Climate is the weather you expect, not necessarily the weather you get!

Looking out of my office window onto a cold, windy February morning, I understand how such weather can make some people wonder how these conditions can be consistent with global warming. The British weather has always been a hot topic of conversation, indeed weather at either end of the scale — scorching temperatures or heavy snow — makes the front pages of our newspapers. The role of climate change and its impact on the extremes in our weather has also been in the news and The Climate Coalition’s ‘Weather Warning’ report looks at how our changing climate is beginning to affect our lives and the landscapes around us.

Determining how climate change impacts our weather is a challenging problem, so, in order to understand it, we exploit several strands of climate science. We analyse detailed historical observations of UK weather and our fundamental physical understanding of how greenhouse gases drive changes in climate, employing one of the latest developments in the statistical analysis of climate data: “event-attribution”.

Detailed observations of temperatures from around the world tell us that surface temperatures have risen by around 1 degree C since the industrial revolution. And 200 years of physical sciences and experimentation tells us that that this warming is driven by human emissions of greenhouse gases. In the UK, we are fortunate to have the longest continuous temperature record available, dating back to the 1700s which indicate a similar amount of warming to the global average. Yet UK weather shows much more day-to-day, month-to-month and year-to-year variation, so we still get some years, such as 2010, when it can be colder than average. Nevertheless, the trend towards warmer temperatures coupled with heavier rainfall events is apparent

Our climate simulations consistently indicate increases in winter-time rainfall across the UK and we have robust physical understanding of how our warmer atmosphere holds more water vapour (around 7% per degree of warming). This leads to heavier extreme rainfall events. We also observe sea-level rise directly linked to warming of the ocean and melting ice-caps. This rise is currently only a few mm per year but is accelerating. There are some aspects of climate change we are less certain about, such as drought and jet-stream changes. Understanding how the jet-stream is affected by climate is crucial to predicting the effects of climate change on the UK and is an ongoing research topic. It is also worth noting that there is no evidence of a climate driven increase in the frequency of storms, only their intensity. Together this science tells us that climate change will have likely exasperated weather-related impacts caused by heavy rainfall events, wetter- warmer winters and/or tidal surges.

The cutting edge science of “event attribution”, developed by colleagues at the Met Office and the University of Oxford, combines these different strands of evidence to determine the likelihood that climate change caused a specific weather event (such as a given heat-wave or storm). Such analyses have only been undertaken on a limited number of case studies so far. Storm Desmond for December 2015 being one and the wet winter of 2013/2014 across the UK being another. For Storm Desmond,the authors examined the return period of wet December’s for the North West of England. Analysis of historical observations indicate that such a wet month is a very rare event in the current climate, typically occurring once every few hundred years. Both regional and global climate simulations predicted that background climate change made the event 50–75% more likely and 40% more likely in the case of December 2015.

At the moment we can’t predict whether any given flood or weather event would have occurred without human CO2 emissions playing a contributory role, but all our science points to these emissions making such events more frequent. This science, combined with the other evidence outlined in the Climate Coalition’s report shows that our countryside and culture are already being affected. It also highlights the steps many individuals and organisations are taking to build greater resilience to future climate shocks. With Britain’s special — and everyday places — from our coastlines, churches, gardens, woodlands, farms to pubs and cricket pitches — feeling the force of likely climate-related impacts it’s fair to say that climate change is not only coming home — it has already arrived.

Piers Forster, Director, Priestley International Centre for Climate

This blog was first published for The Climate Coalition 07.02.17. See also News

Image credit: Ben Salter (https://creativecommons.org/licenses/by/2.0/)