February 27, 2012

Darwin Day Lecture: “Climate Change Past and Present”

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The effects of climate change like melting sea ice and rising sea levels occur naturally, and may not necessarily be due to human impact on the environment, according to some scientists and contrary to popular belief. These changes are part of a natural cycle and have happened before.

Dr. Thomas M. Cronin, a geologist at the U.S. Geological Survey, gave a guest lecture Wednesday, Feb. 15, as part of the University’s Darwin Days. Cronin’s presentation, “Climate Challenges and the Geologic Record,” focused on two main areas: sea ice and sea level.

“The polar bears had it harder during the last ice age, than during interglacial warmth, which is what we’re in today,” Cronin said. Sea ice has been melting and contributing to sea level rising. During the last ice age, the arctic was covered with a kilometer of sea ice and the polar bears would not have been able to fish for food; however, they adapted and moved south, Cronin said. We are in an interglacial period, an era of warmer global average temperatures between ice ages. The current Holocene period began about 11,500 years ago. Based on fossil records and phylogenetic evidence, Cronin marks the divergence of the modern polar bear from the brown bear at 400,000 years ago. This indicated that the polar bears have lived through climate changes in the past and could live through more in the future.

Cronin’s job is to link the past and present and to understand the relationship between current climate changes and past geological events. Part of this research was done through analysis of sediment cores. To examine sediment cores a hollow steel tube was drilled into the Earth to retrieve a sample of deposits. Changes within the core reflect changes over time because the deeper the core, the older the material.

Many short sediment cores of Arctic ice had already been sampled, but this project’s mission was to bring back long sediment cores, something that had not been done before.

Sharing results from his 2004 Arctic Coring Expedition, Cronin revealed that his group was able to drill down 425 meters, which provided 60 million years’ worth of sediment. From the ice cores, data from proxies such as phytoplankton and oxygen isotopes is analyzed to determine properties like carbon dioxide concentration and temperature during specific time periods.

In the last 20 years there has been about a one-degree Centigrade increase in the Arctic Ocean close to the Atlantic Ocean. Scientists have determined that this is not necessarily due to human activity because these variations are natural. Cronin said that although over the past 1,500 years this change is abnormal, it did happen during a deglaciation period around 20,000 years ago. “Time scales are everything,” Cronin said.

As with temperature, sea levels are also rising quickly. Cronin stated that the rising sea level is an important issue but that no one knows how fast the sea level will rise. “This issue is terribly misunderstood and there are enormous amount of unknowns that we are only beginning to understand,” Cronin said.

Satellite records of sea level are relatively recent and date back to the early 1990s. Sea level has been rising faster than anticipated, Cronin said, because old predictions were not accurate. Scientists now know that there is more ice melting. Three-hundred fifty gigatons of ice has to melt to raise sea level one millimeter per year, Cronin said, adding that sea level had risen at an average rate of 3.3mm per year between 1993 and 2009. That equals about 1.15 trillion tons of ice melting in one year.

Cronin emphasized the uncertainty surrounding climate change and human impact in addition to natural cycles. He said the climate system is very complicated and extremely sensitive over long and short time scales.

“Imagine how sensitive these parts of the climate system would be to an abrupt injection of carbon dioxide and greenhouse gases,” Cronin said. Cronin noted that society is not going to stop burning fossil fuels today or make any policy changes soon, and in the future he is looking for a focus on technology to reduce human impact.

Original Author: Lisa Gibson