 I'm Dr Ed Hawkins at the National Centre for Atmospheric Science at the University of Reading in the UK and my interest are in climate variability and climate predictability. I think what a lot of people don't realise is that the history of climate change is a very long one, the history of climate science. It was started with Joseph Fourier back in the 1820s when he first started wondering what set the temperature of the earth. He was trying to work out what should the temperature of the earth be given the information he had about how much radiation we had from the sun, for example. He started some very quick, easy, dirty calculations and he couldn't explain the fact that the earth was not a ball of ice. He ended up with a temperature of the earth well below zero degrees Celsius. He spent a lot of time trying to work out what was wrong with his very simple model of what the temperature of the earth should be. So he tried various things. He first thought that perhaps there's another source of energy in the system and that there must be energy coming from the centre of the earth. Geothermal energy was helping heat the planet up and he quickly ruled that out by doing some calculations about how much energy was coming from below. He eventually concluded that there must be extra energy coming from outer space and he thought the temperature of empty space must be higher than absolute zero and hence providing some energy. But he also noted while doing that that there was a potential for the atmosphere to have some property which increased the temperature instead. And that kind of started the ball rolling on people thinking about what might be setting the temperature of the planet a bit more and whether the atmosphere is involved in that. So one of the next important people is Claude Pouillet who was the first to measure the fact that there was downwelling infrared radiation from the atmosphere. And so this was the first hint that perhaps there's something in the atmosphere which is intercepting infrared radiation coming from the surface and downwelling it back to the earth providing some extra energy effectively in the atmosphere. So the next person in the story is John Tyndall who's an Irishman. He did some very careful experiments in London in the 1860s on the absorption of infrared radiation by various gases. He tried lots of different gases. He had normal air and water vapor and carbon dioxide and methane and other hydrocarbons measuring how much infrared radiation was absorbed by these gases. And he found that various greenhouse gases as they're now called, so carbon dioxide and methane and so on, they absorb infrared radiation in this laboratory experiment. And so this is the basis for what we now call the greenhouse effect. All of these figures who were doing this historical research at the time, they were not really concerned with the human influence on climate. Their entire motivation was explaining ice ages which had been discovered in the 1830s or so. They realised that the earth had been much cold at time because they could see these valleys carved by enormous glaciers but they couldn't explain how the earth could change its temperature so much between these warm and cold phases. And so they were looking for reasons that might cause the earth's temperature to change by such a large amount. And this is why they were looking at various mechanisms such as changes in the earth's orbit. This was an important influence so a man named James Crowell put forward the theory that changes in the earth's orbit would provide different amounts of radiation to the earth and therefore cause changes in temperature. And that was one school of thought on one side and that was further developed by Milankiewicz in the 1920s. They are called Milankiewicz cycles, they should probably be called Crowell cycles. And on the other side there were these other groups who thought that these changes in the earth's temperature were due to things like greenhouse gases and so on. And so there was this discussion going on between these two groups. And now of course we know that both are important of course. We know that the timing of these ice ages are set by the changes in the earth's orbit but those small changes in radiation received by the earth are amplified by the changes in greenhouse gases as well. So what was two theories is in fact one theory coming together to explain these temperatures changes. Yeah, it's important to realise that they were not interested in the human influence side of it. They're doing this science to explain what they could see going on around them and that is important. They weren't trying to prove the carbon dioxide warming the planet for any particular reason apart from trying to explain what the observations that they saw. So in the late 1890s we have Swedish professor Sfante Arrhenius who was the next person to start investigating this problem. And again he was mainly concerned with explaining ice ages what could cause these large temperature changes in earth's history. But also at this time it was becoming realised that the emissions of greenhouse gases from human activity could potentially be changing the composition of the earth's atmosphere. And therefore the role of carbon dioxide could be more than just explaining earth's history it could be potentially changing how things change in the future. And this is becoming more important in the late 1890s. So Sfante Arrhenius he went away and again did some relatively simple calculations on what would happen if the amount of carbon dioxide in the atmosphere was doubled. The kind of thought experiment if you like. And he came up with a number around about four degrees or so for the change in global temperature if you doubled carbon dioxide in the atmosphere. And this is the first estimate if you like of what we now call climate sensitivity. So my scientific hero in this story is an amateur, an amateur meteorologist called Guy Stewart Callender. He was a professional steam engineer and that was what he did as a day job. But in the evenings and weekends he used to collect temperature measurements from around the world and he was a very avid meteorologist. He took measurements in his back garden and he just enjoyed collecting data from temperatures all over the world. And in the 1930s he started to put together a global average temperature. He put together readings from 150 stations all over the world and averaged them all together to try and get an estimate of what the globe was doing instead of just looking at one or two locations. And what he saw was the earth was warming up over the previous 50 years. And he had data from about 1880 up to the mid 1930s, so about 50 years. And he could see the temperature of the planet warming up according to his data. So what Callender also did was actually take the available carbon dioxide observations. So building on the work of Arrhenius who had started to suggest that perhaps human activity might influence temperatures, although the rate of emissions was thought to be far too slow to affect global temperatures back in the late 1800s, but by the start of the 20th century the rate of emissions had gone up an awful lot. And so it had begun to realise that perhaps the rate of human emissions of greenhouse gases could affect the global temperature. And so Callender also collected for the first time the available carbon dioxide measurements. And he showed that carbon dioxide in the atmosphere had gone up because of human activity. And he linked that to the rise in temperatures that he saw. And he did again, did some simple calculations like Arrhenius had done, and showed that carbon dioxide could explain about half of the observed temperature rise that he'd seen in his data. And this is really the first time that someone has suggested that human activity had caused carbon dioxide to increase, and then it had increased global temperatures. This is the first sort of attribution of a change in global temperature to human activity. So Callender presented this as an amateur to the Fellows of the Royal Mitchell's Good Society. And it was generally regarded as very interesting, but it was treated with a lot of scepticism. People didn't really believe that human activity could affect something as big as the Earth's climate. That was a large part of it. A large part of it was we didn't have enough information about the absorption spectrum of the atmosphere to really do the detailed calculations necessary to work out how the radiation propagated through the atmosphere. That had to wait. And so I think there was quite rightly some scepticism because we didn't have all the information we needed. But this really kick-started more research and to try and understand what was going on in the atmosphere. And this really re-sparked the whole interest in this discussion because it had been quite quiet for the previous 30, 40 years. And this really energized the whole discussion. I think he's very under-appreciated. This book describes the life and work of Guy Stuart Callender. And the greenhouse effect was actually called the Callender Effect for many years. So my main interest is in climate variability which has become more and more popular recently because of what we've seen happen to global temperatures with the recent relative slowdown in the rate of global temperature increase. But for many years people have realised that climate variability plays a very important role in how climate will change for a particular person. For example, if they're living in one location, variability is the single most important factor for how they experience the weather and climate. So what is the difference between weather and climate? My favourite saying, which was a chap called Robert Heinlein came up with, is climate is what you expect and weather is what you get. So if you look out the window and you see the weather, that is what you get on a day-to-day basis. The climate is what you might expect to happen given a long-term average or average conditions over a very long period. So understanding how variability may change in the future or how it will affect different people in different locations, I think, is what drives me at the moment. So over the last 50 years or so we've seen global temperatures increase, especially up to the late 1990s. And then since the late 1990s temperatures have increased at a slightly slower rate than before the late 1990s. This has been termed the hiatus or the slowdown or the pause. And it's been much discussed amongst scientists in the media and so on about the various reasons for why temperatures have not increased as fast as we might have expected or as they did before. There are many possible explanations and many possible reasons, but I think the most important point is that it is not unexpected. We know that there is a long-term trend in temperatures, but we fully expect that to have periods where global temperatures are warming very quickly and periods where global temperatures are not warming so quickly. And this is because there is natural variations in how the earth's climate behaves. We see trends, but we also see variability, which will temporarily mask a long-term trend for some periods and enhance it at other times. So we expect to see periods where temperatures increase quite quickly and periods where temperatures do not increase so quickly. And we know that the planet undergoes these changes on decadal timescales and this is what we believe is part of the explanation for what we're seeing now. The role of climate variability is very important for global temperatures, as we've seen with the recent temperature slowdown, but it's also very important for other aspects of the climate system like Arctic sea ice. We've seen Arctic sea ice decline since 1979 since we have very good satellite observations. It declined particularly quickly up to about 2007 when we saw a very extreme low minimum in the summer sea ice extent. And many people expected then the Arctic ice to disappear very quickly after seeing that short trend from about 2000 to 2007. They saw the ice melting very quickly and they said, ah, it's going to disappear very quickly. But of course many others said, hang on a minute, that's not necessarily how things will work because there are these variations in the climate system. And I think the fact the Arctic ice hasn't declined as quickly since 2007 shows that perhaps variability was playing a role in some of the pre-2007 melt and the variability is going the other way now and causing the ice to melt less quickly. So variability is very important. We mustn't forget about it. We must keep communicating the fact that there are long-term trends caused by human influence, but there is variability that happens on top of those trends. Unfortunately, I think that the media jump on the extreme views of course. So when 2007 happened everyone said, ah, there's going to be no sea ice in five years' time. Certain characters are still repeating that. And I think that's very unhelpful because we knew there was potential for decadal variability in the whole climate system and sea ice in particular. And that didn't stop some people making some rather rash proclamations unfortunately. We have to point out when people are being too alarmist as well as being too cautious and that we have to do that more and more because we have to be seen to be fair and rigorous and robust. So I very much enjoy blogging and tweeting about climate change. I go and give public talks, which I find particularly very interesting to talk to people about what they understand about climate change and what people may want to do about it. Going and discussing these issues I think is very important. I would definitely encourage any climate scientist to go and give talks to schools and the public to try and learn how to communicate their very detailed science to a far wider audience. We've known since about 1938 that global temperatures were increasing and that carbon dioxide was likely responsible for at least part of the increase. And that increase has continued to this day. We're continuing to emit greenhouse gas into the atmosphere. We're continuing to see global temperatures increase.