Global Warming 'Irreversible' For 1,000 Years

The pioneering study was conducted by an international team led by senior scientist Dr. Susan Solomon of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder. It shows how changes in surface temperature, rainfall, and sea level are irreversible for more than 1,000 years after carbon dioxide emissions are completely stopped.

"People have imagined that if we stopped emitting carbon dioxide the climate would go back to normal in 100 years, 200 years. That's not true," said Solomon in a teleconference with reporters.

The coal-fired Liddell power plant in New South Wales, Australia emitting greenhouse gases. (Photo courtesy Macquarie Generation)

"In the long run," said Solomon, "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."

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.

Decrease in 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 are among the impacts predicted.

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.

They 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.

The coal-burning Neurath power plant in Nordrhein-Westfalen, Germany emitting greenhouse gases. (Photo courtesy RWE AG)

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 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, they state.

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.

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.

The research was supported by the Office of Science at the Department of Energy.

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.

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