Heavy Weather

Interview with climate change scientist

So let's nail it down. Is global warming a natural phenomenon or a man made cascade? In the first of a new ask-the-expert series, Neil Goodwin interviews Dr Mike Hulme from the Tyndal Centre for Climate Change Research.

28th February 2001

SQUALL: There have been a few predictions in terms of temperature and sea level rise etc. What is the latest view on the actual physical effects of climate change in the 21st Century?

DR HULME: Here in the UK, we anticipate a continuing increase in our average temperature, probably somewhere between an extra 1 to 3 degrees of warming in the century to come. That is most likely to be associated with wetter winter conditions. Summers will probably become dryer. And certainly when you consider the additional increase of evaporation during a hotter summer season, water is going to become more scarce. With regards to our coastlines, with the accelerating increase in sea level that we anticipate in the century to come, the risks of storm surges breaching or overtopping our coastal defences in various parts of our vulnerable coastline - those risks are likely to increase quite substantially.

SQUALL: People point out that there have always been floods, storms and droughts. Aren't we really looking at natural weather variability?

DR HULME: Well, this is one of the challenges we have as climate scientists - to try to disentangle what is just natural weather variability within our climate system, from that part of the change that is due to human pollution of the atmosphere. And it's not an easy challenge. But as we've progressed our understanding, we now understand many of the forces that cause natural variation in weather and climate - such as changes in the activity of the sun, or volcanic eruptions, or just natural oscillation within the system from time to time. And once we've done that, we can begin to pick out a clear distinct signal in climate that is due to human pollution. It's not easy to do. And that's why there are many uncertainties still in our statements about future climate. But we do think that we're at a level of understanding where we can make some useful comments that help organisations and businesses and ordinary community's plan ahead for the century to come.

SQUALL: You recently wrote in a newspaper that there can no longer be a purely natural weather event. What did you mean by that?

DR HULME: Recently, I claimed that there is now no longer such a thing as a purely natural weather event. And by that I mean quite simply that we know that our climate is warming globally - it's about 0.6( warmer now than it was 100 years ago. And we anticipate that warming to increase in the future. And increasingly we're confident that at least part of that warming is due to human pollution of the atmosphere. We're actually creating a semi-artificial climate here on earth due to human, often quite unconscious, pollution. All climate is the sum of individual weather events. It basically means that every single weather event now is in some way tainted or influenced by this background of global warming. We can't necessarily say that all of a particular storm is human caused. It may be that only a particular part of that storm or the particular location of that storm might be human caused. But I think the bottom line is that now we have got to recognise that our climate on earth is not a purely natural climate - it is a semi-artificial one. We are actually fashioning the climate that we and our children are going to be living in, in the decades to come. We're clearly influencing climate, and we're doing it in ways that actually we don't understand, and ways in which the outcomes are very poorly defined. We're actually running quite serious risks of having quite undesirable outcomes with our climate system. And one could say that there is somewhat of a parallel with the current debate about GM crops. That we're actually introducing artificial varieties into the natural world - the consequences of which we don't actually know very precisely how it's all going to turn out. It's very much like that with our climate as well. We don't really know in the end what the long-term consequences of our intervention are going to be.

SQUALL: Do the greenhouse gases we emit today have an immediate effect on our climate, or are present climatic changes occurring as a result of past emissions?

DR HULME: Well, in trying to understand why greenhouse gasses cause global warming, it is important to separate two effects. And this also helps us to understand why climate change is such a long-term problem.

First of all, when we emit greenhouse gasses - for example carbon dioxide - into the atmosphere, then these molecules of CO2 have an immediate radiative effect on the energy balance. In other words, they absorb energy that is trying to leave the planet. And that energy is trapped by the molecules and re-radiated back to the earth's surface, preventing that energy from escaping to space. And that effect is quite immediate. So as soon as you put an extra molecule of CO2 into the air, it will have to start absorbing energy - by its nature it absorbs energy. It has got no choice about that matter. It is the way it has been designed. And that is an immediate effect. So greenhouse gasses that we emit today have an immediate effect on altering the energy balance of the atmosphere.

However, for the full effect of that extra energy to be realised within the climate system, that energy has then got to be dispersed throughout the whole of the atmosphere and in particular dispersed throughout the oceans.

Now, dispersing that extra energy throughout our ocean system can take decades, even centuries to come about. If you think about how large a body of water is represented by the oceans, if you put one extra unit of heat onto the surface of that water, for that unit of heat to be equally distributed throughout all the world's oceans, you can understand why that might take 50, 100 or 200 years. So, although there is an immediate radiative effect, the long-term consequences of this energy trapping are one, two, three hundred years into the future.

In fact, when we look at sea-level rise, the major reason for sea level rise is nothing to do with ice sheets or glaciers. In fact, the major reason for sea-level rise is simply that warmer water in the oceans expands. As it expands it consumes a bigger volume. So it rises. For that process of expansion to happen throughout the world's oceans, it is very likely to take hundreds and hundreds of years. So, sea-level rise, almost no matter what we do to try to limit greenhouse gas emissions, will be with us for many generations to come.

SQUALL: Where does this leave us in terms of the Government's target of a 20 per cent reduction in CO2 emissions by 2010? Considering the fact that emissions hang around in the environment for a long period of time, do you think that that target needs to be more ambitious?

DR HULME: I think that that when it comes to discussing what sort of greenhouse gas emission targets governments should pursue, it depends very much on the objectives that you're really after. On the one hand, the scientist will say that if we want to stabilise climate within the next generation, in other words within the next 50 or so years, we actually need emissions reductions probably in the order of 50 or 60% compared to present day levels. And we need that to happen world-wide. Not just in one country like the UK. So if that's your objective - to actually stabilise climate once again within a generation or two - then very deep cuts are needed, and they're needed world wide.

Now, depending on your viewpoint, you could say that that is politically na•ve. There's just no way that we're going to find the world community agreeing to that level of emissions reduction over a short period of time - 10 or 20 years.

So the whole process of negotiating the Kyoto Protocol - the international protocol that seeks to bring about emissions reduction - was a lot more of a politically charged debate. It wasn't saying that we need a 60% reduction because we have got to stabilise climate in 50 years time. It was actually a balance between various sectarian interests, various industrial interests, and of course balancing various governmental positions between rich and poor countries - East and West, North and South.

And so in the end, the Kyoto Protocol compromised that the first achievable target would be a 5% reduction by 2010, and only by the industrialised countries. Now, in climate terms that is almost negligible. It has very little impact in reducing global warming.

However, the true significance of the Kyoto Protocol is not about its affect on the climate system - we, as scientists know that that affect is negligible. The real significance of Kyoto is first of all that there is an international protocol in existence. And secondly that it's sending out a very strong and clear message to a wide range of business interests saying that: Look, carbon is no longer a neutral by-product of our industrial economies - carbon has got a price tag attached to it. And many of the energy companies are hearing that signal loud and clear and beginning to move away from carbon-based energy production.

And I believe that this will be the greatest legacy of the Kyoto Protocol, that it is actually shifting us gradually away from a polluting carbon-based energy economy, towards seeking more sustainable and renewable forms of energy.

Now, in that context, the UK government, by setting itself its own internal target of a 20% reduction is actually well ahead of the game. And no other government has set themselves that ambitious target. And in a sense I think that the UK is taking a very important and significant lead. And if they can demonstrate that they can achieve a 20% reduction, and it won't be easy, but if they can achieve that by the year 2010, then again I think that that sends out some strong signals that these sorts of things are achievable, if the political will exists to achieve them.

But no one is suggesting that this is solving the climate change problem. It's not. It's simply the first step, pointing us in the right direction.

SQUALL: What kind of extreme impacts could we be facing if the international community fails to curb greenhouse gas emissions?

DR HULME: I mentioned earlier about the long term risks that we are running with our climate system by altering the way in which the system behaves. And we're altering it in ways that we don't fully understand. We don't understand the consequences even 50 years into the future. And certainly if we take a much longer term perspective - 100, 200, even 300 years - there are many factors that could kick in to destabilise the planetary climate system even further.

One or two of these catastrophic changes, or very abrupt changes, have received quite a lot of attention. And they remain definite possibilities. The two most cited would be the slowing down of the Gulf Stream, the circulation that actually brings to our latitude here in the UK the heat that brings us the equitable climate that we have. Now, if that circulation was to shut down there could be some major dislocations in the North West European climate. Equally, if the West Antarctic ice sheet, which is actually ice that is grounded below sea level, if that were to collapse and disintegrate, there could be a rise of up to 5 metres in sea level.

These sorts of things, clearly they're possibilities. And clearly, by destabilising climate or altering global climate, we're raising the stakes. The risks of one or two of these sorts of things happening are clearly increasing. However, trying to quantify just how likely that is an entirely different question. And my own judgement, from listening to various experts and participating in meetings, is that both of these examples that I have cited are actually quite low probability outcomes - at least in the time scale of the next 100 or 200 years.

But I wouldn't say that I am entirely sanguine about that. I mean, clearly there are risks. And even if its a 1 in 100 or 1 in 1000 risk, I think it clearly begs the question: Are these risks ones that we, as a society, are prepared to take with something as fundamental as our climate?

And this, I think, is what motivates many campaigners and lobbyists who wish to see more rapid action on greenhouse gas emissions. They basically take the line: Well look, we don't know what the full consequences are. Clearly there are some really high risks that might materialise. We should take the precautionary approach and do everything we can to actually minimise these risks.