Has the weather become more variable? Studies say 'no'
Much concern has been expressed about climate change brought on by natural or human causes. But even at their best, these climatic perceptions involve a good deal of speculation.
This point is emphasized by a pair of independent studies published in the Monthly Weather Review. They undercut two of the prominent climatic -- namely that the weather has become more harsh and variable and that aerosols thrown into the air by human activity are blocking sunshihe and cooling down the Earth.
The World Meteorological Organization (WMO), among others, has called attention to supposedly increased climatic variability. It has said, for example, that "since the 1940s and 1950s . . . the atmospheric circulation in the Northern Hemiof . . . greater extremes of weather conditions in many areas of the world."
G. J. Boer and K. Higuchi of the Canadian Climate Centre now have looked for such increased variability -- and failed to find it -- in data representatives of average middle atmosphere temperatures and air circulation over the Northern Hemisphere north of 25 degrees latitude for the period 1949 to 1975. They did find a slight long-term cooling trend, which is well recognized already. however, they conclude that their "results do not support the idea that the climate has become significantly more variable during the period . . . nor do they support a significant connection between variability and either mean temperature or north-south variation of temperature."
Boer and Higuchi point out that warning to beware of more variable weather, such as that embodied in the WMO statement, represent a "general belief" rather a systematic study of well-defined measure of "variability." This is the kind of analysis they have undertaken.
Their primary data are measurements taken twice daily at standard locations of a quantity meteorologists call thickness. This is literally the thickness of the lower half of the atmosphere. The warmer this layer of air is, the thicker it is, and vice versa. Thus its thickness is a measure of the average temperature of the lower atmosphere at each site where the data were taken.
The scientists analyzed their data to show how these temperatures have varied over time and from place to place in the Northern Hemisphere. They also use them to estimate the energies avaiable to drive the winds of the air's general circulation. Thus they worked with well-defined climatic indicators -- temperature of the lower atmosphere and measures of the general air circulation -- and analyzed the variability of these according to standard statistical techniques.
In other words, they have replace an impressionistic notion of climatic variability with a study of well-defined indicators of variability computed from a representatives set of data. In this perspective, the much-touted rise of more variable weather disappears. In spite of the impressive succession of droughts and severe winters in the first half of the 1970s, the swings of weather appear to be no wilder or extreme than they were in the preceding 20 years.
The cooling effect of manmade dust seems equally elusive. Here again, the concern has been raised by a number of experts. Just as dust from volcanic eruptions can spread out to veil large parts of Earth, so city smog, industrial pollution, and agricultural hazes are suspected of gradually increasing the general turbidity of the air.
This, in turn, could either heat the air or cool the lower atmosphere depending on whether the heat absorbed by the aerosol veil or its ability to block incoming sunshine were more important. Generally, those concerned with what has been called the "human volcano" effect have warned of possible cooling. Indeed, much of the cooling that has been observed over the Northern Hemisphere since 1940 is attributed to this.
However, any such effect is hard to find in the 37 years of atmospheric transmission data (1940 to 1977) studied by Douglas V. Hoyt and Robert D. Evans of the Cooperative Institute for Research on Environmental Science in Boulder, Colo., working with Wisconsin meteorologist Charles P. Turner. ". . . the [ worldwide) climatological impact of anthropogenic aerosols appears to be insignificant," the three scientists say.
This sweeping conclusion is drawn from a detailed study of only three sites -- Madison, Wis.; Albuquerque, N.M.; and Blue Hill, Mass. The scientists defend this as a straightforward logical extensive of their specific findings.
In these data, it is easy to see the effect of dust veils from three volcanic eruptions -- the outbursts of Agung and Awu in the East Indies in March 1963 and August 1966 respectively, and of Fuego in Guatemala in October 1974. This dust soon fades away.
The data also show the dimming effect of urban pollution. However, the scientists note, this is a local effect that dies out away from the urban area. The three sites, taken individually, show no general long-term trend in air turbidity although, taken together, they do show a slow average decline in the clarity of the air. It amounts to about 3.7 percent decrease in transmitted sunshine from 1940 to 1977. That would imply a corresponding global decrease of about 0.4 percent.
"Although this decrease [in clarity] might appear to be climatologically significant," Hoyt, Turner, and Evans say, "there are good reasons to believe its impact is no more than local."
They point out that the "human volcano" dirties the lower part of the atmosphere. The net effect of dustiness there should be a slight warming over land areas that rather than a cooling. Yet long-term data show a definite cooling. This, they argue, implies that "anthropogenic aerosols would have a negligible impact on climatic change compared to other causes."
Also, they note that pollution maps in the National Air Quality Report of the US Environmental Protection Agency and some other data tend to show that these aerosols are confined to regions in and near urban areas. Thus, they say, the decreases in air clarity found in their study "should only be interpreted as upper limits on what is occurring on a regional or global basis because the measurements are near the urban sources of pollution."
Assuming that manmade turbidity is confined mainly to urban areas and drawing on results of several other studies, Hoyt, Turner, and Evans say, "Because of the limited area of the Earth covered by cities and because of the limited spread of aerosols from these cities, we estimate that the total aerosol loading is increasing by [roughly] 0.05 percent per year on a global scale . . ."
This doesn't mean that urban pollution itself is a trivial problem. The scientists emphasize that there may be serious environmental effects that are not related to the climate problem.
However, as far as that problem is concerned, they conclude: " . . . it will take [about], 1,400 years for the total aerosol burden of the atmosphere to double at present growth rates. Such a slow growth rate implies that anthropogenic aerosols have not been a serious climatic threat in the past nor will they be one in the immediate future."
These two studies are unlikely to be the last word on the "human volcano" or the possible threat of more variable weather. But they do show that alarms raised about such purported dangers have to be taken with reservation for there are sound scientific grounds for skepticism.