Climatologists study 89 years of history to find out whether weather is changing
With record-breaking cold chilling the Eastern half of the United States, people may wonder if the weather is changing. So do climatologists Thomas R. Karl, Robert E. Livezey, and Edward S. Epstein of the US National Oceanographic and Atmospheric Administration (NOAA). They find that the past decade has had an abnormally high frequency of extreme winters -- six years out of eight between 1975 and 1983.
The climatologists estimate that, if this concentration of extremes were a random fluctuation of an otherwise stable climate, it would be expected to occur only once in several hundred years, perhaps only once in a millennium.
Whether this winter also turns out to be extreme remains to be seen. A warm December and a cold January could average out to be a normal winter unless February also is unusually cold.
Nevertheless, the NOAA study suggests that the United States, and perhaps other parts of the world, are experiencing a run of abnormal weather, if not an outright climatic change.
The NOAA scientists base their conclusion on an analysis of US climatic records from 1895 to 1984. As they explain in the December Bulletin of the American Meteorological Society, statistical techniques were used to estimate the likelihood of a run of six extreme winters out of eight given the past century's climatic patterns.
In this analysis, the term extreme means unusually warm as well as unusually cold.
Thus an eight-year period featured three abnormally warm winters -- 1975-76, 1980-81, and 1982-83 -- as well as a run of three abnormally cold winters -- 1976-77 through 1978-79.
The NOAA scientists observe that this ``recent variability is either a moderately rare event in a reasonably stationary climate, or it represents climate change. Which of these is the correct view is beyond the scope of this paper.''
Asked if it is likely that a true climate change is showing up, Dr. Karl said, ``To be honest, we really don't know.''
``Even if it does represent a climatic change, we don't know what would be causing it,'' he added.
Karl explained that he and his co-authors hoped to stimulate many scientists to look at what may be going on.
Various possible causes of climatic change would be one subject to investigate.
It would also be difficult to identify causes because, within the ocean-atmosphere weather machine, everything influences everything else.
Karl notes that some meteorologists point to changes in the jet stream to account for recent weather extremes.
But what changed the jet stream?
Others then cite patterns of sea surface temperature which influence atmospheric circulation.
But sea temperature patterns are, in turn, affected by the overall air flow. ``It's a fascinating problem,'' Karl says, ``but I don't expect any answers in the next six months.''
The recent period of more variable winters needs to be seen in the perspective of the whole time period studied -- 1895-1984 -- Karl says.
There was a 20-year interlude of quite stable climate over the contiguous United States from the mid-1950s up to 1975 -- that is, up to the beginning of the present epoch of more variable winter weather. This too was unusual compared with the preceding 50 to 60 years, which were intermediate in variability.
In summing up, Karl says that, during the recent highly variable period, 30 percent of the winter months have been in the extreme category.
During the 20-year interlude of low variability, only 10 percent of winter months ran to extremes. And, from 1985 to the mid-1950s, about 20 percent of the winter months were unusually warm or cold.
This suggests that some of the hardship that recent winters have brought may be due to the fact that people have adapted to a gentler climate, which was itself abnormally benign. This may be especially true for some types of agriculture, such as citrus growing in northern Florida.
Raising this question, Karl notes that Florida was warmer in the 1930s, '40s, and early '50s than it is today. There has been a cold trend in Florida for the past 20 to 30 years.
``Has citrus growing moved too far north?'' he asks. It is a question the Florida citrus farmers themselves are asking as some of them abandon their businesses in northern Florida.
The NOAA study is limited in time, area, and season. It covers only the United States for the past century. It considers only winters.
Yet, as its authors point out, it strongly suggests that the present weather is abnormal enough to warrant special research attention.
The study has raised many questions, Karl says, not only about the mechanisms and trends of climate, but about the effect that climate trends have on the economy and on society. If Florida citrus growers are in trouble in the northern part of that state, what other farming areas face similar danger from climatic shifts? Such questions, Dr. Karl says, are worth pursuing.