Carbon dioxide levels climbing toward a doubling of the 280 parts per million (ppm) concentration found in the preindustrial atmosphere pose the question: What impact will this increased greenhouse gas load have on the climate? If relatively small changes in CO2 levels have big effects—meaning that we live in a more sensitive climate system—the planet could warm by as much as 6 degrees Celsius on average with attendant results such as changed weather patterns and sea-level rise. A less sensitive climate system would mean average warming of less than 2 degrees C and, therefore, fewer ramifications from global warming.

Human civilization is now running an experiment (and without a control) that will definitively determine the answer. Scientists, however, have also realized that history can be a guide: Two new papers published in Science this week examine the historical record preserved in a stalagmite and microscopic seashells, respectively, to offer some clues.

Earth scientist Aradhna Tripati of the University of California, Los Angeles's Department of Earth and Space Sciences and her colleagues extracted a record of past atmospheric concentrations of CO2 stretching back 20 million years from the shells of tiny creatures known as foraminifera buried in a column of ocean mud and rock. The microscopic animals build shells of calcium carbonate out of minerals in seawater—a process that is affected by the water's relative pH (acidity), which is, in turn controlled by the level of CO2 in the atmosphere. More CO2 in the atmosphere means a more acidic ocean.

"The two species we picked to analyze [Globigerinoides ruber and G. sacculifer] are both ones that are around today, and the living animals actually have photosynthetic algae as symbionts, which means that they live in the surface ocean, since the algae require sunlight to survive," Tripati explains. And that means the fossil record of their shells will reveal the relative acidity of the surface waters in the ratio of boron to calcium as well as the specific chemical signature of the boron itself. "When seawater is more acidic, less boron gets incorporated into the calcium carbonate shells," she adds.

The researchers first matched this fossil record secured by the Integrated Ocean Drilling Program Expedition in the western tropical Pacific to existing records from bubbles trapped in Antarctic ice cores that stretch back 800,000 years, which preserve a precise record of past atmospheric composition. Thus reassured of the technique's accuracy, they plunged back into deep geologic time.

"Modern-day levels of carbon dioxide were last reached about 15 million years ago," Tripati says, when sea levels were at least 25 meters higher and temperatures were at least 3 degrees C warmer on average. "During the middle Miocene, an [epoch] in Earth's history when carbon dioxide levels were sustained at values similar to what they are today [330 to 500 ppm], the planet was much warmer, sea level was higher, there was substantially less ice at the poles, and the distribution of rainfall was very different."

Further, "at no time in the last 20 million years have levels of carbon dioxide increased as rapidly as at present," Tripati adds; CO2 concentrations have climbed from 280 ppm to 387 ppm in the past 200 years. And "our work indicates that moderate changes in carbon dioxide levels of 100 to 200 parts per million were associated with major climate transitions and large changes in temperature"—indicative of a very sensitive climate.

A nearly 400,000-year record of Ice Age transitions preserved in a stalagmite in the Sanbao and Linzhu caves of Hubei Province in China would seem to offer evidence in support of the sensitive climate scenario. The stalagmites, composed of calcium carbonate leached from dripping water, preserve a record of monsoon rainfall in the region by their composition. Paleoclimatologist Hai Cheng of the University of Minnesota and his colleagues then compared this record with climatic transitions, such as the shift into and out of an Ice Age.

The rock record reveals that such rainfall changes occur at the same time as general alterations in the relative strength of sunlight hitting the planet thanks to periodic shifts in Earth's orbit, known as Milankovitch cycles. At the same time as the solar heat increases, according to the monsoon record published in Science, CO2 levels also begin to rise.

"Climate systems are well linked worldwide, such as sea-level, CO2, ice sheet[s], the Asian monsoon, regional temperature and precipitation," Cheng says. "So a change in one of them could trigger changes in others." And that might mean the climate is too sensitive to tolerate current levels of CO2 without changing the conditions that have allowed human civilization to flourish in the past 10,000 years.