The temperature of the atmosphere near the earth's surface is warmed through a natural process called the greenhouse effect. Visible, shortwave light comes from the sun to the earth, passing unimpeded through a blanket of thermal, or greenhouse, gases composed largely of water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Infrared radiation reflects off the planet's surface toward space but does not easily pass through the thermal blanket. Some of it is trapped and reflected downward, keeping the planet at an average temperature suitable to life, about 60°C (16°C).
Growth in industry, agriculture, and transportation since the Industrial Revolution has produced additional quantities of the natural greenhouse gases plus smaller quantities of chlorofluorocarbons and other more potent greenhouse gases, augmenting the thermal blanket. It is generally accepted that this increase in the quantity of greenhouse gases is trapping more heat and increasing global atmospheric and ocean temperatures, making a process that has been beneficial to life potentially disruptive and harmful. Since the late 19th cent., the global average surface temperature has risen about 2°C (1.1°C), with two thirds of the increase having occurred since 1975, and sea level has risen several inches. Some projected, longer-term results of global warming include melting of polar ice, with a resulting rise in sea level and increase in coastal flooding, in part because temperature increases will be greater in the polar regions; disruption of drinking water supplies dependent on snow melt; profound changes in agriculture due to local climate changes; an increase of the land area that, due to high temperatures, is inhospitable to human habitation; extinction of species as ecological niches disappear; more intense hurricanes and typhoons due to warmer ocean water, as well as more intense winter storms; and an increased incidence of tropical diseases. Oceans are expected to become more acidic, to the detriment of sea life, especially shellfish, and there is evidence that this is occurring. The effect of such changes on people and communities would not only be locally disruptive but could also aggravate or cause political, societal, and economic instability, which could contribute to or spark national and international conflict.
Among factors that may be contributing to global warming are the burning of coal, petroleum, and other fossil fuels (sources of carbon dioxide, methane, nitrous oxide, and ozone); deforestation, which increases the amount of carbon dioxide in the atmosphere; and methane gas released in animal waste. The thawing of the tundra as the climate warms may lead to additional increases in methane release.
Much of the debate surrounding global warming has centered on the accuracy of scientific predictions concerning future warming. To predict global climatic trends, climatologists accumulate large historical databases and use them to create computerized models that simulate the earth's climate. The validity of these models has been challenged by some. Some skeptics say that the climate is too complicated to be accurately modeled, and that there are too many unknowns. Some also question whether the observed climate changes might simply represent normal fluctuations in global temperature. However, within climatology, meteorology, and related fields, knowledgeable experts generally accept that human activity is the primary cause for the observed global warming. Most scientists also agree that it is difficult in general to tie the effects of human activity to specific unusual or extreme weather events, but a number of studies released in 2014 agreed that the 2013–14 extreme heat waves in Australia would not have been as severe without the effects of emissions resulting from human activity.
Despite political controversies over global warming, the scientific consensus is increasingly focused on the likely serious consequences of human-caused global warming and the need for urgent and concerted action. Some climate scientists have proposed the use of geoengineering, such as introducing sulfur compounds into the atmosphere to produce global cooling (as volcanic eruptions do); this approach is not without risk, and has been rejected by most environmentalists. In 1992, at the United Nations Conference on Environment and Development, over 150 nations signed a binding declaration on the need to reduce global warming.
In 1994, however, a UN scientific advisory panel, the Intergovernmental Panel on Climate Change (IPCC), concluded that reductions beyond those envisioned by the treaty would be needed to avoid global warming. The following year, the advisory panel forecast a rise in global temperature of from 1.44 to 6.3°C (0.8–3.5°C) by 2100 if no action were taken to cut down on the production of greenhouse gases; a more recent study by the same panel estimated a rise of 3 to 7.5°C (1.8 to 4°C). Even if action is taken, the already released gases will persist in the atmosphere, and a rise of from 1 to 3.6°C (0.5–2°C) is expected to occur. A 2007 IPCC report, based on a three-year study, termed global warming
unequivocal and said that most of the change was most likely due to human activities, and its report five years later restated those findings even more strongly.
A UN Conference on Climate Change, held in Kyoto, Japan, in 1997 resulted in an international agreement to fight global warming, which called for reductions in emissions of greenhouse gases by industrialized nations. Not all industrial countries, however, immediately signed or ratified the accord, known as the Kyoto Protocol. In 2001 the G. W. Bush administration announced it would abandon the accord; because the United States produces about one quarter of the world's greenhouse gases, this was regarded as a severe blow to the effort to slow global warming. Despite the American move, most other nations agreed later in the year (in Bonn, Germany, and in Marrakech, Morocco) on the details necessary to convert the agreement into a binding international treaty, which came into force in 2005 after ratification by more than 125 nations.
In 2002 the Bush administration proposed several voluntary measures for slowing the increase in, instead of reducing, emissions of greenhouses gases. The United States, Australia, China, India, Japan, and South Korea created (2005) an agreement outside the Kyoto Protocol that proposed to reduce emissions through the development and implementation of new technologies. The Asia-Pacific Partnership on Clean Development and Climate established no commitments on the part of its members; it held its first meeting in 2006. Also in 2006, California enacted legislation that called for cutting carbon dioxide emissions by 25% by 2020; the state is responsible for nearly 7% of all such emissions in the United States. In 2007 U.S. President Bush called for the world's major polluting nations to set global and national goals for the reduction of greenhouse gas emissions, but the nonbinding nature of the proposed goals provoked skepticism from nations that favored stronger measures.
The 15th UN Conference on Climate Change, held in Copenhagen, Denmark, in Dec., 2009, failed to lead to a legally binding treaty on reducing global greenhouse-gas emissions. It had been hoped that the meeting would result in a new protocol that would replace that agreed to at Kyoto. The 17th conference, which met in 2012 in Durban, South Africa, agreed to extend the accord (which was extended to 2020 later in 2012) and also agreed to work toward an unspecified new accord; at the same time, however, Canada became the first ratifying nation to withdraw from the Kyoto Protocol.
In 2015, in Paris at the 21st conference, the world's nations agreed for the first time to take measures to hold global warming below 3.6°C (2°C), and all parties, not just developed nations, agreed to reduce emissions; a revision of emissions targets (beginning in 2020) and a review of actual emission reductions (beginning in 2023) were required every five years under the treaty (signed 2016). Technological support and financial aid was also promised to developing nations. U.S. support for the 2015 agreement, however, reversed after the election (2016) of Donald Trump as president; in office he moved to dismantle federal policies intended to address climate change and announced the United States would withdraw from the agreement. The present rate of carbon dioxide emissions has been increasing since 1970, and measures adopted so far have not slowed the increase in emissions. In the mid-2010s the atmospheric concentration of carbon dioxide exceeded 400 parts per million (ppm); in the last 800,000 years carbon dioxide levels have never previously exceeded 300 ppm.
See G. E. Christianson, Greenhouse (1999); T. Flannery, The Weather Makers (2006); E. Kolbert, Field Notes from a Catastrophe (2006); E. Linden, The Winds of Change (2006); P. Conkling et al., The Fate of Greenland: Lessons from Abrupt Climate Change (2011); B. McKibben, ed., The Global Warming Reader (2012); W. D. Nordhaus, The Climate Casino (2013).
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