Daniel Sigman on learning about our climate’s future from past ice ages

Daniel Sigman: We use computer models to predict how the world will respond to carbon dioxide additions to the atmosphere from human activities. 24:40 and one way of testing them is to apply them to past earth conditions.

Princeton earth scientist Daniel Sigman is a recipient of a 2009 MacArthur ‘genius’ grant. He’s been studying past ice ages, and says they can help us better understand our current climate warming. For example, during the last ice age, he said, what was happening in the oceans dramatically affected levels of atmospheric carbon dioxide, or CO2. Today’s global warming is also believed to be caused by CO2.

Daniel Sigman: To the degree that we want to have full information on where the Earth is going, we need to explore this further.

Sigman says ice age data can help strengthen climate models by providing a sort of test. He talked about how this has worked for weather models – the kind experts rely on to make our daily forecasts.

Daniel Sigman: Every day the weather models make a prediction, and every day weather occurs and provides a test of that prediction. This has allowed the models to be improved over time.

Sigman there’s no doubt that climate warming is happening, but, how much and how quickly is uncertain. Sigman said that to improve our models of climate, we need data from long periods of time. Climate data collected over the past century can help, Sigman said, but the geologic past represents much, much more time than this. If reconstructed properly, the past thus holds a great wealth of data with which the climate models can be tested and improved. But he urged people not to discount what today’s imperfect climate models are telling us about global warming.

Daniel Sigman: As humans emit carbon dioxide into the atmosphere, the simplest and most robust understanding that we have of the physics of the atmosphere tell us that warming will occur. We don’t know yet how much warming will occur, and we are just beginning to think about the secondary affects of that warming. Some people argue that our uncertainty means that we should not take this problem seriously. I see it in the opposite way. Because of this uncertainty, we may have less warming than we currently expect, but we could also have much, much more, and we need to have some insurance against that possibility.

Dr. Sigman said that he studied the ocean’s past nitrogen cycle using deep sea sediment cores, and his results about ice age oceans and how they affect climate contradict some predictions of climate models. The point of contention is how the circulation of the ocean responds to climate and in turn affects it. Much further testing is needed on both sides, he said.

Daniel Sigman: The prediction from climate models under the conditions of global warming has been that the ocean circulation will decrease in rate, especially in the part of the ocean circulation that involves the deep ocean. The data that we have for the last ice age may suggest that we should expect the opposite pattern into the global warming future, with polar ocean circulation actually accelerating. We don’t yet know what this will mean for ocean biology, for climate and for carbon dioxide fluxes, but to the degree that we want to have full information on where the Earth is going, we need to explore this further.

March 8, 2010

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