Record heat over North America. A wet summer for Britain. Floods in Sudan. It's weather as usual for meteorologists who take the global view. They aren't ready to say pollution-driven global warming has caused any shift in basic weather patterns. But some of them do urge preparations for a time when such a climate shift may indeed occur.
``There are always weather anomalies around the world,'' Jerome Namias of the Scripps Institution of Oceanography points out. He explains: ``When it's dry here, it's wet there.''
Dr. Namias, who pioneered long-range weather forecasting for the United States National Weather Service, is one of the foremost authorities on weather trends. He notes that, when there is drought over North America, there tends to be drought over the Eastern Atlantic (``which no one notices'') with wetter weather over Britain tied in. This can set up periods of extreme weather that break records in specific regions. But, Namias observes, ``Weather records are always being broken somewhere.''
Meteorologists have generally taken this for granted. The question they now ask is whether a gradual global warming due to accumulation of heat-trapping gases such as carbon dioxide is changing underlying weather patterns. Is there, for example, a climatic shift that could make this summer the norm rather than an anomaly for North America?
Namias says, ``I can't see it and I can't see it particularly tied to CO2.'' This reflects the general assessment of climate scientists that one year's drought doesn't mean general climatic warming. They note that the dust bowl drought of 1934-36 arose long before man-made warming would have been involved.
Climate researchers made this point again last week in a hearing on global warming held by the US Senate Energy and Natural Resources Committee. But, as Stephen Schneider of the National Center for Atmospheric Research at Boulder, Colo., explained, ``there is concern that we are assaulting the environment much faster than we are understanding it.''
The buildup of so-called greenhouse gases - especially carbon dioxide from burning fossil fuels and methane from agriculture - should raise Earth's average surface temperature several degrees over the next half century, according to most projections. This may begin to show up in the next decade. Computer models of climate are not yet refined enough to show how such a gradual warming might change weather in specific regions.
Reflecting general opinion among climatologists, Dr. Schneider said that, in spite of this uncertainty, it would be wise to start now to curb release of the greenhouse gases. Global warming may bring more drought to one place, yet improve agricultural weather somewhere else. Schneider explained, however, that ``it is my opinion that credible details about specific `winners and losers' will not be available before we may have committed ourselves to potentially dangerous, large atmospheric changes.''
While weather scientists can't blame 1988's ``anomalies'' on global warming, they can, with hindsight, link it to recent trends. Four of the hottest Northern Hemisphere summers on record have occurred in the 1980s. Indeed, this present summer may be the hottest ever recorded. For North America, the current drought appears to have been foreshadowed by scattered dry conditions that appeared over the past several years.
Looking at the immediate past trend, Namias says he can clearly trace this year's drought development back six months. He explains that the pattern appears to be locked in by three high-pressure systems over the North Pacific, United States, and North Atlantic respectively. These set the main air circulation pattern. They are linked to a series of hemisphere-encircling high- and low-pressure areas called planetary waves. It is this basic general pressure pattern that guides air circulation so that, with drought over North America, Britain has a cold, damp summer.
Namias further explains that the drought-inducing pattern of three high-pressure cells is, in turn, related to underlying surface conditions. A dry spring meant that seasonal heating tended to warm the land rather than evaporate moisture.
This strengthened the North American high-pressure cell. Meanwhile, the pattern of sea-surface temperatures encouraged formation of strong Pacific and Atlantic highs.
Once set, sea-surface temperature patterns tend to persist. They provide the stability that tells the drought ``You keep on going,'' Namias says. He adds that one can then look further back into last year's weather to see what set up the sea-surface temperatures, and so on. ``Anomalies breed anomalies,'' he says.