If you think greenhouse gas, climate change, tipping points and related issues are the sole preserve of tofunibbling tree-huggers, environmental alarmists and scientists in pursuit of lucrative grants, you’d be wrong. In fact, you should be feeling a little nervous…
Abrupt climate change is bad news. Most people think of climate change as something that occurs only gradually, but it has happened with frightening rapidity in the past and will almost certainly do so again.
Perhaps the most famous example is the reverse hiccup in a warming trend that began 15 000 years ago and eventually ended the last ice age. Roughly 2 000 years after it started, the warming trend suddenly reversed and temperatures fell back to near-glacial conditions; Earth stayed cold for over a thousand years – a period called the Younger Dryas (named for an alpine wildflower). Then warming resumed so abruptly that global temperatures shot up 10° in just 10 years.
Because civilisations hadn’t yet emerged, complex human societies escaped this particular rollercoaster ride. Nevertheless, some form of abrupt climate change is highly likely in the future, bringing with it wide-ranging economic and social effects. If an abrupt climate change caused the rapid break-up of the West Antarctic ice sheet, for example, sea levels could rise by several metres within a century.
Now, it seems, the world is finally waking up to the threat. The US Department of Energy’s Offi ce of Biological and Environmental Research recently launched IMPACTS – Investigation of the Magnitudes and Probabilities of Abrupt Climate Transitions. The programme, led by William Collins of Berkeley Lab's Earth Sciences Division, brings together six national laboratories to attack the problem, calling on experts in physical, chemical and biogeochemical climate processes and in computer simulations of the whole Earth system.
Says Collins: "IMPACTS is one part of a two-pronged approach to studying abrupt climate change, one based in the universities and the other in the national labs. The goal is to understand possible mechanisms of abrupt climate change well enough to build comprehensive computer models, and to make accurate predictions before abrupt climate change strikes again."
One of their first tasks was to define what they meant. In essence, they were looking at a large-scale change that happened more quickly than that brought on by forcing mechanisms . such as the accumulation of carbon dioxide in the atmosphere from the burning of fossil fuels, or widespread changes in land use – on a scale of years or decades, not centuries, and which persisted for a very long time.
The IMPACTS team will initially focus on four types of abrupt climate change:
- Instability among marine ice sheets, particularly the West Antarctic ice sheet.
- Positive feedback mechanisms in sub-arctic forests and arctic ecosystems, leading to rapid methane release or large-scale changes in the surface energy balance.
- Destabilisation of methane hydrates (vast deposits of methane gas caged in water ice), particularly in the Arctic Ocean.
- Feedback between biosphere and atmosphere that could lead to mega-droughts in North America.
Only half joking, Collins refers to these as "the Four Horsemen of the Apocalypse".
Know your enemy
Marine ice sheet instability. Marine ice sheets flow from the land into the sea and are partly grounded below sea level, extending across the ocean surface as floating ice shelves. Greenland and the West Antarctic ice sheets are already losing mass at an accelerating rate, with the primary cause apparently the warming of the oceans, not the air, through melting of ice buttresses below the water's surface. Topographical and other features make the West Antarctic ice sheet particularly vulnerable.
Marine ice sheets are grounded below the water line and are susceptible to ocean warming, not just atmospheric or surface warming. For 40 years, Earth scientists have worried about what would happen if global warming eventually caused the West Antarctic ice sheet, some 3,8 million cubic kilometres of ice, to break up and slide into the ocean.
For starters, sea levels would rise by 4-6 metres. Port facilities worldwide would be submerged; atolls and island chains would vanish; parts of Bangladesh, Brazil, Burma, America's Gulf States and other low-lying areas would be flooded; Venice, New Orleans, and many other cities would sink.
It's now apparent that these events may not be "eventual" – abrupt climate change could cause rapid melting and the subsequent rise of sea level not by centimetres but by metres per century.
Under the direction of Bill Lipscomb of Los Alamos National Laboratory, IMPACTS researchers will undertake the difficult task of realistically modelling the processes of ice-shelf melting, the retreat of a shelf's underwater grounding line, and the calving of icebergs. Concentrating on the West Antarctica ice sheet, they will use the Hybrid Parallel Ocean Programme developed at Los Alamos and other programmes to simulate ocean currents beneath ice shelves up to hundreds of metres thick and hundreds of kilometres in extent; to model the exchange of heat between ocean and ice; to model the shifting grounding lines of the ice shelves; and to model the changing shore lines of the world's oceans as the ice shelves evolve.
Their goal is to assess the likelihood of abrupt ice-sheet retreat under different conditions and to incorporate the regional ice-sheet model into the global model.
Positive feedbacks in boreal and Arctic ecosystems. More than a third of Earth’s terrestrial organic carbon is concentrated in the ecosystems north of the 45th parallel, much of it in soil, peatland basins and permafrost. Positive feedback within ecosystems and among terrestrial ecosystems, climate and ocean currents could rapidly release much of this stored carbon into the atmosphere.
Higher air temperatures and increased precipitation have already reduced snow cover and increased run-off from melting permafrost; vegetation types have started to shift, affecting, for example, how much sunlight reaches the soil or is intercepted by forest canopies. Less snow cover means less reflectivity (albedo); dark soil and trees absorb more heat from the Sun.
The kind of permafrost called yedoma is particularly rich in carbon, an important substrate for methane formation. Methane, a greenhouse gas 26 times more powerful than carbon dioxide in the short term, is released when the permafrost melts. Even ordinary soil will exchange more carbon with the atmosphere as it warms and wets.
In addition, as more fresh water flows into the Arctic, changes in salinity could alter the exchange of water with the North Atlantic and shut off ocean currents that keep Europe warm in winter. Northern Europe may grow colder even as the oceans’ ability to absorb carbon dioxide decreases and the planet as a whole grows warmer, faster.
Positive feedback involving ice-melt has already accelerated the pace of global warming in the far north. It now seems likely that changes in terrestrial ecosystems, which could occur over just 20-30 years, may amplify currently predicted global warming by two or three times, in the Arctic and possibly globally.
The researchers aim to produce dramatically improved predictions of abrupt climate change resulting from positive feedbacks between the climate and ecosystems of the boreal and Arctic region.
Methane hydrate destabilisation. A vast quantity of carbon – possibly more than all the recoverable fossil fuels on Earth – is trapped in frozen methane hydrates under the oceans. Methane gas molecules are locked inside cages of water ice in a form so concentrated that when the ice melts, the gas expands to 164 times its frozen volume.
Over time, this methane, if released into the atmosphere, would be up to 72 times more potent than carbon dioxide as a greenhouse gas. During a thermal maximum 55 million years ago, which marked the boundary between the Paleocene and Eocene epochs, the planet warmed rapidly by five to eight °C; the pattern of deepsea extinctions and spikes in methane concentrations in the fossil record during this 20 000-year period point to methane release as a possible cause.
High pressure and low temperature insure that most deep-water methane hydrate deposits would be stable even with considerable warming of the atmosphere. But in the Arctic, methane hydrate deposits exist near the edge of the safe temperature-pressure zone; in these locales, methane release could be abrupt. The resultant rapid warming would trigger yet more releases of methane: permafrost would melt, the deep sea would become a dead zone, the hole in the Arctic ozone would grow bigger and occur more frequently.
IMPACTS researchers will study a range of rapid-change scenarios by coupling a number of proven models, hoping to make accurate assessments of methane releases and the consequences for oceanic and atmospheric chemistry.
Mega-droughts in North America. “Ordinary” greenhouse warming as forecast by the IPCC will result in warmer and dryer conditions in the subtropics, including Mexico and the southwestern US. More than warming of the air and sea surface are involved. Storm tracks are likely to shift north, and the jet stream will probably stabilise in a new configuration. Dried-out soil and hot, dry atmosphere could interact to start abrupt climate change.
If dried-out soil and hot, dry atmosphere interact to trigger abrupt climate change in the Southwest, mega-droughts such as those that plagued North America a millennium ago could quickly return. Conditions as severe as the Dust Bowl of the 1930s will return and could persist for decades: a mega-drought. Until seven or eight hundred years ago, central North America seems to have been characterised by much longer and more severe droughts than at present. The Dust Bowl of the thirties turned many once-productive fields and rangelands into shifting sand dunes; the past was even worse, with vast sheets of sand on the move all across the High Plains.
IMPACTS researchers will test two hypotheses. The first is that plants modulate soil hydration in the early stages of a drought by redistributing deep ground water through their roots and leaves. But when the water table drops too low, plants accelerate the desiccation of the soil surface by their one-way transpiration of moisture into the atmosphere.
The second hypothesis is that dust storms will alter the North American monsoon, which brings water from the Gulf of Mexico and the Gulf of California during the summer months to contribute one-half to two-thirds of the annual precipitation in the Southwest. Complex interactions of surface conditions, atmospheric stability, dust-particle aerosols and other factors could profoundly alter monsoon circulation in unpredictable ways.
To predict mega-droughts, researchers will have to improve global and regional climate models, including a model of water-energy exchange developed at the University of Washington, as well as land surface and plant models.
What to do next
Says Collins: “IMPACTS starts by using primary scientific research to build detailed models of processes that have been included only roughly, if at all, in climate models – the effects of methane on sunlight, for example, which has never before been incorporated, or the role of vegetation in moisture exchange between soil and atmosphere. We will use our improved models… to make very robust predictions.”
And when they succeed?
“Suppose, for example, our models predict the abrupt onset of a megadrought in the Southwest. The first step will be to assess the prediction by measuring it against the past. We go back in time. If the conditions we think will lead to abrupt climate change would not have produced this result in the past, we still have work to do. On the other hand, if our models can use data from past conditions to recover what actually did happen, it’s an excellent test.”
A believable abrupt climate change prediction, says Collins, would mean “we are facing the largest imaginable negative impacts on human civilisation… conditions that will take society outside all normal modes of adaptation very quickly. The consequences will be especially dire for resource-limited populations. This is a huge threat to the security and stability of our nation and the world…”
Better science, more sophisticated models and more powerful computers like those at the National Energy Research Scientific Computing Centre in the US have only recently made it possible to build comprehensive climate models. Collins says the 30-year quest to make these truly comprehensive is only now paying dividends.
“We used to build in the carbon cycle by telling the model how it would work – but we’ve learned that we can’t assume that; the carbon cycle is extremely sensitive to even relatively small changes in temperature and rainfall. We’ve only recently been able to simulate vegetation. We used to model forests by just placing them into the model arbitrarily; now we allow them to grow themselves.”
One of the great benefits of IMPACTS, in Collins’s view, is the bringing together of parts of the climate community whose communication has traditionally been poor. As he tells it, “the working relationship hasn’t been there”.
How important is their work? Very. How difficult will it be? Ditto.
“Modelling a specific region with a specific set of circumstances in high resolution and making a solid prediction of what’s going to happen in the next few decades is much tougher than modelling what’s going to happen to the whole globe a century from now. We hope that IMPACTS will demonstrate that it can be done.
“Of the major threats to human health from global warming, the worst is malnutrition – but the second worst is extreme weather.”
Source: Berkeley Lab