, and the thawing of the permafrost can impact hugely on the environment. The higher global temperatures rise, the greater the likelihood of extreme weather events. If we can understand and predict them better, we can help society be better prepared.


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