in British Columbia beetle outbreaks have killed such extensive areas of boreal forest that experts estimate 270 million tonnes’ worth of carbon sink have been eliminated.23
All over the world ecosystems face being wiped out as their climatic zones shift rapidly elsewhere – or disappear altogether. Just as polar animals are effectively pushed off the top of the world by the rising heat, so mountain-dwellers are confined to ever-shrinking islands of habitat on the highest peaks. Indeed, what is possibly global warming’s first mammal victim – the white lemuroid possum – may already have disappeared from its habitat of just a few isolated mountaintops in tropical Queensland, Australia. ‘It was quite depressing going back on the last field trip a couple of weeks ago, going back night after night thinking, “OK, we’ll find one tonight,”’ biologist Steve Williams told the Australian Broadcasting Corporation. ‘But no, we still didn’t find any.’24 In Madagascar, another global biodiversity ‘hotspot’, mountain-dwelling species are already being displaced uphill, and some species of frog and lizard may now be extinct because of the changing climate.25
Thermal stress also affects humans, of course, as increasingly intense and frequent heatwaves scorch our cities. Hundreds died in the August 2010 Moscow heatwave. Tens of thousands (and possibly as many as 70,000 in total26) succumbed across continental Europe in the record-breaking summer of 2003. Very hot summers have already become more frequent across the Northern Hemisphere, and the risk of a repeat of the 2003 heat disaster has now doubled, thanks to global warming.27 According to news reports, 2010 saw Japan endure its hottest-ever summer, whilst all-time heat records were smashed in 17 different countries.28 Heatwaves have also increased in the Mediterranean region in number, length and intensity, according to the latest studies.29 This warming and drying trend is repeated across much of the world: in southwestern Australia, for example, rainfall has fallen by a fifth since the 1970s, leading to permanent water shortages in Perth.30
All these lines of evidence – of rising temperatures, thawing ice caps, shifting weather patterns and increasingly dangerous impacts – emphasise that limiting CO2 concentrations at 350 ppm in order to prevent substantial future global warming is the only sensible option. Getting back within this planetary boundary would potentially restore the Arctic to health and prevent the complete thawing of mountain glaciers in the Andes and Himalayas that help sustain freshwater supplies to many millions of people. Limiting the speed and magnitude of the future temperature increase to just a degree and a half this century, the most likely outcome of a 350 ppm pathway, would keep global warming slow enough to allow both natural ecosystems like coral reefs and human societies to adapt to climate change.
350: MODELLING EVIDENCE
Observing the present allows us to extrapolate using educated guesswork towards the future. But perhaps a more scientifically rigorous way to project future climate change is to look at the output of complex computer models that simulate the way the climate operates in incredible detail. Taking months of supercomputer time to crunch all their complex equations, these modelling studies allow scientists to simulate changing conditions on Earth as CO2 rises, ice melts and temperatures climb inexorably. Although computer models are always going to be an imperfect representation of the real planet we live on, they are the only way to run experiments into the future – other than sitting back and watching what really happens to the Earth, by which time it will be too late to do anything about it.
The point of setting a planetary boundary on climate is to enable humanity to keep on the right side of potential tipping points that could mark dangerous and potentially irreversible shifts in the way the biosphere operates. With that objective in mind, two members of the planetary boundaries expert group, Tim Lenton and Hans Joachim Schellnhuber, were co-authors of a landmark study published in 2008 that tried to identify the different tipping points that might exist in the climate system and get some idea of what level of temperature rise might trigger them.31 Top of the list was Arctic sea-ice loss. This is because the Arctic melt is self-reinforcing: as ice disappears, its highly reflective surface is replaced by darker sea 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