Even if all greenhouse gases had somehow been stabilized back in the year 2000, we would still be committed to a warmer Earth and greater sea level rise in the present century, according to a new study performed by a team of climate modelers at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Indeed, say the researchers, whose work was funded by the National Science Foundation (NSF), globally averaged surface air temperatures would still rise one degree Fahrenheit (about a half degree Celsius) by the year 2100, even if no more greenhouse gases were added to the atmosphere. And the resulting transfer of heat into the oceans would cause global sea levels to rise another 4 inches (11 centimeters) from thermal expansion alone.
The team's findings are published in this week's issue of the journal Science.
“This study is another in a series that employs increasingly sophisticated simulation techniques to understand the complex interactions of the Earth,” says Cliff Jacobs of NSF’s atmospheric sciences division. “These studies often yield results that are not revealed by simpler approaches and highlight unintended consequences of external factors interacting with Earth’s natural systems.”
“Many people don’t realize we are committed right now to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere,” says lead author Jerry Meehl. “Even if we stabilize greenhouse gas concentrations, the climate will continue to warm, and there will be proportionately even more sea level rise. The longer we wait, the more climate change we are committed to in the future.”
The half-degree temperature rise predicted by the NCAR modelers is similar to what was actually observed by the end of the 20th century, but the projected sea level rise is more than twice the 3-inch (5-centimeter) rise that was observed then. Moreover, these forecasts do not take into account any fresh water from melting ice sheets and glaciers, which could at least double the sea-level rise caused by thermal expansion alone.
The models also predict a weakening of the North Atlantic thermohaline circulation, which currently warms Europe by transporting heat from the tropics. Even so, Europe heats up along with the rest of the planet because of the overwhelming effect of greenhouse gases.
Though the study finds signs that the temperature rise will level off some 100 years after the greenhouse gases stabilize, it also finds that ocean waters will continue to warm and expand beyond then, causing global sea level to rise unabated.
According to the report, the inevitability of climate change results from thermal inertia, mainly from the oceans, and the long lifetime of carbon dioxide and other greenhouse gases in the atmosphere. Thermal inertia refers to the process by which water heats and cools more slowly than air because it is denser than air.
The new study is the first to quantify future “committed” climate change using coupled global 3-dimensional climate models. Coupled models link major components of Earth's climate in ways that allow them to interact with each other. Meehl and his NCAR colleagues ran the same scenario a number of times and averaged the results to create ensemble simulations from each of two global climate models. Then they compared the results from each model.
The scientists also compared possible climate scenarios in the two models during the 21st century in which greenhouse gases continue to build in the atmosphere at low, moderate, or high rates. The worst-case scenario projects an average temperature rise of 6.3 °F (3.5 °C) and sea level rise from thermal expansion of 12 inches (30 centimeters) by 2100. All scenarios analyzed in the study will be assessed by international teams of scientists for the next report by the Intergovernmental Panel on Climate Change, due out in 2007.
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