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Published online 17 March 2005 | Nature | doi:10.1038/news050314-13


Oceans extend effects of climate change

Sea levels will rise for centuries, even if we stop burning fossil fuels now.
 Even small rises in sea level can cause severe flooding and storms.© AP Photo/Tatyana Makeeva

Even if the world stopped burning fossil fuels tomorrow, the emissions already in the atmosphere would cause global temperatures to climb for the next hundred years and the sea level to keep rising for even longer, scientists have calculated.

Researchers have long known that the oceans delay the full effects of climate change because they heat up more slowly than the land. But until now they have had only a vague idea of how this lag will shape our long-term climate.

“The longer we wait, the worse it gets.”

Gerald Meehl
National Center for Atmospheric Research, Boulder, Colorado

Two studies by researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, use sophisticated computer models to show just how much climate change we have already signed up for.

Gerald Meehl and colleagues1 charted this course using two new models that allow major components of the climate to interact. They froze the atmosphere as it was in the year 2000 and projected how climate change in our century would play out if the concentrations of greenhouse gases were kept constant.

Their models forecast that even in this unrealistically rosy scenario, the earth would warm by an additional 0.5 °C by 2100, a similar rise in temperature to that seen during the previous century.

As ocean waters expand in response to this warming, global sea levels would mount by about 10 centimetres in the next hundred years. But the model does not account for ice cap and glacier melting; a better estimate would be double or triple this value, says Meehl.

"People think that 10 to 30 centimetres is not much, but a relatively small average rise in sea level is manifested in extreme high tide and storm surge events," Meehl adds.

Next-generation modelling

“What's important here is that we start taking steps to adapt to the eventual new climate.”

Vicki Arroyo
Director of policy analysis, Pew Center on Global Climate Change in Washington, DC

Meehl's group is the first to run such complex models on these types of experiment, known as 'climate commitment' studies.

In a complementary study2, Tom Wigley, a climatologist who created some of the first models to predict climate commitment, used a simpler model to come up with similar results. Both studies are published in the journal Science1,2.

The new models developed at NCAR will be used to generate data for the next assessment report of the Intergovernmental Panel on Climate Change, the consensus body that sets benchmarks for future climate change.

No stopping the sea

Both Meehl and Wigley were surprised at how dramatically sea levels continued to surge in their models, long after emissions were fixed. After 100 years at constant greenhouse gas concentrations, temperatures leveled out, but the sea level kept rocketing up.

"The relentless nature of this sea level rise impressed me," Meehl says.

Wigley also looked into the future along a course he thinks comes closer to reality, by fixing the rates of emissions rather than their concentrations. Under this scenario, a range of warming from 2 to 6 °C, and a sea level climb of 25 centimetres per century were predicted.

"In 400 years that's another metre of sea level rise," he says. "The consequences of that are very serious."

Committed to change

Both studies show that some of the damaging effects of climate change are unavoidable.

"We're already committed to a significant amount of climate change, even if we could stabilize concentrations at some point," Meehl says. "And the longer we wait, the worse it gets."

In the two studies' best-case scenarios, the world could be just tenths of a degree from the temperature the European Union has set as 'unsafe' for the world, Schneider points out.


Such a rise, of 2 °C over pre-industrial temperatures, would bring more extreme heat waves, storms and flooding. These would intensify crop failure, droughts and disease worldwide, a recent report by the International Climate Change Task Force indicates.

"What's important here is that we see the urgency of acting not only to reduce emissions or mitigate climate change in the first place but also to start taking steps to adapt to the eventual new climate and sea level rise conditions that we'll be operating under," says Vicki Arroyo, director of policy analysis at the Pew Center on Global Climate Change in Washington, DC.


