Climate Change - it's happening now

The winter 2015-16 floods in the UK have sparked the usual debate - we're not spending enough on flood protection, building on floodplains has made things worse, it's all because we're paving over driveways etc. Many commentators (including my old colleague Reza Ahmadian) have cited climate change as a contributory factor - but how true is this?

Lots of work on climate change seeks to look well into the future - some work I recently did for the Committee on Climate Change (report here) focused on changes by the 2050s and 2080s (although we did look at the 2020s too). But the recent news that we're already approaching 1°C above the pre industrial average prompts a question - should we therefore also see a change in flood risk? The link between global temperatures, rainfall and river flows is well established (in the minds of climate scientists at least), so in this post I'll try to understand what this means practically for flood risk.

It's important first to understand a key concept in climate studies - the baseline. While we're almost at 1°C above the pre industrial average for global mean temperature, that's only around 0.7°C above the temperatures in the period 1961-1990, which is typically used as the baseline in climate studies (like the IPCC). The chart below shows global mean temperature relative to this 1961-1990 baseline, showing that the baseline almost corresponds to a relatively stable period for global temperatures between 1940 and 1980, but that there was significant period of warming before that. I've added a thick red line to show a "cartoon" of running average temperature, which is constant before 1920 and from 1940 to 1980.

This global mean temperature history has two potentially significant effects for how we understand flood risk. Firstly, our records of flows from gauging stations might not be stationary - there might be trends in there which we need to correct for before doing any statistical analysis. Secondly, there has been an increase in temperature since the 1961-1990 baseline which could have caused an increase in flood flows.

What does this mean practically? I've done some calculations for 5 gauging stations across the UK, using the latest advice on climate change impacts on river flows, and an assumption that increases in flows are roughly proportional to global temperature increase. I've expressed the results as the 1 in 100 Annual Exceedance Probability (AEP) flow for the baseline period 1961-1990, and the annual exceedance probability of that flow now in 2016.

The results are shown in the table below - for the Eden at Sheepmount in Carlisle, the baseline 1 in 100 flow of 1405m³/s now has an AEP of 1 in 62 - an increase in frequency of ~70%. The Blackwater in Northern Ireland shows a doubling of frequency for a big flood. The effect is less pronounced for the Tay, and the Severn and the Wye show no increases (because the climate change impact guidance indicates no sensitivity to climate in the short term up to 2025). 

So for some catchments, we're seeing significant increases in the likelihood of large floods compared to the baseline period 1961-1990. We need to think about these effects when we undertake flood risk analysis - rather than assuming our records of flow are representative of conditions now, and that climate change is something that only happens in the future.