On the issue of sea-level rise, the AR4 is essentially in agreement with the IPCC’s previous assessment, published in 2001. That is, we can expect a sea-level rise of between 0.15 and 0.90 metres by 2100, with a best guess around 0.4 m. Currently sea-level rise is increasing at a rate that would lead to an 0.3 m rise by 2100, but the rate has increased over the past 15 years. More recent research, however, has shown that the range of potential sea-level rise should be altered to a minimum of 0.5 m and a maximum of 1.4 m. Thus, many scientists now believe that the best guess would lie in the 0.5 to 1.0 m range. This doesn’t sound like much, but in many parts of the world, the impacts can be significant. For example, with just an 0.5 m sea-level rise, extreme sea-level events (flooding associated with high tides and storm surges) would occur every few days instead of every few years in many places around Australia’s coast, including Sydney, Melbourne and Brisbane. The same would be undoubtedly true for many coastal locations in East Asia.
What has caused such a rapid shift in the scientific outlook? The main culprits are the large polar ice sheets on Greenland and West Antarctica, which together hold enough grounded ice to raise sea-level by 13 m if they melted or disintegrated entirely. Just a few years ago, these ice sheets were thought to be stable for long periods of time, certainly over the 21st century, and that it would take millennia, not centuries for them to melt.
Now there is rapidly growing concern over Greenland (which contains ice equivalent to 7 m of sea-level rise). A few years ago the Greenland ice sheet was in balance – the loss of ice around the edges was compensated by increased snowfall over the interior. The mass balance has shifted dramatically over the past several years, with a net loss now approaching 300 cubic kilometres of ice per year. Of more concern is that the rate of loss is accelerating sharply. The Greenland ice sheet may have crossed a threshold, driven by the strong warming in the North Atlantic region, beyond which it is committed to disappearing. If it hasn’t crossed such a threshold, it is likely getting close to it. The question then turns to how fast can the ice disappear, and thus raise sea-level.
Again, there are some worrying observations. Melting of the ice now seems to be the less important process. Rather, large blocks of ice are breaking away from the outlet glaciers and sliding into the ocean, where they instantaneously raise sea-level. If this process were to continue to accelerate, a global sea-level rise of 0.5 m by 2100 seems likely, and a rise of 1.0 m cannot be ruled out. Such a sea-level rise would have enormous impacts on many coastal regions in the Asia-Pacific region.
The second issue of growing concern is the carbon sink strength of the natural environment. At present only about half of anthropogenic emissions of carbon dioxide stay in the atmosphere; the other half is absorbed by land and oceans. All of the trajectories of emission reductions needed to reach a target atmospheric concentration of carbon dioxide implicitly assume that these natural sinks will maintain their current strengths relative to emissions. And that has been the case for most of the 20th century.
Now there is evidence that the oceanic sink is beginning to weaken. The oceans are still taking up more carbon dioxide every year, but the proportion of emissions that they absorb is slowly dropping. Thus, relatively more of our emissions are now remaining in the atmosphere, where they can affect the energy balance at the Earth’s surface. This implies that anthropogenic emissions will be need to be reduced further and faster to reach a given target atmospheric carbon dioxide concentration.
All of this is, of course, bad news for policy. It raises the fundamentally important question that “dangerous climate change” – however one may wish to define it – probably lies at lower levels of carbon dioxide concentrations that we earlier thought. In fact, well-known American climate scientist James Hansen now believes that a safe level of carbon dioxide (as defined by not losing big ice sheets and avoiding massive loss of biodiversity) is around 325 to 350 parts per million. Carbon dioxide concentration now stands at 383 ppm so, according to Hansen, we are now effectively in overshoot and thus need to become not only a zero-carbon emission global society, but a carbon-absorbing global society.
As a scientist, it is not easy to be the bearer of bad news, but as “planetary physicians” we have an obligation to be absolute straight and up front with the patient.
Great article. With the Obama administration’s climate change policies, hybrid cars will dominate the market soon. Many companies are also working on biofuels as alternatives to gasoline.