LOOKING BACK TEN YEARS

William A. Nierenberg

IIASA, Vienna
October 13, 1993

ABSTRACT -- This paper is (phrase deleted)an update of the science of global warming. The point of departure is 1983 with the publication of the National Research Council/National Academy of Sciences report Changing Climate. The present author was chairman of the report committee and considers this an appropriate time for an update. The advances (or retreats) in the overall science are reviewed with special emphasis on those that have critical policy implications.

It was accepted in 1983 and is accepted today without question that atmospheric CO2 is rising steadily and exponentially since the beginning of the modern industrial era. However, the direct measurement of the rise has only been made with any precision for about forty years. As of ten years ago, the guesses for the value of the concentration about the year 1890 ranged between 260 and 290 parts per million of CO2 in the atmosphere. From a policy viewpoint this variation left much to be desired. The lower value implied, at the time, that the global warming should have been observed, the upper value implied that the rate of growth was too slow for any effects to be seen above the noise. Since that time, and as predicted, the analysis of gas bubbles in the ice cores of the Antarctic have fixed this 1890 value with greater precision but we are still left uncertain as to when there will be a consensus that global warming effects have emerged from the general climate noise.

Another question related to this growth of CO2 in the atmosphere is how long does the increase, which is presumed to be of anthropogenic origin, persist? As of 1983, the literature (not the NAS report) cited long lifetimes of the order of one thousand years, derived from various lines of reasoning, among them being tritium isotope variation with depth but undoubtedly influenced by the measured "C ages of the very bottom ocean waters. Since then, with the advent of coupled atmosphere-ocean climate models, the duration has dropped to between fifty and two hundred years although that statement is an oversimplification. (The present picture is that of a sum of exponential decays with the fastest being of the order of fifty years.)

The originally stated long lisctime meant that, if the effects of an increase in CO2 were serious, they would be remain for a long time. They would be permanent for all practical considerations. If it is assumed that the available fossil carbon fuel would be consumed in a period short compared to this lifetime say two or three hundred years -- a small reduction in anthropogenic output, say twenty per cent, for example, would have a negligible effect on the peak effects. With the short lifetime, however, short compared to this period of total consumption of fossil fuel, the picture changes drastically. On one hand, any reasonable cutback in cmissions would show correspondingly reasonable beneficial effects. On the other, there is reduced urgency for drastic action for if, in fact, adverse climate changes appear, action could be taken in the full expectation that the correction would also appear in reasonable time.

Another serious question whose implications were widely discussed and analyzed ten years ago was that of sea level rise. Putting aside some grossly exaggerated predicted changes that were as high as twenty-five feet, the Academy report settled on a rise of two feet, that is 60 centimeters, as a result of an equivalent doubling of CO2 concentration of greenhouse gases. The IPCC report used this Academy number. This calculated rise was composed of two approximately equal parts. That due to the estimated deglaciation and that due to the thermal expansion of the upper ocean.

1 Changing Climate, US National Academy of Sciences, 1983

However, it appeared that the thermal expansion had not been correctly calculated in the Academy report. The Hamburg atmosphere-ocean coupled model first showed that the properly calculated thermal expansion was much less. In their model this was largely due to the fact that, since the thermal coefficient of expansion of sea water nearly vanishes at 0° Celsius, this near zero number is the correct one to use rather than the average ocean temperature because the polar waters are the primary sinks for the excess heat. The deglaciation effect had been empirically estimated but the improving models all seem to yield increasing precipitation in the Antarctic and that implies increasing, rather than decreasing, ice cap depth which, in turn of course, implies a decrease in occan height. (sentence deleted)

The early concern expressed by geologists and oceanographers related more to the possible melting and disappearance of the Western Antarctic Ice Sheet (WAIS). This would result in an increase in sea level of the order of twenty feet which would severely impact highly developed coastal areas. The principal concern was derived not just from geological deductions that the sheet had disappeared in relatively recent geological times but more that these disappearances may have taken place during a relatively short period of a few hundred years or even less. This possibility was carefully examined in the Academy report with the conclusion that it was unlikely to happen in the next hundred years but it has not been ruled out as a possibility in the longer term.

In the early analyses, like the Academy's, one accepted the expressed view of the geochemists that the disposition of the man made CO2 was well understood, namely that there was a complete balance between the amount of carbon emitted by power plants and other human sources, the fraction that remained in the atmosphere and the amount that went into the oceans. The geochemists believed that the carbon reservoirs represented by the forests, the humus and the inorganic carbon in the oceans changed very slowly compared to the flux generated by man despite the fact that the rate of exchange between the oceans and the atmosphere as well as the exchanges between the forests and the atmosphere were about fifteen times the man made emissions. They were literally dealing with small differences between large numbers.

To complicate matters, from the very beginning many sylvanologists believed that the forests were not constant reservoirs of carbon but were yielding carbon to the atmosphere at a rate commensurate with the other emissions, primarily due to a rate of human destruction of the world's forests greater than natural restoration. Leaving aside some early highly exaggerated rates of forest destruction, the Academy report accepted a rate of disappearance of forests equivalent to between 2 and 4 gigatons of carbon per year or about half the cmissions duc to power generation. The Academy report specifically recognized this dilemma as not resolvable at the time and a necessary subject for further investigation.

While the large divergences are no longer an issue there still are problems and, to mix a metaphor, the carbon balance estimate is in a state of flux and perhaps may have been reversed. The situation has since changed to being described as that of the "missing carbon". The results of direct measurement of the flow of CO2 from the atmosphere into the upper ocean showed a much reduced flow than that needed to add to the airborne fraction to balance the man made input. The original report was quite extreme, proposing that the flow of carbon into the ocean was reduced by two thirds to 1 gigatons per year. Other analysts have since suggested a somewhat lesser reduction but it still represents a substantial amount of "missing" carbon. The complete analysis strongly suggests that the carbon is going into land areas in the northern hemisphere -- presumably the forests. This is in contradiction to all our intuition. Also, and subsequently, papers have been published establishing a 2 gigaton per year growth of forests in western Europe which could help reestablish the balance. Resolving this series of questions is important for policy implications for the manipulation of forests is a frequently proposed mitigation procedure.

2 P. Tans, Takahashi and Fung, Observational Constraints on the Global Atmospheric CO2, Science, 247:1432-1438, Mar. 23, 1990

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It was understood from the very beginning of analysis of global warming, that by far the largest contribution to the warming of the earth by the interaction of solar radiation with the atmosphere was due to the water in the atmosphere. The direct effect of anthropogenic greenhouse gases is small compared to the possible induced feedback on the generation of clouds, their specific physical characteristics such as height and thickness and the change in the water vapor content in the atmosphere. The lack of solid scientific knowledge on the phenomena involved was the most serious issue blocking progress in developing useful models and unfortunately the situation remains the same today. There is wide disagreement among the experts on the feedback, not only the magnitude but even the sign.

Another contributing atmospheric constituent that was noted in the Academy report was the presence of aerosols. Little of numerical value was then available. Knowledge has developed considerably in the intervening ten years and it is has become an active research arca on many fronts. One of the strong incentives is the surmise among many workers in the field that the smaller change in average surface temperature over the last one hundred years as determined from surface stations compared to the model predictions is due to the compensating effect of aerosols. The reduction in the greenhouse effect would simply be due to the scattering of solar radiation by the acrosols. The analysis is complex because a fraction of the aerosol content is anthropogenic, particles from power plants, for example, and some is generated biologically over the oceans. The effects depends not only on the density of the particulate matter but on the size of the particles. It is further complicated by the geographical inhomogeneity of the aerosols. Over the years it has not been possible to achieve a reasonable correlation with the shape of the observed temperature change.

To rationalize the observed temperature rise and its less than predicted value other factors than acrosols have been looked at. An obvious one is the possibility of variations in solar energy output. (sentence removed) Statistical correlation of the temperature change with sunspot number (phrase removed) revealed no connection between the two and it was assumed that that settled the question. Yet a Danish group, using the same data, but correlating the length of the sunspot cycle to the hundred year temperature data found remarkable agreement. This result has been severely attacked, mostly because the agreement is too good! In any event, the sharp divergence of the results of the two methods at the least puts both in doubt.

Somewhat carlier, Newell published an analysis of ocean surface temperatures that showed no change over the one hundred year period and, as a byproduct, established a small solar effect.

To gain further insight into the possibility of solar variations being a factor, a group of solar astronomers have been studying a number of stars similar to the sun and observe changes over time periods commensurate with the ones associated with the greenhouse climate phenomenon.

The Academy report recognized the importance of other greenhouse gases. The two most prominent ones were the CFC's (Chlorofluorocarbons) and methane. The origin of the CFC's was clear. They arc man made. They result from the use of Freon as a refrigerant, others as industrial cleaners and so on. The growing emissions are estimated to account for 25 per cent of the current greenhouse potential. This is considerable in view of the relatively small volume of these atmospheric components compared to CO2. This smallness is compensated by the fact that the CO2 fraction in the atmosphere is so great that there is a saturation effect. The result is that the induced change in greenhouse potential is proportional only to the logarithm of the change in CO2 concentration. On the other hand, the corresponding change due to the change in the CFC's is not only linear but is enhanced due the unusually high absorption activity in the infrared at the molecular level for some of the species.

3 Fris-Christensen, E. and Lassen, K. The Length of the Solar Cyclc:An Indicator of Solar Activity Closely Associated With Climate. Science 254:698-700 1 Nov 1991.

4 Newell, N.E., Hsuing, J. and Zhongxiang, W., 1989, Global Marine Temperature and the Solar Magnetic Cycle, Geophys. Research Letters 16:311-314

The CFC's may disappear as a global warming problem, however, because of international agreements restricting their use to suppress their effect of thinning the protective stratospheric ozone layer. It remains to be seen if the substitutes that are being introduced are more climatically benign. The situation with respect to methane has become more confused. Until recently the methane has been growing at a steady exponential rate. This rate of growth has not only decreased but the actual curve has developed a definite concavity towards the time axis. The source of this atmospheric methane has always been hard to pin down. Rice fields, cattle, termites and the like have been proposed as primary sources but the numbers have never been clear and even less so with the new functional behavior. This dip in rate is not comforting to some for there always is a consideration of an instability due to the worldwide presence of clathrates, in the oceans and in the Arctic tundra. If warming temperatures penetrate deeply enough, sufficient methane could be released at a rate that would feedback positively through the greenhouse behavior of the released methane.

There has been a persistent difficulty in presenting the anatomy of the problem of global climate change and that has been the widely used surrogate measure -- the average global surface temperature. Taken by itself, a small change in this quantity has very little meaning as far as climate change goes. What is relevant are changes in the statistics of regional environmental phenomena such as periods of sustained droughts as has just occurred in California, the statistics of the frequency and intensity of hurricanes in Florida, for example, the frequency and severity of storm surges, as happens in Bangladesh and so on. Nor are we talking about averages but about changes in the statistical distribution, the variance or autocorrelation, for example. In these and other matters there has been very little progress made in the modelling arena. Very great efforts, in time, money and manpower have been and are being expended in developing general circulation models of the globe in many countries. Great ingenuity in programming and use of the most advanced computing power is being used but little progress has been achieved in assessing the effect of the growth of greenhouse gases on predicting the behavior of regional climate or even predicting today's precipitation.

To emphasize the surrogate nature of expressing climate change in terms of changes in average global temperature we can visualize a model where the increased greenhouse forcing has such a strong negative feedback that increased cloudiness results in a change in albedo that just cancels a possible temperature incrcase. There are such proposals. One can hardly say, however, that there has been no climate change. Just the increased cloudiness is a clear climate change and, since it most likely will not be globally uniform, some have indicated, in effect, that there would be a spectrum of regional climate effects.

In fact, the spread in predicted average temperature due to an equivalent of doubling CO2 concentration in the atmosphere as calculated by the models ten years ago was approximately between 1 and 4 degrees centigrade. This was what was reported in the first IPCC report and the spread has not significantly decreased. This discrepancy is generally assigned to the different approaches in dealing with the water in the atmosphere. This may or may not be the only source of the discrepancy but the situation remains fairly much the same today. One result is that massive essorts are being made to deliberately compare the workings of the different models to get at the root cause of this behavior.

The generally recognized deficiency was the lack of an equivalent dynamic ocean model coupled to the atmospheric one. There has been some success in this area in that many of the models now incorporate a coupled ocean model although the occan part is not as advanced as its atmospheric counterpart. This has resulted in the improved understanding and value of the resident lifetime of anthropogenic CO2 in the atmosphere that was put forward earlier.

5 Clathrates, MacDonald, Gordon J., Enc. Earth Science Systems, Academic Press, 1:476-484
6 Raval, A. and Ramanathan, V. Observational Determination of the Greehouse Essect. Nature
342:758 14 Dec 1989, for example.