Even if all industrial pollution and auto emissions suddenly ceased today, Earth's climate will warm at least 1 degree by the year 2100 and seas will rise 4 inches (11 centimeters), according to a new study.
The warming is likely to continue through 2400, another study forecasts.
The worst-case scenario projects the global average temperature rising 6.3 degrees Fahrenheit within this century and the sea level climbing a foot or more.
The outlook is based on greenhouse gases that were in the atmosphere in 2000, with no additional input of the chemicals, which serve as a global blanket to trap solar energy.
"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," said Gerald Meehl, who led the study out of the National Center for Atmospheric Research (NCAR).
"Even if we stabilize greenhouse gas concentrations, the climate will continue to warm, and there will be proportionately even more sea level rise," Meehl said today. "The longer we wait, the more climate change we are committed to in the future."
Trends and forecasts
Results of the computer modeling are reported in the March 18 issue of the journal Science.
Computer models like these are based on what's known about past climate change. Projecting the future involves many variables that are not completely understood, scientists caution. Critics charge that the models do not necessarily represent actual outcomes.
Few scientists doubt that the planet's climate is indeed growing warmer. A report last month confirmed that last year was among the four warmest on record and projected 2005 will be the warmest.
More controversial is whether and how much humans have contributed.
The model through 2100 has the temperature rising at least 1 degree Fahrenheit, or about a half-degree Celsius. That's similar to the warming scientists say occurred during the 20th Century.
Only part of the picture
The sea-level prediction is based solely on thermal expansion -- the oceans physically swell as they get warmer. The projection does not take into account fresh water that other scientists expect to melt from glaciers and ice sheets, a process that appears already underway and which could snowball, some experts say.
Greenland's largest glacier, for example, doubled its forward progress toward the sea between 1997 and 2003. It is also thinning rapidly, adding water to the sea more quickly than realized, a study last year found.
Add the probable melting in, and seas could rise 8 inches through 2100 in the best-case scenario, Meehl and his colleagues say.
The rise could swamp some coastal villages, shrink islands, and make hurricanes and other extreme weather events more catastrophic.
The inevitable change, as Meehl's model has it, is due to two factors.
- The ocean lags far behind the land and the air in temperature changes. This "thermal inertia," as scientists call it, means big changes in the oceans occur over decades and centuries, not years. A warming change seen in the atmosphere in recent decades cannot have fully played out yet in the water.
- Carbon dioxide and other greenhouse gases do not break down into other chemicals rapidly, so what's up there will be there for a long time.
'Impossible to avoid'
A separate paper in the journal, also from NCAR, suggests temperatures and sea levels are bound to rise for the next three centuries even if no more greenhouse gases are added to the air.
"Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels," said Tom Wigley, author of the second study. "For sea level rise, a substantial
long-term commitment may be impossible to avoid."
Unlike the models' assumptions, greenhouse gas emissions continue.
"When and how we stabilize concentrations will dictate, on the time scale of a century or so, how much more warming we will experience," Meehl and his colleagues write in the journal. "But we are already committed to ongoing large sea level rise, even if concentrations of [greenhouse gases] could be stabilized."