The Carbon Cycle and Atmospheric Chemistry

It was accepted in 1983 and is accepted today without question that atmo- spheric C 0 2 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 C 0 2 in the atmosphere. From a policy viewpoint this variation left much t o be desired. The lower value implied, a t 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 t o 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 t o when the global warming effects will clearly emerge from the general climate noise.

Another question related to this growth of C 0 2 in the atmosphere is how long does the increase, which is presumed t o be of anthropogenic origin, per- sist? 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 14C ages of the very bottom ocean waters. Since

then, with the advent of coupled atmosphere-ocean climate models, the du- ration has dropped t o fifty years although that is a n oversimplification. (The present picture is that of a sum of exponential decays with the fastest being of the order of fifty years.)

T h e originally stated long lifetime meant that, if the effects of an increase in C 0 2 were serious, they would 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 short period compared to this lifetime - say two or three hundred years - a small reduction in anthropogenic output, say 20%, for example, would have a negligible effect on the peak effects. However, with the short lifetime - short compared t o this period of total consumption of fossil fuel - the picture changes drastically. On one hand, any reasonable cutback in emissions 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 C 0 2 concentration of greenhouse gases. In fact, that was the number used in the first IPCC report. This calculated rise was composed of two approximately equal parts. One due to the estimated deglaciation and the other due to the thermal expansion of the upper ocean.

However, it appeared that the thermal expansion had not been correctly calculated. The Hamburg atmosphere-ocean coupled model first showed that the properly calculated thermal expansion was much less. This was largely due t o the fact that the thermal coefficient of expansion of sea water nearly vanishes a t O0 Celsius and 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. T h e deglaciation effect had been em- pirically estimated but the improving models all seem to yield increasing precipitation in the Antarctic and that implies increasing, rather than de- creasing, ice cap depth which, in turn of course, implies a decrease in ocean height. Satellite observations, taken so far, indicate increasing ice thickness and little change in sea level, of the order of one millimeter per year.

T h e early concern expressed by geologists and oceanographers related more t o 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 t o happen 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 t o 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 t o the atmosphere a t a rate commensurate with the other emissions, primarily due t o a rate of human destruction of the world's forests greater than natural restoration. Leaving aside some early highly exagger- ated rates of forest destruction, the Academy report accepted a rate of dis- appearance of forests equivalent to between 2 and 4 gigatons of carbon per year or about half the emissions due t o power generation. The Academy report specifically recognized this dilemma as not resolvable a t the time and a necessary subject for further investigation.

T h e status has not changed and, t o mix a metaphor, the carbon balance lysts 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 contradicts the earlier deductions of the syl- vanologists. 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.

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 greenhouse gases is small compared to the possibly 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 at the time was the aerosol. Little of numerical value was then available. Knowledge has developed considerably in the intervening ten years and it has become an active research area on many fronts. One of the strong incentives is the surmise among many workers in the field that the smaller change in aver- age surface temperature over the last one hundred years as determined from surface stations compared to the model predictions is due to the compensat- ing effect of aerosols. The reduction in the greenhouse effect would simply be due to the scattering of solar radiation by the aerosols. 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 depend not only on the density of the particulate matter but on the size of the particles. Over the years, it has not been possible to achieve a reasonable correlation with the observed temperature change.

To rationalize the observed temperature rise and its less than predicted value, other factors have been looked at. An obvious one is the possibility of variations in solar energy output. In the early stages, it was generally excluded as a possibility by most researchers. Statistical correlation of the temperature change with sunspot number, for example, revealed no connec- tion between the two and it was assumed that that settled the question. Yet a Danish group (Fris-Christensen and Lassen, 1991), using the same data,

but correlating the length of the sunspot cycle t o the hundred year tem- perature d a t a found a 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 a t the least puts both in doubt.

Somewhat earlier, Newell et al. (1989) published an analysis of ocean surface temperatures that showed no change over the one hundred year pe- riod 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 t o 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. T h e two most prominent ones were the CFC's (Chlorofluorocarbons) and methane. The origin of the CFC's was clear. They are man-made. They result from the use of Freon as a refrigerant, others as industrial cleaners and so on. T h e growing emissions are estimated t o account for 25% of the cur- rent greenhouse potential. This is considerable in view of the relatively small volume of these atmospheric components compared t o C 0 2 . This smallness is compensated by the fact that the C 0 2 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 t o the logarithm of the change cause of international agreements restricting their use to prevent their effect of thinning the protective stratospheric ozone layer. It remains t o be seen if the substitutes that are being introduced are more climatically benign. The situation with respect t o methane has become more confused. Until recently, the methane has been growing a t a steady exponential rate. This rate of growth has not only decreased but the actual curve has developed a definite concavity toward the time axis. The source of this atmospheric methane has always been hard t o 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 t o some for there always is a consideration of an instability due t o the worldwide presence of clathrates in the oceans and in the Arctic tun- dra (MacDonald, 1992). If warming temperatures penetrate deeply enough,

sufficient methane could be released a t a rate that would feedback positively through the greenhouse behavior of the released methane.

2 . Average Temperatures vs. Climatic Extremes

There has been a persistent difficulty in presenting the anatomy of the prkb- lem 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 environmen- tal 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 hap- pens in Bangladesh and so on. Nor are we talking about averages but about changes in the statistical distribution, the variance for example. In these and other matters, very little progress has been made in the modeling arena.

Very great efforts, in time, money, and manpower have been and are be- ing expended in developing general circulation models of the globe in many countries. Great ingenuity in programming and the most advanced comput- ing 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 increase. There are such proposals. One can hardly say, how- ever, 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 (Raval and Ramanathan, 1989, for example) that there would be a spectrum of regional climate effects.

In fact, the spread in predicted average temperature due t o an equiva- lent of doubling C 0 2 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 t o the differ- ent 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 efforts are being

made to deliberately compare the workings of the different models t o get a t counterpart. This has resulted in the improved understanding and value of the resident lifetime of anthropogenic C 0 2 in the atmosphere that was put forward earlier.

T h e major advances in our thinking relative to global warming in these last ten years have been largely made outside the model environment. They have been accomplished via field measurements, some of which have been very ingenious. Among them are the ice core work in the Antarctic (now also in Greenland), the measurement of aerosol concentrations (much more is needed), the direct measurement of the flow of COz into the ocean from the atmosphere, vertical temperature probes into the Canadian Shield (Lauchen- bruch and Marshall, 1986; Lewis, 1992) and the now continuing measurement of the surface temperature at the earth's surface (Spencer and Christy, 1990) via satellite observations.

T h e latter series of measurements now cover a slightly larger time span than 10 years t o the present. They show no change in average surface tem- perature over that period of time which is in contradiction to the behavior of the ground station averages. This difference has to be resolved. The mea- surements, in a sense, cost nothing. They are a byproduct of measurements on spectral emission lines of molecular oxygen. They have the virtue of being composed of a huge number of observations blanketing the entire surface of t h e e a r t h with virtually nogaps.

The results from the bore holes in the Canadian Shield present still another view on what may be happening. The authors used old and new vertical logging temperature data from abandoned oil prospecting holes t o calculate the surface temperature backwards in time for a few thousand years from its diffusive penetration into the earth's surface. The analysis is com- plicated but they do indeed observe a recent significant rise that, however, difference and this had been held to be still another model deficiency.

Included in this category of recent advances in understanding the set of phenomena are the aerosols and the changes in visualization of the carbon cycle that were mentioned earlier. These subjects are by no means closed large complex of instruments that eventually is t o be established in a variety of locations t o measure the radiation patterns under the normal variety of weather conditions. Again we must wait and be patient.

A totally different set of problems are now being examined that go t o the mathematical foundations of the equations that are fed into the computers.

These equations are composed of many independent variables connected in a highly non-linear environment. From the famous work of Lorenz on the similar equations for predicting weather we already know that there are problems with the solutions t o these equations. But we are also aware of the possible chaotic behavior of coupled non-linear equations. Just what that means in this application is not a t all clear. T h e worst scenario is that viable solutions may not exist.

W h a t is clear is that solutions often tend t o drift and artificial boundary conditions have t o be employed to stabilize the solutions. When the coupled ocean model was introduced, it was found necessary t o use altered values for the wind stress on the ocean surface t o obtain non-drifting solutions. As an aside, there is a very difficult problem t o overcome in that the time scales for the ocean are far longer than those for the atmosphere and, conversely, the length scales for the ocean are far shorter than those for the atmosphere.

A good boundary match is very difficult to achieve. Withal, the computer models achieve a surprisingly good picture of the earth's climate. But this method of "tuning" the models t o obtain today's climate is probably not adequate t o evaluate changes in climate due t o perturbations such as those due t o the anthropogenic introduction of greenhouse gases.

This extra detail on models is intended t o advance t o the next topic which is that of mitigation. To have a successful mitigation program it is necessary t o be assured with certainty that the program carries no harmful or unacceptable side effects. To put forward a general statement - although a mitigation procedure is an intended perturbation unlike that of global warming due t o CO;! emissions, it is, nevertheless, a perturbation that has t o be analyzed with the best analytic tools available, namely the GCM's.

However, from the preceding discussion, we can only be sure that we cannot foretell the regional climate consequences with any degree of reliability and, we may, in fact, unleash a whole series of "unintended consequences", t o intentionally borrow a term from the economists. This makes the entire concept of mitigation a precarious one.

This thesis can be made a bit clearer by examining a popular e x a b - ple, that of planting large expanses of trees sufficient in area t o take up the increase in C 0 2 via the excess of photosynthesis over respiration. It is feasi- ble and has certain additional attractiveness if the forests are systematically harvested for fuel, particularly in the form of methyl alcohol. However, there have been several analyses of this stratagem indicating that the change in albedo that would result would just about balance the resultant decrease in greenhouse forcing due to the uptake in C o n . This would appear t o be harmless enough except that the reforesting would be concentrated in

This thesis can be made a bit clearer by examining a popular e x a b - ple, that of planting large expanses of trees sufficient in area t o take up the increase in C 0 2 via the excess of photosynthesis over respiration. It is feasi- ble and has certain additional attractiveness if the forests are systematically harvested for fuel, particularly in the form of methyl alcohol. However, there have been several analyses of this stratagem indicating that the change in albedo that would result would just about balance the resultant decrease in greenhouse forcing due to the uptake in C o n . This would appear t o be harmless enough except that the reforesting would be concentrated in