Reflections on the stabilization of Earth’s climate by life.
People frequently believe the claim that basic physics, established in the 19th Century, is sufficient to predict that Earth will warm in response to increasing CO2. However, I argue here that negative feedbacks due to life (‘Gaia’) may have stabilized the planet’s climate — on geological timescales and in recent decades. The biology of any such stabilization is far from settled, with a mechanistic understanding delayed by evolutionary debate. I conclude that even with such advanced biology we have little power to predict global climate changes.
There is a basic flaw in the basic physics argument of climate change: biology. Indeed, just one word should be enough to cast doubt on all models of the atmosphere: “oxygen”. No educated person is unaware of one aspect of Earth’s basic biology: most atmospheric oxygen results from living organisms. Physics and chemistry therefore cannot explain atmospheric composition or properties. Basic chemistry would leave the planet a rusty ball (like Mars or Venus). So, as James Lovelock articulated in his Gaia hypothesis in the 1970s, the properties of our atmosphere result from the tight coupling of living and non living components (biota and abiota). Earth’s obvious and massive departure from chemical equilibrium is unique in the solar system. So, if it’s easy to understand that life is central to atmospheric chemistry, why have many people found it much harder to understand life could be pivotal in atmospheric energy and climate? And if life is so intimately involved, predictive models would need to include it — which I’ll argue they can’t because the biology is too complex.
An initial response, I anticipate, will be that oxygen is not a climatically-active gas, because it is not radiatively active. However, that does not weaken the argument that life changes Earth far from the state which non-biological “basic” science would predict — an example of the planetary power of life. Moreover, few realise that oxygen could have major implications for the long-term temperature trajectory of the planet, if it is helping to keep Earth wet. This controversial idea was discussed in meetings on Gaia in Oxford in the 1990s, postulating that in the absence of life and oxygen, the splitting of water by sunlight would eventually lead to desiccation of the planet (as hydrogen bled away into space). Photo-dissociation might be offset by the presence of atmospheric oxygen, scavenging hydrogen and restoring water. If so, the dominant climatically-active gas in the atmosphere — water — also owes its abundance to life.
Whether the planet is wet due to life requires further study and discussion. Fortunately my argument — that life is largely missing from the models — does not depend on this. What is more important is that people who believe basic physics is sufficient to predict climate should consider cloud condensation.
It is very widely accepted that clouds are hard to model, yet central to understanding climate sensitivity to CO2. It is not even known if the overall cloud feedback effect in a warming world is positive or negative. Indeed, the IPCC (2013) state: “Clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget….some aspects of the overall cloud response vary substantially among models…”.