Some media coverage of a report released earlier this week suggests shrinking water supplies caused by hotter and drier summers could cause problems for food production in England. But a closer look at the report suggests that the reason why water could be in short supply is more complicated. Changes in rainfall might have an effect, but rising demand for water is likely to be a more important and more predictable factor.

Hotter and drier

Earlier this week the Committee on Climate Change released a report looking at how the UK might have to adapt to the effects of climate change. The media coverage mostly focused on one message from the report – that climate change could damage UK food production. The Guardian, for example, suggested that as summers become hotter and drier, this will make it more difficult to irrigate crops.

But there’s an important caveat here. Climate models show with high levels of certainty that UK temperatures will rise as the climate changes. As it gets hotter, soil will get drier, leaving less water for plants. Overall demand for irrigation will probably rise with temperatures.

But how summer rainfall will change is less certain. Professor Richard Betts, head of climate impacts at the Met Office Hadley Centre, explained to Carbon Brief:

“We don’t know for sure whether the UK will get drier or wetter in individual seasons in response to anthropogenic climate change, and moreover for the next few decades we’d expect natural internal variability to dominate – that could mean dry for a while then wet for a while, or the other way round. This is very difficult to predict.”

In fact, UK rainfall could increase or decrease because of climate change. The report from the Committee on Climate Change (CCC) lays out this uncertainty pretty clearly. It says that by the 2020s, summer rainfall in the UK could decrease by as much as 30 per cent, or increase by as much as 20 per cent. In the winter, rainfall could fall by as much as 10 per cent, or increase by as much as 30 per cent.

So it’s quite possible that UK summers could be wetter in the future, although evidence from climate models suggests that, on balance, drier summers are more likely.

If we do see drier summers, summer river flows and groundwater levels could fall, with consequences for irrigation and agriculture. However, changing rainfall is only one part of the picture. There are other more predictable factors that could lead to a shortfall in water for farming.

Supply or demand?

The CCC’s modelling suggests that demand for water will be a bigger factor than supply when it comes to the amount of water available for agriculture in the near term.

One statistic from the report that was featured in the media was that by the 2020’s, farmers could face irrigation shortfalls of 115 billion litres a year.

Here are two indicative scenarios – A and B – that the CCC used to calculate that figure:

You can see that whether rainfall falls or remains the same, demand for water is the dominant factor in determining the supply/demand balance.

Some of the extra demand is because temperature rise makes soils drier, the CCC says. The rest is down to projected population growth and more demand for food. Ironically, perhaps, warmer temperatures and longer growing seasons resulting from climate change may provide farmers with opportunities to use more water, meaning demand will increase as an indirect result of climate change.

What about the long run?

Growth in water demand appears to be the biggest reason Britain is facing “shrinking water supplies” which risk “devastating food production” – in the short term at least. What about the longer term though? Is there more certainty about what will happen to the UK’s rainfall?

Professor Betts told us:

“In the UKCP09 projections, and IPCC AR4 projections, it is true that more of the models  simulate a summer drying trend than wetting trend, but this is by no means unanimous between the models.”

Future projections of rainfall are uncertain for a number of reasons, he cautioned.

While climate models are a useful tool, they have limitations. There may be links between parts of the climate system models haven’t accounted for, or at least not very well. So the future of rainfall in the UK remains couched in scientific uncertainty.

But according to the CCC’s modelling, the main driver of water shortages – increasing demand – is something we can act on. Using water more efficiently, growing less thirsty crops, and throwing away less food could help manage the future balance between water supply and demand.