[This article is extracted from a chapter of the same name from the book, Global Warming: The Science and the Politics, The Fraser Institute, forthcoming.]

Any argument for an expensive and potentially disruptive policy to reduce greenhouse-gas emissions based upon forecasts of global warming requires strong scientific backing. In the course of the last year, the requisite science largely disappeared. Yet, many nations—especially the European Union and the United States—continue to propose policies such as mandatory reductions in greenhouse-gas emissions that are increasingly hard to justify in light of what is now known about greenhouse science. This article details the scientific evolution of this issue.

Global temperature history based upon measurements from satellites that became operational in 1979 (figure 1) leaves little doubt that a dramatic warming of the atmosphere is not occurring. While these satellites do not measure ground temperatures, they faithfully reproduce global mean temperatures measured between 5,000 feet and 30,000 feet by weather balloons. This history shows a statistically significant net cooling when averaged over the 18.5 year period of record. As reported in the July 19, 1997, issue of New Scientist, "This is not what is predicted by the computerized climate models on which all estimates of global warming depend" (Pearce 1997). Yet on June 27, 1997, President William Clinton told the United Nations: "The science is clear and compelling: we humans are changing the global climate." Mr. Clinton is probably correct, but the changes are so small and of such a benign nature that they are insufficient to support any expensive or disruptive policy. If anything, they indicate that the best policy is probably to do nothing.

Climate change: an inevitable result of human activity

Ever since the first hominid cleared a patch of land, the climate has been changed. Only the scale of the effect has increased with technological evolution. Today, the earth's landscape bears the marks of a large-scale modern human civilization; so does its climate. Large, expansive cities, built with billions of tons of concrete, store the heat of the day, raising temperatures by several degrees. Downstream of these heat islands, thunderstorms become larger and more violent.

In areas that were once desert, oases now exist with water piped in from hundreds of miles to keep them green. This water eventually makes its way into the air where there was none before. Fully one-third of the small rise in sea level during the last century—glibly attributed to "global warming"—is actually a result of the "dewatering" of continents by man's activity (IPCC 1995). Huge areas of monocrop agriculture stretch across previously diverse regions, changing the colour of the land, changing the moisture balance, changing the regional climate, and feeding increasing billions of people an increasingly better diet.

These influences on the climate are mainly local, in most cases not extending far past the reaches of the underlying landscape change. Over the course of the last 150 years, however, human influences on the climate have increased greatly. Primarily through the burning of fossils fuels for energy, we have altered the chemical make-up of the earth's atmosphere. Carbon dioxide and other radiatively active gases, such as methane (from bovine flatulence and rice-paddy agriculture), and chlorofluorocarbon refrigerants, have increased to the point where they may have subtly changed the climate. The change has been inadvertent—a necessary result of human technological advancement.

These gases absorb infrared radiation emanating from the surface of the earth, which is warmed by solar heating. Since these gases redirect (re-radiate) this energy, an increasing fraction goes back down towards the surface of the earth as the concentrations of these gases increase. This phenomenon is called the greenhouse effect, a natural process that keeps the surface of the planet some 33oC warmer than it otherwise would be. Humans have only very slightly enhanced this effect.

The prime greenhouse gas in the atmosphere is water vapour, the concentration of which is determined by the planetary mean temperature. Over 30oC of the natural greenhouse effect results from water, and only about 1.5oC of the total of 33oC is caused by carbon dioxide, which is the primary greenhouse gas produced by human activity.

The scientific ferment about climate change has never been about whether human beings could (or would) change climate. That is an inevitable consequence of known physics. But, the more important questions—how much and in what fashion—is clearly a matter of great debate. The facts that have emerged in the last year, in spite of a public climate of fear, are very reassuring. How much? Relatively little. How? In a benign or perhaps even beneficial fashion.

Background

In 1990, the first scientific assessment of the United Nations' Intergovernmental Panel on Climate Change (IPCC) stated that "when the latest atmospheric models are run with the present atmospheric concentrations of greenhouse gases, their simulation of climate is generally realistic on large scales."

This statement created considerable disagreement in the scientific community. As later shown by Mitchell et al. (1995), the most popular type of climate model referred to in the IPCC's statement as producing simulations that were "generally realistic" predicted that, by now, the earth's temperature should have warmed between 1.3oC and 2.3oC (the larger figure refers to the northern hemisphere), as a result of changes in greenhouse gases caused by human activity.

Clearly this has not been the case. Figure 2 shows the surface temperature history of the northern hemisphere, calculated from land-based thermometers and published by the same United Nations panel. The warming in the record is about 0.6oC, or less than one-third what was predicted. At least half of that warming was prior to major changes in the greenhouse effect, i.e., before 1940. Everything else being equal, then, the maximum contribution of the enhanced greenhouse effect was 0.3oC.

The satellite temperature history from the southern hemisphere shows that the statistically significant decline in global temperatures is largely a result of this negative trend in the southern hemisphere.

As noted earlier, weather balloon temperature anomalies in the 5000 foot to 30,000 foot layer and satellite readings show a remarkable annual correspondence since the two records became concurrent in 1979. Through 1996, both showed no warming trend whatsoever. None of the three temperature histories (and they are the only three that exist), surface, satellite, or radiosondes in the analogous level to the satellite, show any net warming in the last decade.

While there has been no overall warming in these records, regional warming patterns are present. These patterns are particularly amplified when seasonal differences are examined. The difference in temperature trends (winter minus summer) in the satellite data shows that the warming has been predominantly occurring in the coldest air masses—over Siberia in the wintertime. The IPCC data on surface temperatures, which has sufficient coverage since the mid-1940s over most of the northern hemisphere, shows a similar pattern of relative (winter minus summer) warming over a large part of northern Eurasia and northwestern North America, which are the source regions for the coldest air masses.

Greenhouse theory is consistent with this observation, inasmuch as the water vapour and carbon dioxide behave similarly over a considerable portion of their absorption spectra, and these air masses are virtually devoid of water vapour. Small changes in the absorption in these wavelengths have a logarithmically decreasing effect on temperature as the carbon-dioxide concentration increases. In other words, the first increments of either water vapour or carbon dioxide to these very dry air masses result in substantial warming. In general, warm air masses contain much more moisture, so they are not as sensitive to changes in the concentration of carbon dioxide.

Observations, therefore, confirm that what little, if any, greenhouse warming has occurred, is characterized by the following:

• according to the satellites, warming is not "global," inasmuch as there is a slight (but statistically significant) negative trend
• according to ground-based thermometers and the satellites, the character of observed warming is a change in the coldest air masses.

These data undermine the IPCC's 1990 statement about the correspondence between observed climate and what was predicted by climate models.

Five years later, the IPCC (1995) produced its second assessment of climate change. It contains a remarkable sentence that I have seldom seen quoted in the mainstream media:

When increases in greenhouse gases only are taken into account . . . most [general circulatory models] produce a greater mean warming than has been observed to date, unless a lower climate sensitivity is used. . . . There is growing evidence that increases in sulphate aerosols are partially counteracting the [warming] due to increases in greenhouse gases."

The new consensus might be paraphrased as follows: either it is not going to warm up as much as earlier indicated, or something, like sulphate aerosols, is hiding the warming.

Why observed warming was less than predicted: the sulphate hypothesis

The sulphate explanation for the lack of warming is simple in principle. The finely divided sulphate particles directly reflect away solar radiation and also serve to brighten existing clouds. They do not reside in the atmosphere for long, so their cooling is primarily in the northern hemisphere, where almost all are produced by the combustion of fossil fuel, which contains small amounts of sulphur.

Recently, however, the sulphate hypothesis has been challenged. Hansen et al. (1995) reported that the direct cooling effect of sulphates was too small to account for the lack of observed warming. Further, the magnitude of the indirect cooling (from cloud brightening) is highly speculative, as Hansen et al. (1997) argued that a "semi direct" heating of the lower atmosphere by aerosol absorption may minimize the indirect cloud effect. Most recently, Hobbs (1997) reported that air samples from the eastern United States showed a predominance of soot (carbon), which should have created a net warming, rather than sulphates, which should have caused cooling.

Further, the internal inconsistency of the attempts to explain the lack of planetary warming leads inexorably to the default argument: the planetary surface temperature is simply not as responsive to small changes in the natural greenhouse effect as it was once thought (modeled) to be.

Without sulphate aerosols, modelling results indicate that the planetary mean temperature should have risen between 1.3oC and 2.3oC as a result of the greenhouse enhancement (Mitchell 1995). The observed surface rise of 0.6oC in the last 100 years and the lack of any warming observed by the satellites demonstrate quite decidedly the lack of connection between observations and forecasts.

It is generally assumed, based upon a series of empirical arguments, that the sensitivity of the planetary surface to greenhouse changes is about one degree (C) for every one watt change in the radiation affecting the surface. But the United Nations 1995 statement opens the possibility that this is wrong when it says that climate models for changes in the greenhouse effect tend to produce too much warming "unless a lower climate sensitivity is used."

Assuming again that half of the observed warming (0.3oC) is from greenhouse changes, either the sensitivity to greenhouse changes is much lower than current estimates, or the thermal lag of the ocean is enormously long; i.e. the 2.25 degrees of warming would take hundreds of years to appear. If that is the case, any policy attempt to mitigate or prevent global warming is simply futile.

Schlesinger and Jiang (1991) and others have argued that response times for ocean heating are fairly short, on the order of 40 to 60 years. The relatively small observed warming that can be ascribed to greenhouse changes would then yield a lower sensitivity. The implication is that, unless sulphate aerosols are exerting a much greater effect than is now thought, the sensitivity is about 25 percent of the previously accepted value. This would reduce warming projections for doubled concentrations of atmospheric carbon dioxide from a mean ultimate value of nearly 4oC down to 1oC, assuming a continuation of small sulphate cooling.

Figure 3 gives the results of a model produced by the National Center for Atmospheric Research both as published, and as adjusted for the more realistic greenhouse changes. It is noteworthy that the result is achieved without sulphate cooling. Clearly the implied sensitivity is much less than previously assumed—approximately 25 percent of the previously accepted value, in fact.

Why observed warming was less than predicted: the solar hypothesis

Recently, researchers have found some rather striking correlations between the temperature history of northern hemisphere during the twentieth century and the history of solar activity during the same period. Lean et al. (1995) estimate that approximately one-half of the warming of the last 130 years was a result of solar changes. In particular, much of the warming of the early twentieth century—prior to the greenhouse enhancement—appears to be a product of the sun.

The solar argument also favours the concept of reduced sensitivity to greenhouse gases produced by human activity. It leaves the same 0.3oC of warming in the last 100 years as a residual, possibly to be explained by greenhouse changes. This gives the same low implied sensitivity and reduced estimates of overall warming that are consistent with the lowest forecasts now being generated by general circulatory models with realistic greenhouse changes.

Conclusion

No doubt attempts will continue to be made to explain the lack of the planet's warming with some compensating emission like sulphate aerosol. In fact, it is also likely that there will be considerable resistance to the alternative explanation—that the atmosphere's sensitivity was simply overestimated. Nonetheless, the balance of evidence suggests there is, at best, a very small human influence on global climate. President Clinton was correct when he stated that humans change the climate. But the fact that the changes are very small, primarily in the coldest air, and likely to remain small, spells the end of the greenhouse scare—at least in a world controlled by reason.

References

Hansen J.E., M. Sato, and R. Ruedy (1995). Long-term changes of diurnal temperature cycle: implications about mechanisms of global climate change. Atmos. Res. 37: 175–209.

_____ (1997). Radiative forcing and climate response. J. Geophy. Res. 102: 6831–64.

Hobbs, P.V. (1997). Interviewed in London Sunday Times, June 8, 1997.

Intergovernmental Panel on Climate Change (1990). The IPCC Scientific Assessment. Cambridge: IPCC.

Lean, J., Beer, J., and R. Bradley (1995). Reconstruction of solar irradiance since 1610: implications for climate change. Geo. Res. Lett. 22: 3195–98.

Mitchell, J.F.B., et. al. (1995). "On surface temperature, greenhouse gases, and aerosols: models and observations." Journal of Climate 8: 2364–85.

Pearce, F. (1997). "Greenhouse wars." New Scientist 139: 38–44.

Schlesinger, M., and Jiang (1991). Revised projection of future greenhouse warming. Nature 350: 219–21.