2009 News & Events

A new scientific study to be published in the Proceedings of the National Academy of Sciences this week reaches a powerful conclusion about the climate change caused by future increases of carbon dioxide (CO₂): to a large extent, there's no going back.

The pioneering study, led by CSD senior scientist Susan Solomon, shows how changes in surface temperature, rainfall, and sea level are largely irreversible for more than 1000 years after carbon dioxide emissions are completely stopped. If carbon dioxide is allowed to rise to 450-600 parts per million from its current value of 385 parts per million, and then CO₂ emissions completely cease, the results would include persistent decreases in dry-season rainfall that are comparable to the 1930s North American "Dust Bowl" in zones including southern Europe, northern Africa, southwestern North America, southern Africa, and western Australia. The study notes that decreases in rainfall that last not just for a few decades but over centuries are expected to have a range of impacts that differ by region, including human water supplies, effects on dry-season wheat and maize agriculture in regions of rain-fed farming such as Africa, increased fire frequency, ecosystem change, and desertification.

The scientists emphasize that increases in CO₂ that occur in this century essentially "lock in" sea level rise that would slowly follow in the next thousand years. Considering just the expansion of warming ocean waters as a lower bound, the authors find that the irreversible global average sea level rise by the year 3000 would be at least 0.4 - 1.0 meters if CO₂ peaks at 600 parts per million, and about double that amount if CO₂ peaks at 1000 parts per million. Rising sea levels would cause "...irreversible commitments to future changes in the geography of the Earth, since many coastal and island features would ultimately become submerged," the authors write.

Added carbon dioxide and its climate effects linger because of the ocean.

"In the long run, both carbon dioxide loss and heat transfer depend on the same physics of deep-ocean mixing. The two work against each other to keep temperatures almost constant for more than a thousand years, and that makes carbon dioxide unique among the major climate gases," said Solomon.

Geoengineering to remove carbon dioxide from the atmosphere was not considered in the study. "Ideas about taking the carbon dioxide away after the world puts it in have been proposed, but right now those are very speculative," said Solomon.

The authors relied on measurements as well as many different models to support the understanding of their results. They focused on drying of particular regions and on thermal expansion of the ocean because observations suggest that humans are contributing to changes that have already been measured.

Besides Solomon, the study's authors are Gian-Kasper Plattner and Reto Knutti of ETH Zurich, Switzerland, and Pierre Friedlingstein of Institut Pierre Simon Laplace, Gif-Sur-Yvette, France.

Background: It has long been known that some of the carbon dioxide emitted by human activities stays in the atmosphere for thousands of years, but the new study advances the understanding of how this affects the climate system. The authors used measurements and several models to examine the consequences of allowing carbon dioxide to build up to several different peak levels beyond present-day concentrations, and then completely halting the emissions after the peak. The authors found that the scientific evidence is strong enough to quantify some irreversible climate impacts, including rainfall changes in certain key regions, and global sea level rise.

Significance: This study gives a clear demonstration that the climate change that takes place due to increases in carbon dioxide is largely irreversible, and would have large consequences for agriculture, ecosystems, and coastal environments.