Published: 28.01.09
Climate change

Irreversible climate change

Once climate change is realized and its consequences become noticeable, there is no way back for over 1000 years. This is what an international team of scientists is telling society, politics and science with its new study.

Simone Ulmer
Archipelagos like the Seychelles could soon disappear off the world map completely due to rising sea level. (Photo: flickr/guebosch)
Archipelagos like the Seychelles could soon disappear off the world map completely due to rising sea level. (Photo: flickr/guebosch) (large view)

The scientists paint a threatening picture: Even if all carbon dioxide emissions caused by burning fossil fuels could be stopped from a certain point in time onwards, the global temperature would not fall for 1000 years. These findings are not really new, but Reto Knutti, professor at ETH Zurich’s Institute of Atmosphere and Climate and co-author of the paper that appeared in PNAS, explains: “Many members of the public, politicians and experts are unaware of this. If we want to prevent disastrous consequences within bounds of the UN Framework Convention, we have to predict what is going to happen, rather than wait and observe, because then it may be too late.”

CO2 residence time prevents damage limitation

The scientists attribute the fact that the damage will remain irreversible for such a long period of time to the fact that the greenhouse effect is dominated by carbon dioxide; CO2 has a very long residence time in the atmosphere and is on a multi-century timescale only removed from the atmosphere by absorption by the ocean and land biosphere. “The key issue for us was to demonstrate the extent of impact the long residence time of CO2 in the atmosphere can have”, explain Knutti and Gian-Kasper-Plattner, research scientist at ETH Zurich’s Institute of Biochemistry and Pollutant Dynamics, and also co-author of the study.

Oceans preserve temperature rise

As the CO2 content in the atmosphere increases, the average global temperature rises. The researchers’ models reveal that if all CO2 emissions could be curbed overnight, the temperatures would still remain at the level they have reached now. “The thing is that people generally think that all the problems will be solved as soon as steps have been taken to combat global warming and that the negative effects will just disappear. In reality, this couldn’t be further from the truth”, the two researchers warn. After the CO2 emissions cease and the CO2 levels in the atmosphere gradually fall, more heat can be radiated from the Earth without being reflected back by the gas. Through the change in the system’s equilibrium, the oceans, which heat up slower than the atmosphere, do not cool but rather continue to heat up, only more slowly, states Knutti. This compensation effect ultimately means that there will not be a noticeable drop in atmospheric temperature for a period of 1000 years or more.

The researchers only considered aspects in their modeling that are scientifically well-understood. Melting of polar ice sheets and continental glaciers was not taken into consideration in the modeled sea level rise, for example, as these processes are still uncertain. However, according to their models, the expansion of the seawater, which warms up purely as a result of the CO2 levels, causes the sea level to rise by at least 0.4 to 1.0 meters with a CO2 concentration of 600 parts per million (ppm). If the concentration reaches 1000 ppm, sea level will even rise by 0.6 to 1.9 meters. “Melting glaciers could cause the sea level to rise by a another 0.2 to 0.7 meters and the melting of Greenland and the Antarctic could even mean an increase of several meters”, claims Knutti.

Acidification of the oceans

The oceans also become increasingly acidified through the CO2, 80% of which is taken up by the ocean in the long run (see ETH Life article). This has far-reaching consequences for this particular ecosystem. The habitats where it is still possible for calcifying organisms to form shells and skeletons out of CaCO3 become severely limited, for instance. These organisms are often at the beginning of the food chain and are essential for the ecosystem to remain intact. The rise in sea levels, however, will also alter the map: Islands will disappear and coastlines will be redrawn.

Additional consequences of global warming are the fact that the “normal” circulation pattern of precipitation and evaporation, i.e. the water supply all over the world, will be altered in a way that arid regions will become even drier and new arid zones will be created. “The rainfall in regions of the world that are already dry could decrease by up to 20 or 30 percent”, says Plattner. For comparison, during earlier drought periods in North America, Europe or Australia, the average rainfall was reduced by about 10 percent for 10 to 20 years only; this affects the water supply and increases the risk of fire.

Radical measures required

According to the recommendations of the fourth report of the Intergovernmental Panel on Climate Change (IPCC), released in 2007, the CO2 levels in the atmosphere should not exceed 450 ppm. It is assumed that humans and the Earth can adapt to the changes this would inevitably entail. That said, Plattner stresses that even stabilizing the CO2 concentrations at 450 ppm would require radical steps to be taken. “We would have to rethink our entire energy system within a few decades and completely eliminate fossil fuel emissions in the medium term before we can even start thinking about stabilizing CO2 concentrations at 450 ppm.” However, this will be no easy task as our annual CO2 emissions alone increased by three percent each year between 2000 and 2005.

The researchers’ studies do not include geoengineering measures that would remove CO2 from the atmosphere, using geotechnical procedures for example, and storing it in used salt mines or oilfields. Such procedures are already being explored but the question as to whether they work effectively on a large scale is still open. Consequently, we do not know whether the CO2 could leak back out or cause damage underground through chemical reactions.

Reto Knutti states: “The economic principles of discounting say that all the effects in the distant future, i.e. after 2100, are irrelevant (see ETH Life article). Our results now show that the negative effects will persist for at least 1000 years. It is hard to justify that our behavior can harm the next 30 generations or so and that they will be unable to do anything about it.”


Solomon S, Plattner G-K, Knutti R & Friedlingstein P: Irreversible climate change due to carbon dioxide emissions. PNAS-Online publication:; doi:10.1073/pnas.0812721106