Carbon dioxide increase threat to world climate?

Press, 14 April 1982, Page 10

Carbon dioxide increase threat to world climate?

From the "Economist.” London , .One group .will be delighted if lower oil prices discourage the shift to coal, synthetic fuels and such unconventional :energy sources as shale oil, A growing number of scientists are worried that a build-up of carbon dioxide in the atmosphere could, drastically alter the earth’s climate. CO2 emissions come primarily from the burning of fossil fuels. As it happens, synthetic fuels and the like are particularly CO2-dirty. The retorting and 'burning of shale oil is reckoned to release IVz to 5M> times more CO2 than the burning of conventional oil for an equal amount of ergyThere is a snag however. Economic and strategic good sense argues in favour of carrying on with synthetic fuel development and the like. Political realism discourages any early, big switch to (CO2-free) nuclear energy as an alternative. Before climatologists can hope to convince governments in oil-poor countries to change course, they will have to produce hard evidence that the CO2 scare is real. The scientists think they can. They are not at all sure they can do so before it is too’late to do much about the problem. Some already think a CO2-driven climate change "is likely to be the most fascinating global geophysical experiment that man will ever conduct." The basic reason for the fuss over CO2 is simple. Once it gets into the atmosphere, the gas can persist for centuries. Once there, it changes the earth’s heat balance. While transparent to incoming solar radiation. CO2 effectively blocks some infrared radiation that normally returns heat from earth to space. The result is higher temperatures — the so called "greenhouse effect.” Something of a consensus is emerging that a doubling of the amount of CO2 in the

trations for the early decades of the last 100 years atmosphere could lead to a rise in the mean global temperature on the order of 1 to 44deg. That may not sound dramatic, but its consequences would be. The global averages mask big regional changes: e.g,, shifts in climatic belts, that could wipe out America’s bountiful bread basket. Of course, any doubling of atmospheric CO2 concentrations will-not happen overnight. Nor does the earth’s average surface temperature respond instantly to an increase in CO2 concentrations. The huge capacity of the earth’s oceans to absorb heat introduces lags — perhaps on the order of a decade or two. ' There is lots of room for argument about scenarios for the future: shortcomings in the climate models used, uncertainties about where all the CO2 released by fossilfuel burning actually goes (historically only about half seems to have ended up in the atmosphere and climatologists tend simply to extrapolate this past trend into the future), and so on. So why not look at the past instead? Although estimates of atmospheric CO2 concen-

are somewhat shaky, it is generally reckoned that the rise from 1880 to 1980 was about 14 per cent. Admittedly. much of that rise occurred relatively recently — over a quarter in the 1970 s alone. Still, if the greenhouse models are correct, you would expect to see some impact by now. Awkwardly, for the greenhouse theorists, although there was a global warming between 1880 and 1940, thereafter temperatures fell (especially in the Northern Hemisphere) until the early 19705. The dip can be explained as a temporary aberration caused by sun-shield-ing dust from volcanic eruptions and (perhaps) some variation in the sun’s own energy output. There is general agreement that temperatures are rising again. Over the century as a whole, the global mean temperature is reckoned to have risen by just under 4deg. That is consistent with the greenhouse effect. It is not, in itself, very impressive. So the search is on for more convincing evidence of a "signal” of a CO2-induced climatic change. Two studies, both American, have caused excitement. In the first, researchers at the LamontDoherty Geological Observatory used satellite data to track the extent of polar pack ice. The data showed that the extent of the Antarctic ice in summer months decreased by 2.5 million square kilometres between 1973 and 1980. The question is how much significance can be attached

to this. The answer is not much. Average ice conditions in 1973-1980 seem to have been lighter than earlier in the century, in the 19305, but the earlier data came only from observations by ships, which cannot be considered very reliable. There was no similar decline in Arctic ice in the 19705. The second study, done by researchers at NASA's Goddard Institute for Space Studies, looked not only at CO2 but also at other trace gases capable of absorbing infrared radiation from earth (chloro-fluorocarbons, methane and nitrous oxide). Because emissions of these other trace gases have become important (and have begun to be monitored) only recently, the team looked at data only for the 19705. Calculating the temperature rise to be expected 'from the combinations of CO2 and the other trace gases (which could enhance the greenhouse effect by 50 to 100 per cent) and allowing for the buffering effect of the world’s oceans' the team reckoned the mean global temperature should have risen by O.ldeg. or so in the 10 years to 1980. Which recent analyses suggest that it did. A clear signal? Not at all. As the team itself points out, normal fluctuations in global mean temperature are on the order of O.ldeg for decadal time scales. But the team’s calculations predict a further warming of 0.2 to o.3deg. in the 1980 s. If that comes to pass, it will not be dismissed so easily.