By the end of the 19th century, sociologists were uneasy at the social costs of industrial progress, and scientists were becoming more and more anxious about the thoughtless manner in which the waste products of mining and manufacturing were left to look after themselves.

In the 1890s, the Swedish physicist Svante Arrhenius (known for his ionic theory and for the hypothesis that life on earth had come from outer space) calculated that if the concentration of carbon dioxide in the air was doubled from its pre-industrial value, the global mean surface temperature would increase by about 5 degrees Celsius. But he thought that such a doubling would only be reached in many hundreds of years. He was not reckoning with the exponential growth of industry.

The Russian geochemist Ivan Vernadsky had been entrusted with the task of mobilising Russian industrial output during the First World War. In the 1920?s he published a paper claiming that the amount of material being moved by man was approaching geological proportions.

He cautioned that the steadily increasing activity in the mining of coal and the pumping of oil was adding more and more carbon dioxide to the air and an increase of the temperature at the earth?s surface was inevitable.

Many scientists considered that life had always been a geophysical force and had been responsible for many changes in the biosphere. Topsoil was the result of the activity of early-earth fungi and bacteria, later aided by small burrowing animals (the annelidae, for instance) on weathered rock material.

Chlorophyll-bearing algae had appeared long back in the history of the earth, and at the beginning of the palaeozoic era (500 million years ago) there already was 1% of oxygen in the atmosphere, which in the early stages is thought to have consisted of carbon dioxide, water vapour, oxides of nitrogen, sulphur compounds and probably hydrogen.

The first traces of charcoal from forest fires found in ancient sedimentary rocks have been dated to around 375 million years ago. This indicates that by that time the plants had already produced enough oxygen to make up at least 13% of the gas in the atmosphere, the minimum required to light and sustain forest fires.

The percentage of oxygen apparently reached the present value of 21 some 350 million years ago, and exceeded 35 some 280 m. years ago. It is thought to have come back to present-day values in a few million years and stayed in the neighbourhood of this value thereafter.

Climatic instability

In the nineteen thirties, the British meteorologist W. J. Humphreys expressed the idea that the ocean-atmosphere climate system was inherently unstable and liable to change rapidly from a stable phase of warmth to another stable phase of ice. The climate expert C.E.P.Brooks also came up with similar opinions a few years later. Many of their colleagues thought there was sense in the theory, but the majority rejected it as fanciful, even preposterous. At the time of Humphreys, it was only known that there had been an ice age a long time ago, but he believed that the instability of the climate could one day cause another one, maybe millions of years hence.

It took three more decades and the extraction and examination of kilometres of deep-ocean mud cores to discover that in the past two million years the ocean-atmosphere system had known a series of alternating ice ages and warm periods.

Effect of human activity

Each ice age lasted about 100,000 years, and each warm period a mere 15,000years or so. A few decades later, more detailed information about temperatures was obtained from ice cores drilled out from locations in Greenland. From these data it could be deduced that the climate was indeed unstable, and changes from cold to warm or warm to cold could be achieved in a few decades.

From the thirties right through the sixties, the main worry was that we were approaching the next ice age, and society should start preparing to face it. But in the seventies meteorologists again started to think of a warming brought about by the increase of carbon dioxide in the atmosphere.

With the knowledge that living things do alter conditions in the biosphere, it seemed appropriate that we should be asking ourselves whether the activities of our own species are not altering the environment in a dangerous manner.

The temperature at the earth?s surface has exceeded present-day values by only about 2 to 3 degrees over the past 200,000 years. Modern man has been on the move only about 125,000 years, so that the 3- to 6-degree rises being forecast for the third quarter of the present century will be taking us into unknown territory.

The graph here represents mean temperatures over the earth from 250,000 years ago to our own times. The time-scale is logarithmic, and the labelling is in thousands of years. Here it is plain that all through the period covered, temperatures have been subject to sudden changes and that the duration of any set of conditions has been of the order of three to five thousand years, sometimes even less.

But during the current Holocene beginning 10,300 years ago, the temperature has been rather too equable, with the exception of a small dip 8,300 years ago, which did not go too far down and from which the climate recovered very soon. Meteorologists would like to know if the start of agriculture around 8,000 years ago, with the primitive system of clearing land by burning, the rearing of herds and flocks, and probably the rice paddies, should be thanked for this respite. So far so good, but the question now arises as to how far the activities of man, the new species of ape with a voluminous brain and exceptional intelligence, will take us along the path of climate change.

And if he continues to load the air with carbon dioxide, methane nitrogen oxides etc., will this behaviour take us along the path of accelerating warming, or will the unstable climate system be pushed into another cold phase?

B. M. PADYA (formerly director Meteorological Services)