Since the industrial revolution sparked the widespead burning of fossil fuels, climatologists have been preoccupied with measuring the effect of carbon dioxide (CO₂) on the Earth's climate.

In the first of our series of programmes on climate change, Earth Report takes a look back over 200 years of, sometimes confusing and contradictory, evolving scientific thought that has shaped the global warming debate.

Founding fathers

In 1827, French scientist Jean-Baptiste Fourier first used the analogy of a greenhouse trapping the sun's warmth to explain why the Earth was not as cold as Mars. In 1863, Irish scientist John Tyndall identified that water vapour, CO₂ and hydrocarbons were responsible for this 'greenhouse' effect by absorbing and reflecting heat.

But the real founding father of the global warming debate was a Swedish professor called Svente Arrehuis who published an article in 1896 putting forward the idea that temperature was directly linked to the amount of atmospheric CO₂. He also warned that burning fossil fuels would lead to an acceleration of the natural greenhouse effect - what he didn't know was that it had already started.

Early warnings

Between 1890 and 1940 the Earth's surface air temperature had increased by quarter of a degree centigrade which scientists believe caused the devastating American dust bowl of the 1930s. It doesn't sound like much but, put into context, just four degrees centigrade separates this century from the last ice age.

But where was the proof that atmospheric CO₂ levels were rising dramatically?

The Keeling Curve

In 1958, US scientist Charles D Keeling began a study of atmospheric CO₂ levels. Over the next forty years readings were taken every 10 seconds. His results, known as the 'Keeling Curve', provided dramatic new evidence for global warming - showing that CO₂ levels had risen alarmingly since 1958.

But the idea that the Earth was warming was soon to be challenged by a new theory.

The ice age commeth

In 1974, after a string of unusually cold winters, scientists claimed the earth was not warming but cooling.

Scientists had discovered that the 'warm' interval between ice ages only lasted for around 10,000 years and that, statistically, another big freeze was imminent. Coupled with an increase in man-induced atmospheric smoke and dust that was blotting out the sun, global warming seemed unlikely.

Then a string of hot summers in the 1980s melted away any ideas of a new ice age.

Going, going

To those who doubted that man's activities could alter the composition of the atmosphere, 1985 came as a big shock. Jo Farman, a scientist with the British Antarctic Survey, was the first to discover a hole in the ozone layer the size of the United States.

The link between ozone depletion and global warming is confusing. Ozone is an atmospheric gas which filters out the sun's harmful ultra violet rays which, in too great a quantity, lead to eye damage and skin cancer. Depletion of the ozone layer does not cause global warming - but the chemicals that destroy the ozone - chloro fluoro carbons (CFCs) which are found in aerosols, packaging and cleaning products - are potent greenhouse gases.

Finally there was proof that man was having a direct and dramatic effect on the atmosphere.

A planet in peril

Three years later US scientist James Hansen reported to the US Senate Energy Committee that recent global warming was connected to the greenhouse effect.

For the first time governments were confronted with hard evidence that the world was getting warmer. Within months the United Nations had set up the Intergovernmental Panel on Climate Change (IPCC).

But politicians weren't too concerned - surely climate changes occurred very slowly?

In 1991 the massive eruption of Mt Pinatubo in the Philippines threw an enormous cloud of dust into the atmosphere which cooled the planet for the next two years - calling into doubt the traditional view that climatic changes took place over thousands of years. At the same time, a study of ice cores from the Greenland ice cap were to stun the scientific community.

From these cores, Richard Alley was able to reconstruct past temperature records with unparalled accuracy. They showed that past temperatures fluctuated over much shorter timescales than previously thought.

In the late 1990s oceanographer Wally Broecker came up with the mechanism behind these fluctuations.

The 'Atlantic Conveyor'

Broecker's 25-year study of the Atlantic Ocean made him realise it had the potential power required to cause these rapid climate changes. He realised that there must be a deep water current that brought warm water from the tropics to Northern Europe. The mechanism behind this 'conveyor-belt' was 'convection'. When warm salty water from the tropics travels north it cools, becomes denser and sinks - producing a strong convection current which drives the conveyor.

With this evidence he demonstrated that Northern Europe experienced warmer temperatures when the conveyor was 'on', and rapidly colder temperatures when it was 'off'. But what switched the conveyor off?

The culprit was melting freshwater ice. A warming atmosphere causes an armada of ice bergs to break away from the poles and melt - diluting the salt water so that it's no longer dense enough to sink which forces the conveyor to shut down. Only when fresh water has sufficiently diluted over thousands of years can the ocean achieve the salinity required to start the conveyor again.

Political acceptance?

By the time ministers met at the 1997 Kyoto Summit the scenario of a big freeze in Northern Europe was clearly backed up by credible scientific evidence and political acceptance - and a real understanding that something had to be done.

In next week's Earth Report we take a look at how governments are responding to the threat of climate change, and what impact the big climate conferences such as Kyoto and the Rio Earth summit have had in controlling greenhouse gas emissions.