What is Global Warming?

First of all let me explain what global warming is about. Around 1900, the French artist Claude Monet visited London and enjoyed the city enormously. He loved the light coming through the smog and if any of you went to the Monet exhibition last year you would have seen paintings with varieties of smog and lots of variations of light. He must, I think, have worn a handkerchief over his nose or had extremely good lungs because London was not a very pleasant place to be in at that time. It is still a polluted city, much more so than it need be, but a great deal better than in 1900.

The problem in London is local pollution largely arising from vehicles that affect the air around them. But we now know there are forms of pollution - global pollution - which individuals in one place may emit and which then affect the whole world. One example of this is ozone depletion by chlorine-containing chemicals. Very small quantities of these emitted into the atmosphere, for instance from leaking refrigerators or from aerosol cans, can reach the stratosphere. This may be only perhaps in parts per trillion, but free chlorine is released that catalytically destroys ozone, rapidly affecting the whole atmosphere.

Global warming is a second and a more important example of this global pollution. Carbon dioxide that I cause to be emitted, because I drive my car or use electricity or in many other ways, enters the atmosphere, and rapidly spreads around the whole atmosphere, much of it remaining in the atmosphere for 100 years or more. Now, because carbon dioxide is a greenhouse gas, it causes the average global temperature to increase, significantly affecting the climate. So everybody in the world is affected. North Atlantic.

But you will see that the nature and rapidity of the change in temperature over the 20th Century is very different from that over the previous 1000 years. In particular the recent years have been the warmest over that entire period. 1998 was the warmest year in the global instrumental record, and a more striking statistic is that each of the first eight months of 1998 was the warmest of those months in the instrumental record - suggesting that the earth really is warming up.

Why is the earth warming? Fig 3 illustrates solar radiation travelling through the atmosphere on its way to warm the earth's surface. This incoming energy is balanced by infrared radiation leaving the surface. On its way out through the atmosphere, this infra red is absorbed by greenhouse gases - water vapour, carbon dioxide and methane are the principal ones - that act as a blanket over the earth's surface keeping it warmer. Increasing the amount of these gases increases the greenhouse effect and so increases the average temperature of the earth's surface.

We know for certain that carbon dioxide is increasing because of the burning of fossil fuels - the isotope signatures of atmospheric carbon confirm that. Its increase since the end of the industrial revolution has been about 30% (Fig 4). Also shown in fig 4 is that methane has approximately doubled since the industrial revolution, very much in line with the growth of human population.

Fig 5 shows the global average surface temperature over a much longer period, including the last ice age which finished about twenty thousand years ago. The last warm period occurred about 120 thousand years ago. The temperatures for these curves are determined from an ice core bored out by Russian scientists from the Antarctic icecap. Such cores are very long - a kilometre or more - and the temperature at which the ice was laid down is deduced from measurements of the ratio of the concentrations of different isotopes of oxygen in the core. The carbon dioxide concentration can be measured from the composition of air in bubbles trapped in the ice. You will notice that the curves of temperature and CO2 content track each other well. Part of this is because carbon dioxide influences the temperature, but it is also because other factors that depend on temperature are controlling the carbon dioxide content in ways that are not yet well understood.

Carbon dioxide levels now are about 365 parts per million. By the year 2100, if we carry on burning fossil fuel in a "business as usual" way without caring about its effects, carbon dioxide concentrations will rise to 600 or 700 parts per million. If the whole world decided to work very hard indeed so as to stabilize carbon dioxide concentrations, we could possibly stabilize at about 450 parts per million. But that is still a very dramatic increase, taking carbon dioxide concentrations far beyond any level they have had in the atmosphere for millions of years.

You may say, "But, of course we'll be due for another ice age sometime soon and such an increase of carbon dioxide could perhaps help to keep us warm or even prevent it coming". We understand quite well what triggers ice ages; it is variations in the earth's orbit round the sun that causes variations in the incidence of solar radiation, particularly in polar regions. These orbital variations are well known from astronomical data and can be precisely predicted; the next ice age is expected in about 50,000 years. As we are likely to use up most of the available supply of fossil fuels in a few hundred years, we will not have much effect on the next ice age!

I will now explain briefly how the influence of increased greenhouse gases on the climate is calculated. Fig. 6 shows the external forcing on the climate system over the last 150 years or so from a number of factors, of which the largest is the increase in the well-mixed greenhouse gases shown at the left of the diagram. Other factors are changes in ozone that is also a greenhouse gas and changes in particles (known as aerosols) in the atmosphere especially, for instance those resulting from emissions of sulphur dioxide from power stations. These sulphate particles tend to reflect sunlight and cool the system; other particles like soot and mineral dust could well be absorbing radiation and warming the system. Careful studies by solar physicists have also been carried out to estimate the range of possible changes in the amount of radiation from the sun over this period; this is shown at the right of the diagram. You will notice that some of the external forcing factors are well known (for instance the increase in greenhouse gases) while others are quite uncertain (for instance the indirect effect of aerosols).

An important question to ask is whether, given the information in fig 6, the global average temperature over the 20th century can be simulated with climate models. These are computer models that include descriptions of the physics and dynamics of the whole climate system (atmosphere, ocean, land and ice) and that integrate the fundamental equations of motion as a function of time from appropriate initial conditions. A lot of research has gone into such simulations over the last five years that is described in detail in the latest IPCC report. Fig 7 shows the results from four or five of the best models in the world. On shorter time scales, the models, similar to the real atmosphere, show substantial variability independent of variations in the external forcing. For the longer term variations there is actually quite good agreement. Most of the reason for the recent increase in global average temperature, as I said earlier, is the increased greenhouse gas concentrations. The fact that models can reproduce the observed data with some sophistication provides confidence in the use of models for projection into the future. Confidence in models also comes from model studies of more detailed phenomena (such as the effect on the weather and climate of particular volcanic eruptions) and of climates of the past, for instance of the period 6000 thousand years ago when the radiation conditions were different because of the changes in the earth's orbit.

Fig 8 illustrates the simplest calculation that we can make about the global effects of increased carbon dioxide and it illustrates what we know and what we don't know. On the left of the diagram is shown on average the balance for the earth between the incoming solar radiation, and the outgoing infrared radiation, 236 watts per square metre coming in and 236 watts per square metre going out. If there were no greenhouse gases in the atmosphere, the average temperature of the earth's surface would be -18ºC or thereabouts. That means the earth's surface would be covered by ice and this lecture would be held in an igloo! The average temperature of the surface is actually about +15ºC so that there is about 20-30ºC of natural greenhouse effect due to water vapour, the strongest greenhouse gas, naturally occurring CO2, and methane. Now suppose the carbon dioxide concentration in the atmosphere is doubled instantaneously, we can easily calculate what the change in the radiation balance would be. The outgoing radiation at the top of the atmosphere is reduced to 232 watts per square metre, leading to an imbalance of 4 watts per square metre. To bring the system back to balance, the temperature at the surface and in the lower atmosphere has to increase by about 1.25ºC. That is on the assumption that nothing else changes. But, of course, other things do change: for instance, as the surface temperature increases there is more evaporation, hence more water vapour in the atmosphere. Now, since water vapour is a powerful greenhouse gas, the temperature increases further. In fact there is a positive feedback of about 60%. Also the increased temperature will melt some ice allowing the surface underneath to absorb some sunlight instead of reflecting it back to space - another positive feedback of around 20%.

What about changes in clouds? We still don't know very much about how clouds are influenced by increased temperature. Low clouds provide a negative feedback, because they reflect sunlight rather well; high clouds, because they blanket the earth, like the greenhouse gases possess a positive feedback. We do not know the net effect of clouds with any certainty; it probably varies from place to place and with season. It is the largest unknown in projecting climate for the future. And then we also need to include the effect of increased temperature on the circulation of the ocean and on the way it moves heat around, which could cause substantial regional climate changes.

Taking all these effects into account, the IPCC's best estimate of the global average temperature rise for doubled carbon dioxide is 2.5ºC, with a range of 1.5 to 4.5ºC that allows particularly for the cloud uncertainty. At first sight, such a temperature rise does not seem very large. I suspect the temperature in this lecture theatre has gone up by a degree or two since we walked into it. But here we are not talking about a local rise of temperature but a rise in the global average. The difference in global average temperature between the middle of an ice age and the warm periods in between, is only about 5 or 6ºC (see fig 5). So 2.5ºC is about half an ice age in terms of climate change and we're talking about this occurring in about a hundred years; that's a very rapid change in climate compared with changes in the ice ages.

Projections of temperature for this century as estimated by the IPCC from a range of models (fig 9) vary between 1.4ºC and 5.8ºC dependent on what assumptions are made about the amount of carbon dioxide and other greenhouse gases that will be emitted due to human activities. The range also includes allowance for our uncertainty about the response of the climate to increased carbon dioxide. A rise anywhere in this range is very likely to represent a more rapid change of climate than the earth has seen for 10,000 years.