How Is the Climate Changing?

Temperature Changes
Figure 3 shows how the global average surface temperature (over sea and land) has risen from 1850 to the present. Because temperatures vary naturally from year to year, climate scientists must compare several years’ temperature records with long-term averages to be sure the suspected temperature change is significant. The data are set out in terms of the “anomaly”—that is, the difference between each year and the average temperature in the period 1961–90. Before 1978, it was generally colder, with all later years warmer. These data are shown in the figure in degrees Celsius, not Fahrenheit: A rise of 0.47°C is equivalent to 0.8°F.

Global temperature, established from millions of individual measurements taken from around the world, rose by about 1.1°F during the last century, with two-thirds of this warming occurring since the 1970s. The 1990s was the warmest decade, with 1998 being the warmest year since 1860, when world temperature measurements were first recorded. Scientists at the University of East Anglia in the United Kingdom, in one of the most comprehensive studies to date of climatic history, have confirmed that the planet is now warmer than it has been at any time in the past 2,000 years. And there is no sign of this trend reversing: nineteen of the twenty hottest years in the past 150 years have occurred since 1980, and 2005 was the warmest year on record in the Northern Hemisphere. Worldwide, temperatures on land have warmed more than the oceans.

The United States reflects these trends. Observations from over twelve hundred weather stations across the country have shown that the average temperature rose by almost 1°F during the twentieth century, with annual temperatures in the coastal Northeast, the upper Midwest, the Southwest, and parts of Alaska experiencing increases four times that average. The largest observed warming has been seen in winter.


Other Climate Changes
Rising temperatures increase the amount of energy in the atmosphere, and this has consequential effects: Climate models predict changes in rainfall amounts and patterns, and a rise in the occurrence of storms, heat waves, and other extreme events. Some of these changes, including periods of sustained drought from higher than average temperatures and well below average rainfall, have already been experienced in the United States and around the world. The most populated state of Australia has recently experienced four years of drought. When persistent droughts occur, there is little reserve water to weather them, leaving people, livestock, and crops at risk. The growing season is then reduced or even lost completely. Because of this, food production in some parts of the developing world is already in decline. The well-known variability of the climate can make it difficult to ascertain that any individual event is unusual and specifically caused by climate change. But the accumulating evidence from studies over the last forty years makes it highly unlikely that the data can be interpreted as the outcome of a natural cycle uninfluenced by human activity.

In the United States, signs of water stress that used to be limited to the West are now increasingly common in the East—from dried-up rivers to shrinking lakes and falling water tables. On the other hand, average precipitation has increased by 5 to 10 percent over the last century, with much of that due to an increase in the frequency and intensity of heavy rainfall. Precipitation increases have been especially noteworthy in the Midwest, southern Great Plains, and parts of the West and Pacific Northwest.

Of particular concern has been the increased frequency and intensity of extreme events, such as storms and hurricanes, as these tend to have severe effects on people and on the man-made environment as well as the natural environment. This concern was intensified as a result of the very active hurricane season and particularly the severe impact of Hurricane Katrina on New Orleans and the surrounding area in 2005. Indeed, 2005 was the most destructive year on record.

Copyright © 2007 by Mayer Hillman with Tina Fawcett and Sudhir Chella Rajan. All rights reserved.