WASHINGTON, D.C.--Climate researchers who worry that they may have overlooked something the first time around have retested the links between the burning of fossil fuels, greenhouse warming, and the warming of the deep oceans. With greater attention this time to uncertainties, they have come up with the same answer but with even greater confidence: Humans are indeed warming the world, right down to the depths of the sea.
In 2001, climate researcher Tim Barnett of Scripps Institution of Oceanography in La Jolla, California, and colleagues ran a state-of-the-art climate model to gauge the impact of a century's worth of greenhouse gas emissions on atmospheric warming and the effect this warming has had on the oceans. When they compared the model's predictions of where and when the greenhouse's heat would have entered the oceans to actual observations of ocean temperature, they found a good match. With a confidence of 95%, they calculated, human-produced greenhouse gases are behind real-world warming. Three other studies using three other models have since found matches between model predictions and ocean observations.
But uncertainties lingered, prompting Barnett and colleagues at Scripps and Lawrence Livermore National Laboratory in Livermore, California, to revisit their model. They used a revised and updated set of ocean observations, this time avoiding a quirk in the data processing that had skewed temperatures in the data-poor Southern Hemisphere. They also took account of model-to-model variations by including detailed results from a second, independent model and by incorporating in their uncertainty analysis the range of ocean warmings from eight models. Finally, they followed the heat deep into separate ocean basins, which most other studies had not done.
In the end, the agreement among the two models and the observations "was really strong," says Barnett. The models "absolutely nailed the greenhouse signal" seen in the ocean. This time, statistical confidence is much greater than 95%, he reported here today at the annual meeting of the American Association for the Advancement of Science.
Most climate scientists are reassured. "The fact that multiple models simulate a comparable [ocean] warming gives a robustness to the results," says climate modeler Thomas Delworth of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Not that uncertainty has been banished from greenhouse warming. Climate researcher and modeler Gerald North of Texas A&M University in College Station still wonders whether the models have realistic enough oceans. More tests no doubt await.