The entire West Antarctic ice sheet might collapse

Surely that's a bit sensationalist?

Not really. Ice cores reveal growing evidence of sudden, dramatic shifts in climate over the past 10 000 years that have occurred within a few decades as a result of "flips" in ocean circulation.

Are there any other cataclysms in the offing?

One fear is that the entire West Antarctic ice sheet might collapse, sliding into the ocean and raising sea levels worldwide by several meters. Even the most pessimistic experts say this is only a worry if the world warms by about 4 degrees C, which is outside the range of mainstream predictions for the next century. And even then, what glaciologists mean by a collapse is still a slow process. It would take several hundred years for all the ice to slide into the sea.

Will there be global warming everywhere?

Maybe not. Climate modellers admit to being very uncertain about how global warming will affect particular regions. This is because much of our weather depends on circulation patterns, which could alter in unexpected ways. 

Crudely, however, modellers expect many coastal regions to become wetter, while continental interiors will become drier, causing some deserts to expand. Warming will probably be greatest in polar regions, mirroring climate changes this century in both the Arctic and Antarctic.

Local climate could also be altered by changes in ocean circulation. Western Europe could be particularly vulnerable. At present, it is kept exceptionally warm in winter by the Gulf Stream, which is part of the ocean conveyor belt. 

Take that away and British weather would be like the Hudson Bay, which is at the same latitude. If the conveyor belt slackens, or the path of the Gulf Stream shifts, that is precisely what could happen. So British hopes of a climate like Bordeaux in the 21st century could be cruelly dashed!

Sulphate particles? Don't we make them, too?

Right again. One of the nice ironies of this story is that sulphate particles (acid rain) from burning coal and oil help to shield the more industrialised countries from the full impact of global warming. In some places, such as central Europe and parts of China, they may have overwhelmed the warming, producing a net cooling. 

Other aerosols, such as dust from soil erosion and "desertification" can also moderate warming. But even if you find the idea of using one form of pollution to protect us from another, there is a problem. Whereas the average CO₂ molecule in the atmosphere lasts for about a century, sulphates and other aerosol molecules persist for only a few days. 

This means two things. First, if you turned down the power stations, the world would get much hotter within a few days. Second, aerosols do not accumulate in the atmosphere in the way that CO₂ does. If you carry on burning a given amount of fossil fuel, the cooling effect of the sulphates will remain constant, while the warming effect of CO₂ will keep on increasing. So sulphates are not a solution.

Anything else that could shield us from global warming?

Yes, volcanoes. When Mount Pinatubo erupted in June 1991, it threw a huge amount of debris into the stratosphere that partially shielded the surface of the Earth from incoming solar energy. Particularly effective was Pinatubo's emissions of large amounts of sulphate particles that scatter sunlight. 

Computer models successfully predicted that in the short term, the debris would cool the Earth's atmosphere. The models also predicted that as the volcanic debris cleared in 1992 and 1993, average temperatures would swiftly return first to the level of the 1980s, and then, by the middle of the 1990s, to the slightly higher levels that would be expected with the ongoing buildup of greenhouse gases. 

See "Blowing hot and cold" feature.

This is all very pessimistic.

Couldn't a warmer planet absorb more of our pollution?

Yes indeed. Warmer temperatures and the fertilising effect of more carbon dioxide in the air will stimulate faster growth of trees and other vegetation, which in turn will help to soak up some of the CO2 in the atmosphere. This can already be seen in some places. But plants need other things besides CO2 to grow. 

They need water, which could be in short supply as greater evaporation dries out soils. They need space, which we are eating up. They also need climatic stability. Recent studies by the Intergovernmental Panel on Climate Change (IPCC) suggest that climate change could soon be so fast that many forests, particularly in northern latitudes, will be unable to adapt and could die off.

What about organisms in the oceans?

Once dissolved in surface waters, a lot of CO2 is absorbed by plankton and other marine organisms and turned into organic compounds. Most of this eventually falls to the ocean floor. The strength of this sink for carbon depends on how much life the ocean is producing. 

It is not clear to what extent global warming will affect the oceans' biological productivity--it could rise or fall. Cooler seas tend to produce more life, but iron dust from expanding deserts could make warm seas more fertile. And some scientists have investigated whether we could boost this effect artificially by seeding the oceans with iron.

The concentration of greenhouse gases in the atmosphere.

Are there other complications?

Yes. A whole series of other feedbacks will influence the concentration of greenhouse gases in the atmosphere. Not all the CO2 that we put into the atmosphere stays there. Some is absorbed by vegetation on land, mostly forests, while more is taken up by the oceans. 

Change these uptake rates and the rate of buildup of the gas in the atmosphere from human activities will also change--speeding up or slowing down global warming. One way this uptake might be reduced is if we chop down all the tropical forests. Another could be the impact of warming on ocean currents, particularly the global "conveyor belt" that begins in the North Atlantic. 

Here the formation of ice leaves the remaining water increasingly saline, and so more dense. As a result it descends to the ocean floor, where it begins a long journey through the oceans that lasts an estimated thousand years. This water carries dissolved CO2 with it on its long journey. 

Some oceanographers believe that as warming takes hold and ice formation is reduced, these currents could slow down or carry less water, which could mean less CO2 is removed from the atmosphere. If they are right, global warming could happen faster than predicted.

The global warming has failed to penetrate the atmosphere

Why do sceptical scientists think that?

One reason is that something strange has been happening to warming trends in the past couple of decades. While temperatures at ground level round the world have gone up, the warming has failed to penetrate the atmosphere. In wide areas some three kilometers above Earth, the atmosphere has actually been cooling. 

This is not what is predicted by computerized climate models, which all say the warming should spread right through the troposphere, the bottom ten kilometers or so of the atmosphere. Skeptics argue that if the models are wrong about how surface warming influences temperatures in the troposphere, they are also likely to be wrong about the movement of water vapor between the surface and the free troposphere.

That in turn may mean they are wrong about water-vapor feedback--one of the vital mechanisms behind global warming. (See "Greenhouse Wars" Article)

So does this mean there are some scientists who don't believe in the greenhouse effect or global warming?

No, this is a myth. All scientists believe in the greenhouse effect. Without it the planet would be largely frozen. And all scientists accept that if humans put more greenhouse gases in the atmosphere it will tend to warm the planet. The only disagreement is over precisely how much warming will be amplified by feedback.

As the world warms, ice caps will melt.

What are the main feedback likely to be?

One of the easiest to estimate is the "ice-albedo" feedback. As the world warms, ice caps will melt. As this happens parts of the Earth's surface once covered with ice will be replaced by water or land. Ice is very efficient at reflecting solar radiation into space, whereas water and land absorb far more. 

So the Earth's surface will trap more heat, increasing warming--a positive feedback. Less clear-cut is the impact of the extra water vapour likely to enter the atmosphere because of greater evaporation in a warmer world. This added water vapour itself contributes to the greenhouse effect, another positive feedback. 

But it may also increase cloud cover. The dominant effect of some kinds of cloud is to shroud the Earth--a negative feedback--but other clouds, such as cirrus, may trap heat at low levels--another positive feedback. 

Disputes about how water vapour and clouds will influence global warming are at the heart of many of the disputes between mainstream scientists and the handful of greenhouse sceptics. Overall, the mainstream view is that feedbacks will amplify warming by perhaps 2.5 times. But some sceptics believe the feedback effect could be neutral or even predominantly negative.

How do we know what these levels were?

The most informative measurements have come from bubbles of air trapped in Antarctic ice dating back to preindustrial times. These show that for at least the past 160 000 years, CO₂ levels in the atmosphere have almost exactly matched the record of temperatures, as revealed in ice cores, tree rings and elsewhere.

If it's all so precise, why is there so much confusion and uncertainty about global warming?

Are water and CO2 all we have to worry about?

No. Other gases can absorb in the infrared and contribute to greenhouse warming, including methane, ozone, CFCs (chlorofluorocarbons) and nitrous oxide (released by nitrogen-based fertilizers). 

Of these, methane is the most important. Its concentration in the atmosphere has more than doubled since preindustrial times. Sources include the biological activity of bacteria in paddy fields and the guts of cattle, as well as the release of natural gas from commercial oil and gas fields and landfills. 

Molecule for molecule, other substances are even more potent greenhouse gases. A single molecule of either of the two most common CFCs has the same greenhouse warming effect as 10 000 molecules of CO₂.

the greenhouse effect is a thoroughly bad thing?

What is the greenhouse effect?

Warmth from the Sun heats the surface of the Earth, which in turn radiates energy back out to space. Some of this outgoing radiation, which is nearly all in the infrared region of the spectrum, is trapped in the atmosphere by so-called greenhouse gases. 

For instance, water vapor strongly absorbs radiation with wavelengths ranging from 4 to 7 micrometers, and carbon dioxide (CO₂) absorbs in the range from 13 to 19 micrometers.

The trapped radiation warms the lower part of the Earth's atmosphere, the troposphere. In turn, this warmed air then radiates energy--again, largely in the infrared--in all directions. 

Some of this radiation works its way upward and out, but some finds its way back down to the Earth's surface, keeping it hotter than it would otherwise be. This is the greenhouse effect.