New evidence of global warming in Earth’s past supports greenhouse climate theory

New evidence of global warming in Earth’s past supports current models for how climate responds to greenhouse gases

CA–Scientists have filled in a key piece of the global climate picture for a period 55 million years ago that is considered one of the most abrupt and extreme episodes of global warming in Earth’s history. The new results from an analysis of sediment cores from the ocean floor are consistent with theoretical predictions of how Earth’s climate would respond to rising concentrations of greenhouse gases in the atmosphere.

The new study, led by James Zachos, professor of Earth sciences at the University of California, Santa Cruz, will be published online by Science Express on October 23, and will appear in a later print edition of Science magazine.

The researchers analyzed sediments deposited on the seafloor during a period known as the Paleocene-Eocene Thermal Maximum, when a massive release of heat-trapping greenhouse gases is thought to have triggered a runaway process of global warming. Climate theory predicts that the increase in greenhouse gases would have caused temperatures to rise all over the planet, with greater increases in sea surface temperatures at high latitudes than at low latitudes.

Zachos and a team of researchers at UCSC and several other institutions have now obtained the first reliable estimates of the change in tropical sea surface temperatures during this period. When combined with existing records of sea surface temperatures at high latitudes, the findings fit well with the predictions of computer simulations based on current climate theory.

The study provides important backing for the climate models that scientists are using to predict the effects of the current rise in atmospheric carbon dioxide due to industrial emissions, Zachos said.

“The predictions from the models seem to be consistent with the geologic record, so I’d say greenhouse climate theory is alive and well,” he said. “People have raised questions about how accurate these models are in terms of handling heat transport in response to rising greenhouse gases, but this study indicates that the climate people have got it right or close to right.”

The Paleocene-Eocene Thermal Maximum, starting about 55 million years ago and lasting about 150,000 years, is marked by dramatic changes in the fossil record of life in the ocean and on land. Average global temperatures increased by about 5 degrees Celsius (9 degrees Fahrenheit). The increase in sea surface temperatures at high latitudes was 8 to 10 degrees Celsius, and the new study shows a 4- to 5-degree Celsius increase in tropical sea surface temperatures.

“This event is the best example of greenhouse warming in the geologic record, and for the first time we have been able to document the climate response on a relatively broad planetary scale, from the tropics to polar latitudes,” Zachos said.

The temperature estimates were derived from chemical analyses of the shells of microscopic plankton preserved in the seafloor sediments. The chemical composition of the plankton’s calcite shells reflects the temperature of the water in which they were formed. A key measurement examined in this study was the ratio of magnesium to calcium, which increases exponentially with the temperature at which the shells formed.

“The ratio of magnesium to calcium in seawater is relatively constant over the timescale of this event, so the ratio in the shells is really only sensitive to one variable, the calcification temperature,” Zachos said.

UCSC graduate students Michael Wara and Steven Bohaty performed most of the chemical analyses. The researchers analyzed sediment cores recovered from a site called Shatsky Rise in the tropical Pacific during an expedition of the ship JOIDES Resolution in 2001 (Leg 198 of the Ocean Drilling Program). The cores provided a complete sequence of deposits representing the boundary between the Paleocene and Eocene epochs.

“There aren’t many places in the Pacific where you can recover sediments of this age in which the fossils are not so recrystallized that they’ve lost their original geochemical signatures,” Zachos said.

ODP Leg 198 and a complementary drilling expedition in the Atlantic earlier this year (ODP Leg 208) were designed to test the leading explanation for the Paleocene-Eocene Thermal Maximum, which attributes it to a massive release of methane. Methane, a potent greenhouse gas, accumulates in frozen deposits known as clathrates found in the deep ocean near continental margins and also in the Arctic tundra. For reasons that remain unclear, the clathrates suddenly began to decompose, releasing an estimated 2,000 gigatons (2 trillion tons) of methane.

Once released, the methane would have reacted with dissolved oxygen in the ocean to produce carbon dioxide, another greenhouse gas. Large amounts of both carbon dioxide and methane would have entered the atmosphere, raising temperatures worldwide.

In addition to Zachos, Wara, and Bohaty, the coauthors on the Science paper are Margaret Delaney, professor of ocean sciences at UCSC, Maria Rose Petrizzo and Isabella Premoli-Silva of the University of Milan, Amanda Brill of the University of North Carolina, and Timothy Bralower of Pennsylvania State University. Bralower and Premoli-Silva were co-chief scientists on ODP Leg 198.