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If you shout "fire" in a crowded theatre, when there is none, you understand that you might be arrested for irresponsible behaviour and breach of the peace. But from today, I smell smoke, I see flames and I think it is time to shout. I don't want you to panic, but I do think it would be a good idea to form an orderly queue to leave the building.
Because in just 100 months' time, if we are lucky, and based on a quite conservative estimate, we could reach a tipping point for the beginnings of runaway climate change. That said, among people working on global warming, there are countless models, scenarios, and different iterations of all those models and scenarios. So, let us be clear from the outset about exactly what we mean.
The concentration of carbon dioxide (CO2) in the atmosphere today, the most prevalent greenhouse gas, is the highest it has been for the past 650,000 years. In the space of just 250 years, as a result of the coal-fired Industrial Revolution, and changes to land use such as the growth of cities and the felling of forests, we have released, cumulatively, more than 1,800bn tonnes of CO2 into the atmosphere. Currently, approximately 1,000 tonnes of CO2 are released into the Earth's atmosphere every second, due to human activity. Greenhouse gases trap incoming solar radiation, warming the atmosphere. When these gases accumulate beyond a certain level - often termed a "tipping point" - global warming will accelerate, potentially beyond control.
Faced with circumstances that clearly threaten human civilisation, scientists at least have the sense of humour to term what drives this process as "positive feedback". But if translated into an office workplace environment, it's the sort of "positive feedback" from a manager that would run along the lines of: "You're fired, you were rubbish anyway, you have no future, your home has been demolished and I've killed your dog."
In climate change, a number of feedback loops amplify warming through physical processes that are either triggered by the initial warming itself, or the increase in greenhouse gases. One example is the melting of ice sheets. The loss of ice cover reduces the ability of the Earth's surface to reflect heat and, by revealing darker surfaces, increases the amount of heat absorbed. Other dynamics include the decreasing ability of oceans to absorb CO2 due to higher wind strengths linked to climate change. This has already been observed in the Southern Ocean and North Atlantic, increasing the amount of CO2 in the atmosphere, and adding to climate change.
Because of such self-reinforcing positive feedbacks (which, because of the accidental humour of science, we must remind ourselves are, in fact, negative), once a critical greenhouse concentration threshold is passed, global warming will continue even if we stop releasing additional greenhouse gases into the atmosphere. If that happens, the Earth's climate will shift into another, more volatile state, with different ocean circulation, wind and rainfall patterns. The implications of which, according to a growing litany of research, are potentially catastrophic for life on Earth. Such a change in the state of the climate system is often referred to as irreversible climate change.
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