Perfect harmony

Ridiculed by some, Gaia theory - the idea that all living and non-living components on earth work together to promote life - is gaining support.

Earth is a perfect planet for life but, according to Gaia theory, this is no coincidence. From the moment life first appeared on Earth it has worked hard to make Earth a more comfortable place to live. Gaia theory suggests that the Earth and its natural cycles can be thought of like a living organism. When one natural cycle starts to go out of kilter other cycles work to bring it back, continually optimising the conditions for life on Earth. Named after the Greek Earth goddess, Gaia, the theory was developed in the 1960s by scientist Dr James Lovelock. At the time, Lovelock was working for Nasa, looking at methods of detecting life on Mars. The theory came about as a way of explaining why the Earth's atmosphere contains high levels of nitrogen and oxygen.

Initially, Gaia theory was ignored, and then later ridiculed by scientists such as Richard Dawkins and Stephen J Gould. However, in recent times stronger evidence for the theory has emerged and Gaia has started to gain support. The theory helps to explain some of the more unusual features of planet Earth, such as why the atmosphere isn't mostly carbon dioxide, and why the oceans aren't more salty. In its early years Earth's atmosphere was mostly carbon dioxide - the product of multiple volcanic burps. It wasn't until life arrived that the balance began to change. Bacteria produced nitrogen, an inert gas, and photosynthesising plants produced oxygen, a very reactive gas. Ever since that time, about 2,500m years ago, Earth's atmosphere has contained significant amounts of nitrogen and oxygen, supporting life on this planet. The nitrogen helps to keep things stable, preventing oxygen levels from climbing too high and fuelling runaway fires. Meanwhile, the oxygen supports complex life.

Gaia also helps to explain how the oceans are kept in balance. Rivers dissolve salt from rocks and carry it to the ocean, yet ocean salinity has remained at about 3.4% for a very long time. It appears that the salt is removed again when water is cycled through cracks on the ocean floor. This process keeps the oceans' salinity in balance and at a level that most lifeforms can tolerate. These processes are not thought to be conscious ones, or to favour any one life form over another. Gaia theory simply maintains that Earth's natural cycles work together to keep the Earth healthy and support life on Earth. Lovelock argues that humans have now pushed Gaia to her limit. In addition to filling the atmosphere with carbon dioxide, we have hacked our way through the "lungs" of the planet (the rainforests) and driven many species to extinction. He thinks we are heading for a very warm world, where only polar regions are comfortable for most life forms. Eventually, he suspects, Gaia will pull things back into check, but it may be too late for the human race.

Explainer: Feedback loops

Feedback loops often appear to keep the planet in balance. One good example of this is the way in which atmospheric carbon dioxide is kept in check. Carbon dioxide is pumped into the atmosphere by volcanoes, and removed by the weathering of rocks (encouraged by bacteria and plant roots in the soil). When it reaches the sea, the dissolved carbon dioxide is used by tiny organisms, known as coccolithophores (algae), to make their shells. When coccolithophores die they release a gas - dimethyl sulphate - which encourages the formation of clouds in the atmosphere. When atmospheric carbon dioxide levels become too high, coccolithophores get busy, locking up more carbon dioxide in their shells and pumping dimethyl sulphate into the atmosphere when they die - producing clouds which reflect back sunlight and help the Earth to cool. Conversely, if atmospheric carbon dioxide levels become low, coccolithophores reduce their activity.

Over the past 200 years mankind has greatly increased atmospheric carbon dioxide levels, and recently there has been evidence that algal blooms in the ocean are increasing. Could Gaia be trying to correct our mistake?

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