Scientists have long warned that some level of global warming is a done deal - due in large part to heat-trapping greenhouse gases humans already have pumped skyward.
Now, however, researchers are fleshing out how much future warming and sea-level rise the world has triggered. The implicit message: "We can't stop this, so how do we live with it?" says Thomas Wigley, a climate researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
One group, led by Gerald Meehl at NCAR, used two state-of-the-art climate models to explore what could happen if the world had held atmospheric concentrations of greenhouse gases steady since 2000.
The results: Even if the world had slammed on the brakes five years ago, global average temperatures would rise by about 1 degree Fahrenheit by the end of the 21st century. Sea levels would rise by another 4 inches over 20th-century increases, just from expansion of the warming water. Rising sea-levels would continue well beyond 2100, even without adding water from melting glaciers and ice sheets. The rise highlights the oceans' enormous capacity to absorb heat and its slow reaction to changes in atmospheric conditions.
The team ran each model several times with a range of "what if" concentrations, as well as observed concentrations, for comparison.
Temperatures eventually level out, Dr. Meehl says in reviewing his team's results. "But sea-level increases keep on going. The relentless nature of sea-level rise is pretty daunting."
Dr. Wigley took a slightly different approach with a simpler model. He ran simulations that capped emissions, as well as concentrations, at 2000 levels. And he ran his calculations out to the year 2400. If concentrations are held constant, warming could exceed 1.8 degrees F. by 2400. If annual emissions are held at 2000 levels, warming could range from nearly 4 degrees to roughly 11 degrees F. The outcome depends on how sensitive the climate truly is to changes in greenhouse gases.
Wigley also calculates rates of sea-level rise. These range from 4 inches per century when limiting greenhouse- gas concentrations, to nearly 10 inches if annual emissions are held at 2000 levels. In this case, melting glaciers and ice sheets are taken into account.
The two researchers add that far from holding steady, concentrations of greenhouse gases continue to rise. Thus, at best, the results point to the least change people can expect, they say.
Both studies were published in Friday's edition of the journal Science.
The idea that some level of global climate change from human activities is inevitable is not new. But the word has been slow to make its way into the broader debate.
"Many people don't realize we are committed right now to a significant amount of global warming and sea-level rise. The longer we wait, the more climate change we are committed to in the future," Meehl says.
These studies are part of a wider effort to build the basis for the Intergovernmental Panel on Climate Change's next set of reports, slated for release in 2007.
While the concept of climate-change commitment isn't new, these fresh results "tell us what's possible and what's realistic" and that for the immediate future, "prevention is not on the table," says Roger Pielke Jr., director of the Center for Science and Technology Policy Research at the University of Colorado at Boulder.
To Pielke and others, this means adaptation should be given a much higher priority that it's received to date. "There's a cultural bias in favor of prevention," he says. But any sound policy includes preparation as well, he adds. "We have the scientific and technological knowledge we need to improve adaptation" and apply that knowledge globally.