(Discover Magazine, October 1989).
This is now also recognized by James Hansen from NASA’s Goddard Institute.
In 1988 he initiated the greenhouse warming doomsday scenario with his
testimony to the U.S. Senate Committee on Energy and Natural Resources,
but in 1997 he and 42 co-authors stated that
(Journ. Geoph. Res. 102, D.22, 25,679-25,720, November
1997).
However, particularly among geologists, even more scepticism exists about the indictment of atmospheric carbon dioxide as a dominant climate forcing factor. For geologists, climate is a manifestation of how the incoming energy from the Sun is absorbed, redistributed by air and oceans, and eventually re-radiated to space as long-wave radiation. The outcome is determined by many interacting and in part interrelated and interdependent processes, involving the incoming solar radiation, the atmosphere (including the warming by greenhouse gases), the hydrosphere (particularly the oceans), the biosphere, the surface of the crust, and soils on land. The geologic record shows that all components of this system are in constant motion and continual change. Continuous climate change is thus a normal phenomenon in Earth’s history. Nevertheless, in spite of the steadily increasing heat output from the Sun, Earth managed to maintain air temperatures within a narrow range throughout the four billion years of recorded geologic history.
Of course, the greenhouse gases in the atmosphere, including carbon dioxide, do play a role in the climate system. They trap infrared radiation from the ground, thus keeping the surface air warmer than it would otherwise be. This natural ‘greenhouse blanket’ has been in operation for billions of years and is an important factor in making our planet a fit place for life.
But are changes in the concentration of carbon dioxide really such a major factor in bringing about climatic change? Does there exist a simple cause-and-effect relationship between changes in the atmospheric concentration of carbon dioxide and global temperature change? Does the enhanced greenhouse effect due to man’s injection of carbon dioxide warrants the alarm for an imminent calamitous climatic warming? Evidence from the historical and geological record does not lend support to such a governing role of atmospheric carbon dioxide as a climate forcing factor.
The concentration of carbon dioxide in the atmosphere is determined by a global system of supply and extraction. On the one side there is a continuous addition through degassing of the solid Earth, mainly by volcanoes and mid-ocean ridges, while on the other side there is a continuous removal through biological, chemical and geological processes. The volatiles exhaled by volcanoes include about 80 percent water, 20 percent carbon dioxide, and minor amounts of a wide variety of other components. The emanated water vapour condenses at the saturation point and adds to the oceans, but most of the exhaled carbon dioxide would accumulate if it were not continuously pulled from the air by three natural ‘pumps’. Two of these, the ‘carbonation pump’ (chemical weathering enhanced by bacterial activity) and the ‘carbonate pump’ (biological precipitation of carbonate in the oceans) lead eventually to the storage of carbon dioxide in enormous deposits of limestone and other carbonate rocks.
The third, the ‘biomass pump’ (photosynthesis, followed by the burial of organic matter in sediments), produces deposits of fossil hydrocarbons, coal, peat and dispersed biogenic carbon in sediments. If all that carbon had not been removed, the carbon dioxide pressure at Earth’s surface today would amount to about 70 bars, some 200,000 times the present value. Our atmosphere would then resemble very much that of Venus – both planets posess a similar amount of carbon dioxide on or near the surface, but whereas on Venus it is a constituent of the atmosphere, on Earth most of it is stored in sediments. Still, all carbon in sediments, biosphere, atmosphere and oceans is only a tiny fraction of the total amount of carbon in the Earth. Most of it occurs in core and mantle – from this reservoir stems the juvenile carbon dioxide in volcanic exhalations.
Over the last century there also is a substantial anthropogenic input through industrial activities and changes in land use. Since 1958, when continuous monitoring started, the atmospheric concentration of carbon dioxide has grown from 315 ppmv to the present 368 ppmv, a rise largely attributed to man-made emissions. As earlier air analyses are subject to uncertainty, air bubbles trapped in glacier ice that formed when snow fell on Antarctica and Greenland are usually taken as the best estimates of the concentrations of carbon dioxide and other greenhouse gases before 1958. They indicate that the atmospheric carbon dioxide level has been rising from a pre-industrial level of 280 ppmv. The increase was slow until about 1900, but has accelerated dramatically during the twentieth century. Going further back in time, the bubbles indicate a growth since the last glacial minimum, some 15,000 years ago, from an initial level of about 190 ppmv to the pre-industrial level of 280 ppmv. The growth was not steadily, but with oscillations: at times it leveled off or was even negative, while at other times it accelerated.
The increase from 280 to 358 ppmv in 1994 relates to an addition of 607 gigatons CO2, but the cumulative release from fossil fuel burning, cement manufacturing and the oxidation of organic matter exposed by the tilting of soils and deforestation has been estimated by IPCC at about 1,166 gigatons. Natural processes have thus already removed nearly half of this cumulated input through the absorption by ocean water, living organisms (mostly forests in the boreal and temperate zones and plankton in the oceans), and one or more unidentified sinks. Though it has so far been assumed that most of the excess atmospheric CO2 would be removed over about a century, the biological-geological control system is – at a higher CO2 level causing higher sink flows – able to respond even quite faster in counteracting a rise in atmospheric carbon dioxide. Within 30 years about 40-60% of the CO2 currently released to the atmosphere is removed (IPCC, 1995).
A crucial question is: do the air bubbles in glacier ice really always reflect the atmospheric composition during precipitation? There is reason to doubt this. Some investigators have argued that a selective uptake of carbon dioxide in the crystal lattice may cause fractionation of the gases in the bubbles, leading with time in an unpredictable way to a change of the carbon dioxide concentration. This finds support in the fact that the carbon dioxide values obtained when the whole ice sample is vaporized, are usually higher than those from the air that is liberated when the ice is grinded or abrased – the conventional practice in analysing the air bubbles. Also, studies in the palaeobotanical laboratory of Utrecht University show that the stomatal indices of leaves grown between 11,500 and 10,500 years ago, at the end of the last glaciation, indicate atmospheric carbon dioxide concentrations considerably higher than the values obtained from bubbles in ice.
Nevertheless, there can be little doubt that considerable fluctuations in the atmospheric carbon dioxide concentration did occur in the time interval covered by the ice cores. In addition, the oxygen isotope composition of the ice reflects fluctuations in the global temperature during the precipitation of the snow that in time conversed to the glacial ice. The fluctuations in the carbon dioxide concentration appear to track those in temperature to a remarkable degree, but a closer look reveals that the fluctuations generally leg behind those in temperature. Never does a changing carbon dioxide concentration precede that of temperature. This pattern can be explained by release of carbon dioxide from the oceans during warming and solution during cooling. A changing carbon dioxide concentration was thus not the cause of the change in temperature, but could have amplified it in a positive feedback.
For the last century, the available temperature data indicate a global warming of about 0.5 degrees centigrade, but this rise is not undisputed. It is deduced from measurements by a network of ground-based stations, where urban and industrial heating contaminates the record, while data are lacking from many remote areas, including the oceans. Although care is taken to overcome these disturbing factors, it remains speculative to derive from this temperature record something like the ‘mean global temperature’. Moreover, a warming trend of the lower atmosphere seems to be at odds with measurements since 1979 by orbiting satellites of microwave emissions from oxygen molecules, which allows the determination of their temperature to an accuracy of 0.01 degree centigrade. These data are confirmed by measurements from weather balloons and do not show a warming trend, rather a slight cooling.
Still, retreating glaciers, shifting latitudinal vegetation zones and other circumstantial evidence do point to an increase in global temperatures over the last century. Ardent greenhouse advocates insist that this is clear evidence of the enhanced greenhouse warming due to the steadily rising concentration of atmospheric greenhouse gases, particularly carbon dioxide. However, a warming of the order of 0.5 degrees lies well within the observed natural short-term variability in the past, but far beneath the enhanced greenhouse warming that, according to the climate models, the added carbon dioxide should have invoked. Moreover, much of the warming was already realised before 1940, when only 30 percent of the increase in atmospheric carbon dioxide over the last two centuries has taken place. Finally, the warming has been far from steady. Between 1940 and 1970 there even was a transient cooling, which led some climatologists in the 1970s to claims that an ice age is imminent. This period of cooling is difficult to reconcile with the role attributed to carbon dioxide as the dominant climate forcing factor, as there was not any interruption in the ever-rising level of carbon dioxide. Nor was there an increase in volcanic activity that could have caused a transient cooling by putting a veil of dust into the stratosphere.
Going back in time, the last millennia are characterised by a succession of cool and warm periods, until the steep drop when approaching the end of the last Ice Age about 10,000 years ago. A fairly detailed picture of annual and decade-to-decade variations in global temperature is emerging from studies of historical records, ice cores, tree rings, and growth rings of corals. There were several long cold spells, for example the famous ‘Little Ice Age’ in the seventeenth century. A well-known warm spell was the ‘Medieval Warm Period’, which lured Vikings into colonising Greenland. None of these climatic changes shows a clear correlation with changes in atmospheric carbon dioxide.
During most of the four billion years of recorded Earth’s history, warmer conditions than today prevailed, with no or only small polar ice caps. These overall warm conditions were interrupted by five cold spells, each lasting millions of years, during which the polar ice caps expanded. For the last three million years or so Earth has been in such a cold spell, but the extent of the polar ice caps varies through time: the ice sheets advance down to the lower latitudes during glaciations, and retreat again during interglacial stages. The rhythmic fluctuations appear to be governed by periodic changes over time scales of thousands of years in the orbit of the Earth around the Sun, the tilt of Earth’s axis, and the wobble of Earth’s spin axis. This cyclic astronomical climate forcing is operating all over Earth’s history – it is not only reflected in the advances and retreats of the polar ice sheets, but also in sedimentary sequences throughout the whole of geologic time.
When penetrating deeper into the past the geological record reveals numerous fluctuations in both global temperature and atmospheric carbon dioxide, sometimes correlated and sometimes not. For example, in the Middle Pliocene, 3.0-3.5 million years ago, when the Earth’s surface did not much differ from today’s, it was globally much warmer than at present. Still, the stomata of fossil leaves reveal that the carbon dioxide level at that time was about the same as now, whereas, according to the climate models, the level should have been at least twice that of today if this gas would be the prime arbiter of global temperature.
For the Middle Cretaceous, about 100-120 million years ago, the sedimentary record indicates that the air contained some eight times as much carbon dioxide as today, probably due to the enormous volcanic activity in the Pacific region. Because of the outflow of massive volumes of basaltic lava at the Pacific Ocean floor the sea level was higher and the dry land area smaller than today. The land surface had little relief, so there was little removal of carbon dioxide through weathering. On the other hand, the enormous limestone deposits from this period testify that the biological ‘carbonate pump’ was working at full capacity. The sedimentary record indicates a warmer climate than today, but much less than the climate models dictate for such a high carbon dioxide pressure. The warmer climate can to some extent be attributed to Earth’s lower albedo because of the greater area covered by sea. Oxygen isotopes show that the tropics were about six to seven degrees centigrade cooler than today, while polar temperatures were about 13 degrees colder than the tropics, less than half the modern difference. The Mid Cretaceous was thus primarily characterised by a rather flat latitudinal temperature gradient, probably because the low relief of the continents allowed an unimpeded heat transfer from the tropics to the poles.
Further back in time sediments from the Late Ordovician show that 450 million years ago the atmosphere contained even some 16 times more carbon dioxide than today. Still, this was not accompanied by unusual warm temperatures. On the contrary, at higher latitudes there was widespread glaciation. The average global temperature did not much differ from today’s.
There are, of course, many environmental differences between the geological past and the present, the more so when going further back in time. Nevertheless, the geological record clearly shows that the climate models are not able to reproduce past climates realistically, if carbon dioxide is taken as a major factor in climate regulation. This casts doubt on the assumption that changes in atmospheric carbon dioxide should be a basic driver of global temperature change. Obviously, the temperature effect of such a change is arrested by a host of competing feedbacks.
If not carbon dioxide, what other natural factors may have enforced
the climatic changes in the past? On time-scales of millions of years the
surface expression of plate-tectonic processes must have been important,
but these cannot play a significant role in forcing climatic changes over
decades or centuries. Among earth scientists it is a long-held suspicion
that changes in the behaviour of the Sun are in some way responsible. For
example, historical records report that the coldest epoch in the ‘Little
Ice Age’ coincided with a prolonged period when sunspots were almost absent.
The global cooling between 1940 and 1970 also coincided with a period of
low sunspot activity. Further back in time, a relation between the sunspot
cycle and climate change is shown by high-resolution sedimentary sequences
such as those covering the last 13,000 years in the maars of the Eifel
– they display a rhythmic climatic fluctuation with an average period of
eleven years.
A relationship between solar activity and Earth’s climate is also
apparent from isotope studies of tree rings and ice core samples. Tree
rings high in carbon-14 grew and snow high in beryllium-10 precipitated
when the climate was cool, and vice versa. Both radioactive isotopes are
produced by cosmic rays entering the Earth’s atmosphere. During periods
of increased sunspot activity, the magnetic fields in the solar wind have
a larger shielding effect on cosmic rays, preventing them from entering
and thus causing a reduced cloud coverage. Carbon-14 is taken up in growing
trees and beryllium-10 in precipitating snow. Their abundances in tree
rings and ice cores, respectively, reflect the solar activity during growth
and precipitation over the past 8,000 and 100,000 years.
How changes in the solar activity drive changes in Earth’s climate system, is still a matter of debate. Anyhow, the close match between changes in the length of the sunspot cycle and mean temperatures on land in the northern hemisphere, is a clear sign that changes in the climate over the past century were mainly governed by changes in the Sun’s activity. This excellent correlation contrasts sharply with the poor correlation between the steadily increasing carbon dioxide level and the average global temperature.
Even James Hansen from NASA’s Goddard Institute no longer considers the emission of carbon dioxide as a major factor in bringing about the apparent surface heating in our time. (PNAS 15 Aug 2000) He now attributes the biggest role to the emission of black carbon (soot) and greenhouse gases other than carbon dioxide, particularly tropospheric ozone, methane and chlorofluorocarbons (CFCs) – the CO2 effect itself being mostly compensated by aerosols. However, man’s industrial and agricultural activities emit not only greenhouse gases such as carbon dioxide, ozone, nitrous oxide and methane that are added to the natural reservoirs and join in the global biological and geochemical cycles, but also pollutants that are alien to Earth’s environment. Some have an exceptionally large infrared absorption. For example, the current atmosphere contains very small amounts of a number of long-lived fluor compounds of exclusively anthropogenic origin*, with a warming potential up to tens of thousands of times larger than carbon dioxide. They did not occur before their invention and industrial production in the 20th century, but are building up since then at an accelerating rate. Their cumulative radiative forcing has not yet been evaluated properly.
However, humankind influences the climate system above all, at first on a regional scale, through the disruption of the natural interaction between land surface, atmosphere and biosphere. For example, through the large-scale destruction of tropical forests, overgrazing, soil erosion, desertification, changes in land use through agriculture and urbanization, urban heat production, industrial emission of aerosols, construction of storage lakes and other changes in the surface water cycle, pollution, and so on. All these disruptions of the natural environmental system increase in our time at an alarming rate as the inevitable consequence of the rapidly growing world population, accompanied by an ever rising level of consumption.
Over the last decade the insight in the climate system has grown. The advocates of the greenhouse warming were forced to water down their initial predictions from an average global warming of some five degrees centigrade by the end of the 21st century to about one to two degrees, and from a sea level rise of up to eight metres to about half a metre. However, pressure groups of environmentalists, supported by a number of climatologists who claim to speak for all relevant science, had already managed to command the attention of governments and media. They succeeded in bringing about international commitments to curb the emissions of carbon dioxide. Although this may be seen as a case of man’s neurotic capacity to worry about the wrong things, there still are excellent reasons otherthan the dreaded climatic warming for such a reduction. The reserves of fossil fuels are not inexhaustible, so their use should anyhow be economised. Mankind will also benefit from planting more trees than there are felled. Moreover, many scientists who are sceptical about the validity of the greenhouse warming scenario, even so do support measures to curb the emission of carbon dioxide on the basis of the so-called ‘Precautionary Principle’: prevent any action that might have serious consequences, how improbable they may be.
But there never was and is anything like a ‘scientific consensus’ on which the policy makers based their commitments. On the contrary, the initial claim that the man-made emission of carbon dioxide would lead to a calamitous global warming, is increasingly contested, both on methodological grounds and on the basis of data from the real world. In ordinary scientific discourse the adverse evidence produced by a single researcher is sufficient to destroy any cherished theory – when in Nazi Germany a bunch of scientists put forward their consensus rejection of the ‘Jewish doctrine’ of relativity in the ill-famed book Hundert gegen Einstein, the master in Princeton replied ironically that one would have been enough. But, unfortunately, the greenhouse warming scenario has been seized upon and made to a political issue by environmental interest groups, no longer subject to scientific scrutiny.