Frontiers

Carbon Counter

April 2006

We know that the amount of CO2 going into the atmosphere is increasing. We know that the physical chemistry of CO2 gas means that the more we pump into the atmosphere, the warmer it's going to get. ...And we know that all of this becomes detectable starting around 1850, at the beginning of the Industrial Revolution when the burning of fossil fuel became more prevalent."

Andréa Grottoli is cataloging the knowledge base that shores up the "overwhelming consensus in the scientific community" that global warming is a growing threat. Grottoli is an assistant professor in the Department of Earth and Environmental Science and the principal investigator in the Marine Biogeochemistry Lab. She knows all this because her ongoing research is adding to the mounting pile of evidence that scientists find so convincing. "There is no other time in the last 400,000 years that we know of," she states, "when the rate of increase in CO2 in the atmosphere has been greater than it has been in the last century."

With both hands, she lifts a piece of evidence from the drawer of a laboratory cabinet. It's a chalk-white bar of calcium carbonate. She drilled it with a hollow bit from a coral head beneath nine feet of water near the Palau islands in the Pacific. Corals build up a skeleton of calcium carbonate, one layer at a time, like the annual growth rings in a tree. For researchers like Grottoli who know what to look for � and have the patience to shave a two-foot coral core into thousands of slices � the bands of skeletal matter can yield information on climate going back some 400 years.

"When I talk about climate," Grottoli explains, "it's not just air temperature. It's an ocean-atmosphere coupled system." Earth is largely an interactive, air-water entity, despite being named for terra firma. The carbon dioxide dumped into the atmosphere makes its way into the chemistry of seawater and eventually shows up in plants and animals that live in the world�s oceans. Grottoli looks for those signal traces in the skeletons of corals and sclerosponges. Sclerosponges are slow growing filter feeders whose skeletons also accrete layers of calcium carbonate. She takes a mushroom-size specimen from a jar. "Everything marine stinks," she apologizes. "They smell like wet dog or low tide." By studying carbon isotopes in the strata of bone-matter in these creatures, some of which can grow for centuries, Grottoli can reconstruct the history of carbon dioxide buildup in the ocean.

In her most recent work, she looks at the ratios of carbon-13 to carbon-12 in sclerosponges, which are known to parallel the carbon isotopic composition of seawater. "Fossil fuel has very little carbon-13, and it has lots of carbon-12," she explains. "As we burn more fuel, the carbon dioxide gets taken up by the ocean. So you have a dynamic exchange — and an isotopic exchange as well. The ratio of carbon-13 to carbon-12 in the ocean starts to go down, and it turns out, the ratio of carbon-13 to carbon-12 in sclerosponges goes down too." Scientists have measured the decrease in the carbon-isotope ratio in seawater since 1990. Grottoli�s preliminary findings suggest that isotopic analysis of sclerosponges can extend that data back by another 50 years or more. "It's an evolving science," she says, "but the slope dramatically changes as soon as we start having the combustion engine."

Grottoli's research encompasses a range of other studies, all related to climate change arising from the global interaction of oceans, atmosphere and human activity. "It's the same reason why I think we should all recycle and why we should all drive fuel-efficient cars," she says of her research interests. "It's a sense of responsibility for the planet and our children and our future."