Why was 2014 warm AND wet?

Colour-coded Map of UK showing how each region of the UK exceeded the 1981-2010 average temperature. Crown Copyright

Colour-coded Map of UK showing how each region of the UK exceeded the 1981-2010 average temperature. Crown Copyright

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

The World Meteorological Organisation (WMO) also report that 2014 is likely to have been the warmest year in Europe and indeed over the entire Earth for at least 100 years.

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.

Colour-coded Map of UK showing how each region of the UK exceeded the 1981-2010 average rainfall. Crown Copyright

Colour-coded Map of UK showing how each region of the UK exceeded the 1981-2010 average rainfall. Crown Copyright

Why Wet?

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.

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11 Responses to “Why was 2014 warm AND wet?”

  1. Ed Hui Says:

    Evaporation = rainfall if average humidity is constant. Is it constant if the average temperature goes up? In other words does a warmer atmosphere carry exactly the same amount of water as a cooler one? Or is the difference negligible? (I would have thought warmer might mean more water is carried, maybe more cloud?)

    • protonsforbreakfast Says:

      Good point. Actually there is some slight evidence of ‘global moistening’ and that is expected. But the rate of increase in the atmosphere is very slow compared to the rate at which water cycles through the atmosphere. If we have 1 degrees Celsius rise in ocean temperature (about right) then the rise in the amount of moisture in the atmosphere will be about 6% (ish). In contrast, the average residence time of water in the atmosphere is still only 11 days. So the additional water vapour required to saturate the atmosphere will be provided with just a few days evaporation – after which the amount in the atmosphere is at a new equilibrium. And then the evaporation = rainfall equation applied again.

      Happy New Year and all that.

  2. telescoper Says:

    Reblogged this on In the Dark and commented:
    It’s certainly a wet start to 2014 here in Brighton, but did you know that 2014 was the warmest year since records began as well as one of the wettest?

  3. jauntytraveller Says:

    Reblogged this on jauntytraveller.

  4. markpmcc Says:

    Hi,

    Your statement about precipitation is not correct. The 6%/degC relates to the water vapour holding capacity of the atmosphere in response to warming, but the response of precipitation is rather more complicated and constrained by changes in energy balance. The precip response is more like 1 – 3 % rise per degC. For a more comprehensive discussion of the processes involved see section 7.6 of IPCC Ar5 http://www.ipcc.ch/report/ar5/wg1/.

    In contrast the increased water vapour holding capacity does change precip during extreme events by a greater amount closer to the Clausius Clapeyron – but I would argue that 2014 as a “wet” year was more due to persistence of rain than daily extremes. Evidence of this from high number of “rain days” but not too many extreme daily totals.

    Cheers,
    Mark

    • protonsforbreakfast Says:

      Thank you Mark. I will re-read the assessment report but I can’t see what’s wrong my argument.

      As I understand it, the relative humidity of air above the ocean surface is a pretty constant 80% and is expected to remain at a similar value as ocean’s warm or cool. All moisture which leaves the ocean surface passes through this surface layer, and if the rate of evaporation increases due to increased surface temperature, I can’t see how the amount of evaporation – and therefore rainfall – cannot simply scale.

      I will re-read and amend he post if required. Thanks

      • markpmcc Says:

        The fundamental change here is the holding capacity of the atmosphere that changes in response to warming as constrained by Clausius-Clapeyron (C-C) at approx. 6%/degC. The oceans provide an essentially limitless supply of water to maintain RH and balance the water that is removed from atmosphere through precip, but the precip rate itself is limited by a more complex interaction of processes so precip (and consequently net evaporation) do not follow direct C-C scaling but atmospheric moisture content does.

  5. Why was 2014 warm AND wet? - Ask a scientist Says:

    […] contributor Michael de Podesta takes a look at the weather we experienced in the UK last year in his own blog, exploring the facts behind the statistics and why this is […]

  6. Warmer and wetter. But not as wet as I thought. | Protons for Breakfast Blog Says:

    […] Making sense of science « Why was 2014 warm AND wet? […]

  7. ravisubbie Says:

    Reblogged this on More Known Than Proven and commented:
    This will written blog post explains why 2014 was was and wet. And concludes that the UK’s long term weather patterns are going to be more of the same.

  8. Joseph Mitchell Says:

    It might have been warm and wet in southern england, but up here in central Scotland, last summer was just warm, hot at times, we got below average rainfall, barely any rain fell the whole summer, with September 2014 being the driest on record, we got 2 days of rain out of the whole month, spring last year was very warm and very dry as well, very little rain, and few cloudy days.

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