While the question of whether our planet is warming is settled among climatologists (they say it is), other equally important matters remain unresolved. How much of the warming can be attributed to human activity? What are the long-term effects of overloading greenhouse gases into Earth's atmosphere? And just how smooth will planetary warming be?

The last question receives a partial response in this month's Geology , as Indiana University Bloomington geochemist Simon Brassell and graduate student Mirela Dumitrescu and Royal Netherlands Institute for Sea Research colleagues report a very changeable climate during the late Mesozoic Era, a period dominated by dinosaurs and non-flowering plants 120 million years before human beings existed.

As part of the National Science Foundation's Ocean Drilling Program, the geoscientists voyaged in 2001 to Shatsky Rise, a study site 1,600 kilometers (1,000 miles) east of Japan and 3,100 meters below the ocean surface. Shatsky Rise is known to have formed at the end of the Jurassic Period immediately prior to the beginning of the Cretaceous, the last period of the Mesozoic Era.

The scientists' vessel, the JOIDES Resolution, is specially outfitted with a drill that can be lowered to the sea floor for the collection of rock samples.

The drill bit was driven 566 meters into Shatsky Rise. Rocks freed by the drill were transported directly to the surface for analysis. The rocks corresponding to early Aptian time were extremely rich in organic material. By analyzing the carbon and nitrogen content of the samples, the geochemists found evidence for changes in carbon cycling and in nitrogen fixation by ocean biological communities associated with changing climate. A special analysis method targeting certain complex carbon-containing molecules provided values for a measurement called TEX86 that revealed mean temperature variations between 30 deg C (86 deg F) and 36 deg C (97 deg F) with two prominent cooling episodes of approximately 4 deg C (7 deg F) in tropical surface temperatures during the early Aptian. By comparison, today's tropical sea surface temperatures typically lie between 29 and 30 deg C.

The finding is relevant to the ongoing climate change discussion, IUB geologist Simon Brassell says, because it portrays an ancient Earth whose temperatures shifted erratically due to changes in carbon cycling and did so without human input.

"One of the key challenges for us is trying to predict climate change," Brassell said. "If there are big, inherent fluctuations in the system, as paleoclimate studies are showing, it could make determining Earth's climatic future even harder than it is. We're learning our climate, throughout time, has been a wild beast."

More information about the Ocean Drilling Program can be found at http://www.oceandrilling.org/.