Human Induced Climate Changes

Few scientists would disagree that the planet has been warming dramatically over the past century and that human contributions to climate change have been accelerating to critical levels as the world becomes increasingly industrialized. Much of what is known about these short-term climate changes has been described in a report issued in late 2007 by the Intergovernmental Panel on Climate Change, an international group of hundreds of scientists who have analyzed all available data, assessed the causes of these recent, short-term changes to the global climate, and made predictions of what the climate of Earth may look like at various times in the future. Much of the information in this chapter is based on the findings of the Intergovernmental Panel on Climate Change.

Eleven of the 12 years between 1996 and 2006 were the warmest on record since weather recording instruments were widely used starting in 1850. The rate of temperature increase seems to be increasing, with polar areas affected more than equatorial regions. Sea levels are also rising at an increasing rate. Between 1961 and 1993 global sea level was rising at a rate of .05-.09 inches per year (.13-.23 cm/yr.), and since 1993 they have been rising at .09-.11 inches per year (.24-.28 cm/yr.). Some of the sea level rise is due to melting glaciers, ice caps, and snow, and some is from thermal expansion of ocean water as the water warms. Glaciers are shrinking in both the Northern and Southern Hemispheres, and the ice caps on the Arctic Ocean and over parts of Antarctica are shrinking rapidly.

Global precipitation patterns are observably changing on the century scale, with much of eastern North and South America, northern Europe, and north and central Asia seeing increased rainfall but other areas such as the Sahel, Mediterranean, southern Africa, and southern Asia seeing decreased precipitation. On a global scale, areas that are experiencing drought or less precipitation are greater than areas receiving greater precipitation.

This chapter examines these and other trends in the short-term climate, focusing on changes since 1850, when the world was going through the later industrial revolution and people began increasing the amount of carbon dioxide and other greenhouse gases released into the atmosphere. Changes to different Earth systems are assessed, then possible causes of these changes are examined. Possible relative contributions of natural and man-made or anthropogenic changes are discussed, and different time-scales of climate forcing are presented.

Temperature Variations during the Past 1,000 Years

Understanding changes to Earth's climate in the past 100-200 years, or the slightly longer interval extending back through the last glacial interval, rely on several types of data. Instrumental records of Earth's climate extend back in time to about the year 1850, when recording devices were put into widespread use. Long cores of ice obtained from Greenland and other locations are also widely used to measure past climate conditions, with this record extending back for about 650,000 years.

The Intergovernmental Panel on Climate Change issued a statement in November 2007 that "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level." This bold statement was based on rigorous analysis of data from the past 1,000 years, showing that temperatures remained fairly steady at about 0-0.5 degrees below the 1990 average value from the year 1000 to about 1910, then began a sharp upward turn that flattened off for a short time in the 1950s and has turned sharply up again since about 1976. Temperatures are now about 0.5-1.0 degrees above the 1990 value and expected to rise 2-5 degrees above this value by 2100.

To measure global average temperatures, groups of meteorologists, such as the World Meteorological Organization (WMO) and the Global Climate Observing System (GCOS), have a large number

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Plot of the average global temperature variations on the planet in the past 1,000 years, and what different models predict the temperature will be by the year 2100. All models show a predicted temperature rise, ranging between 1.5 and 5.5 degrees. (IPCC 2007)

of observation points on the continents, which are then gridded into equal areas to assign a temperature for each box in the grid. Where local observations are not available, and for calibrating the model, observations from satellites are widely used. Local effects, such as any urban heat island effect from cities, are accounted for in these types of models. With rapid improvements in computer modeling, it has been possible to make more and more detailed and accurate models for the globe.

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