1. What is the greenhouse effect?
The Sun's warmth heats the surface of the Earth, which in turn radiates energy back to space. Some of this radiation, which is nearly all in the infrared spectrum, is trapped in the atmosphere by greenhouse gases. For instance, water vapour strongly absorbs radiation with wavelengths between 4 and 7 micrometres, and carbon dioxide (CO2) absorbs radiation with wavelengths between 13 and 19 micrometres.
The trapped radiation warms the lower atmosphere, or troposphere. Some heat then finds its way back down to the Earth's surface, making it hotter than it would otherwise be. This is the greenhouse effect.
2. Are water vapour and carbon dioxide all we have to worry about?
No. Other gases can absorb infrared radiation and contribute to greenhouse warming. These include methane, ozone, CFCs (chlorofluorocarbons) and nitrous oxide (released by fertilisers). Methane is the most important of these. Its atmospheric concentration has more than doubled since pre-industrial times. Methane sources include bacteria in paddy fields, cattle guts and natural gas from landfills and rotting vegetation. 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 CO2 molecules.
3. Is the greenhouse effect a thoroughly bad thing?
Not quite. Without it, the planet would not be warm enough to support life as we know it. The problem is that pre-industrial greenhouse gas levels are being boosted by burning fossil fuels. If nothing is done to curb emissions, the amount of CO2 in the atmosphere will probably be more than double pre-industrial levels by the end of this century.
4. How do we know what pre-industrial greenhouse gas levels were?
The most informative measurements have come from air bubbles trapped in Antarctic ice. These show that, for at least 400,000 years, CO2 levels in the atmosphere have closely followed the global temperatures as recorded in ice cores, tree rings and elsewhere.
5. If measuring greenhouse gas levels is so precise, why is there so much confusion and uncertainty over global warming?
There is no easy formula for predicting what CO2 increases will do to global temperatures. While we can calculate that a doubling of atmospheric CO2 will force roughly 1°C of warming, the planet is more complex than that. It could magnify the effect, but it could also conceivably dampen down warming. Global processes such as the formation of ice and clouds, the circulation of the oceans and biological activity all interact to provide feedback effects.
6. What effects are global warming feedbacks likely to have?
One of the easiest to estimate is the "ice-albedo" feedback. As the world warms, ice caps will melt, to be replaced by water or land. Ice is very efficient at reflecting solar radiation, whereas water and land are less so. Therefore, 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 higher evaporation rates. This added water vapour itself contributes to the greenhouse effect, another positive feedback. But it may also increase cloud cover, shrouding and cooling the Earth - a negative feedback.
Disputes about how water vapour and clouds will influence global warming are at the heart of disputes between mainstream scientists and the handful of greenhouse sceptics. Most believe that positive feedbacks could amplify the warming effect by between two and five times. But some sceptics believe the feedback effect could be neutral or negative.
7. Are there scientists out there who do not 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 frozen. And all scientists accept that if humans put more greenhouse gases into the atmosphere then it will warm the planet. The only disagreement is over precisely how much the warming will be amplified by planetary feedbacks.
However, there is a growing consensus that the average global warming of 0.6°C seen in the twentieth century - and particularly the pronounced warming of the past three decades - is due to the greenhouse effect.
8. Are there other greenhouse gas 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 and a lot is taken up by the oceans. If CO2 absorption rate changes, then the rate of build-up in the atmosphere will also change, potentially speeding up, or slowing down, global warming.
One way to increase the build-up of CO2 would be to 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. This water carries dissolved CO2 with it on a centuries-long journey across the ocean floor.
Most oceanographers believe that as warming takes hold, and ice formation is reduced, these currents - which lock CO2 up in the depths - could slow down or carry less water, meaning that less CO2 is removed from the atmosphere.
9. Is there any evidence of a speed-up in the accumulation of CO2 in the atmosphere?
Yes. Since the start of the 21st century, the rate of accumulation has accelerated. It is now at twice the 1990s level. Nobody is sure why. It is not because emissions have accelerated. It could be temporary natural variability. Or it could be that the forests and oceans are losing the ability to absorb our pollution. If so, then global warming could shortly gather pace.
10. This is all very pessimistic. Is it not true that a warmer planet will absorb more carbon dioxide?
That is correct. Warmer temperatures and the fertilising effect of CO2 in the air will stimulate faster plant growth, which in turn will soak up some of the CO2. But plants need other things too. They need water, which could be in short supply as greater evaporation will dry out soils, and space, which urbanisation is taking up.
11. How do organisms in the oceans affect global warming?
Once dissolved in surface waters, a great amount 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.
12. Is there anything else that could shield us from global warming?
Yes, volcanoes. When Mount Pinatubo erupted in 1991, it threw masses of sulphate particles and dust into the stratosphere that partially shielded the Earth from solar energy. Computer models successfully predicted that the debris would temporarily 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 mid-1990s, to the higher levels expected with the ongoing build-up of greenhouse gases.
13. Volcanoes produce cooling sulphate particles, but do we make them, too?
Yes we do. Ironically, burning fossil fuels produces sulphate particles. These particles - which make acid rain - help to shield industrialised countries from global warming's full impact. In some places, such as central Europe and parts of China, they may have even produce a net cooling effect. Dust from soil erosion and desertification can also curb local warming effects.
But even if you are comfortable with 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 their like persist for only a few days. If you turned down the power stations, the world would get much hotter within a few days. So sulphates are not a solution.
14. How are temperatures predicted to rise over the next few centuries?
This depends on whether we halt the growing concentration of greenhouse gases in the atmosphere. Some warming is inevitable - there are time lags in the natural systems which store up warming for future decades. CO2 concentrations are currently about 35% above pre-industrial levels, storing up perhaps another degree of warming.
If we can stabilise atmospheric CO2 concentration by the end of this century, below twice pre-industrial levels, we can probably limit warming to under 5 degrees. But because the gas stays in the atmosphere for a century or more, stabilisation requires cutting emissions by 70% to 80%. A tall order. However, some models predict temperature rises of 8 to 10 degrees within 200 years if we do not kick the carbon habit.
15. What are some of the most significant effects global warming will have on the human race?
Unusual droughts are causing serious problems for farming in many regions. Whole countries could get swallowed up by this process, triggering poverty and mass migrations. Super-hurricanes could make other places uninhabitable.
As rainfall patterns alter, rivers will dry up in some regions, while others will flood. Rising sea levels will wipe out many islands and flood low-lying areas, from Bangladesh to the US. As ever, the poor will be most vulnerable. At least in the early decades, rich nations may cope, but ultimately even they could be undermined. Human civilisations have developed over the past 10,000 years - since the end of the last ice age - in an era of generally stable climate. We just do not know how well we will cope with a radical change to the climatic status quo.
16. Will there be global warming everywhere?
Maybe not. Climate modellers admit to uncertainties over how it will affect particular regions. This is because much of our weather depends on circulation patterns, which could alter unexpectedly. Crude estimates suggest that coastal regions may become wetter, while continental interiors become drier, causing deserts to expand. Warming will probably be greatest in polar regions, mirroring climate changes already seen this century in both the Arctic and Antarctic.
Local climate could also be altered by changes in ocean circulation. Western Europe is particularly vulnerable. At present, it is kept exceptionally warm in winter by the Gulf Stream, which is part of the ocean conveyor belt mentioned above. Take that away and British weather would be more like Canada's frigid Hudson Bay, found at the same latitude.
Ice cores reveal growing evidence of sudden shifts in climate over the past 10,000 years that have occurred within a few decades as a result of "flips" in ocean circulation. But most models suggest that the Gulf Stream will not turn off for at least another century.
17. Are there any cataclysmic events in the offing?
One fear is that the entire West Antarctic and Greenland ice sheets might disappear into the oceans. According to projections by the UK's Hadley Center for Climate Prediction, before the end of the century a warming of at least 3°C would probably trigger the eventual melting of the entire Greenland ice sheet. The glacial collapse would take hundreds of years, but could raise sea levels by 6 metres.
The Sun's warmth heats the surface of the Earth, which in turn radiates energy back to space. Some of this radiation, which is nearly all in the infrared spectrum, is trapped in the atmosphere by greenhouse gases. For instance, water vapour strongly absorbs radiation with wavelengths between 4 and 7 micrometres, and carbon dioxide (CO2) absorbs radiation with wavelengths between 13 and 19 micrometres.
The trapped radiation warms the lower atmosphere, or troposphere. Some heat then finds its way back down to the Earth's surface, making it hotter than it would otherwise be. This is the greenhouse effect.
2. Are water vapour and carbon dioxide all we have to worry about?
No. Other gases can absorb infrared radiation and contribute to greenhouse warming. These include methane, ozone, CFCs (chlorofluorocarbons) and nitrous oxide (released by fertilisers). Methane is the most important of these. Its atmospheric concentration has more than doubled since pre-industrial times. Methane sources include bacteria in paddy fields, cattle guts and natural gas from landfills and rotting vegetation. 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 CO2 molecules.
3. Is the greenhouse effect a thoroughly bad thing?
Not quite. Without it, the planet would not be warm enough to support life as we know it. The problem is that pre-industrial greenhouse gas levels are being boosted by burning fossil fuels. If nothing is done to curb emissions, the amount of CO2 in the atmosphere will probably be more than double pre-industrial levels by the end of this century.
4. How do we know what pre-industrial greenhouse gas levels were?
The most informative measurements have come from air bubbles trapped in Antarctic ice. These show that, for at least 400,000 years, CO2 levels in the atmosphere have closely followed the global temperatures as recorded in ice cores, tree rings and elsewhere.
5. If measuring greenhouse gas levels is so precise, why is there so much confusion and uncertainty over global warming?
There is no easy formula for predicting what CO2 increases will do to global temperatures. While we can calculate that a doubling of atmospheric CO2 will force roughly 1°C of warming, the planet is more complex than that. It could magnify the effect, but it could also conceivably dampen down warming. Global processes such as the formation of ice and clouds, the circulation of the oceans and biological activity all interact to provide feedback effects.
6. What effects are global warming feedbacks likely to have?
One of the easiest to estimate is the "ice-albedo" feedback. As the world warms, ice caps will melt, to be replaced by water or land. Ice is very efficient at reflecting solar radiation, whereas water and land are less so. Therefore, 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 higher evaporation rates. This added water vapour itself contributes to the greenhouse effect, another positive feedback. But it may also increase cloud cover, shrouding and cooling the Earth - a negative feedback.
Disputes about how water vapour and clouds will influence global warming are at the heart of disputes between mainstream scientists and the handful of greenhouse sceptics. Most believe that positive feedbacks could amplify the warming effect by between two and five times. But some sceptics believe the feedback effect could be neutral or negative.
7. Are there scientists out there who do not 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 frozen. And all scientists accept that if humans put more greenhouse gases into the atmosphere then it will warm the planet. The only disagreement is over precisely how much the warming will be amplified by planetary feedbacks.
However, there is a growing consensus that the average global warming of 0.6°C seen in the twentieth century - and particularly the pronounced warming of the past three decades - is due to the greenhouse effect.
8. Are there other greenhouse gas 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 and a lot is taken up by the oceans. If CO2 absorption rate changes, then the rate of build-up in the atmosphere will also change, potentially speeding up, or slowing down, global warming.
One way to increase the build-up of CO2 would be to 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. This water carries dissolved CO2 with it on a centuries-long journey across the ocean floor.
Most oceanographers believe that as warming takes hold, and ice formation is reduced, these currents - which lock CO2 up in the depths - could slow down or carry less water, meaning that less CO2 is removed from the atmosphere.
9. Is there any evidence of a speed-up in the accumulation of CO2 in the atmosphere?
Yes. Since the start of the 21st century, the rate of accumulation has accelerated. It is now at twice the 1990s level. Nobody is sure why. It is not because emissions have accelerated. It could be temporary natural variability. Or it could be that the forests and oceans are losing the ability to absorb our pollution. If so, then global warming could shortly gather pace.
10. This is all very pessimistic. Is it not true that a warmer planet will absorb more carbon dioxide?
That is correct. Warmer temperatures and the fertilising effect of CO2 in the air will stimulate faster plant growth, which in turn will soak up some of the CO2. But plants need other things too. They need water, which could be in short supply as greater evaporation will dry out soils, and space, which urbanisation is taking up.
11. How do organisms in the oceans affect global warming?
Once dissolved in surface waters, a great amount 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.
12. Is there anything else that could shield us from global warming?
Yes, volcanoes. When Mount Pinatubo erupted in 1991, it threw masses of sulphate particles and dust into the stratosphere that partially shielded the Earth from solar energy. Computer models successfully predicted that the debris would temporarily 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 mid-1990s, to the higher levels expected with the ongoing build-up of greenhouse gases.
13. Volcanoes produce cooling sulphate particles, but do we make them, too?
Yes we do. Ironically, burning fossil fuels produces sulphate particles. These particles - which make acid rain - help to shield industrialised countries from global warming's full impact. In some places, such as central Europe and parts of China, they may have even produce a net cooling effect. Dust from soil erosion and desertification can also curb local warming effects.
But even if you are comfortable with 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 their like persist for only a few days. If you turned down the power stations, the world would get much hotter within a few days. So sulphates are not a solution.
14. How are temperatures predicted to rise over the next few centuries?
This depends on whether we halt the growing concentration of greenhouse gases in the atmosphere. Some warming is inevitable - there are time lags in the natural systems which store up warming for future decades. CO2 concentrations are currently about 35% above pre-industrial levels, storing up perhaps another degree of warming.
If we can stabilise atmospheric CO2 concentration by the end of this century, below twice pre-industrial levels, we can probably limit warming to under 5 degrees. But because the gas stays in the atmosphere for a century or more, stabilisation requires cutting emissions by 70% to 80%. A tall order. However, some models predict temperature rises of 8 to 10 degrees within 200 years if we do not kick the carbon habit.
15. What are some of the most significant effects global warming will have on the human race?
Unusual droughts are causing serious problems for farming in many regions. Whole countries could get swallowed up by this process, triggering poverty and mass migrations. Super-hurricanes could make other places uninhabitable.
As rainfall patterns alter, rivers will dry up in some regions, while others will flood. Rising sea levels will wipe out many islands and flood low-lying areas, from Bangladesh to the US. As ever, the poor will be most vulnerable. At least in the early decades, rich nations may cope, but ultimately even they could be undermined. Human civilisations have developed over the past 10,000 years - since the end of the last ice age - in an era of generally stable climate. We just do not know how well we will cope with a radical change to the climatic status quo.
16. Will there be global warming everywhere?
Maybe not. Climate modellers admit to uncertainties over how it will affect particular regions. This is because much of our weather depends on circulation patterns, which could alter unexpectedly. Crude estimates suggest that coastal regions may become wetter, while continental interiors become drier, causing deserts to expand. Warming will probably be greatest in polar regions, mirroring climate changes already seen this century in both the Arctic and Antarctic.
Local climate could also be altered by changes in ocean circulation. Western Europe is particularly vulnerable. At present, it is kept exceptionally warm in winter by the Gulf Stream, which is part of the ocean conveyor belt mentioned above. Take that away and British weather would be more like Canada's frigid Hudson Bay, found at the same latitude.
Ice cores reveal growing evidence of sudden shifts in climate over the past 10,000 years that have occurred within a few decades as a result of "flips" in ocean circulation. But most models suggest that the Gulf Stream will not turn off for at least another century.
17. Are there any cataclysmic events in the offing?
One fear is that the entire West Antarctic and Greenland ice sheets might disappear into the oceans. According to projections by the UK's Hadley Center for Climate Prediction, before the end of the century a warming of at least 3°C would probably trigger the eventual melting of the entire Greenland ice sheet. The glacial collapse would take hundreds of years, but could raise sea levels by 6 metres.