The fact that carbon dioxide is a ‘greenhouse gas’ – a gas that prevents a certain amount of heat radiation escaping back to space and thus maintains a generally warm climate on Earth, goes back to an idea that was first conceived, though not specifically with respect to CO2, nearly 200 years ago. The story of how this important physical property was discovered, how its role in the geological past was evaluated and how we came to understand that its increased concentration, via fossil fuel burning, would adversely affect our future, covers about two centuries of enquiry, discovery, innovation and problem-solving.
In the beginning…
To pick up the scientific trail of what is today known as the Greenhouse Effect, we need to travel back in time to France in the 1820s. Napoleon, defeated at the Battle of Waterloo just a few years previously, had just died, but somebody who had at one time undertaken significant engineering and academic projects for the late Emperor was now busily engaged on his investigations of the physical world, with a specific interest in the behaviour of heat. This was Jean Baptiste Joseph Fourier (1768–1830).
Fourier had calculated that a planetary object the size of Earth should, quite simply, not be as warm as it is at its distance from the Sun. Therefore, he reasoned, there must be something else apart from incoming solar radiation, some other factor that keeps the planet warmer. One suggestion he came up with was that the energy coming in from the sun in the form of visible and ultra-violet light (known back then as “luminous heat”) was easily able to pass through Earth’s atmosphere and heat up the planet’s surface, but that the “non-luminous heat” (now known as infra-red radiation) then emitted by the Earth’s surface could not make it back in the opposite direction quite so readily. The warmed air must, he reasoned, act as some kind of insulating blanket. That was about as far as he got with the idea back then, as the detailed measurements required to explore this hypothesis were not available, given the technology of the day.
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