SciDev.Net

Summary

The 2001 report by the Intergovernmental Panel for Climate Change, the body set up in 1988 to advise governments on the latest science of climate change, concluded that “most of the warming observed over the past 50 years is likely to have been due to the increase in greenhouse gas concentrations”.

The report reflects general agreement among scientists that, because the world’s growing population has led to an increase in activities such as burning fossil fuels and expanding agriculture and deforestation, the concentration of greenhouse gases in the atmosphere has grown significantly.

Most climate experts also believe that there is now sufficient scientific evidence to indicate that this process has had a detectable effect on the climate, and will continue to do so in future.

Evidence for the growth of greenhouse gas emissions

There is a large body of evidence for the growth of greenhouse gas emissions. Typical of this is the fact that measurements over the past 40 years of atmospheric carbon dioxide - the most significant such gas whose emissions from human activities have increased - as well as much longer records of carbon dioxide levels obtained from bubbles of air trapped in Antarctic ice, show that carbon dioxide levels today are more than 30 per cent higher than before the Industrial Revolution.

Indeed, records of carbon dioxide levels obtained from the longest Antarctic ice cores show that the concentration of carbon dioxide in the atmosphere is higher than it has been for the past 420,000 years, and probably the highest in the past 20 million years.

The IPCC reports that other greenhouse gas concentrations have also increased since the middle of the eighteenth century: methane levels have more than doubled (151 per cent) due to activities such as rearing cattle, landfills and burning fossil fuels; and nitrous oxide levels have increased by 17 per cent due to activities such as agriculture and chemical industries. We have also introduced new greenhouse gases to the atmosphere that didn’t exist until the 20th century, such as the chlorofluorocarbons (CFCs), formerly used as a propellant and still widely used as a coolant in refrigerators and air conditioning units.

Evidence for global warming

In parallel with this increase in the emissions of greenhouse gases, there is also clear scientific evidence that temperatures near the Earth’s surface have been rising. Satellite measurements show an overall warming of the free atmosphere of about 0.15 ºC since 1979.

Longer records of temperature near the ground and at the sea surface show that the Earth is about 0.6 ºC warmer than it was a century ago, with most of the warming having occurred between the two periods 1910 to 1945 and 1976 to the present (with slight cooling in the intervening period). And the 1990s were the Earth’s warmest decade ever measured, with the ten warmest years since 1860 occurring since 1990. Indeed, the warmest year ever recorded was 1998.

Scientists have carefully checked these observations to exclude observational errors and other influences — such as the urban heat island effect and localised warming due to land surface changes.

In addition, proxy records from many areas around the world show temperatures are warmer than they have been for a much longer period: for example, hundreds of years of temperature records can be inferred from melting glaciers, the temperature of the upper layers of the Earth’s crust, coral growth, sea floor sediments and the widths of tree rings. This evidence agrees with the warming of the 20th century shown in the thermometer records, and also shows the 20th century to have been the warmest century in the past millennium.

There is also clear evidence that many other aspects of the world’s climate have also changed over the past 100 years. Some countries have experienced more rainfall and more frequent storms in the second half of the twentieth century compared to the first, while other areas have suffered drought.

Also in the latter part of the twentieth century there have been fewer frosts and more heatwaves in many parts of the world. Mountain glaciers have shrunk, snow cover has decreased in some areas, and areas of permafrost have been thawing. Sea level rose by between 10 and 20 cm during the century. [1]

What is causing global warming?
Although many of these changes are consistent with a warming world, it is not possible to say for certain that all these changes have been caused by the human activities alone. It is the number of unusual changes in many elements of the climate system, together with the increase in greenhouse gases, that make these observations significant.

Some critics have argued that the observed temperature change has been due to natural effects, such as changes in solar output. Indeed, in addition to changes in climate documented above, there have been fluctuations in climate due to natural events, such as very long term to variations in the amount of energy emitted by the Sun, perturbations in the Earth’s orbit around the Sun and natural variations in greenhouse gas concentrations.

There are also shorter term fluctuations due to ocean currents such as El Niño and the cooling effect of dust emitted by volcanoes. Any warming due to human activities is superimposed on, and can at times be partly hidden by, these natural variations.

The use of climate models
To help explain the observed changes in climate and link them to increases in greenhouse gases, scientists have designed climate models that create global three-dimensional representations of the atmosphere, land and oceans, based on the physical laws of how the atmosphere behaves.

Model simulations where the temperature is forced to change due to natural effects alone (such as solar changes and volcanic eruptions) do not produce the increase in temperature seen in the observed temperature record. In particular, the warming in the latter part of the twentieth century cannot be explained only by natural causes.

In contrast, most model simulations where the temperature is forced to change due to human increases in greenhouse gas concentrations show a similar, or even larger, warming than that which has been observed over the past century. These models include the localised cooling effect of sulphate aerosols. The simulations that agree best with the observed changes in temperature combine both the natural and human effects. [4]

Although these models are still being refined - in particular to include feedbacks from the biosphere - these findings further suggest that global warming over the past 100 years is unlikely to be only due to natural variability.

Conclusions
In summary, records of greenhouse gas concentrations, carefully studied temperature measurements over many years, observed changes in other aspects of the climate, and model simulations of human and natural changes to the climate, all combine as evidence that human activities have changed global climate.

The IPCC report concludes that these past increases in temperature and sea level due to human activities will continue - by 1.4 to 5.8 ºC and 9 to 88 cm respectively - by 2100. And scientists expect the rises to continue well into the future. How fast, and for how long, depends in part on what we do to manage climate change.

Mike Hulme and Simon Torok are, respectively, director and external communications manager at the Tyndall Centre for Climate Change Research, based at the University of East Anglia in the United Kingdom.