CHARLES Miller rides plumes of greenhouse gases ''like a roller-coaster'' at the top of the world. The NASA scientist is one of a handful of researchers taking part in a remarkable experiment that few people have heard of, but which could prove to be one of the most crucial pieces of scientific field work so far this century.
For now, the findings are under wraps. ''But I think 'tantalising' is probably the right word,'' he says.
Miller's mission has fresh urgency in light of a new report that was due to be released on Tuesday night at the UN climate change negotiations in Doha, Qatar.
The Arctic permafrost is thawing. Ancient forests locked under ice tens of thousands of years ago are beginning to melt and rot, releasing vast amounts of greenhouse gases, according to the report, Policy Implications of Warming Permafrost.
While countries the size of Australia tally up their greenhouse emissions in hundreds of millions of tonnes, the Arctic's stores are measured in the tens of billions of tonnes.
Human emissions now appear to have warmed the Arctic enough to unlock this vast carbon bank, with stark implications for international efforts to hold global warming to a safe level.
This is a ''tipping point'' - a scenario predicted by scientists where the climate becomes trapped in a vicious cycle of warming. That process now seems to be starting.
''The permafrost carbon feedback is irreversible on human time scales,'' the report says. ''Overall, these observations indicate that large-scale thawing of permafrost may already have started.''
It estimates the greenhouse gases leaking from the Arctic will eventually add more to emissions than last year's combined carbon output of the US and Europe - which means current global plans to hold climate change to an average 2-degree temperature rise this century are now likely to be wrong.
Until very recently, permafrost was thought to have been melting too slowly to make a meaningful difference to temperatures this century, so it was left out of the Kyoto Protocol, and ignored by many climate change models.
''Permafrost emissions could ultimately account for up to 39 per cent of total emissions,'' says the report's lead author, Kevin Schaefer, of the University of Colorado. ''This must be factored in to treaty negotiations expected to replace the Kyoto Protocol.''
What isn't known is the precise rate and scale of the melt, and Miller's NASA experiment is designed to answer this question. Permafrost exists in areas of the US, Siberia and Canada that are so remote it has been impossible to conduct detailed surveys until now.
His office is a rugged little Sherpa passenger aircraft, stripped of seating and packed with electronics and sensors. Each day, the plane criss-crosses the ice fields, forests and tundra of Alaska, skimming along at low altitude, hugging the contours of the ground at 100 metres a second.
''I've seen the annual migration of the caribou - thousands of animals in a single line stretching for 10 kilometres along a ridge, led by a bull with giant antlers,'' Miller says. ''There are grizzly bears in the forests, and moose wallowing in lakes - it's just incredibly beautiful up here.''
But it isn't the scenery that brought them to Alaska. What the scientists are searching for is invisible to the human eye - the haze of methane and CO2 that hovers low over the landscape in summer as the permafrost melts on the ground.
''We have to go in low because that's where the new information is,'' says Miller, the principal investigator in NASA's Carbon in Arctic Reservoirs Vulnerability Experiment, or CARVE. When high levels of methane or carbon dioxide are detected on the plane's sensors, the Sherpa swoops into action.
''We fly like a roller-coaster, in a flight line that touches the 'boundary layer' [a layer where the air from the ground mingles with air from higher altitudes] and then we fly down, and come straight back up. We keep doing that repeatedly.''
The plane dips in and out of the methane plumes, sucking up data about their composition that hints at the extent and speed of the permafrost melt.
''We're finding very, very interesting changes, particularly in terms of methane concentrations,'' he says.
''When scientists say 'interesting', it usually means 'not what we expected'. We're seeing biological activity in various places in Alaska that's much more active than I would have expected, and also much more variable from place to place
''There are changes as much as 10 to 12 parts per million for CO2 - so that's telling us that the local biology is doing something like five or six years' worth of change in the space of a few hundred metres.''
Methane, a gas made up of one carbon atom and four hydrogen atoms, is not present in the frozen soil but is instead created as the earth thaws and organic matter is consumed by tiny organisms. It is a potent but relatively short-lived greenhouse gas - 25 times as effective at trapping heat as carbon dioxide over a century, but about 75 times as potent over 20 years.
''If the Arctic becomes warmer and drier, we will see it released as carbon dioxide, but if it is warmer and wetter it will be released as methane.''
The findings of the first year of the experiment are so complex that Miller and his team at NASA's Jet Propulsion Laboratory are still trying to work out what they've found. The results are being kept under wraps, which is standard practice while the numbers are crunched and the work is submitted to a peer-review process before publication.
''What we can say is that methane is significantly elevated in places - about 2000 parts per billion, against a normal background of about 1850 parts per billion,'' he says. ''It's interesting because the models are predicting one thing and what we are observing is something fairly different.''
The rate of melt was ''deeply concerning'', says Andy Pitman, the director of Australia's Centre of Excellence for Climate System Science, an adviser to the Climate Commission, and a lead author of the reports by Intergovernmental Panel on Climate Change.
''It had been assumed that on the timescale of the 21st century, that the effects of methane release would be relatively small compared to other effects - that's why it has been largely left out of the climate models,'' Pitman says.
He says work is under way to better integrate faster Arctic thawing into climate models.''I think it's fair to say that until recently climate scientists underrated the rate at which permafrost melt could release methane. I think we've been shown to be over-conservative.
''It's happening faster than we had thought. This is not good news.''
According to the report presented to the UN on Tuesday, a tipping point could still be averted if the world moved quickly to cut emissions from fossil fuels. ''The target climate for the climate change treaty is not out of date,'' Schaefer told Fairfax Media. ''However, negotiation of anthropogenic emissions targets to meet the 2-degree warming target must account for emissions from thawing permafrost. Otherwise, we risk overshooting the target climate.''
The report points out that permafrost carbon feedback had not been included in the fourth IPCC report, the most recent comprehensive update from the UN's climate body, published in 2007.
''Participating modelling teams have completed their climate projections in support of the Fifth Assessment Report, but these projections do not include the permafrost carbon feedback,'' the report says.
''Consequently, the IPCC Fifth Assessment Report, due for release in stages between September 2013 and October 2014, will not include the potential effects of the permafrost carbon feedback on global climate.''
The cost of this omission could be high if measured in financial terms, according to Dr Pep Canadell, a CSIRO scientist and executive director of the Global Carbon Project, which tallies up how much CO2 humans can release before the climate can be expected to warm to dangerous levels.
''If you were to take the price of a tonne of carbon to be $23 like Australia does, you are looking at an extra cost of about $35 billion for the permafrost,'' Canadell says. ''That's on top of the hundreds of billions we already know it will cost to slow emissions to reach a 2-degree level. It's a significant problem in the carbon budget.''
The evidence that major change is already happening is trickling in not just from the NASA measurements, but from ground-based tests.
''There is compelling evidence, not just that permafrost will thaw but that it is already rapidly thawing,'' says Ben Abbott, a researcher at the Institute of Arctic Biology at the University of Alaska, Fairbanks.
''Borehole measurements, where temperature readings are taken at multiple depths within the soil, show more than 2 degrees soil warming in some areas of Alaska. While that may not sound like much, a lot of permafrost is at or just below freezing. The difference between minus 1 degree Celsius and 1 degree is the difference between a fresh frozen meal and a rotten mess.''
In a recent article in Nature, Abbott and fellow researcher Edward Schuur, of the University of Florida, summarised recent findings from experts in the field.
About 1700 billion tonnes of organic carbon is held in frozen northern soils, they said - about four times more than all the carbon emitted by human activity in modern times and twice as much as is present in the atmosphere now.
They calculated the impact of thawing soil on the speed of climate change would be similar to the total rate of logging in all forests around the world.
''Our collective estimate is that carbon will be released more quickly than models suggest, and at levels that are cause for serious concern,'' they write. ''We calculate that permafrost thaw will release the same order of magnitude of carbon as deforestation if current rates of deforestation continue.''
Like Miller, Abbott's job involves long expeditions into the Alaskan tundra to measure greenhouse gas releases. ''I think it's easy for people to feel that the Arctic is just a faraway place that will never have any direct effect on their life,'' he says. ''[But] the last time a majority of permafrost carbon was thawed and lost to the atmosphere, temperatures increased by 6 degrees Celsius. That's a different world.
''Too often climate change is depicted as a story of drowning polar bears and Third World countries. Human-caused climate change has the potential to change our way of life. Mix in the potent feedbacks from the permafrost system and it becomes clear that we need to act now.''