Introduction

There is no doubt that the amount of carbon dioxide in the air is increasing, and there is almost universal agreement among scientists that this will have significant effects on the climate. However there are uncertainties and differences of opinion on the magnitude and timing of the effects.

The climate of the Earth results from a complex set of interactions, largely driven by the heat from the sun. Over geological time temperatures have varied, sometimes warmer and sometimes colder, while the composition of the atmosphere has also altered. Animals have adapted, moved, or become extinct. Very sudden changes in climate, some of which have been ascribed to the impact of enormous meteors or to vast volcanic eruptions, have been associated with widespread extinction. However, the last 4,000 years have been a period of relative stability, which is likely to end due to the impact of human ingenuity. The reason is that we are using fossil fuels to generate the energy needed to drive our inventions: electricity, motor cars, aeroplanes, central heating, refrigeration, air conditioning, televisions, computers, and others. This has increased the amount of carbon dioxide and other so-called greenhouse gases in the atmosphere. Other human activity, notably deforestation, adds to the problem.

Nowadays, the phrase global warming mainly refers specifically to the increase of average temperatures over the globe in recent years and what will happen in the future. This is also referred to as climate change, since there are other effects in addition to rising temperature.

When scientists started to see evidence for global warming, in the last few decades of the 20th century, sceptics first questioned whether the temperature increase was real. As the evidence became undeniable, the sceptics instead tried to deny that the increase had been caused by human activity. Again, the evidence became more and more convincing. Human activity dumps tens of billions of tons of carbon dioxide into the atmosphere every year, enough to increase the amount of carbon dioxide significantly. Carbon dioxide is a greenhouse gas – that is a scientific fact. Having seen many other effects of human activity changing the Earth, the question should perhaps be the other way around: "How can we put so much greenhouse gas into the atmosphere without having a significant effect?"

Global warming is unlikely to destroy human life on Earth. But it could have very serious effects on many places where people live, flooding coastal communities and cities, and have devastating effects on food and water supplies. The disruption could lead to wars, poverty, disease, and nowhere to live for huge numbers of people. We are already beginning to see significant changes due to global warming. The big question is whether we can act quickly and decisively enough to minimise the effects, or whether people will just look after their own short-term interests and problems so that the worst effects of global warming are allowed to happen.

The role of carbon

The chemical element carbon is the basis and key element for all life on the earth, due to the vast number of complex organic compounds it can form. Only a very small proportion of the world's carbon is in the atmosphere, in the form of two of the most important greenhouse gases: carbon dioxide (CO₂) and methane (CH₄).

Carbon is released into the atmosphere in several ways.

• Rocks that form the continents include significant quantities of carbon in limestone as well as in fossil fuels. This carbon is being recycled into the atmosphere continuously by volcanoes. Human activity is currently speeding up the release of carbon by extracting fossil fuels and burning them, and by extracting and processing carboniferous rocks to create cement.
• Land animals breathe oxygen and eat plants, and generate carbon dioxide and methane.

Carbon is removed from the atmosphere in several ways.

• Land plants extract carbon dioxide, to grow wood and other vegetable matter. Some of this is returned to the atmosphere when the plant dies, but some can be locked into the soil. Human activity is reducing this by clearing tropical forests, which contain about half the volume of the world's plants. As these forests are cleared, to use as agricultural land and for their timber, the amount of carbon being removed from the atmosphere decreases.
• Oceans contain about 100 times as much carbon as the atmosphere. The carbon in the sea can be converted to shells by living things, and their bodies eventually fall to the bottom and become limestone or chalk.

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Greenhouse Gases

Greenhouse gases are so named because they have an effect on the Earth similar to that of the glass of a greenhouse. They let the sun's visible rays in to warm the Earth, but they then absorb some of the infrared energy being radiated out again and radiate some of this back to Earth. This is illustrated in the diagram.

The existence of some greenhouse gas in the atmosphere is essential. If there were none at all the climate would be very much colder, an average of about –18 °C (the temperature inside a freezer). The Earth would be very different: water would be frozen, and without carbon dioxide in the atmosphere trees and plants could not grow.

The main greenhouse gases whose abundance has been increased due to human activity are carbon dioxide, methane, nitrous oxide, and fluorocarbons such as CFCs. Of these, carbon dioxide is by far the most abundant but other gases have much greater effect at a given concentration. Water vapour is also a very important greenhouse gas. Since 1750 the concentration of carbon dioxide in the atmosphere has increased by about 35%. The amount of methane has increased by 150%, nitrous oxide by 16%, and all the fluorocarbons are new. The two graphs below at the left show the increases in carbon dioxide and methane in the atmosphere over the past 10,000 years, while the insets expand the time since the beginning of the industrial revolution. The pie chart (top right) illustrates the relative greenhouse effect of the different gases at present. The effect of the other greenhouse gases is usually expressed in terms of carbon dioxide equivalent, in order to make comparisons easier and to give overall figures.

The source of these increases is almost entirely human activity. The human input of carbon dioxide into the atmosphere, and the breakdown of what it comes from, is illustrated in the last graph. Not shown in the graph is the effect of large-scale clearance of tropical forests – these forests soak up carbon dioxide.

Aerosols

The burning of fossil fuels has also increased the density of very small particles, called aerosols, in the atmosphere. These have an overall cooling effect as they block and reflect incoming sunlight. In the 1970s it was thought that this effect might more than cancel out the warming effect of the greenhouse gases, but this has not proved to be the case.

Does this have anything to do with the ozone layer?

No! People often confuse the effects of greenhouse gases such as carbon dioxide, which are spread throughout the atmosphere, with the ozone layer because it too has been in the news in recent years. The ozone layer is located far up in the Earth's atmosphere and contains relatively high concentrations of ozone (O₃), which is a form of oxygen. This ozone absorbs most of the sun's ultraviolet light, which otherwise would be very harmful to life on earth. It was in the news because fluorine compounds known as chlorofluorocarbons, which were widely used in refrigerators, air conditioners, aerosol sprays, and industrial cleaning, were found to be destroying the ozone layer, most obviously by producing a big hole in it over Antarctica. Under a very successful international treaty called the Montreal Protocol, use of the most damaging chlorofluorocarbons began to be phased out in 1987 and was banned in 1996. The ozone layer now seems to be recovering gradually.

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Effects of Increased Greenhouse Gases

Rise in average temperature

The climate of the Earth is a result of so many different complex interactions that it is difficult to predict exactly what the effect of adding greenhouse gases to the atmosphere will be. However, computer climate models have become much more realistic and sophisticated, reproducing past and present behaviour more and more accurately. Nearly all scientists studying the problem agree that the effect will be an increase in average temperatures. This is already happening – the graph on the left indicates that the Earth is now warmer than at any time for thousands of years. The graph on the right zooms in on the past 150 years, for which we have more accurate records. There has been a significant rise of about 0.8 °C in average temperatures over the last 100 years. Although 0.8 °C does not sound like a very big rise, it has already had clear effects that will be mentioned below. In the 20th century the steady rise was interrupted from about 1950 to 1980, and this is believed to be due to variations in the energy output of the sun and also ash from volcanoes. Since the mid-1990s we have had 11 of the warmest years on record.

Global warming does not mean that each year will always be warmer than the previous year. This can be seen clearly in the right-hand graph below. The reason is that there are also short-term world-wide climate fluctuations. The best-known and understood of these is due to temperature changes in the surface waters of the tropical eastern Pacific Ocean and is called El Niño (warm) and La Niña (cold). These cause some years to be unusually warm or cold, as well as being associated with floods, droughts, and other disturbances around the world. Other "oscillation" effects, notably in the North Atlantic, must still be verified and understood; in some cases they may even lead to as much as a decade or more of apparent cooling. In terms of global warming, this means that what is important is the long term trend, not whether one specific year or a few years in a row are unusually cold or hot.

The overall rise in temperature is the easiest effect to predict, and the predictions of average global temperature made by the Intergovernmental Panel on Climate Change in 1991 have proved very accurate. It is believed that the current high level of greenhouse gases has not been seen for 200 million years. No study which ignores the effect of this increase in greenhouse gases has predicted or explained the sharp rise in temperatures since the mid-1990s.

It is possible that part of this global warming is caused by other effects than greenhouse gases – for example changes in the sun, long term cyclic changes in the Earth's orbit, or the effect of cosmic rays on cloud cover. However, none of these alternative explanations for the rise in temperatures has been able to explain it fully.

There is a general consensus that the average temperature of the Earth will rise, but estimates of the size of the rise differ. Estimates are in the range of 1.1 to 6.4 °C over the next 100 years, and depend strongly on the level of future greenhouse gas emissions. The effects will also be different in different parts of the world. A rise of 2–3 °C, in the middle of this range, would cause a larger climate change than the Earth has experienced for at least 10,000 years.

Even relatively small changes in climate and the environment can cause serious disruption to the way we live. A rise of 3 °C in global average temperature over the 21st century (a mid-range estimate) would have a pace and impact that would be difficult for people and ecosystems to adapt to. If the rise in temperature can be limited to 3 °C or less, predictions are that some regions will benefit from the warmer climate while others will suffer. Some cooler regions (northern Canada, for example) may experience longer growing seasons for crops, and yields might be further boosted by the fertilising effect of having more carbon dioxide in the atmosphere. But many other regions would suffer badly. Total global crop yields are expected to fall, and this will cause many problems.

If the rise is more than 3 °C then it is predicted that virtually all regions will either be worse off or will suffer due to the regions that are worse off. The effects would be most severe in the tropics and subtropics. The figure illustrates this, showing how an average rise of 3 °C might vary according to location. In some areas the rise in temperature is far above the average.

Severe heat waves (already occurring in some recent summers), storms and flooding (see below) will all be much more frequent. Many of the world's hotter areas will become even less tolerable to live in. Areas that now grow large fractions of the world's staple foods, such as the American mid-west, may become arid semi-deserts. In some places, such as central Spain, a rise of average temperature of up to 7 °C is predicted. These effects, and others described below, will tend to affect poorer countries disproportionately, and therefore fall on populations who are the least able to adapt, especially in terms of health, and access to food and clean water.

Ecosystems will not always be able to adapt to the changes, and the result would be large numbers of species becoming extinct and loss of biodiversity. For example, the southern part of the Arctic region has seen a temperature rise of 1 °C to 3 °C over the past 50 years, far more than the global average of 0.7 °C. Canada, Alaska and Russia are experiencing initial melting of permafrost. This disrupts ecosystems, and by increasing bacterial activity in the soil could lead to these areas becoming carbon sources instead of carbon sinks, thus accelerating global warming.

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Extreme weather and changes in precipitation

Another climate prediction is an increase in the intensity of extreme weather events. A well-known example is that hurricanes and other tropical storms can acquire more energy and become more violent due to tropical oceans being warmer. There is already some evidence for increasing numbers of category 4 and 5 hurricanes, but the evidence is not yet totally convincing. Note that individual violent storms cannot be directly attributed to global warming.

Predicted changes to the amount and pattern of precipitation would leave some areas that now have abundant rainfall much drier, while some arid regions would get more rain. A prediction for northern Europe, including the UK, is for wetter winters and hotter, drier summers. The Middle East, sub-Saharan Africa and China could lose significant rainfall, while other locations would see more. The extended drought in Australia might be due to climate change.

Possible consequences of weather changes

Predicted increases in extreme weather are not limited to the tropics. Very severe storms cause flooding, and this can also lead to epidemics of infectious diseases. High winds damage buildings and other structures that were built to withstand milder conditions. However, the most disruption and damage is likely to be from tropical storms in poor countries with high population densities in coastal regions and low-quality housing.

Changes in precipitation would have a serious effect on food supplies. For example, Stephen Mwakifwamba, national co-ordinator of the Centre for Energy, Environment, Science and Technology in Tanzania, has said that this is already happening: "In the past, we had a drought about every 10 years. Now we just don't know when they will come. They are more frequent, but then so are floods. The climate is far less predictable. We might have floods in May or droughts every three years. Upland areas, which were never affected by mosquitoes, now are. Water levels are decreasing every day. The rains come at the wrong time for farmers and it is leading to many problems."

Drought, desertification and overpopulation can cause severe problems, as populations move in the search for water and come into conflict with other people. This may be a cause of the Darfur conflict in Sudan. The Intergovernmental Panel on Climate Change has estimated that 150 million environmental refugees may exist in the year 2050, mainly due to coastal flooding and agricultural disruption.

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Melting ice

Melting ice is another serious, and widely publicised, effect. Pictures of glaciers in mountainous regions as they are now compared with only a few decades ago show dramatic shrinkage.

On a larger scale, most climate prediction models assume that the Arctic ice sheet remains in place as it thins. Predicting ice movements is very difficult. But in fact we are seeing major losses of sea ice in the Arctic. As shown in the figure, in 2005 the extent of summer ice was 40% less than the 1979–2000 average, and in 2007 it had decreased by a further 23%. The loss of ice cover leads to even more warming. This is because ice reflects more of the sun's rays back into space than open ocean or exposed land, which absorb more sunlight and convert it to heat, warming the Earth further.

Feedback

This sort of effect, in which the effects of warming cause even more warming, is called feedback. A wide variety of other possible feedback effects are being studied, for example:

• As arctic permafrost melts there may be huge releases of methane from the melting of extensive peat bogs, notably in Siberia. That will add much more greenhouse gas to the atmosphere.
• As the oceans warm, deposits of methane compounds buried in deep ocean-floor sediments could suddenly release methane gas, again adding to what is already in the atmosphere.
• There is a huge amount of carbon dioxide dissolved in ocean water. As the oceans warm, the capacity of the water to hold carbon dioxide decreases, so carbon dioxide is released into the atmosphere.

At present we simply do not know enough to predict accurately how much feedback effects like these will worsen the situation, but some of the possible effects might be extremely serious due to passing levels that trigger big changes.

Possible consequences of melting glaciers

Mountain glaciers in the Andes, Alps, Pyrenees, Himalayas, Rocky Mountains and other areas (not including the Arctic and Antarctic ice sheets) have decreased in area by 50% in the past century. Since 1980 the retreat of ice has sped up, and it has increased even more since 1995. An example of a glacier in Alaska is shown in the photos.

This can cause landslides and flash floods, but the most serious effect for people living in the valleys of glacially fed rivers is that the water flow could be greatly reduced during much more of the year. Glaciers store snowfall from winter and from earlier years of high precipitation, and provide water in summer that is used for drinking and irrigation of crops. For example, the region of the Himalayas and Hindu Kush feeds Asia's biggest rivers: the Ganges, Indus, Brahmaputra, Yangtze, Mekong and Yellow. Floods followed by droughts could affect 2.4 billion people living in India, China, Pakistan, Bangladesh, Nepal and Myanmar. This would affect water supplies and farming, and the resulting disruption could have very serious social and political consequences.

Melting polar ice

Recent rapid increases in the rate of melting of Arctic sea ice have been beyond expectations. Melting of Arctic sea ice does not affect sea levels in a direct way. However, we mentioned earlier the feedback process by which loss of Arctic sea ice causes even more global warming.

Another effect which may occur earlier than expected is that by 2030 the Arctic seas could be completely free of ice in summer. In September 2007, Arctic sea ice retreated sufficiently to allow the fabled Northwest Passage to become navigable to shipping for the first time in recorded history.

In the Antarctic there has been significant melting of the West Antarctic ice sheet, but not of the main bulk of the Antarctic ice. In fact, ice cover was actually at its largest recorded extent in 1979. However, recent observations seem to indicate that this will change. Melting of either the Greenland and Antarctic ice sheets would cause very big rises in sea level, and this is discussed below.

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Rise in sea level

Increasing global temperature causes a rise in sea level, even without considering melting of polar ice, because as water warmer than 4 °C increases in temperature it expands. Although the water deep in the ocean is at about 4 °C, the upper levels of the sea are at higher temperatures, and so the water will expand if it becomes warmer. Sea levels will rise further if the Greenland or Antarctic ice caps melt.

Since 1900, sea level has risen by an average of about 1.7 mm per year, but since 1993 this has increased to about 3 mm per year. In 2007 the Intergovernmental Panel on Climate Change prediction for the year 2100 was a rise of 28 to 43 cm above levels in 2000, but other predictions go higher. A report in November 2009 from the Scientific Committee on Antarctic Research (SCAR, see below) predicts that the rise could be up to 1.4 metres by 2100, due to melting in the Antarctic being caused by warmer oceans and air.

A rise of even 20 cm in sea level is serious for low-lying countries such as Bangladesh and Vietnam, many islands, and for very low-lying coastal cities such as New Orleans. Coupled with more violent storms, many other coastal cities such as London and New York could also suffer from severe flooding. (The Thames Barrier is already being used much more often than a few years ago.) Other coastal regions would also suffer, due to breaches in sea defences and the sea level rise.

Increased melting of Greenland and Antarctic ice

Greenland – About 99% of all glacial ice is in the ice sheets of Greenland and Antarctica. The ice in Greenland is retreating and thinning, more rapidly since about 2000. A major effect is to contribute to rising sea levels. If the average temperature in Greenland rises by 3 °C, then the entire Greenland ice sheet might eventually melt. This would produce a far more serious rise in sea level of about 7 metres, which would inundate low-lying coastlines, islands, and river deltas. Predictions said that this would take many centuries, but the accelerating rate of melting has led to concerns that it may happen much sooner. The figure shows how much the extent of surface melting of the Greenland ice cap in summer grew between 1992 and 2002.

Antarctic – Although the main bulk of Antarctic ice does not yet show signs of large-scale melting, some recent reports predict that this might happen sooner than had been expected. An analysis by SCAR says that Antarctic warming has been slowed down by the effect on climate of the ozone hole, but this will end now that the main chemicals causing the ozone hole have been banned.

In addition, a recent paper predicts that at atmospheric carbon dioxide levels that are expected within a few decades (roughly 450 parts per million) the Antarctic ice sheet may start to disappear. Until recently it was believed that a limit of 450 parts per million would not cause such a catastrophic effect. The new prediction, based on geological history, continues by saying that if we do not get the carbon dioxide level back down to below the present level (which is about 385 parts per million, and rising by about 2 parts per million each year) quite soon after reaching the higher level, the Antarctic ice will be completely and permanently lost. This would raise sea levels by perhaps 70 metres, which would be catastrophic for human civilisation. (To illustrate the extent of the effect, note that Blewbury is about 70 metres above the present sea level.)

As explained below, some of the warming effects of greenhouse gases are immediate, but others take years and can therefore be at least partially avoided if the increase in greenhouse gases is only temporary.

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Acidification of the oceans

A less frequently discussed effect is acidification of the oceans. As the amount of carbon dioxide in the atmosphere increases, so does the amount dissolved in the oceans. This reacts with the water to produce carbonic acid, which makes the oceans more acid. There is already evidence of a small change in pH (which measures acidity) of seawater from 8.3 to 8.2, and predicted carbon dioxide emissions by 2100 might reduce it by a further 0.5 to 7.7. This is probably a lower pH than has been seen for hundreds of millennia.

The increasing acidification could have a serious effect on corals, which are already in trouble in many places due to increasing water temperatures. Possibly even more serious is the effect the increased acidity might have on marine organisms with shells, which are made of calcium carbonate and would tend to dissolve. This could have major effects on the entire marine ecosystem.

Ocean current circulation

Currents of warm water near the surface of the ocean transport heat from tropical regions northwards, and deeper currents of cold water flow from the north to the tropics. This large-scale circulation, due to differences in temperature and salinity, is known as the thermohaline circulation. The best-known segment is the Gulf Stream, which helps keep north-western Europe warmer than would otherwise be expected for its latitude. The Gulf Stream is driven by dense, cold, salty water sinking at the northern end. Global warming causes more ice in the Arctic to melt, and this would add a lot of less dense fresh water to this flow. There are worries that this could slow or even stop the circulation system. There are indications that this has probably happened in the past, triggered quite suddenly. The consequences of losing the Gulf Stream would include colder and more violent weather over much of northern Europe. So far, evidence that this might already be happening is unclear, and we do not know yet how serious the threat is.

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Frequent questions

What about our recent cold, snowy winters?

The past two winters have been very cold and snowy in northern Europe: 2009/10 was the coldest for three decades in the UK. This has misled many people to conclude that global warming has slowed down or stopped, but that is not correct. Global warming is based on the average over the entire planet, and despite our cold winter 2009 was one of the warmest years of recent times. For example, just when northern Europe was having very cold weather, Vancouver's Winter Olympics were suffering from the warmest winter on record. It would be helpful if we could understand why our local weather was so different from the overall climate.

A recent study (led by Prof. Mike Lockwood of Reading University and the Rutherford Appleton Laboratory) may explain this exceptionally cold winter weather. They compared temperature records of central England since 1659 with astronomical observations of sunspots. Sunspots are related to the sun's magnetic field and its total activity – in 2008/9 there were virtually no sunspots and solar activity was at its lowest for a century. The study showed that cold winters in the UK, especially when not following overall temperature trends in the northern hemisphere, tend to be associated with very low levels of sunspots, and hence low solar activity. And as solar activity is still very low, we might expect more cold winters in the near future.

An important point is that this is a purely local and seasonal phenomenon in northern Europe, and not related to overall global warming. Variations in solar activity have now been ruled out as a major cause of global warming, as there is no clear relationship between solar activity and overall warming trends. So how does solar activity have a local effect on northern Europe?

During the recent cold winter the jet stream, a strong wind pattern high in the stratosphere, developed a large, persistent kink to the south over the north-eastern Atlantic. This blocked warm winds from the west, and so our weather tended to be dominated by cold winds from Siberia. The cause of this is believed to be related to how much heating occurs in the stratosphere, where the ozone layer absorbs solar ultraviolet radiation. If there is less heating than usual the jet stream tends to develop this southerly kink, via a mechanism that is not yet fully understood.

This analysis may also explain the so-called Maunder Minimum of 1650–1700, when there were virtually no sunspots and "frost fairs" were held on the River Thames. Those frigid winters were accompanied by cold winds from the east, and were also a purely European phenomenon.

What if all emissions stopped now?

Finally, suppose that by some miracle we could stop all emissions of greenhouse gases tomorrow – would that stop global warming? The answer is no, not entirely. This is because increased carbon dioxide already in the atmosphere produces some of its effects immediately, but the rest of the warming it causes takes many years to happen. The main reason is that the oceans take a long time to warm up, and in addition ice sheets take a long time to disintegrate. Once the carbon dioxide is in the atmosphere, it will take centuries for the "excess" carbon dioxide from human activity to be removed. (Other greenhouse gases, such as methane, will disappear more quickly.) Thus the predictions for this "miracle" are for a continuing temperature rise of at least 0.5°C over a century or more, and a rise in sea level for several centuries.

References

The Wikipedia has comprehensive articles on the greenhouse effect, global warming, and its effects among many others.

The Intergovernmental Panel on Climate Change (IPCC) provides a variety of authoritative and comprehensive reports.

The Royal Society has much useful information and some important statements.

The US Environmental Protection Agency provides summaries of the current state of scientific thinking.

The US Energy Information Administration has energy statistics covering the world in some detail.

The American Institute of Physics has a comprehensive website on how climate change was discovered, and many useful links and references.

RealClimate, run by climate scientists, has a wealth of information. Its start here section has good articles rebutting the arguments of sceptics.

National Geographic's website has some very interesting and dramatic interactive displays and videos.

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