Maybe all they need is a bigger computer
Guest post by by Paul Homewood
Following the wet summer in the UK last year, the Met Office provided the Environment Agency with a briefing document, giving an overview of the weather. This was discussed at the September Board Meeting of the Environment Agency, which Met Office officials attended.
As far as I know, this document, which I obtained through FOI, has never entered the public domain. It is brutally honest in admitting how little the Met’s scientists understand about what affects our climate, and, in particular, what caused the unusual weather last year. This is in stark contrast to many of the hyped up claims, made in public statements in the recent past by, among others, the Met Office themselves.
The full document is reproduced below, but there are four particular areas I wish to focus on.
The document has this to say about droughts in the UK (my bold):-
Prior to April 2012 the UK was experiencing hydrological drought associated with a prolonged period of below average rainfall. The hydrology of the UK is such that replenishment of water reserves occurs predominantly during the autumn, winter and spring (October – April) referred to as the recharge ‘winter’ season; conversely, the summer period (May – September) is a time when the balance between precipitation and evaporation means that replenishment of water reserves is small. Summer is also the time when temperature can play a significant role in determining evaporation and soil moisture availability.
Although there is not a unique definition for major hydrological droughts it is generally agreed that 1975/76, 1963/65 and 1933/34 come into that category. They extend over 12 months in duration and can take in at least two failed replenishment cycles.
The drought of 2010/12 is similar in severity to these historical events and, as with past events, built up over 2-3 years. The 2010/12 rainfall deficit was not as intense as 1975/76, which ranks as the most severe for the past 100 years in many respects. Much of England and Wales received less than 65% of average rainfall, with sizeable areas receiving only 55 to 60%. Rainfall deficits in the recharge ‘winter’ of 1975/76 were particularly severe and widespread, with the effect that the UK entered the summer of 1976 with severely depleted soil moisture in many regions. In addition to the lack of rain, summer 1976 was also the equal warmest in the series from 1910 across England and Wales, and the sunniest in the series from 1929. The hot, sunny conditions would have significantly increased evaporative demand, and the dry ground would have also influenced the extreme high temperatures experienced during the summer.
Neither the development nor the severity of the 2010/12 drought was exceptional compared with historical events, and its climatological drivers have several similarities with past droughts.
It reinforces this message in the conclusion:-
Neither the development nor the severity of the 2010/12 drought was exceptional compared with historical events, and its climatological drivers have several similarities with past droughts.There is therefore, as yet, no evidence that it was due to climate change and not part of the natural variability of the climate.
We are constantly told how “climate change” will lead to more severe droughts. DEFRA’s own Climate Change Risk Assessment talks about “a reduction in summer rainfall of up to 60% by 2080”, while Environment Minister, Lord Henley, told us in 2011 “the recent exceptionally dry weather is a snapshot of what we might expect from climate change.”
It might, therefore, come as a surprise to many to find that recent droughts are in fact perfectly normal, and indeed much less than severe than some earlier ones.
2) Jet Stream Changes
It is now well known that that last year’s wet weather, (and the drought that preceded it), was the result of changes in the position of the jet stream. The Briefing Document has this to say:-
What is causing this summer’s wet weather?
The jet stream has been displaced southwards compared to its climatological summertime position. The jetstream is the fast-moving ‘river’ of air at altitudes of around 30,000ft which forms in the mid-latitudes at the boundary between the cold air surrounding the poles and the much warmer air in the tropics. It usually runs from west to east, and acts to develop and steer the low pressure systems which are responsible for much of the UK’s rain. On average, these systems pass to the northwest of the UK, and hence northwestern parts of the UK – particularly higher ground such as in Western Scotland and Cumbria – receive the most rain.
However, when the jetstream dips to the south of the UK, the distribution of rainfall is skewed away from the climatological average, and southern areas can see periods of significantly above average rainfall and associated higher risk of river and surface water flooding. Not only do the low pressure systems steer across southern areas, but the following factors act to increase the risk of heavy rain and flooding:
· different prevailing wind direction means that different windward slopes will be subject to enhanced rainfall
· the frequent southerly to easterly component to the airflow means that warm, thundery air from the near Continent may be drawn towards the UK, increasing the potential for heavy rainfall
· fronts are more likely to become slow-moving, giving persistent rain in some areas
· between the low pressure systems themselves, the dominant low-pressure (‘cyclonic’) environment is conducive to formation of heavy showers during summer. Again, these may be slow-moving, with an increased risk of intense downpours and surface water flooding.
Low pressure systems of this nature are unusual in summer and because the atmosphere is warmer it can hold more water than in other seasons resulting in significant amounts of rainfall.
The $64000 question, of course, is why has it moved. The Met Office are admirably frank. They admit they do not have a clue. This is what they say:-
The jet stream, like our weather, is subject to natural variability – that is the random nature of our weather which means it is different from one week, month or year to the next. We expect it to move around and it has moved to the south of the UK in summertime many times before in the past. It has, however, been particularly persistent in holding that position this year – hence the prolonged unsettled weather.
This could be due to natural variability – a bad run of coincidence, if you will – but scientific research is ongoing research to investigate whether other factors at play.
Factors which might contribute include:
· North Atlantic Sea Surface temperatures are warmer than normal. These can drive low pressure during summer over NW Europe, and have been a consistent feature of the last five summers (June, July August), all of which have been wetter than the climatological average for 1971-2000;
· It has been suggested that the decline of Arctic Sea Ice may drive low pressure over the UK, although this remains very uncertain at present. Record loss of summer Arctic sea ice cover has also been a consistent feature of the last five summers;
· Recent summers have been under the influence of La Nina-type forcing from the tropical Pacific. Although the tropical East Pacific has warmed in recent months and there are indications of a transition to El Nino conditions, the recent weather patterns in the tropical Pacific are still representative of La Nina conditions, with very disturbed weather over Indonesia and the West Pacific. La Nina drives an increased risk of low pressure over the UK and predisposes the jetstream to shift southwards.
· There is evidence that the circulation changes over the UK are part of a pattern of changes which circumnavigates the whole of the northern hemisphere mid-latitudes.
So, while they are researching various factors, they actually have no evidence on any of them, and certainly none which can link jet stream changes to “climate change”.
But none of this appears to have stopped Julia Slingo telling the Telegraph “The trend towards more extreme rainfall events is one we are seeing around the world, in countries such as India and China, and now potentially here in the UK. “
Or head of the Environment Agency, Lord Smith, informing us “We are experiencing a new kind of rain. Instead of rain sweeping in a curtain across the country, we are getting convective rain, which sits in one place and just dumps itself in a deluge over a long period of time.”
Or DEFRA warning us that “The climate is changing. This means we are likely to experience more flooding”.
3) Madden-Julian Oscillation
The Met posed the question – What caused the shift from very dry weather to very wet weather in 2012? This was what they had to say:-
The reason for this sudden change is still being investigated. There is a suggestion that it could be linked to disturbed weather patterns over the Indian Ocean and tropical Pacific caused by a strong Madden-Julian oscillation (MJO) – a large scale tropical phenomenon – in March. Understanding the initiation of an MJO event is, however, largely unpredictable, and remains one of the great unsolved challenges of tropical meteorology. It is therefore very unlikely that the MJO and its impact upon our own weather could have been anticipated in forecasts produced in early and mid-March. However, by the end of March, once the MJO had been observed, short range forecasts were able to predict the wetter and more unsettled weather the UK experienced during April.
To some extent, it may be that they are using this as an excuse to cover up their failure to predict the change. Nevertheless, as they make clear, climate science really does not understand this phenomenon.
It is a pity that their public statements do not admit this.
4) Decline of Arctic Ice
There have been many attempts recently to blame just about every bit of bad weather on declining Arctic sea ice. Julia Slingo, herself, told a Parliamentary Committee last year:-
“There is increasing evidence in the last few months that depletion of ice, in particular in the Bering and Kara seas, can plausibly impact on our winter weather and lead to colder winters over northern Europe”.
(This, of course, came a few months after previous predictions of warmer, wetter winters, and a few months before Slingo decided Arctic ice was responsible for heavier rainfall).
The private briefing document totally demolishes her argument and that of others:-
It has been suggested that the decline of Arctic Sea Ice may drive low pressure over the UK, although this remains very uncertain at present.
In the long term, most climate models project drier UK summers – but it is possible there could be other influences of a changing climate which could override that signal on shorter timescales.
If low levels of Arctic sea ice were found to be affecting the track of the jet stream, for example, this could be seen as linked to the warming of our climate – but this is currently an unknown.
The Met Office Hadley Centre, working with climate research centres around the world, is making strides in determining how the odds of extreme weather happening have been influenced by climate change. However, it is very difficult to do this type of analysis with such highly variable rainfall events, so it may take many years before we could confirm how the odds of this summer’s wet weather happening have been altered by greenhouse gases.
So why did Slingo give the testimony she did to Parliament?
The Met openly admit that neither they, nor climate science in general, have any real understanding about the basic processes that affect our climate.
It is surely time that they, DEFRA and others admitted this in public, instead of continually repeating the same old speculations that every bit of bad weather is linked to global warming.