Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Examination of Witnesses (Questions 549 - 559)

WEDNESDAY 12 DECEMBER 2007

DR ANN CALVER, MR TERRY MARSH, PROFESSOR JOHN MITCHELL, MR STEVE NOYES, DR CHRIS WEST AND MS JACQUI YEATES

  Q549  Chairman: We move on to the second part of this afternoon's evidence session. May I welcome the star-studded cast before us. We are very grateful to have you with us and also for your written submissions. Are you able to help us at the Met Office because we would not mind knowing what the forward forecast is in terms of winter and summer. It might just give us a way of evaluating your forward forecasting techniques. In your written evidence, and indeed in some of the material you have been sending to Members of Parliament, you give a mouth-watering array of different forms of forecasts that you are involved in, even up to trying to forecast 100 years ahead, which seemed a bold challenge, bearing in mind that some people say you cannot even tell us what is happening tomorrow. Just give us a flavour for the winter period and summer. Are we in for a good one or a bad one?

  Mr Noyes: In terms of the winter ahead, it looks as though it is going to be colder than last year but not colder than normal. We do issue a seasonal forecast for the winter and then again for the summer. For the coming winter, it looks like being colder than last year—remember last year as particularly mild—but not colder than normal and in terms of rainfall, round about normal rainfall amounts. We have not yet produced our seasonal forecasts for the summer; that will be something to look forward to in the spring.

  Q550  Chairman: At least it sounds like normal. I think one of the problems about the science in meteorology obviously which underpins some of our discussion is this question of deviation from what you describe as "normal" and over what kind of time periods "normal" is established. Looking at the factors which you describe that affected the summer's weather, I think you were indicating the opposite of the El Nino effect had occurred this summer and we had things blowing in from further south than they should have been and therefore we had an atypical condition in the United Kingdom. Of course, as a result of that, you will have heard a lot of discussion as to whether this is the beginning of some kind of stream of predictable or unpredictable weather, extreme weather events: is this climate change? Perhaps you could just set the scene for us on the question about normality and whether you need something abnormal, like the events to which you referred, to occur for us actually to get the severity of flooding which has occurred. One of the points that we want to pursue later in our inquiry is the difference between the probabilistic effect of predicting flood events and the severity with which they are occurring. Perhaps we could start with the Met Office but others can pitch in if you want.

  Professor Mitchell: You have alluded to the wide range of timescales that we cover, so the daily forecasts that you see on the television for the next few days is very much what we call a deterministic forecast. We would expect to see those events unfold as they are forecast. We do take into account uncertainties by running a set of forecasts from slightly different initial conditions, and that gives a range of forecasts that allows for uncertainty, but for the daily forecasts, we would expect things to evolve very much as directly produced by the model. When you go to seasonal forecasting, what we are looking at is much more on a statistical basis. You cited El Nino as being one of the world-wide, global variations in climate that we see on anything from a three to seven year timescale. Certain phases of that are associated with more common frequencies of certain kinds of weather. This is particularly strong in the tropics, so for seasonal forecast in the tropics, we will look at sea surface temperatures. That is a very powerful indicator, particularly of how rainfall around the tropics will be distributed. But for our latitudes, the signal is much weaker, and that is why I think Steve was quite right to be slightly more cautious in making a forecast for winter, where we are looking very much at a statistical approach. What we are saying is that this is what the most likely outcome is. The third timescale you alluded to is forecasts for 10 to 100 years and there we are looking very much at the long-term effect of increases in greenhouse gases and other factors. There is an uncertainty there and we have to decide what the concentration of those gases is going to be over the next 50 to 100 years, and that is a socio-economic forecasting problem. Given a scenario of greenhouse gases, we can then put those in our models and see statistically how that changes the distribution of rainfall and rainfall extremes into the future temperature and so forth. As you go to the very long scales, they are very much statistical in nature. We are not forecasting that on Christmas Day in 2055 there will be a particular temperature; we just say, "This is the range of temperatures that we would expect at that time", and the range of rainfall and so forth.

  Q551  Chairman: Can you help me to get weather forecasting timescales into some perspective? When are you satisfied that a new norm has been established, because obviously normal is made up of a series of short-term events against a previously determined trend. At what point do you say: "We are now in a new weather scenario"?

  Professor Mitchell: Traditionally, we have taken a period of 30 years. That tends to be the norm specified in the World Meteorological Organisation, and of course if you have a climate that is not moving, is not changing, it is not very critical what period you choose. Now that we have strong evidence that we are influencing climate on a decadal timescale, it then becomes important to reconsider, as you have noted, what period you do use. So the current normals are from 1971 to 2000. Previously we used 1961 to 1990, so we do tend to move them on. Where it has really become an issue, again as Steve has alluded to, is that in giving our seasonal forecasts, we now have to say, "It will be a cold winter but not as cold as the previous one", and the meaning of "cold" has changed in people's minds. People tend to remember the last few years. So if we have had a succession of mild winters, and that is part of an underling warming trend and we predict that the next winter will be colder than perhaps the last few winters but not colder than average, you have to be very clear that because of the long underlying trend, what was a mild winter will appear as a cold winter. However there is no particular methodology for choosing what the reference period is.

  Mr Noyes: If I could add to that, this is a particularly important point in the context of how we communicate our forecasts because traditionally, as John says, we would refer to the 30-year period from 1971 to 2000. If you take the recent summers and recent winters, all of them have been above the average for that period, or most of them have. So, whilst in communicating our forecasts to the public and to our customers, we might well say that it is going to be warmer than normal, one might say that that is the most likely scenario in an environment where climate is gradually warming up. What we find, in terms of making it much more relevant to the people who use our seasonal forecasts, as John implies, is that we have to really link it back to the most recent summers and most recent winters so that people can put that into context. The reason for that is partly because people's memories are relatively short and a 30-year mean from seven years ago does not mean an awful lot, particularly for people who have not lived that long, and also because the climate is changing, so the backdrop is changing. It is becoming quite difficult to explain what a warm summer really means if you refer back to a 30-year period of some time ago.

  Dr West: Could I add something to what you said about a normal climate? I think we fail to appreciate what is normal. Even without the fact that climate is changing, we have short memories, so that in this room I guess some of us can remember the 1962 winter, which was very cold, and rather fewer of us the 1947 winter, which was very cold, and yet it is those extreme hot and cold, wet and dry years that describe and define "normal". What we perceive in the climate we are familiar with is only a subset of the true range of what is normal.

  Q552  Chairman: Part of the reason why I think this is an interesting area is that the investments which have to be made in flood defences, or shall we say "response to flooding", are of a very long-term nature. What you have to deal with in terms of capacity is, by definition, related to the weather events which cause the flooding. Just give us a hint as to how much you can contribute to that debate in territory in which you feel comfortable rather than just taking a punt, bearing in mind it could affect the expenditure of many millions of pounds.

  Professor Mitchell: Certainly in terms of looking at longer term infrastructure, one would be prudent to take future climate change into account. I think there are a number of points to make on that. The first is that in terms of the size of the projected changes over the next 100 years relative to the year-to-year variability, in terms of temperature we have quite a strong signal to noise; in other words, the trend and the change over 100 years is large compared with the year-to-year or decade-to-decade variability in temperature. With rainfall that is much less true. I think in England/Wales rainfall, if you look at the year-to-year totals, they can be anything from an average of about 60 mm, varying from 10 to 100, so there is huge variability from year to year. Defining what happens in the future, particularly in terms of extremes, is quite difficult because of what in statistics we would call a low signal to noise ratio. That is the first point to make. The second point is that there are some seasons over the UK where we could have a fairly good idea of what the changes are going to be. In winter we expect increases and precipitation; that is true of all the models that we looked at in the Hadley Centre, but also going to the recent IPCC report, that of the Intergovernmental Panel on Climate Change, pretty well all the models there show increases in rainfall over the UK in winter. The difficulty comes with the summer, and it is particularly relevant to this committee, and there we find about half the models get an increase over the UK, particularly the southern UK, and half the models get a decrease. The reason for that is that when you look globally, you find at high latitudes there is a consistent increase in rainfall and snowfall. Around the Mediterranean region in particular, there is persistent decrease, and in summer that divide just happens to fall in the middle of the United Kingdom. That is why we are unable to give particularly clear indications of what is going to happen in summer. I think the other thing that we should bear in mind is that an event such as we had in the summer was certainly quite a large anomaly in the large-scale circulation and the ability of models to simulate those anomalies is limited. For example, in UKCIP 08—this is a series of scenarios which we are producing in conjunction with Defra for UKCIP—those will certainly be very much the best available predictions that we can make at the current time, but there is still room for improvement. Essentially, as to where we are on climate change, we have established that climate change is happening; what we are unable to do at the moment is to make reliable, accurate predictions on a regional scale, and that is something which unfortunately will need more investment.

  Mr Marsh: It is clear that the models are strong in relation to temperature and there is a fair degree of confidence about where we will be moving. The uncertainty in rainfall is particularly important in relation to flood risk, and that uncertainty in a sense is at its broadest in relation to extreme events. I would make the point that modelling is very much a complementary activity to monitoring and what we need to capitalise on is the information that we have in the field, not just contemporary but historical, to see if there have been any tendencies or trends over time which are sensitive to climatic variations.

  Q553  Chairman: Can we move to what did happen this summer. Perhaps you could give us the official version of the events that led to these by our standards—and I suppose by normal standards as well—quite extreme weather conditions.

  Mr Noyes: The meteorology at the time during the summer period from late spring through to July was atypical in the sense that there were the sorts of weather conditions in terms of the way that the atmosphere was established over north-west Europe that one might find, for example, at this time of the year. In that regard, it was not what one normally would expect. The particularly heavy rainfall events of 25 June and 20 July fell on top of a particularly wet period. If you look at the period from May through to July, across many parts of England and Wales there was three or possibly four times as much rainfall as would normally fall during those months, and so the ground was very wet. Then we have had these particularly intense rainfall events on those two particular days. In terms of trying to explain a little bit about the atmosphere and what was going on, at this time of the year (December) in this part of Europe most of our weather comes typically from frontal systems that come in off the north Atlantic, bringing with them cloud and rainfall and sometimes snow and sometimes heavy winds. During the summer months, what tends to happen more is that those frontal systems tend to sit further north and west, often affecting parts of Scotland and Northern Ireland but less so the southern parts of the UK where our weather is affected more by continental Europe. In that regard, what we were seeing this summer was that the frontal systems were much further south. Why do we not get those sorts of rainfall amounts in the winter if that is the norm for winter? It is because typically in the winter the air is colder, the sea is colder. To try to create, if you like, a context in which one can think about the rainfall, in the tropics the atmosphere is very warm and the seas are very warm; there is lots of energy and the air can hold much more moisture when it is hot. In the summer, if you get these frontal systems coming further south over the UK, as we saw consistently week after week during those months, then you have warmer seas and warmer air and more moisture that can be put into the atmosphere and the atmosphere can hold more, and therefore you get more intense rainfall. We have done a bit of investigation in terms of these sorts of events. If you go back to the last century, when do we get more exceptional rainfall? What we discovered, and what we really knew but we made sure we double-checked on that, is that the summer months are when you get most exceptional rain. On the one hand, the systems we saw this summer were atypical in the sense that we would normally see them in the winter months, but getting very heavy rain is normally, yes, a summer phenomenon, and that is linked to the fact that the atmosphere can hold more moisture. There is more energy in the atmosphere, so you get larger rain droplets and more rainfall falling.

  Q554  Chairman: One of the things that is very interesting is where rain falls. One of our colleagues who gave evidence earlier was talking about, for example, Sheffield, the sodden nature of the uplands and the fact that when the second deluge came the uplands were already full and therefore the water ran off very quickly. Are you required or do you as a matter of course inform anybody, any organisations, about the amount of rain that has fallen in key areas like that, so that they might be aware of the fact that areas that traditionally had acted as soak-aways were filling up rather rapidly?

  Mr Noyes: We do a number of different things in that context. On the one hand, we have a weather radar network, which we operate jointly with the Environment Agency, and the equivalent in Scotland, with SEPA, and that radar network is able to make an equivalent estimate of the rainfall that is falling at any moment in time. That data is passed in real time to the Environment Agency that uses that to look at making estimates of the amount of rain that has actually fallen in river catchment areas. They then use that in their own models to understand what is happening to the rivers. Some of our colleagues will be able to explain a little more about that in detail afterwards. The Met Office's primary role is to provide forecasts of how much rainfall is going to fall and then in the context of any warnings that we issue, we certainly take account of what has happened before. So we modify our advice in the context of the prevailing conditions. Our forecasters are very much aware of saturated ground when rivers are at a higher state of alert and they are in regular dialogue with their colleagues in the Environment Agency. In the run-up to both of the major events of this summer, we have, for example, initiated daily conference calls with our colleagues from the Environment Agency each morning when we would ask them about the states of the rivers and the state of the ground and understand what they think is going to happen from a flood point of view, and we would be talking to them about the amounts of rainfall that is likely to fall. Clearly, as I think you are implying, 50 mm of rain at this time of the year (December) probably would not be an exceptional thing. However, 50 mm of rain falling a day or two after what we saw on 25 June and 20 July would be significantly important. Indeed, with regard for example to the COBR environment which we were involved in, we were asked with the Environment Agency to run some "what if" scenarios because, in the context of probability, we were forecasting that there was a small risk of a high rainfall event for the weekend following the 20 July event. It was not the most likely forecast. The most likely forecast was for around 20 mm of rain to fall, which the Environment Agency believed would not lead to any further flooding. However, we were indicating that there was a small probability of a heavier rainfall event. Together we worked out whether if we got 50 mm of rainfall, which was a 15% probability of happening, that would lead to more over-topping of the banks in the Severn area in particular. We were able to advise the Gold Commands in Gloucester and also the COBR community that this probability of further high rainfall was unlikely to lead to further flooding. That is how we do it.

  Q555  Mr Williams: During that period of time, in Wales we did not get much flooding. Was that because the intensity of the rain was less or was it because actually Wales has got better resilience in terms of flood prevention than the areas that did experience flooding?

  Mr Noyes: Could I deal, first of all, with the rainfall that actually fell and then perhaps colleagues from CEH could deal with what then happens to it on the ground? You are right that if you look at the difference from normal, Wales had significantly less difference from normal in terms of the amount of rain that fell on both occasions. Further east from Wales, particularly if you take the July event, the Worcestershire and Oxfordshire area had a much greater than normal amount of rainfall than you experienced in Wales. Perhaps I can pass to my colleagues and see what happens on the ground.

  Mr Marsh: I would agree with that. The rainfall in Wales was less and, relative to the long-term average, Wales is a wetter country obviously than the lower Severn basin and the Avon itself. It was principally a function of the rainfall. I can say a bit more about what happened to the rivers in 2007 if it is appropriate. Localised intense downpours in the summer are not rare in this country and localised severe flooding is not that rare. We remember Boscastle, which was extremely damaging. In 1952, the Lynmouth flood in the summer sadly took 34 lives, so that was a disaster, and that was a summer event. What distinguished this year's flooding was its regional character—indeed, its multi-regional character. That just does not happen in the summer. What is the reason for that? We have heard the synoptic background in terms of the weather, the weather patterns being unusual. It is often reported that the May to July period was the wettest on record. That is true but it under-sells the wetness of that period. The May to July period registered 400 mm for England and Wales. That may not mean too much but the highest in the 20th century was about 100 mm less than that. What you got this year was seven weeks additional rainfall, if you like on top of the highest rainfall in that period, and only in that period, in the 20th century. This rainfall was outstanding. With rainfall of that magnitude, you are going to get extensive flooding. If you look at the rivers of the country as a whole and try to estimate the outcomes for England and Wales in June and July of this year, then as a reasonable estimate it was twice what we have ever had in June and July before. That is only over a period from 1961. We have data over that successive period. It really was outstanding. In those areas in a broad belt from Yorkshire down through to the Bristol Channel embracing much of the rivers and the upper Thames catchment, many rivers registered their highest flow on record. Again, that is remarkable for the summer period. For much of the rest of the country, the experience of river flows was very notable for the summer but not outstanding in relation to winter flows. Just to pin a couple down, in the Thames for example where the flooding was severe in the headwaters and not so severe lower down, the flow was the highest since 1903 for this summer whereas where the flows were most severe, in the Doncaster area, south Yorkshire, Sheffield and in the Warwickshire/Avon and the lower Servern, the flood magnitude there matched or exceeded that of the 1947 event. The 1947 flood was the largest; it was the most extensive flood in the 20th century in England and Wales. It was entirely different in character to the summer flood we have now, but in a few areas, 2007 exceeded that. We are talking about a very singular event.

  Q556  Chairman: Let me try to put together, if I may, the scenarios that you have painted. Mr Marsh, you have given me what I call a forthcoming attraction picture. Here in the spring and the summer there was this increasing amount of rain. You guys in the Met Office are being asked to make certain predictions. As a layman looking at this and with the benefit of hindsight, I am thinking to myself: if they knew that the volumes of water that had been falling in various locations was so far above the normal level, should not bells have started to ring somewhere that if it carried on like that, there might be some problems? One of the parts of the analysis that we are looking at is: how did authorities react to the flooding? One of the factors, for example, was: should the Environment Agency in the specific circumstance have moved flood defence barriers to a certain location? No, they could not because the flooding had already started; the roads were blocked and they could not get the kit in the right place. I think what I am asking is: do we learn any lessons from marrying up, Mr Marsh, if you like, your scenario of "oh look, what's happening; this is not really the normal thing" with, Professor Mitchell, your guys saying, "We seem to be seeing something rather abnormal in the way that weather is happening" so as to be able to say to local authorities, the Environment Agency and water companies, "Something may be out there which is not right. Be prepared". I do not think I am getting the message that these two bits of information were put together ahead of events. It was only afterwards that everybody started to think about what was coming next.

  Mr Noyes: Can I deal with the Met Office's forecast, if you like, and the warnings that we issue? In regard to saying that these types of things happened only after the event, I would certainly challenge that in that the Met Office for both events gave early warning in advance that the two particular events that led to the flooding were likely to happen. We were particularly pleased with how accurately they were forecast. There has been a significant step-change in capability over the last five years. If we go back five years, it would have been much more difficult for us to have given so much confidence to that early warning as we were able to do this year. We have seen an improvement in our capability. There is still more improvement that we want to be able to deliver to enable even earlier warning.

  Chairman: Forgive me for interrupting. Coming back to Mr Marsh's very interesting picture, the message I get, hearing him describe if you like the run-in to this, for the first time in a very objective way is that if the kind of accuracy of forecast that you were dealing with was visited upon the scenario that Mr Marsh has enunciated, you might have said, "If we are going to get this amount of rain and we are forecasting it just a little bit ahead here, it is going to come on top of what Mr Marsh says is already a pretty serious abnormal situation and it will amplify the effect on the territory where this rain is going to come down." The question I am groping towards is: should those who had access to this information have spotted what was coming earlier than they did? Patrick Hall wants to follow up on that.

  Patrick Hall: I understood Mr Marsh to be describing the events after they had taken place. I was tempted to think that one might say, in the face of the very exceptional nature of those events that Mr Marsh so succinctly described, that there is a limited amount one might be able to do anyway. That does not mean you do not try. That was the message I received. He was talking about what had happened after the event. That, he said, did not contribute to forecasting. It is the Met Office's job to forecast.

  Q557  Chairman: I appreciate that. Let me ask this question. At what point could you have, if you like, produced the data which said: we have already had an awful lot of rain and it is above the norm?

  Mr Marsh: I should stress that we have no operational role here, and so it is correct that this is post hoc after the event and after the event you can see that it was extraordinary. If during the event it was possible to recognise a synoptic pattern which tells you something about how the condition is likely to evolve, clearly that is something that has be capitalised on, but it is not my area of expertise.

  Q558  Chairman: You are not saying, as an expert who looks at the way that rainfall and rivers interact, that during the course of the spring and the early summer when this lot is starting to build up you could not have said, "Something unusual is occurring"?

  Mr Marsh: I would not run with the term "build up". I was describing the period as a period. With that totality of rainfall, you would get extensive flooding.

  Q559  Chairman: I am leaping ahead in my analysis of where you can contribute. I accede to Mr Hall's cautionary tale that I should not build too much on what Mr Marsh has said.

  Mr Noyes: I would not accede too much in the sense that I think there is more that could be done. I think the Environment Agency will probably have to answer for themselves in terms of how much they were aware of the situation that was happening on the ground. Because they monitor the rivers in real time, I am sure they were very much aware of the state of the rivers during those few months. What we were able to do during the summer, which we would not perhaps have been able to do so effectively a number of years ago, was work much more closely with the Environment Agency to look further ahead to what might be going to happen a week in advance. The difficulty we have is that at the moment, if you think of the environment as being a system in itself—and you are probably familiar with the water cycle where the water falls from the atmosphere, lands on the ground and goes back up again—is that typically at the moment we model that system in two different parts. We forecast the atmosphere and then forecast separately what is going to happen to the water once it has landed on the ground. At the moment, with the way that we are able now to forecast a long way in advance and from that produce early warnings, the way the Environment Agency system works is that they then model the rainfall once it has fallen. So their models are dealing very much with more of a real time nature rather than a long-term early warning. Where I think there is room for gaining on capability is to integrate those two capabilities much more into a whole system where you are extending the longer range ability to project what is going to happen to the rivers when the rainfall falls five days hence.


 
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