Thursday, March 3, 2011

Six Minutes to Midnight - Part 4

Early Climatologists

This phenomenon of the greenhouse effect is not a recent discovery. People have thought about our climatic system for many years, even as far back as the Greeks. They believed that deforestation in mountainous regions affect local climate. As with many Greek ideas they did not pursue them with any vigour or scientific methodology. To find the true birthplace of this theory we travel back to early 19th century France.

Jean-Baptiste Joseph Fourier (1768-1830) who is now renowned for his work in mathematics also worked in the field of thermodynamics. He wondered as to the process by where a planet retained heat. He correctly believed that we got most of our heat from solar radiation. He also understood that we get heat from other sources such as geothermal and life itself. He theorized that the atmospheric gases somehow store this incoming energy (1827). He established that there must be a balance within this system, we absorb heat and we then must re-emit this heat back into space. Without this balance we simply overheat.

Fourier likened this process of absorption and emission to the process by which a greenhouse functions. The glass he compared to our transparent atmosphere which allowed light to pass freely, the heat transferred in this manner was trapped causing a heating effect. To maintain balance we must radiate heat back into space. Fourier did not have the expertise to find the exact method by which this process takes place but he did inspire others to follow in his footsteps.

The next crucial step in this theory was developed by Irish physicist John Tyndall (1820-1893). Prior to Tyndall, many believed as Fourier had that there was some form of greenhouse effect in place but they had no idea as to the process by which this should be. Tyndall developed an experiment to measure the absorption of infrared radiation in specific gases (circa. 1860). He tested all the gases he had available to him at that time and found that of all the gases he tested water vapour was the most absorbent.

From these experiments he came to the conclusion that the varying amounts of these gases may be responsible for the evidence of ice ages which geologists of time were starting to discover. Tyndall enjoyed mountaineering as a pastime, so he got first hand views of the glaciers present in the Alps. It was a belief among geologists that at one time these glaciers extended across a much greater range. To Tyndall it made sense that these gases which provided this blanket effect could, if concentrations dropped, lead to an ice age or vice versa.

With these two results we now had the framework from with scientists would discover a startling conclusion. In 1896 a Swedish chemist Svante Arrhenius (1859-1927) used the results provided by Tyndall and estimated the effect of doubling the concentration of a specific greenhouse gas, carbon dioxide. From his estimate a global average temperature increase of 5-6 K was the outcome (this estimate is close to present day estimates). Arrhenius, like Tyndall, believed that these gases held a great influence over the ice ages the Earth experienced in the past and from his calculations it was an obvious conclusion to arrive at. 

Arrhenius now arrived at the admission that we could not enter an ice age again to due to human activities. From his measurements he believed that the rate of increase in carbon dioxide levels was such that we would only get warmer to an extent that even if the precession of the Earth as described by Milankovitch occurred it would not be enough to trigger an ice age. This brought a lot of opposition to Arrhenius. At this time Arrhenius believed that the effect of double carbon dioxide levels would take a period of about 3000 years, however today we know that this is happening at an ever increasing rate and may only take a matter of 40-50 years. Many believed as did Arrhenius that this warming was a positive effect, they believed that it would increase crop growth and open up more land to the farming sector. This led many to be not concerned with human activity affecting the climate. As a result climate change would not be back on the scientific agenda until the later part of the 20th century.


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