Since the late 1800s, average global temperatures have risen by about 0.75 degrees Celcius. The evidence for this is overwhelming, and not subject to significant sources of error, because it comes from land-based as well as mid-ocean weather stations, many of which were established over 100 years ago. Thus, we can be certain that the effect is real. The amount of evidence available for the more recent changes in global temperatures has increased dramatically, both through a vast increase in monitoring stations (many intended to improve global weather forecasting), and also through satellite measurements.
Because we now have a record of 30 years of accurate, worldwide satellite observations, our ability to measure global temperature has improved to an unprecedented extent. We can now see the whole surface of the planet, and measure variations in temperature over almost every part of it on a continuing basis. So far, in the last 30 years, the satellite record has shown a warming trend of about 0.25 degrees / decade over land, and about half that over the sea.
So, what evidence is there that we are the cause of all of this? Firstly, we know that carbon dioxide, methane and other so-called greenhouse gases should have an effect on global temperatures. The reason for this is to do with the spectral absorption properties of these gases. When sunlight hits the surface of the Earth, part of it is not reflected, but absorbed. This raises the temperature of any un-mirrored surface exposed to sunlight. The surface of the Earth then radiates some of this heat back upwards in the form of infrared radiation. Some of this radiation makes it back into space, cooling the Earth overall. Greenhouse gases, however, tend to block this upwards transfer of infrared radiation, and thus keep more of it trapped in the form of heat. It is therefore clear that raising the concentration of greenhouse gases should increase global temperatures. Much work has been done in the past few decades in researching the extent to which man-made greenhouse gases influence the climate, and scientists are now virtually certain that these are the prime cause of recent global warming.
So why is there all this debate and controversy? The answer is that there isn't any among scientists. Almost all scientific institutions now accept that manmade greenhouse emissions are responsible for the majority of recent climate change, as can be seen from the following: Wikipedia: Scientific opinion on climate change It is, however, in the short-term interests of certain businesses and politicians to convince us otherwise. Because the true state of consensus among scientists is not publicly understood, people often believe there to be intelligent debate and dissent over the whole issue of climate change, where in reality the scientists are pretty much talking with one voice, and the politicians are sticking their fingers in their ears because they don't want to hear it. And because politicians have more public exposure than scientists, most of what people hear is what the politicians are saying.
So, accepting that the problem is real, what do we actually have to worry about? Some nice hot, sunny weather in the summers? What's so bad about that? In reality, however, hot summers are a significant threat to public health, even in Europe. The summer of 2003, one of the hottest summers on record in Europe, claimed many thousands of lives. In the UK, it is estimated that over 2,000 deaths occurred as a direct consequence of the increased temperatures that summer. In France, the number was over 10,000. Wikipedia: 2003 Heatwave
While it is impossible to attribute the extremes of any one event to global warming, the frequency of such events increases with increasing global temperatures. So we are killing ourselves, right now, by doing this. And we are doing it in numbers that outstrip most major social disasters. In the US, the consequence of rising sea-levels combined the increasing incidence of severe weather events will render coastal areas of Mississippi and Louisiana uninhabitable in the near future. Already, in the wake of hurricane Katarina, 250,000 people have permanently left the area (Earth Policy Article). Clearly, the disaster of Katarina is down to more than just sea-level rises. In particular, the loss of the Mississippi Delta, due to irresponsible engineering projects, is equally to blame. Nevertheless, the higher sea-surface temperatures created by global warming make weather events of this sort more likely, and the rising sea levels due to global warming (20 centimetres in the past 100 years) make them proportionately more devastating.
This of course only outlines the impact on Europe and the US. In reality, the vast majority of the early effects will be felt in marginal places, where the weather determines people's survival from year to year. Professor Martin Parry, of the UK Met Office, estimates that a global temperature rise of just 2 degrees could expose more than 2 billion people to water shortages. The UN Food and Agriculture Organisation warns that the same number could be effected by agricultural losses due to climate change, leading to a significant famines.
All these projections are understood to be very likely consequences of current warming trends. So at the very least, we are going to kill millions by being too lazy to act. But this is the bottom end of the scale. How bad does it get if the worst happens? Firstly, there is the possibility of the Antarctic Ice Sheets melting. This may already be happening (see this article), but if so, it is happening slowly. There is no reason to suppose, however, than it will continue to happen slowly in the future. During the warming period at the end of the last Ice Age, there was a significant discharge of meltwater from Antarctica, which happened very quickly indeed, raising global sea levels by about 20 meters. There is no reason to suppose that the ice cap will respond predictably to temperature rises, when in the past it has responded significantly unpredicably. 39% of the Earth's population lives within 100 km of the coast. Can you imagine the entire population of London moving inland? How about New York, Los Angeles, Tokyo, Beijing, Shanghai, Seattle to name but a few?
We simply do not know what change in global temperatures would be necessary to melt the Antarctic icecap. Yet, even though we don't have this information, we continue to commit ourselves to ever greater increases in global CO2 concentrations, and therefore temperatures. We're effectively playing russian roulette with the whole of human civilisation. If both Greenland and Antarctica melt, we are looking at a global sea-level rise of something like 75 meters. All UK coastal cities would be drowned. Even places as far inland as Manchester and Reading would be underwater. In Europe, Berlin and Rome would be underwater. In the US, all of Florida, and about half of the land area of the states on the Eastern seaboard would be lost. Is this likely to happen? We don't know. This is a long-term threat. It isn't going to happen in 50 years or even 100 years, but we could be making the changes today that will inevitably cause it to happen in the long term.
The Greenland icecap is even more sensitive to climate variations, and it seems very likely that it will melt at an accelerating rate over the next centuries. In total, the melting of Greenland could result in a sea-level rise of up to 7 meters.
So, is that the worst that can happen? In his book Heat, George Monbiot presents what is probably a worst-case scenario for the planet. He writes:
"The Permian period came abruptly to an end 251 million years ago... The marine sediments deposited at the time show two sudden changes. The rock laid down in the presence of oxygen is replaced by rock laid down in the absence of oxygen. A instant shift in the ratio of isotopes of carbon ... suggests a very rapid change in the concentration of atmospheric gases... At the point when the sediments change, 251 million years ago, the fossil record very nearly stops dead. The reefs die instantly and do not reappear on Earth for 10 million years. All the large and medium sized sharks disappear, most of the shelly species, even the majority of the plankton. Among the classes of marine animals, the only survivors were those adapted to the near absence of oxygen.
Plant life was almost eliminated from the earth's surface. Four footed animals, the group to which humans belong were nearly exterminated: so far only two fossil reptile species have been found anywhere earth which survived the end of the period... Altogether, some 90 percent of the species appear to have been wiped out... These events coincided with a series of volcanic eruptions in Siberia. The volcanoes produced great quantities of two gases, sulphur dioxide and carbon dioxide. The rising temperatures caused by the gas appear to have warmed the world sufficiently to have destabilised a super-concentrated form of methane which was found then (and is still found today) in large quantities in the sediments beneath the polar seas... The temperature rose by between 6 degrees and 8 degrees."
Now, as he points out, this change is unlikely to have been the consequence of the temperature rise alone, but also of the acid rain which would have resulted from the sulphur dioxide. On the other hand, it gives us an idea of how bad things can get. To give a basis for comparison, the IPCC predicts that the world will warm between 1.1 and 6.4 degrees in the 21st century. The lower end of this range is if we do everything possible to stave off climate change. Their best estimate if we continue as we have been is a warming of 4 degrees. It is very likely that human activity is already leading to a mass extinction to rival the Permian, of which global warming is one of the contributing factors. What the long-term effects of this may be, we cannot say, but they are potentially dire.
In conclusion, the problem is real. At the very least, we are already killing tens of thousands of people, and making millions homeless. At the most, we are risking the survival of our species, and most of the life on the planet. Given that this is the case, the prevention of manmade global warming must be our first and greatest priority, above all other political, economic or moral considerations. We are playing russian roulette with the future of our economy, our wellbeing, our civilisation and our species itself.