Best Weather Moving North, Study Finds : Rainfall Shift Could Have Big Effect on Agriculture

Rainfall in the United States and the Soviet Union has increased by more than 10% in the last 30 to 40 years, while precipitation in regions closer to the Equator has declined by about the same amount, scientists reported in a study published today.

The changes in rainfall patterns may have profound long-term effects on agriculture because if the shift continues, as many scientists believe it will, the best weather for growing food would shift northward from the United States into Canada, while farming in more southerly regions would require massive additional irrigation.

By the same token, the regions in the Soviet Union where most food is now grown would have a more favorable climate. The study, published in Science magazine, did not examine precipitation data for the Southern Hemisphere.

It said the rainfall changes may be caused by the so-called greenhouse effect, a warming of the Earth caused by the accumulation in the atmosphere of carbon dioxide, which is produced by the burning of fossil fuels. Carbon dioxide traps the Earth’s heat, and it has been estimated that the Earth’s surface temperature has increased by 0.5 to 1.25 degrees Fahrenheit since 1850.

Global warming also can be expected to increase total rainfall, but there is no evidence yet that this is occurring, experts said.

The new report was prepared by a team headed by climatologists John K. Eischeid and Henry F. Diaz of the National Oceanic and Atmospheric Administration’s Environmental Research Laboratories in Boulder, Colo.

They gathered data on precipitation throughout the Northern Hemisphere from 1850 to the present and then found that rainfall in a band between 35 degrees and 70 degrees north has increased by nearly 20% since 1850, with half of that increase occurring in the last 40 years. In addition to the United States and the Soviet Union, nearly all of Europe falls within this band.

Rainfall in the band between 5 degrees and 35 degrees north, they found, was more or less constant until about 40 years ago, but has fallen 10% since then. Mexico, India and many countries in Southeast Asia are within this area.

In the band between the Equator and 5 degrees north, rainfall has remained constant. Scientists are not sure what caused the changes before 1940.

Rainfall Variations

In the United States, rainfall peaked in the late 1800s, fell back to normal, and then increased about 15% in the last 40 years. In the Soviet Union, rainfall increased by about 25% between 1850 and 1900 and another 15% after 1950.

Rainfall has generally been decreasing in Northern Africa and the Middle East. That area includes the Sahel region of northern Africa, which suffered a severe drought during most of the 1970s, and Ethiopia, which has been drought-stricken through much of the 1980s.

Climatologists have praised the authors of today’s Science report, but some, such as Syukuro Manabe of Princeton University, cautioned that it may be too soon to detect long-term trends in rainfall because of the large natural fluctuations.

“We’ve no firm proof yet that warming is due to the greenhouse effect,” said climatologist David Hind of the Goddard Space Flight Center’s Institute for Space Studies in New York City, “so we can only be less sure about rainfall changes.”

Scientists believe that the concentration of carbon dioxide in the Earth’s atmosphere was about 270 parts per million in the mid-1800s. It is now about 345 ppm.

Fuel Burning Slows

A 1983 National Academy of Sciences report predicted that the level could reach 600 ppm within 50 to 100 years if the use of fossil fuels continued to grow at then-current rates. The rate of increase of fossil fuel use has slowed since that report was prepared, however, so that a doubling of carbon dioxide levels may take somewhat longer.

An increase in carbon dioxide levels to 600 ppm, the academy report had predicted, could cause global temperatures to rise by as much as eight degrees Fahrenheit--to temperatures that were last experienced during the age of the dinosaur. Los Angeles might then have 27 days each year with temperatures over 90 degrees, compared to the current average of five days, according to James E. Hansen, director of the Institute for Space Studies.

Another effect of global warming might be a partial melting of the polar ice caps, which could cause flooding in coastal cities. Since the 1850s, sea level has increased by one to two inches.

Global warming would also increase evaporation of the ocean, leading to a worldwide increase in rainfall of as much as 11%, according to the academy report. But the patterns of rainfall would change even more, it said.

Much of the ocean’s evaporation occurs in the tropics because of the higher temperatures there. Convection currents then carry the moisture-laden air toward the poles at high altitudes. If the air temperature increases due to global warming, more moisture will evaporate and more will be carried north.