Completely eliminating atmospheric carbon dioxide emissons may not bring down the global warming it is bringing about. This is the conclusion of Susan Solomon (National Oceanic and Atmospheric Administration, Colorado) and coworkers, using computer models, who also predict two other detrimental global consequences of this warming (precipitation loss and seal level rise) that are already occurring, are well-understood, and are predicted by numerous current climate models.
Carbon dioxide lasts a long time in the atmosphere.
On a thousand-year time scale, some atmospheric carbon dioxide remains in the atmosphere, and the remaining finds its way into the ocean. The scientists deliberately do not consider the fact that other processes (e.g, geological) could remove the remaining excess carbon dioxide from the atmosphere, because these processes take place over many thousands of years.
Given this knowledge, the scientists set out to determine the expected drop of atmospheric carbon dioxide levels, assuming emissions stopped completely and abruptly. They considered scenarios in which levels increased by 2% per year (the approximate increase observed within the past ten years), up to a maximum of 450-1200 parts per million by volume, before termination.
The scientists found that an equilibrium level of atmospheric carbon dioxide is reached in all scenarios. Approximately 40% of the maximum levels are retained in the atmosphere for at least one thousand years.
This means that atmospheric carbon dioxide levels will remain well above preindustrial values, in every one of these scenarios, for at least one thousand years. Once it's in the atmosphere, it's not going anywhere quickly.
Global surface temperature increases.
What do elevated atmospheric carbon dioxide levels mean for global surface temperatures? The scientists found that average temperatures initially increase with carbon dioxide emissions.
However, after emissions cease, temperatures will stay roughly constant for the next thousand years. The reason is that there will be a balance between two effects, one due to removal of atmospheric carbon dioxide, the other due to a reduced ability of the oceans to absorb excess heat.
This means that even if atmospheric carbon dioxide emissions were to suddenly cease, the planet will not cool down. Once temperatures warm up, they will stay that way.
Global rainfall changes.
What do elevated atmospheric carbon dioxide levels mean for global rainfall patterns? The scientists begin by noting that increased temperatures lead to more water vapor in the atmosphere.
Other scientists have predicted that global warming will lead to precipitation shifting northward, drying out the subtropics. This is currently in evidence in the southwest United States and in the Mediterranean.
The scientists used the available data to predict dry-season precipitation levels, per degree Celsius increase in global warming. Globally, the result is uncertain in many regions; however, there are many regions where predicted changes in rainfall patterns are more certain.
If atmospheric carbon dioxide levels peak at 450 parts per million per volume, many regions face an 8-10% drop in dry season precipitation. These include northern Africa, southern Europe, and western Australia.
It is important to put this in perspective. The United States "Dust Bowl" of the 1930s, which saw widespread crop failure, was caused by a 10% drop in average rainfall.
This 1930s dry period was relatively short lived. In contrast, the dry period discussed by these scientists is predicted to last for centuries, and will have long-term consequences.
Some, but not all, of this predicted loss in dry season precipitation would be offset by increases in rainy season precipitation. However, many of these regions are already arid, and will be especially adversely affected by a drop in precipitation.
Sea level rise.
What do elevated atmospheric carbon dioxide levels mean for rising oceans? It is already known that a warmer world will cause the oceans to expand and surface glaciers to melt, contributing to a rise in ocean levels.
Although the observed effect may be slower than the others (temperature and rainfall), the scientists predict an eventual 0.2-0.6 meter increase in sea levels per degree Celsius of temperature increase. Complete loss of surface glaciers and ice sheets have the potential to raise the ocean by several meters, but this estimate is more uncertain.
This means that many low-lying areas of the world will be flooded, even in optimistic scenarios. This will disrupt lives the world over.
Policy implications.
These results clearly tell us that the long-term effects of climate change are dependent on the maximum levels of carbon dioxide emitted into the atmosphere. The detrimental effects will be seen worldwide, and will last at least one thousand years.
Combating global warming, by mitigating carbon dioxide emitted into the atmosphere, requires strong action now. The only other option is to pin our hopes on finding a method to actively reduce carbon dioxide levels, at some unknown point in the future, an extremely risky strategy that puts the future of the planet and our civilization at long-term risk.
for more information:
Solomon, S.; Plattner, G.-K.; Knuttic, R.; Friedlingstein, P.
Irreversible climate change due to carbon dioxide emissions.
Proc. Natl. Acad. Sci. USA 2009, 106, 1704-1709.