NEWS FEATURE CLIMATE CHANGE 2007 NATURE|Vol 446|12 April 2007 A world melting from the top down Despite years of speculation, little can be said for sure about the future of the Arctic’s permafrost. But that’s no grounds for complacency, reports Gabrielle Walker. ome of Phil Camill’s trees are drunk. has brought with it a trebling of the thaw rate. For decades environmentalists have worried Once, the black spruce trees on the In some places the permafrost’s perimeter is about the possibility of this great putrefaction. plots of woodland that he monitors in retreating by 30 centimetres a year1. If this trend It has become perhaps the most cited exam- Snorthern Manitoba stood as straight continues, Camill estimates that no permafrost ple of the biogeochemical feedback that could and honest as pilgrims. Now an ever-increas- will be left in any of his five sites by the end of drastically worsen the effects of anthropo- A. COOPER/ALAMY ing number of them loll about leaning like the century. genic climate change. The idea is that humans lager louts. The decline is not in the moral Thawed-out permafrost has already under- increase levels of carbon dioxide in the atmos- standards of Canadian vegetation, but in the mined buildings, highways and other infra- phere, warming the permafrost, which in turn shifting ground beneath their roots. Once it structure from Alaska to Siberia. The damage is releases yet more carbon, warming the world was all hard, solid permafrost. Now much of it one of the most visible effects of warming tem- — and the permafrost — further still in an has thawed into a soggy sponge that no longer peratures on human activities. But the effects ever-escalating positive-feedback loop. provides a steady footing for the trees. Some on natural systems are to some extent more However, although such feedback has been contrive to grow at screwball angles; others have worrying. Buildings can be rebuilt, asphalt discussed for almost as long as the threat of drowned and been replaced by floating mats of relaid and agricultural practices changed global warming has been taken seriously by mosses or sedges. “It is really easy to tell when through adaptation, given the right policies scientists, the lack of firm data on the subject the permafrost has gone,” says Camill. “The and priorities (see page 716). But changes in is striking. “There is a lot that we don’t know at vegetation changes right before your eyes.” the vegetation and, crucially, in the soils of the this point,” says Walter Oechel from San Diego Camill, an ecologist from Carleton College frozen northern landscapes might not be so State University in California. “People haven’t in Northfield, Minnesota, has used those easy to cope with. The soils of the Arctic are quite pulled the whole picture together yet changes to trace the rate at which the perma- crammed with organic matter — a frozen res- — but what we do know is that the potential frost is disappearing. In their desperate attempts ervoir of beautifully preserved roots, leaves and amounts are huge and very, very scary.” to buttress themselves upright, his leaning other raw material that may contain as much spruces put on extra wood on the downslope carbon as the whole atmosphere. They are The big picture side of their trunks. Counting the asymmet- quite unlike soils from more temperate regions, There is no doubt that the Arctic is heating up. rical tree rings that result and measuring the which are mostly made up of the parts that the Vladimir Romanovsky from the University of distance of each tree from the current bound- bacteria cannot digest. “We are unplugging Alaska Fairbanks has collated borehole- and air- ary of the permafrost gives a measure of the pace the refrigerator in the far north,” says Camill. temperature data from throughout the Arctic2. of change. The results are shocking. An average “Everything that is preserved there is going to He found that only one region in the Arctic warming across his sites of 1.3 °C since 1970 start to rot.” had not warmed over the past 30 years — and 718 NATURE|Vol 446|12 April 2007 CLIMATE CHANGE 2007 NEWS FEATURE in the 1990s even that region joined the trend. flourish and decompose the defrosted organic its future course is even harder. In 2005, two Some places are warming at more than twice matter. The probability that the active layer researchers from Boulder, Colorado — David the global average rate. Romanovsky recently will deepen — putting a larger stash of carbon Lawrence from the National Center for Atmos- received a US$1 million grant to take this moni- up for grabs — or that the permafrost will thaw pheric Research and Andrew Slater from the toring work further with a network of stations completely depends on the type of vegetation Cooperative Institute for Research in Environ- in North America and Russia. and soil. Thus, thawing can accelerate rapidly mental Sciences — published the results of the The effect that this warming will have on the if a fire passes through a dried- first attempt to project the fate permafrost and its stored carbon will vary from out forest in the uplands, or if of the permafrost through the region to region. Not surprisingly, the most the soil contains enough ice “We are unplugging twenty-first century using the dramatic signs of thaw have come from the that thawing causes it to col- the refrigerator climate predictions of a gen- fringing, southerly regions of discontinuous lapse, creating a crater-scarred in the far north. eral circulation model (GCM). permafrost — such as Camill’s research sites ‘thermokarst’ landscape. Their results made dramatic — where the frozen layer is only a few metres Researchers expect to see the Everything that is headlines. Using figures from thick and average temperatures are already first signs of thaw in a deepening preserved there is one of the ‘high emissions’ sce- within a whisker of the melting point. In the of the active layer. But compli- going to start to rot.” narios developed by the Inter- colder Arctic the permafrost can be hundreds cations from terrain, vegetation governmental Panel on Climate of metres thick and it is harder to know what and other local conditions mean — Phil Camill Change (IPCC) in the model to expect. In principle, the thawing might be that data need to be collected resulted in 90% of the northern quite slow, with the warmth at the surface being continuously over several dec- permafrost disappearing by transmitted gradually into the colder depths. ades to pick up such a trend. Unfortunately, 2100. Of 10.5 million square kilometres of per- In practice, things are probably more complex, the relevant measurements so far have been mafrost around today, only about 1 million made and in places more precipitous. patchy and sporadic. In 1998, various organiza- it through the century. Even more worryingly, tions involved in permafrost research banded running the model with a ‘low emissions’ sce- Patchy progress together to receive funding as the Circumpolar nario still wiped out 60% of the permafrost, sug- Permafrost is defined as ground in which the Active Layer Monitoring (CALM) programme, gesting that severe losses are inevitable no matter temperature is less than 0 °C for at least two hosted at the University of Delaware in Newark, which policies are followed. And the model did successive years. But the ground in question to tackle the problem. The network now includes not take into account any further warming from does not have to be at the surface. In most places 125 active sites and has participants from 15 carbon given off in the thaw3. the top part of the soil thaws during the countries. Still, it is unlikely to bear fruit in the summer, providing plants form of spotting unequivocal Thick and thin and microbes with an trends for some time. The model has since attracted some criticism, ‘active layer’ above If measuring the most notably because its permafrost is a mere the permafrost damage done so 3.4 metres thick throughout the Arctic, which in which far is hard, is far from the hundreds of metres present in they can predicting some regions. However, Lawrence says that he has since re-run the model for a mid-way emis- sion scenario and dealt with some of the other criticisms at the same time, and the results remained more or less the same. He says that the model captures many important aspects of the Arctic system — including the hydrology and physical properties of the soil — and that the point is not so much the actual percentage of loss but the overall principle that a fright- eningly large amount of permafrost could be vulnerable to quite small changes in climate. “In this field you have to accept that we won’t have a perfect knowledge of what’s happening up there,” he says. “But we should be able to capture the fundamental properties. And so far the model shows that a major change is going to happen to the Arctic.” Perhaps the most intriguing of the com- plications that Lawrence’s original work did not address is the suspicion in the minds of some researchers that the permafrost itself is putting up a defence against the thaw — a set of negative feedbacks. For instance, Oechel points out that warmer temperatures lead to a thicker layer of moss on the surface of his research sites in the Alaskan tundra. Because 1.0. V. (CAPS), SYST. PERMAFROST ACTIVE-LAYER 1998. CIRCUMPOLAR ASSOC., PERMAFROST INT. UNEP/GRID-ARENDAL; REKACEWICZ, P.