Sigma Xi lecturer Cook speaks on climate change

In a two-part lecture that took place last Thursday and Friday, Mea Cook, assistant professor of geosciences, discussed the impacts of ocean and climate change as part of the annual lecture series sponsored by Sigma Xi, a national society that honors and encourages scientific research.

On Thursday, Cook discussed how the ocean influences greenhouse gases and heat distribution in the atmosphere. She also examined past climate events and considered what might happen in the future.

Cook opened with satellite images showing the Earth’s surface in 2004 and how it has changed month by month. “The Earth has not always been [in] this perfect range of climates with both liquid and solid water,” she said, showing the snow cover moving south and then north again as the year progressed.

The ocean is a very thin skin covering most of the earth, Cook said. It has a depth of just four kilometers within the Earth’s 6400-km radius, but it has “a really profound effect on the climate system,” she said. The sun heats up the ocean and then the ocean warms the atmosphere; latent heat in water vapor makes up most of the heat in the atmosphere.

Similarly, half of global poleward heat transport happens in the oceans, with the other half occurs in the atmosphere. Cook talked about surface currents, like Finding Nemo’s East Australian Current, and deep water currents, which account for 25 percent of heat transportation each. Northern water becomes cold, salty and dense and forms currents to low latitudes with the North Atlantic Heat Transport, being “as large as all the other basins combined,” she said.

The ocean-atmosphere relationship also has “chaotic, irregular variability,” Cook said, causing events such as the North Atlantic Oscillation, which accounts for this year’s warm winter, as well as La Niña, unusually cold ocean temperatures in the Equatorial Pacific, and El Niño, unusually warm ocean temperatures in the Equatorial Pacific.

Cook went on to discuss how humans can “take what we know now about the ocean’s effects on climate and apply it to the ice ages.” She stated that we can reconstruct temperature and greenhouse gas content from ice cores. She used this data to talk about the “Younger Dryas” event, a short cooling period after the last glaciation. She discussed the theory that rapid flooding of the Atlantic with cold fresh water from melted ice sheets disrupted the North Atlantic deep water and caused a cooling effect. At the moment, a similar event due to the Greenland Ice Sheet melting is “very unlikely,” she said, because the freshwater flux is much smaller.

Despite this, “global temperatures have risen 0.7 percent due to human greenhouse gases and aerosols,” which is a dramatic rise above natural variability, Cook said. So far, anthropogenic carbon dioxide has only invaded the surface ocean, but this has reduced the surface ocean pH by 0.1, which is “making living more difficult for organisms that make shells out of calcium carbonate,” according to Cook. Cook constructed a scenario in which this situation continued, and which resulted in all known fossil fuel reserves being burned by the year 2500. In this scenario, pH would decline 0.4 in the deep oceans. The closest analogy to the current system is the Paleocene Eocene Thermal Maximum (PETM). Humans are emitting carbon at 10 times the rate of the PETM, but the PETM produced a warm, ice-free world with a five- to nine-degree increase globally.

“The rapidity of emissions is unprecedented in Earth’s history, but a reduction of greenhouse gas emissions can reduce the impact,” Cook said in her concluding remarks on Thursday.

The second part of Cook’s lecture on Friday focused on “reconstructing past ocean circulation and greenhouse gases using deep-sea sediments.” This talk focused more on her research as opposed to detailing the problem at hand.

Cook started by introducing a specific ice core drilled in Antarctica, which she called a “climate archive of the last 800,000 years,” which shows temperature through isotopes of water. It also has fossil air bubbles of carbon dioxide and methane. Cook’s research explores the question of “why carbon dioxide concentration was low during [the] ice ages.” Carbon moving throughout the atmosphere can be traced using radiocarbon; Cook gave a simplified description of the method used to detect such carbon.

She then presented a hypothesis that increased carbon dioxide concentration  resulted from “expanded sea ice [that] reduced deep water ventilation.” Then, when the ice receded, the carbon was released, which accounts for the raise in atmospheric carbon during de-glaciations.

This hypothesis depends on finding evidence of an “old abyssal water mass during the last glacial maximum,” Cook said, which is done by examining sediments for foraminifera, a type of organism that makes calcium carbonate shells. The shells hold a record of the water chemistry they were formed in.

Cook’s research data indicates that in some North Pacific areas, water deeper than 2.2 km was depleted in radiocarbon compared to today, and that “water was very well ventilated during the de-glaciation,” all data which supports her hypothesis.

Cook then moved to a discussion of the unexplained unstable temperatures and methane levels of the last glaciation. Methane is a very powerful greenhouse gas but exists in relatively low levels. During the last glaciation, there were cases of abrupt warming accompanied by abrupt rises in atmospheric methane.

One hypothesis, titled the Kennett Hypothesis, says this was due to climate warming that disrupted the vast methane reserves in deep-sea sediments.

The scientific community is largely opposed to this hypothesis, but Cook’s data supports it. Low carbon-13 concentrations are found at similar timing and spacing to the warming periods. Species of benthic foraminifera that favor methane seeps also appear more abundant during these periods. However, it is unclear whether the time periods match up.

Lastly, Cook referenced the work of thesis student Nari Miller ’12 on the penultimate glaciation, which showed a similar methane flux to the last glaciation.

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