The 4th Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) has a finely calibrated lexicon of certainty. "Virtually certain", it blares when it assigns a 99% probability to hot days
getting hotter and more frequent. "Very likely", or more than 90% probable, are heavier rains. And so on down the list including the wishy-washy "more likely than not" when assigning a greater than 50%
probability, such as the chance that human activities are affecting the intensity of hurricanes.
Such care is crucial in a field that is still, in some areas, shot through with uncertainty. The IPCC has gone far in tightening up some key scientific unknowns about climate change (see `From words to action'), but many still remain. Some conclusions such as the effect on particular regions of the world, or exactly how much sea level will rise remain more uncertain than others. This means that there's plenty of work left for the climate scientists on whom the IPCC process depends.
Perhaps most critically, researchers know relatively little about feedback effects that might enhance or weaken the pace and effects of climate change. The complex flow of carbon between soils, plants, the oceans and the atmosphere is still being pinned down by large-scale climate experiments. Some experts predict that, in a warmer world, ecosystems that are currently sinks for carbon, such as the Arctic tundra, may turn into carbon sources. But no one can yet accurately predict how this might pan out, and feedbacks among land
and air could end up putting far more carbon dioxide into the atmosphere than currently forecast.
Other big unknowns are the effects of the take-up of carbon dioxide by the oceans, which removes the gas from the atmosphere and locks it away in the calcium carbonate of the shells and skeletons of marine organisms. Higher levels of atmospheric carbon dioxide are expected to make the seas more acidic and slow down the rate of calcification, ultimately reducing the ocean's ability to absorb more carbon dioxide. But precisely how the biology of marine creatures would play into that effect is unknown. Nor is it known how changes in plankton composition and coral reefs, for example, might affect carbon dioxide concentrations.
Pinning down biological feedbacks will be critical for future reports, says Richard Bellerby, a chemical oceanographer at the Bjerknes Center for Climate Research in Bergen, Norway. "We're going blindly into the
future," he says.
Another major source of uncertainty and of debate at the Paris meeting where the IPCC report was finalized is the rise in sea level. In 2001, the IPCC predicted a rise of between 9 and 88 centimetres by 2100, as a result of melting ice caps and the thermal expansion of the ocean. This time around, the group has narrowed that range to between 19 and 58 centimetres. But some scientists say that this is an underestimate.
Stefan Rahmstorf, an oceanographer at the Potsdam Institute of Climate Impact Research in Germany, believes, for instance, that global sea level could rise by much more than that. In a paper published online
the day before the IPCC report's release, Rahmstorf and his colleagues argue that sea-level rises will be close to the worst-case predictions of climate models (S. Rahmstorf et al. Science doi:10.1126/ science.1136843; 2007). "If anything, the IPCC has been conservative, " he says.
Key sticking points include the inability of global climate models to produce the amount of sea-level rise observed over the past couple of decades and whether ice flow at the bases of glaciers is accelerating
or not. How volatile Antarctic and Greenland glaciers might become in a warmer world is therefore pretty much guesswork. For the first time, the IPCC report predicts how changing climate might affect particular regions of the world. But these forecasts are only in their infancy, modellers warn. For some areas, models predict specific and well understood effects, such as hotter summers in Spain and smaller snowpacks (the accumulation of snow each season) in the Rocky Mountains in the United States. But improved analyses that
incorporate clouds, snow and ice into the models must be developed if regional predictions are to become more accurate, says Rasmus Benestad, a climate modeller at the Norwegian Meteorological Institute
in Oslo.
Extreme weather is another example of the remaining uncertainties. Climate researchers believe that storms and heavy rainfall will become more frequent as the planet warms. But pinning down where and when
that might happen is not so simple. In the tropics, rising sea-surface temperatures can be linked in a
relatively straightforward manner to storm formation, and the case for more intense storms seems more or less settled. But in the mid-latitudes, where atmospheric processes are more complex, some climate models predict more storms whereas others do not.
Improving the models, experts say, requires better data. Gaps and errors in observations are attributable to many causes: snowfall gauges that ice up, oceanographic floats that get lost, and changeovers in satellites that throw off carefully calibrated trends, to name but a few. Cloud and storm records urgently need to be
reprocessed using uniform techniques, says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado, and coordinating lead author of the report's chapter on
surface and atmospheric change.
"The IPCC report is a consensus report, and one that develops over nearly three years," he says. "This means that it is not the leading or bleeding edge of the science."
Earth's Polar Regions are priceless repositories of information about
past climates, as well as harbingers of our planet's future. To get a
better picture of where the Arctic and Antarctica have been and where
they are going, and how changes at the poles might affect humanity's
temperate perches, scientists from dozens of nations earlier this
month launched a 2-year research initiative called the International
Polar Year (IPY). This special issue helps raise the curtain on the
IPY with an exploration of some of the more vibrant research under way
at the ends of the Earth.
Polar processes exert a tremendous influence on many of our planet's
ecological and biogeochemical cycles. Sea ice helps control ocean
circulation, thereby influencing heat transport from low to high
latitudes, rainfall patterns, ocean biology, and the composition of
the atmosphere. Serreze et al. (p. 1533) examine the causes of the
decrease in Arctic sea-ice coverage in recent decades and offer
forecasts for the next century. Reduced sea ice could spark an Arctic
version of the California Gold Rush. Krajick (p. 1525) discusses how
nations are staking claims in the Arctic in the hopes of exploiting
minerals and hydrocarbons locked beneath the sea floor.
How fast sea levels rise over the coming century--potentiall y one of
the most serious consequences of global warming--will depend on how
fast the polar ice sheets melt. Shepherd and Wingham (p. 1529)
synthesize studies of the world's ice sheets to present a global
picture, with emphasis on recent changes in the mass of the Greenland
and Antarctic ice sheets. The polar seas are rich sources of marine
productivity, and millions of people depend on their bounty. Warming
is expected to have huge consequences for high-latitude denizens, as
Bohannon (p. 1520) and Stokstad (p. 1522) show. Change is also in the
air (literally) at the poles. Law and Stohl (p. 1537) examine the
progress made in understanding the Arctic's atmospheric chemistry and
discuss how anthropogenic and natural factors have affected, and may
affect, the Arctic's role in mediating regional and global climate.
The Arctic and Antarctica offer some of the most extreme environments
on the planet. For scientists, this poses logistical challenges,
especially with the stepped-up research activities taking place at the
poles during the IPY, as Mervis explains (p. 1514). Clery (p. 1523)
discusses how astronomers are overcoming hostile conditions to install
ever-bigger telescopes high on the Antarctic Plateau. At the other end
of the globe, scientists are teaming up with indigenous people in the
Arctic to fill gaps in their knowledge, writes Couzin (p. 151.
The burst of scientific activity during the IPY should yield vital
insights into the state of our planet for many years to come. If
present trends continue, the message from our polar regions could be
quite alarming indeed.
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