The natural greenhouse effect was first described by Joseph Fourier (1768–1830) in 1827. Solar energy reaches the earth in the entire spectrum. The earth reemits this energy as infrared radiation. Greenhouse gases in the atmosphere, such as water vapor and carbon dioxide, absorb outgoing infrared radiation and reemit it in any direction. As a result the earth holds more energy than it would have without the presence of greenhouse gases. The natural greenhouse effect raises the global annual surface air temperature by some 90 degrees Fahrenheit (30 degrees Celsius) and therefore enables life as we know it.
The enhanced or anthropogenic greenhouse effect was first described by Svante August Arrhenius (1859–1927) in 1896. Deforestation and coal burning release carbon dioxide, which accumulates in the atmosphere. As a result the greenhouse effect is strengthened, and temperatures rise.
The enhanced greenhouse effect was long a curiosity in the natural sciences. In the 1970s the first climatologists were more worried about global cooling—as the next ice age was overdue and temperatures were falling, probably because of sulphur emissions (now reduced because of concerns over acid rain). Global warming returned to the scientific agenda in the early 1980s and got on the political agenda a decade later.
The enhanced greenhouse effect is clouded by large uncertainties caused by imperfect data, limited computing power, and the vast complexity of the earth system. There is virtual agreement, however, that temperatures will rise, and more so at the poles, during winter and at night. Rainfall patterns will change and may well intensify— leading to droughts in some places and in some seasons and floods in others. Storms may become more frequent and intense, but the evidence for this is weak where this matters most, namely in the tropics. Rising temperatures will cause sea levels to rise, mostly through expansion of seawater but perhaps also through ice melt.
The uncertainty about how the climate would respond to greenhouse gas concentrations is compounded by the uncertainty about future emissions, which depend on the number of people, their economic activities, and the types of energy they will use. High future emissions would put the earth into a state it has not been in for millions of years, perhaps with strongly nonlinear consequences.
Impacts of climate change are many. Less heating would be needed in winter, but more cooling would be needed in summer. Heat-related diseases would increase, but cold-related ones would decrease. Tropical diseases may spread or intensify. Lands would drown unless costly dikes are built. Crops would benefit from the higher carbon dioxide concentration but may be hurt by drought. Tourists would seek different holiday destinations. Urban infrastructure, particularly for water discharge, would need to be redesigned. Vegetation patterns would change, probably at the expense of many specialized plants and animals. Aggregate estimates suggest that the initial warming may be positive but that greater warming would bring net damages. Poorer countries would suffer greater damages.
Carbon dioxide is the main anthropogenic greenhouse gas, followed by methane and nitrous oxide. Carbon dioxide is emitted by the burning of fossil fuels and by deforestation. Methane comes from agriculture, waste, and fossil fuel production; nitrous oxide come from agriculture and industry. Chlorofluorocarbons (CFCs) are greenhouse gases too, but they have been phased out to protect the ozone layer. Their replacements, hydrofluoro-carbons (HFCs), are even stronger greenhouse gases.
Greenhouse gases can be reduced by slowing or even reversing economic growth. This was shown to be particularly effective when the Soviet Union collapsed, but it is also responsible for the limited emissions growth in western Europe and Japan. Alternatively energy efficiency can be improved, alternative fuels can be used, or emissions can be captured and stored (biologically or geologically). Substantial emission reduction can be achieved with proven technologies. The costs would be small if policies are implemented gradually and implementation allows for flexibility.
Because greenhouse gases stay in the atmosphere for decades and longer, it does not matter where they are emitted. International coordination is needed for an effective solution. The oceans respond only slowly to changes in greenhouse gas concentrations. Energy infrastructure lasts for decades. As a result both the climate change problem and its solutions span many electoral cycles. Climate policy is therefore weak. The United Nations Framework on Climate Change is universally accepted, but it establishes obligations only to report emissions and to negotiate. Its first implementation, the Kyoto Protocol, sets targets for a limited number of countries only; of these, some have abandoned the treaty, while others are likely to meet their targets by coincidence rather than design.
Climate change has become a cultural phenomenon too. The press and television devote considerable attention to climate science and policy. Major motion pictures are devoted to the topic, and climate change regularly features in advertisements for a wide range of products, sometimes without an obvious connection.
SEE ALSO Change, Technological; Deforestation; Economic Growth; Energy; Energy Industry; Energy Sector; Global Warming; Pollution, Air; Resource Economics; Resources; United Nations
World Meteorological Organization and United Nations Environment Programme. The Intergovernmental Panel on Climate Change. http://www.ipcc.ch/. Produces regular and authoritative assessment reports.
Richard S. J. Tol