Rummaging through piles of neglected data, physical oceanographers have turned up millions of old, deep-ocean temperature measurements that confirm suspicions that Earth is heating up as a result of greenhouse gases. Indeed, the data, published in the 24 March Science, suggest that global warming may eventually be more serious than many scientists had assumed.
Greenhouse skeptics often point to the relatively modest atmospheric warming of the past few decades as evidence that the buildup of greenhouse gases isn't doing much harm. Climate modelers respond that much of the heat trapped by greenhouse gases should be going into the ocean, delaying but not preventing some of the atmospheric warming. But oceanographers have been unable to say who was right, because records of deep-ocean temperature have been too spotty to pick out clear trends. Many old measurements were handwritten on paper, captured in a photograph, or recorded in analog form on magnetic tape. Everything from mold to mice was devouring the data.
That's why, under the auspices of the United Nations-sponsored Global Oceanographic Data Archeology and Rescue project, oceanographers have spent the past 7 years seeking out ocean temperature data around the world and digitizing them for archiving on modern media.
After adding 2 million profiles of ocean temperature to the previously archived 3 million profiles, an international team led by Sydney Levitus of the National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland, could see a clear change. Between 1955 and 1995, the world ocean--the Pacific, Atlantic, and Indian basins combined--warmed an average of 0.06°C between the surface and 3000 meters.
The international data search-and-rescue effort "adds credibility to the belief that most of the warming in the 20th century is anthropogenic," says climate modeler Jerry D. Mahlman of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. As climate models implied, rising ocean temperatures have delayed part of the surface warming, says climate modeler James Hansen of NASA's Goddard Institute for Space Studies in New York City, but that can't continue indefinitely.
Mainstream climatologists have long estimated that a doubling of greenhouse gases, expected by the end of the 21st century, would eventually warm the world between 1.5° and 4.5°C. Some greenhouse contrarians have put that number at 1°C or even less. Now, the ocean-warming data "imply that climate sensitivity is not at the low end of the spectrum," says Hansen. He and some others now lean toward a climate sensitivity of about 3°C or a bit higher.