Changes in climate have occurred in the distant past as the distribution of continents and their landscapes have changed, as the so-called Milanko – vitch changes in the orbit of the earth and the earth’s tilt relative to the ecliptic plane have varied the insolation received on earth, and as the composition of the atmosphere has changed, all through natural processes. Recent evidence obtained from ice cores drilled through the Greenland ice sheet indicates that changes in climate may often have been quite rapid and major and not associated with any known external forces.
Observations of surface temperature show a global mean warming of approximately 0.7°C during the past 100 years. The warming became noticeable from the 1920s to the 1940s, leveled off from the 1950s to the 1970s, and increased again in the late 1970s. The calendar year 1998 is the warmest on record. The 1990s are the warmest decade on record. Information from paleodata further indicates that these years are the warmest in at least the past 1000 years, which is as far back as a hemispheric estimate of temperatures can be made. The melting of glaciers throughout most of the world and rising sea levels confirm the reality of the global temperature increases.
Climate modeling suggests that solar variability has contributed to some of the warming of the 20th century, perhaps 0.2°C, up to approximately 1950. Changes in aerosols in the atmosphere, both from volcanic eruptions and from visible pollutants and their effects on clouds, have also contributed to reduced warming, perhaps by a couple of tenths of a degree. A temporary cooling in 1991 and 1992 followed the eruption of Mount Pinatubo in June 1991. Heavy industrialization and associated pollution following World War II may have contributed to the plateau in global temperatures from approximately 1950 to 1970. Interactions between the atmosphere and the oceans, including El Nino, have contributed to natural fluctuations of perhaps two – tenths of a degree. It is only after the late 1970s that global warming from increases in greenhouse gases has emerged as a clear signal in global temperatures.
Projections have been made of future global warming effects based on model results. Because the actions of humans are not predictable in any deterministic sense, future projections necessarily contain a ‘‘what if’’ emissions scenario. In addition, for a given scenario, the rate of temperature increase depends on the model and features such as how clouds are depicted so that there is a range of possible outcomes. Various projections in which the concentrations of carbon dioxide double 1990 values by the year 2100 indicate global mean temperature increases ranging from 2 to 4°C higher than 1990 values. Uncertainties in the projections of carbon dioxide and aerosols in the atmosphere add to this range. However, a major concern is that the rates of climate change as projected exceed anything seen in nature in the past 10,000 years.
Climate Change and Energy, Overview • Climate Change: Impact on the Demand for Energy • Climate Protection and Energy Policy • Ecosystems and Energy: History and Overview • Environmental Gradients and Energy • Greenhouse Gas Emissions from Energy Systems, Comparison and Overview • Lithosphere, Energy Flows in • Ocean, Energy Flows in
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