The UK is quite famous for its rainy weather. Then a simple question occurring to me was whether climate change would make it worse or not. To answer this question, historical data were compared with projections for the period 2080-2099.
The average annual precipitations observed in the UK from 1991 to 2016 are 1,244 mm.
According to the worst-case scenario for climate change ("RCP 8.5"), the projected precipitations in the UK for the period 2080-2099 are 1,304 mm.
Comparing the results shows that forecast annual precipitations will increase by 61 mm, which is a 4.9% increase.
Then, the answer to the initial question is "yes": Climate change will likely bring more rain to the UK. A 4.9% increase seems like a rather small overall change.
However, the overall amount of precipitation over the year does not allow us to deduce the variation in precipitation over the seasons. Since precipitation varies over the year, we suspect that the increase in precipitation is not spread evenly over the year. If this is not the case, what would the pattern be?
Now, if we have a look at the monthly projected precipitations, we realize that the change in precipitations varies throughout the year.
While there is less rain projected from May to August...
... more rain is projected during the rest of the year.
What would be the consequences of these changes in monthly rainfall for the UK? Let's look at monthly variations in details.
The 4.9% increase observed on an annual basis can be broken down into monthly variations that range from −16.2% to 15.9%.
During months with less rain, it could mean longer dry spells impacting sectors such as agriculture and water production.
During months with more rain, it could lead to more intense rainfall, that could result in increased risks of floodings.
The monthly precipitations alone do not provide the full picture, as rain intensity is critical to understand the impact of rain on human activities. The number of days with rainfall above 20 mm would also be a useful indicator, since 20 mm is the threshold beyond which precipitations are considered very heavy.
The variability in precipitations is quite high, and monthly precipitations from one year to another might vary significantly. Therefore, even though monthly precipitations are projected to increase during certain months, it doesn't mean that during these months, there will always be more rain than what was observed historically.
Here, the median of the results from an ensemble of climate change models has been used, after reading on the Climate Knowledge Portal that: "The median is a fairly robust measure as it reduces the lower quality simulations of precipitation still prevalent in many models". This means that 1/ uncertainty is fairly high when it comes to the results of the models, and 2/ if we need to consider the worst-case scenario, we should directly use the models that result in worst-case situations for the UK, not only the median of the results from all the models.
Another limitation is that we consider the UK as a whole, while climatic conditions have spatial variability as well. Regional rainfall are for example influenced by the distance to the Atlantic Ocean and latitude.
Back to the initial question: Yes, climate change is projected to bring more rain to the UK on average by the end of the 21st century (4.9% increase). The rainfall pattern is also expected to change over the year, and this could potentially lead to more intense rainfall, and longer dry spells.
Further investigations would be needed to better understand the potential impacts on human activities, especially in worst-case scenarios. These investigations should in particular include rainfall intensity, spatial variability, and the climate change models considered.