Contents Updated: Monday, September 13, 1999
The greenhouse effect is another recurrent theme in the demise of the dinosaurs. The planet certainly got warmer. From the ratios of oxygen-16 to oxygen-18 in limestone laid down at the time of the K-T boundary layer, the ocean temporarily warmed by between one and five degrees Celsius then a long term cooling set in.
Most climatologists expect major changes in our climate during the next fifty to one hundred years. The concentrations of greenhouse gases in the air are increasing and the temperature of the earth is rising at a corresponding rate. In 1985 scientists released figures based on the analysis of air trapped in polar ice sheets. The concentration of atmospheric carbon dioxide was 270 parts per million (ppm) by volume from the 15th century till the beginning of the 20th century, enough to keep the temperature of the earth comfortable rather than the -25 degrees Celsius it would be without it. Since then it has risen to 350 ppm by volume in less than a century.
Methane is a greenhouse gas twenty times more powerful than carbon dioxide. Over the last 300 years the amount in the air has doubled and it is now increasing at a steady rate of one per cent per annum.
The UN Environmental Protection Agency in 1984 tentatively predicted a rise of five degrees Celsius by 2100 AD. Tom Widgley of the Climatic Research Unit of the University of East Anglia, England, calculates that the natural greenhouse effect will have doubled by 2027 at the latest. Temperatures would rise between two and four degrees as a consequence.
The direct effect of greenhouse heating is that water will evaporate faster: it will have less time to penetrate into the soil and more water will evaporate from rivers, streams and lakes. Consequently the water table will fall and the land will get much drier. Climate will be more continental with hotter, drier summers and colder, harsher winters. Extended land masses, including the wheat growing areas of the US and the former USSR, will become more arid—the bigger the land area, the more pronounced the effect. A rise of only four degrees Celsius could destroy the wheat producing areas.
In contrast lands adjacent to ocean margins, especially where there are mountains, will be exposed to very heavy rainfall due to the high evaporation from the warm sea surface.
A warmer planet will mean that the permafrost of Siberia and Canada will melt releasing methane trapped there forming a positive feedback loop pushing temperature higher still. A rise of four degrees Celsius might be enough to start to melt the polar icecaps. The reflectivity of the earth near the poles will fall, more heat will be absorbed and another disastrous positive feedback loop will form accelerating the melting. The icecaps contain 98 per cent of the earth's fresh water and could lift sea levels by 160 feet. The rise could be even greater because the earth beneath the Antarctic would rise as the weight of ice was released from it displacing even more water.
Besides fossil fuels burnt for energy the main source of increasing atmospheric carbon dioxide is the destruction of forests for grazing. The tropical wet forests cover almost 10 million square miles and keep vast amounts of carbon bound up that would otherwise be atmospheric carbon dioxide. Yet satellite photographs show that farmers are clearing 0.5 per cent of these forests each year, an area the size of Wales. In August 1987, the US satellite NOAA detected 8000 separate fires at least a million square yards in size.
The release of this bound carbon is building up carbon dioxide in the air at the same rate as the burning of fossil fuels.
Clearing the forests also alters the reflectivity of the surface, changing convection currents and air circulation, decreasing rainfall in the tropics, increasing it in latitudes up to about 40 degrees, and decreasing it in the temperate latitudes beyond. The earth's climate will be more like it was in the Cretaceous period!
In Cretaceous times the anthroposaurs had plenty of fossil fuels because the great coal making era was the Carboniferous starting 300 million years earlier. Plenty of oil lay in the rocks from the Jurassic Period almost 100 million years before.
And Cretaceous forests were also burnt. Thick deposits of carbon have been discovered in New Zealand and elsewhere associated with the Cretaceous terminal event. Was this carbon from forests being consciously burnt?
Carol Greitner and William Winner of Oregan State University have shown that, although plants normally take up more carbon-12 than carbon-13 during photosynthesis, in polluted air the carbon-13 take up is more closely in balance. The ratio of C-12 to C-13 remains as a permanent record in the tissues and can be used to test conditions at the time. Is the carbon dust found with some late Cretaceous dinosaur fossils richer in C-13 than would be expected? Is Cretaceous oil richer in C-13?
Carbon released from burnt forest might give an apparently contradictory picture. Limestone laid down at the time has less C-13 than C-12 apparently signifying that the air was less polluted. But it could be that there was more C-12 in the air. If anthroposaurs burnt forests and fossil fuels then a large amount of old carbon of biological origin would enter the atmosphere. Because the carbon would have been fixed by photosynthesis at a previous period when the air was unpolluted, it would be relatively richer in carbon-12. The proportion of carbon-12 in carbon dioxide circulating in the air and the oceans would therefore increase. Naturally then the proportion of carbon-12 incorporated into limestone from the shells of the foraminifera would also increase.
A similar phenomenon is occurring today. Old carbon is entering the air and old carbon contains less carbon-14, the isotope of carbon used in carbon dating, because it decays radioactively. Very old carbon in fossil fuels contains none. Carbon-14 is produced continuously from the impact of cosmic rays with nitrogen in the air and therefore is always present at a steady concentration. But its concentration in the air is starting to drop. Old carbon from the burning of forests is beginning to dilute it.
Though the average Brazilian consumes less meat each year than a domestic cat in the United States, he has to slash and burn forests to satisfy our demand for steaks and hamburgers. A habitat which harbors perhaps 50 per cent of all species is destroyed to make pastures for one species, cattle, and food for one other, man. The fall in variation of the hadrosaurs at the end of the Cretaceous might indicate they were herded. Could the anthroposaurs have burnt their own forests to provide more nutritious browsing for their cattle?