Richard A. Kerr
Greenhouse Warming Passes One More Test
Are humans indeed warming the world? If so, will future warming be big enough to matter· Confident answers depend in large part on the credibility of climate models. Greenhouse critics claim modelers can get any answer they like about warming simply by adjusting any of the numerous inputs whose values in the real world remain uncertain. Climate model running on the warm side· Crank in a bit more pollutant haze to shade the planet and cool it down, they say, and everything will look fine. Modelers have long argued that constraints such as the need to simulate current climate and the history of atmospheric warming keep their models more honest than that. Now a new, independent reality check from the ocean has strengthened their case.
On pages 267 and 270 of this issue, two groups of climate researchers report that two climate models have passed a new test: simulating the warming of the deep oceans during the past half- century. Their success "provides stronger evidence climate is changing," says climate modeler Simon Tett of the Hadley Centre for Climate Prediction and Research in Bracknell, United Kingdom, "and it's likely due to human influence." However, a conflict between the two studies underscores the difficulties in gauging how bad greenhouse warming could be.
Why worry about the ocean, when greenhouse warming of the atmosphere is what life on the surface will have to deal with? "The ocean is the flywheel of the global climate system," explains climate modeler Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla, California. The ocean holds so much heat that it tends to steady the rest of the climate system. "If there's one place you want to get it right, it's there," he says. Last year, oceanographer Sydney Levitus of the National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland, and his colleagues reported that the top 3000 meters of oceans worldwide had gained 18.2 x 1022 joules of heat between 1955 and 1996 (Science, 24 March 2000, p. 2126). In their new paper, they calculate that less than a tenth as much heat as that went into warming the global atmosphere and melting sea ice and glaciers. Their conclusion: If you're keeping track of the heat trapped by the strengthening greenhouse, the ocean is almost all that matters.
With that pivotal role in mind, both Barnett and Levitus tested greenhouse warming in a climate model against how the ocean has actually warmed. Barnett used the Parallel Climate Model developed at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and Levitus used the model from NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey. Both groups drove the warming with the increasing greenhouse gases of the past century, and both found that the models' world oceans warmed by just about as much as observed. And it appears that the ocean warming was likely due to increasing greenhouse gases, not the random oscillations of the climate system that modelers call internal variability. "The rising heat content of the past 50 years is way out of the bounds of internal variability" produced in a model, says modeler Thomas Delworth of GFDL, a co-author of the Levitus paper. The warming in the model ocean so closely matched the strength and geographical distribution of the actual warming that Barnett calculated with confidence exceeding 95% that human-produced greenhouse gases are behind real-world warming.
While the results of the two models further support the emerging consensus that humans are warming the world (see main text), they also drive home problems with making predictions from models. Just how bad warming will get by the end of the century, say, will depend on how much greenhouse gas--principally carbon dioxide--enters the atmosphere and how strongly the climate system reacts to it, a property called climate sensitivity. For more than 20 years, researchers have been estimating that climate sensitivity to a doubling of carbon dioxide is between a modest 1.5ºC warming and a searing 4.5ºC. Indeed, the NCAR and GFDL models reflect that recalcitrant uncertainty in their climate sensitivities of 2.1ºC and 3.4ºC, respectively.
How, then, can the two models agree about the past century of ocean warming· The explanation may lie in one of the remaining knobs on the climate machine: aerosols, the microscopic particles of sulfate, soot, and organic crud produced by fossil fuel burning, biomass burning, and volcanoes. Researchers are still figuring out how much aerosol of each sort is up there, how effectively each absorbs solar energy or reflects it back to space, and how each affects the number and size of cloud particles, another potent player in the climate system. The two models assumed different fossil-fuel aerosol histories, and the NCAR model ignored volcanic aerosols--discrepancies that may have compensated for differences in the models' sensitivities. As a result, says climate modeler Myles Allen of the University of Oxford, "both models could be right for the wrong reason." But whichever is more realistic, Allen says, the finding that real-world warming is not likely due to internal variability stands--although clearly, some better informed knob twiddling is still in order.
Related articles in Science:
- Anthropogenic Warming of Earth's Climate System.
Sydney Levitus, John I. Antonov, Julian Wang, Thomas L. Delworth, Keith W. Dixon, and Anthony J. Broccoli
Science 2001 292: 267-270.
- Detection of Anthropogenic Climate Change in the World's Oceans.
Tim P. Barnett, David W. Pierce, and Reiner Schnur
Science 2001 292: 270-274.
Issue of 13 Apr 2001,
Copyright © 2001 by The American Association for the Advancement of Science.