The God Species: How Humans Really Can Save the Planet.... Mark Lynas

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СКАЧАТЬ or land, that absorbs more of the sun’s heat, allowing the melt of even more ice. The problem here is that models generally underestimate the observed loss of ice – in other words, what is happening in the real world tends to be worse than anything projected by the models. Given this, the experts concluded, ‘a summer ice-loss threshold, if not already passed, may be very close’. Only a 350 ppm target would likely prevent it, corresponding to 0.5 to 2˚C future global warming. But even this may not be enough.

      Second on the tipping points list came the melting of Greenland’s vast ice sheet. Thick enough to raise the global oceans by seven metres if it melted entirely, the stability of Greenland matters hugely to faraway nations like Bangladesh and the Maldives, which face partial or total inundation (in the case of the latter) if it melts because of global warming. So where does the tipping point lie that might doom the Greenland ice cap to eventual destruction? Between just 1 and 2 degrees above today’s temperatures, the experts concluded, meaning that a 350 ppm trajectory is once again the least we will need to achieve to protect it. Here too the process could become self-reinforcing. The centre of Greenland is extremely cold because the thickness of the ice sheet means that it extends into high altitude: Greenland’s Summit Camp is located 3,200 metres above sea level. But as global warming nibbles away at the edges of this enormous ice body, more of it comes into the lower altitude zone, exposing the ice to higher temperatures and increasing the melt rate. Although eliminating a whole continent’s worth of ice will take time, the process could be completed in as little as three centuries, dramatically changing the coastal geography of the planet. Once again, this is a tipping point that humanity would be wise not to trigger.

      Greenland is not the only vulnerable polar ice sheet, of course. Third on the list came the West Antarctic Ice Sheet, again of serious concern because – like Greenland – its loss could trigger multi-metre rates of sea-level rise. The West Antarctic also could be subject to a positive feedback process once a serious melt got under way, not just because of the change in altitude but because most of the ice sheet is actually grounded well below today’s sea level. As warming waters penetrate underneath the ice mass they could trigger a collapse that would be unstoppable, and would eventually raise global sea levels by another 5 metres. Here we may be on slightly safer ground, as the experts conclude that a global warming of 3–5˚C will likely be necessary to lead to complete collapse. So the 350 ppm boundary would appear to be well within the safety margin according to the models.

      As with the Arctic sea ice, however, the real world may prove the models of Greenland and the West Antarctic to be overly conservative. The most recent satellite data from the GRACE (Gravity Recovery and Climate Experiment) mission shows a doubling in ice mass lost from both Greenland and Antarctica over the last decade³² – despite a thickening of Greenland’s higher interior where warmer winds have increased snowfall rates. Until recently the massive East Antarctic ice sheet was probably stable, but it too began losing ice in coastal areas after about 2006.³³ In total the Earth’s great ice sheets are now shedding a few hundred billion tonnes of ice annually, and sea levels rising by slightly more than 3 mm per year as a result – nearly double the rate for most of the twentieth century.³⁴ A rise in sea levels by 2100 of somewhere between 60 cm and 1.6 metres is now on the cards,³⁵ substantially more than was suggested just a few years ago by the IPCC.³⁶

      A more familiar tipping point was examined next, one that has even been made into a dramatic Hollywood film. In The Day After Tomorrow, a sudden ice age is seen flooding and then freezing New York (why is it always New York?) after global warming destabilises the circulation of the Atlantic Ocean. Although the flash-freezing depicted in the movie is thermodynamically impossible, the scenario of a collapsing Atlantic current is not complete science fiction. All the models examined by the expert group led by Tim Lenton showed a tipping point in the North Atlantic where warmer, fresher waters could shut down the circulation pattern that brings comparatively balmy temperatures to the eastern US and high-latitude Western Europe. This shutdown would not trigger a new ice age, but temperatures in these regions could fall for several decades, causing serious impacts on societies and ecosystems alike.³⁷ Again unlike the Hollywood movie, which showed temperatures dropping in seconds, the full transition towards an Atlantic Ocean circulation shutdown would likely take a century or more. More good news is that avoiding this tipping point is still possible: the scientists conclude from studying their models that a global warming of 3–5˚C would be needed to put us in the danger zone, well above the 1.5˚C maximum warming implied by our 350 ppm planetary boundary.

      Another candidate on the tipping-point list is the Amazonian rain-forest. For years now many scientists have warned that global warming could trigger a collapse of the forest if rising temperatures lead to severe drought in western Brazil. This scenario seems even more of a danger given the recent droughts experienced in Amazonia in both 2005 and 2010, where entire river systems in this normally wet forest dried up for hundreds of kilometres. The problem here is that models don’t concur: some show a warmer Amazon getting wetter, whilst the most pessimistic forecasts for Amazon die-back are based on the projections of just one model, the HadCM3 model produced by the UK Met Office’s Hadley Centre. However, half of the 19 different models examined by a team of scientists led by Oxford University’s Yadvinder Mahli in 2009 did show a shift towards more seasonal forest, and a quarter showed that the rainforest could dry out sufficiently to collapse into a savannah-type ecosystem instead.³⁸ Keeping global temperatures below 3˚C – very likely if our 350 ppm planetary boundary is achieved – should be enough to avoid this transition, but just as important will be respecting the other planetary boundaries on land use and biodiversity loss. The Amazon rainforest today is probably more threatened by deforestation and agriculture than it is by rising temperatures.

      If the Amazon rainforest did collapse, huge quantities of carbon would be released in the process, giving a further boost to global warming. But the biggest carbon stores of all lie not in the tropics, but in the sub-polar continental regions where frozen permafrost holds enormous carbon stores tens of metres thick in Siberia and other high-latitude land areas. The threat to permafrost stability is possibly global warming’s biggest tipping point, because if this frozen carbon store begins to thaw, vast quantities of both carbon dioxide and methane will be released. According to a 2008 study in the journal BioScience, the carbon locked up in the Northern permafrost zone totals more than 1.5 trillion tonnes, double the entire carbon content of the atmosphere.³⁹ Even if only 10 per cent of this permafrost thaws, another 80 ppm of CO₂ will have accumulated in the atmosphere by 2100, raising the planet’s temperature by an additional 0.7 degrees⁴⁰ – and making the eventual attainment of the 350 ppm climate change boundary much more difficult.

      Scientists have already begun watching with some alarm a recent upward trend in atmospheric methane, some of which may be coming from the Arctic.⁴¹ Not all this methane – a greenhouse gas 25 times more potent than CO₂ – is likely to bubble out of swamps on land; vastly more is contained in subsea sediments in the form of ice-like methane hydrates. If these hydrates melt rapidly as the oceans warm up, then all global warming bets are off – a scenario that has already sparked scary newspaper headlines. So how afraid should we be? Researchers have already reported seeps of methane leaking from the seabed offshore from eastern Siberia and the Norwegian Arctic islands of Svalbard, in both cases possibly in response to warmer ocean waters.⁴² But the experts are cautious. ‘Methane sells newspapers, but it’s not the big story,’ writes David Archer on the excellent RealClimate blog.⁴³ ‘CO₂ is СКАЧАТЬ