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Is the Earth's Climate Warming or Cooling?

As Congress scrutinizes new energy and climate legislation, many seem to be asking: Is it getting cooler or warmer?

The answer, according to a new study, is that we need to concentrate on the long-term trend, which points to an overall warming tendency over these past hundred years.

The great majority of climate scientists agree that it's getting warmer in many places around the world. The cause, they also agree, is heat-trapping carbon dioxide produced by human technology.

But how does this square with the observed fact that over the past decade world temperature has actually stayed the same, or even gone down?

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Two scientists, Michael F. Wehner and David R. Easterling, show that such decade-long fluctuations are quite common in weather history. From day to day, season to season, and year to year, the weather shows great variability thanks to natural factors like capricious wind patterns and ocean currents.

Changes in climate -- that is, changes in typical weather conditions over long periods of time -- are more difficult to assess.

These short-term changes, say the scientists, must be differentiated from long-lasting, consequential trends in order to determine the role of human activities in shaping climate and to formulate industrial policy -- such as imposing a tax on carbon emissions.

Wehner, who works at the Lawrence Berkeley National Laboratory in Berkeley, Calif., says that the long-term trend really is toward a warmer planet, but that a single year, 1998, has temporarily thrown off the overall upward march in temperature.

In that year, an immense transfer of heat from the western to the eastern Pacific occurred: El Nino (Spanish for "Christ Child"), which often coincides with Christmas time. An El Nino event can have a major impact on rainfall patterns and temperatures over several continents.

El Nino and other weather factors can cause a short reversal in the warming trend for a year. A 10-year reversal is less likely, but still possible. Just as in throwing a coin, seven heads in a row is unexpected, but it does happen now and then.

In the journal Geophysical Review Letters, Wehner says that even a period of 20 years of modest cooling -- the equivalent of throwing 20 heads in a row -- would not reverse the scientific finding that long-term world temperature is trending upward; the trend is based on data now stretching back more than a century.

Does this mean that after 10 years of relative cooling the next few years will be particularly warm in order to make up the cool years?

No -- scientists can only say that the overall trend, over many years, is toward higher temperatures.

No unusually warm day in January or unusually cool day in July can negate the overall trend. Neither can a cooling decade negate a century-long warming trend.

In making policy decisions, argues Wehner and Easterling, don't let today's weather or even this year's weather influence your judgment. Concentrate on the long-term trend.

This story was reported by Inside Science News Service.