global warming

The warming effect of the Earth's atmosphere is called the greenhouse effect. Radiation from the Sun enters the atmosphere but is prevented from escaping back into space by gases such as carbon dioxide (produced for example, by the burning of fossil fuels), nitrogen oxides (from car exhausts), and CFCs (from aerosols and refrigerators). As these gases build up in the atmosphere, the Earth's average temperature is expected to rise.

Increase in average global temperature, amounting to approximately 0.74°C/1.3°F from 1906 to 2005. Much of this is thought to be related to human activity. Global temperature has been highly variable in Earth history and many fluctuations in global temperature have occurred in historical times, but this most recent episode of warming coincides with the spread of industrialization, prompting the suggestion that it is the result of an accelerated greenhouse effect caused by atmospheric pollutants, especially carbon dioxide gas. The melting and collapse of parts of the Larsen Ice Shelf, Antarctica, in two distinct events in January 1995 and February 2000, have been attributed to global warming, as has the reduction in the Arctic polar ice cap. Melting of land-based ice is expected to raise the sea level in the coming decades (melting of floating ice does not alter sea level).

In 1988, the World Meteorological Organization (WMO) and the United Nations (UN) set up the Intergovernmental Panel on Climate Change (IPCC), a body of more than two thousand scientists, to investigate the causes of and issue predictions regarding climate change.

In 2007, the IPCC's Fourth Assessment Report stated that: most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increased in anthropogenic (human) greenhouse gas concentrations. The IPCC predicts that a doubling of carbon dioxide concentrations, expected before the end of the 21st century, would probably increase the average global temperature by 2–4.5°C/3.6–8.1°F.

Assessing the impact of humankind on the global climate is complicated by the natural variability on both geological and human time scales. The present episode of global warming has thus far still left England approximately 1°C/1.8°F cooler than during the peak of the so-called Medieval Warm Period from 1000 to 1400. The latter was part of a purely natural climatic fluctuation. The consensus view of climatologists is that the Medieval Warm Period was not on a global scale, but this is still a contentious topic.

In addition to a rise in average global temperature, global warming has caused seasonal variations to be more pronounced in recent decades. Examples are the most severe winter on record in the eastern USA 1976–77, and the record heat waves in the Netherlands and Denmark the following year. Mountain glaciers have shrunk, late summer Arctic sea-ice has thinned by 40%, and sea levels have risen by 10–20 cm/4–8 in. Scientists have predicted a greater number of extreme weather events, and sea levels are expected to rise by 9–88 cm/4–35 in by 2100. 1998 was the warmest year globally of the last millennium, according to US researchers who used tree rings and ice cores to determine temperatures over the past 1,000 years.

A 1995 UN summit in Berlin, Germany, agreed to take action to reduce gas emissions harmful to the environment. Delegates at the summit, from more than 120 countries, approved a two-year negotiating process aimed at setting specific targets and timetables for reducing nations' emissions of carbon dioxide and other greenhouse gases after the year 2000. The Kyoto Protocol of 1997 committed the world's industrialized countries to cutting their annual emissions of harmful gases. However, in July 1997, the US Senate resolved unanimously that the USA should not ratify any protocol that did not place comparable burdens on developing countries. In June 2001 US president George W Bush announced that the USA would not ratify the Protocol because it exempted China from limiting its emissions.

Since 1958 scientists have been measuring the amount of carbon dioxide in the atmosphere on mountain tops in Hawaii and in Antarctica, over which time the concentration has risen by about 15%. Longer records of carbon dioxide levels, obtained from bubbles of air trapped in Antarctic ice, extend back hundreds of thousands of years and show that carbon dioxide levels are about 31% higher than before the Industrial Revolution – higher than they have been for the past 420,000 years. Other greenhouse gases have also increased: methane levels have more than doubled and nitrous oxide levels have increased by 17%.

In Australia, the National Climate Centre of the Australian Bureau of Meteorology showed that between 1910 and 1999, Australia's mean annual temperature rose by 0.8°C/1.4°F to 28.43°C/83.17°F, the hottest three decades of the 20th century being the 1990s, 1980s, and 1970s. The Centre said that this supported their prediction of global warming and suggested that there has been a discernible human influence. In October 2000, a report by marine biologists estimated that 50–95% of the coral reefs in the Indian Ocean had died since 1998 as a result of warmer seas.

Under the 1992 Convention on Climate Change, the industrialized countries pledged to stabilize emissions of six greenhouse gases: carbon dioxide, methane, nitrous oxide, sulphur hexafluoride, hydrofluorocarbons, and perfluorocarbons. By 2012, they are committed to reduce their overall emissions by 5.2% compared with 1990 – this would represent a cut of 29.9% relative to the levels of emission expected in the absence of the Protocol. Different nations are required to cut by different amounts, ranging from 8% reductions for the European Union to 6% for Japan and no cuts at all for Russia, while increases were permitted of 8% for Australia and 10% for Iceland. By 2007, only the UK and Sweden were on course to meet their treaty obligations. In the UK, this reduction was largely due to the collapse of the coal industry and subsequent increase in the use of gas, a less polluting fuel.

A UN conference on global warming held in November 2000 failed to produce agreement between the USA and European countries. After days of strongly resisting measures aimed to combat global warming, the USA made some last-minute concessions, but these were not accepted. The main arguments for not imposing stricter controls on carbon dioxide emissions are both economic and political. By most estimates, only severe reductions in global carbon dioxide emissions – in the order of 60% – will have an impact on scenarios envisioned by computer simulations. To do this without adequate controls in the developing nations would simply result in severe economic downturns for the nations imposing restrictions, followed by a relocation of emitting countries to other developing nations who cannot afford to implement carbon dioxide restrictive practices. However, in July 2001, the Kyoto Protocol was finally ratified in Bonn, Germany, with 174 countries agreeing to reduce levels of six gases known to enhance global warming by an average of 2% below their 1990 levels over a period of 11 years, despite the non-acceptance of the USA.