This NASA image shows minimum concentration of Artic sea ice occuring in 2005, the lowest extent ever shown in the satellite record.

Washington — Cutting emissions of greenhouse gases like carbon dioxide (CO2) by 70 percent during the 21st century could help nations worldwide avoid the most dangerous potential consequences of climate change, according to a new study by scientists at the U.S. National Center for Atmospheric Research (NCAR) in Colorado.

The planet already is committed to some changes in surface temperature, rainfall and sea level for hundreds of years or more into the future, scientists say, but if CO2 concentrations in the atmosphere can be held to 450 parts per million, the climate system would stabilize by about 2100 instead of continuing to warm, Warren Washington, NCAR scientist and lead study author, told America.gov.

Today, according to the National Oceanic and Atmospheric Administration, the average global concentration of atmospheric CO2 is 383.9 parts per million by volume of air. If nothing is done to decrease greenhouse gas emissions, CO2 concentrations could reach 750 parts per million by 2100.

“When you burn a molecule of CO2, it has a lifetime of 90 to 100 years in the atmosphere,” Washington said. “So even if you suddenly stopped all emissions, it would take a long time for the system to lower the amount of carbon dioxide. We’re already on a warming path, so the point of the study is that if we start taking steps to cut back on emissions by roughly 70 percent, we can probably avoid the worst effects.”

The work was funded by the U.S. Department of Energy and the National Science Foundation.

CLIMATE THRESHOLD

The study, to be published April 21 in Geophysical Research Letters, a publication of the American Geophysical Union, comes as the nations of the world prepare for a December 7–18 meeting in Copenhagen to draft an ambitious global climate agreement for 2012, when the first commitment period under the Kyoto Protocol expires, and beyond. Officials from 192 countries, including the United States, will participate.

The Kyoto Protocol is an addition to the United Nations Framework Convention on Climate Change (UNFCCC) in force from 2005 to 2012 that establishes legally binding commitments for reducing CO2 and other greenhouse gases produced by industrialized nations and general commitments for all member countries. Several developed nations, including the United States, declined to ratify the agreement.

Greenhouse gases will be the focus of the 15th conference of the parties to the UNFCCC in Copenhagen, where major questions will include what greenhouse gas reductions industrialized and major developing countries will commit to and how industrialized countries will help finance developing-country obligations.

Average global temperatures have gone up by nearly 1 degree Celsius in the last century and much of the warming is believed to be due to human activities. The burning of coal, oil and natural gas; deforestation; and industrial and agricultural processes all emit greenhouse gases, mainly CO2.

Research shows that warming of about another 1 degree Celsius may be the threshold for dangerous climate change effects, and the European Union has called for dramatic cuts in emissions of CO2 and other greenhouse gases. The U.S. Congress is debating the issue.

RISING TEMPERATURES

A man and dog walk along a dry reservoir bed in Alcora, eastern Spain, in 2005.

To examine the impact of 70 percent CO2 cuts on global climate, Washington and colleagues ran a series of global studies with the NCAR-based Community Climate System Model. They assumed that CO2 levels could be held to 450 parts per million at the end of the century.

That figure comes from the U.S. Climate Change Science Program, which cited 450 parts per million as an attainable target if the world quickly adapted conservation practices and new green technologies to cut emissions dramatically.

The research team used supercomputer simulations to compare a business-as-usual scenario to one with dramatic cuts in carbon dioxide emissions beginning in about a decade. They showed that if CO2 is held to 450 parts per million, global temperatures will increase by 0.6 degrees Celsius (about 1 degree Fahrenheit) above current readings by 2100.

The study also showed that if CO2 emissions are not constrained, temperatures will rise by almost four times that amount, by 2.2 degrees Celsius (4 degrees Fahrenheit) above today’s readings.

Holding CO2 to 450 parts per million would also have the following effects, according to the modeling study:

• Sea-level rise would be 14 centimeters (5.5 inches) instead of 22 centimeters (8.7 inches). The 14 centimeters level is from thermal expansion by warmer water. Add another 14 centimeters or more to this from melting ice sheets and glaciers, Washington said. The total sea level rise, even given constrained CO2 concentrations, could be on the order of one meter.

• Arctic ice in summer would shrink by about a quarter in volume and stabilize by 2100, rather than shrinking at least three-quarters and continuing to melt.

• Arctic warming would be reduced by almost half, helping preserve fisheries and populations of sea birds and Arctic mammals in such regions as the northern Bering Sea.

The greatest uncertainty involves the ice sheets of Greenland and Antarctica and their potential to change sea levels. An ice sheet is a kilometers-thick, continentwide pile of snow that has been squeezed to ice and spreads out under its own weight. As a result of spreading, the edges of the ice sheet thin, become ice shelves, then break off and become icebergs.

In a warming world, the ice shelves, already in contact with the ocean, can melt very easily from the bottom up, but scientists do not yet have a good understanding of the process.

“If we don’t do anything about climate change, things will be a lot worse,” Washington said. “If we do something that is feasible and reasonable in a scientific sense, we can mitigate this danger to the world in a very significant way.”

More information about the National Center for Atmospheric Research is available at its Web site.