A global warming emergency
Global carbon dioxide emissions, the main cause of global heating, are rising at an increasing rate. Half of all human atmospheric carbon emissions have occurred in the last 30 years and the world is now producing double the atmospheric carbon the biosphere can absorb. How far are we from the point of no return when runaway climate heating and further dramatic consequences become unstoppable?
James Hansen, director of NASA's Goddard Institute for Space Studies, says that "we must close that gap (between the science and the policy-makers) and begin to move our energy systems in a fundamentally different direction within about a decade, or we will have pushed the planet past a tipping point beyond which it will be impossible to avoid far-ranging undesirable consequences." Global warming of two to three degrees, he warns, would produce a planet without Arctic sea ice, a catastrophic sea level rise in the pipeline of around 25 metres, and superdrought around the globe. "Such a scenario threatens even greater calamity, because it could unleash positive feedbacks such as melting of frozen methane in the Arctic, as occurred 55 million years ago, when more than ninety per cent of species on Earth went extinct."
How far away is two to three degrees? In addition to the 0.8°C rise so far, there is about one-half degree global warming “in-the-pipeline”, because of the ocean’s thermal inertia, due to gases already in the air. Hansen says energy infrastructure such as power plants will add "enough gases for another half degree... even if we decide now to replace business-as-usual with an alternative scenario". So we are effectively committed to almost two degrees. In addition, aerosols (sun-blocking smoke, dust and other polluting particles put into the air by human activity) have a short-term cooling effect in the atmosphere and are masking temperature rises of up to 1.5°C, and their emissions are being reduced by tightening pollution laws because they include sulphates, a source of acid rain.
The widespread scientific view is that temperature rises should be kept below two degrees, but a lower target of 1.5 degrees is necessary if we are to adopt the precautionary principle, which implores us to stay on the statistically safe side of triggering uncontrollable events. This is not a bet where losing means we are simply out of pocket; it is Russian roulette where a lack of suitable precaution is deadly. Practically speaking, more than 1.5°C is already inevitable, so the need is urgent for the level of greenhouse gases to be reduced from its present level; talk about letting it rise further is a suicide note, even if written by Sir Nicholas Stern, author of the recent influential report to the British government on climate change.
Stern says that keeping greenhouse levels to 450 ppm (parts per million in carbon dioxide equivalence), just above the present level of 430 ppm, gives us an even money chance of keeping global increases below 2°C. But because this is a big call, Stern puts the short-term needs of business before the scientific imperative and advocates policies which aim to constrain the increase to 550 ppm, at which "there is around a 50:50 chance of keeping increases below 3°C".
Stern's bet on three degrees is calamitous and, according to Hansen, a death warrant for the biosphere as we know it, even if the execution takes some time. It is estimated that global average temperatures of 2.7°C will trigger runaway melting of the Greenland ice sheet and around 3°C is the threshold for the mass destruction of ocean algae, which both pump down CO2 and produce dimethyl sulphide, a key compound connected to the formation of clouds. The capacity of soils to absorb carbon dioxide is decreasing as temperatures rise. Positive feedbacks will push the increase past 4°C, destabilizing the tropical rainforests into scrub or desert and adding to atmospheric carbon. Melting permafrost will release of huge volumes of methane, a powerful greenhouse gas, from thick layers of thawing peat. Each positive feedback event amplifies the previous event and triggers the next.