2014 was the warmest year in the UK ‘since records began’ – and most probably the warmest since at least 1659. You can read the Met Office Summary here
This was briefly ‘news‘ but somehow this astonishing statistic seems to have disappeared almost without trace.
In fact there are three astonishing things about the statistic
- Firstly – we know it, and it is likely to be correct.
- Secondly – the warmest year was ‘warmer all over’ but did not include the ‘hottest month’.
- Thirdly- the warmest year was also overly wet – both in the UK and world wide.
This article is about why this third remarkable fact is actually not so much remarkable, as inevitable.
1. How do we know this?
We know this because the Met Office maintain a network of observing stations in which they take great pains to ensure that thermometers are calibrated, and that they are read in a consistent way. This allows a meaningful comparison between measurements taken today and historical measurements.
This work is tedious and expensive. But it is also indispensable if we want to know what is happening to our climate.
2. Not hot, just not as cold as usual.
I experienced this year mostly in the south-east of England, and I did notice that things didn’t seem to get as cold as I expected. For example, I remember walking in central London in a T-shirt at 10 p.m. on the evening of 31st October – Halloween – and thinking “This is a bit odd“.
But it did not feel like a record year. Intuitively I would have expected that the ‘hottest year ever’ would probably have included a ‘long hot summer’ – perhaps with a hosepipe ban for good measure! In fact August was the only month of the year which was below average temperature.
So if we didn’t have reliable measurements (See Point#1), it would be perfectly possible that we might not have noticed that this was an exceptional year.
3. Warm and Wet.
And 2014 was also wet. The Met Office said it was:
…the fourth wettest year in the UK series from 1910, behind 2012, 2000 and 1954. This means that five of the six wettest years in this series have occurred since 2000.
And although I understand why a trend of rising temperatures can quite plausibly make for more rainfall, it is not the first association that many people will make.
Of the roughly 1.338 billion cubic kilometres of water on Earth, on average the atmosphere contains ‘only’ about 12,900 cubic kilometres – approximately 0.001% of the total.
This fraction is the result of a balance between evaporation and rainfall. If the fraction is constant – no matter what its value – then the rate at which water evaporates must be balanced by the rate at which rain falls.
So if a warming world causes the ocean surfaces to warm then the rate of evaporation will increase. And necessarily the rate of rainfall must increase by exactly the same amount.
This is not a prediction of a climate model – but an inevitable consequence of the fact that the amount of water on Earth is constant.
What is surprising is just how rapidly evaporation increases with rising temperature. Roughly speaking, every 1 ºC causes a 6% increase in the rate of evaporation – which gives rise to an additional 6% of global rainfall.
Erratum: 21 January 2015: This fact is wrong! The explanation is in the next blog article but in fact to the best of our knowledge, for every 1 ºC rise, there is a 2% increase in global rainfall.
So in a warming world…
- warmer ocean surfaces create more rainfall.
- warmer land surfaces dry out.
Together these two trends mean that – approximately – dry places (like Southern California) are likely to get warmer and drier. And wet places – like the UK – are likely to get warmer and wetter.