Increased carbon dioxide in the atmosphere drives global warming. That is what scientists have been saying about the ongoing warming in the 21st century (with the decade of 2001 to 2010 the warmest since 1850, for example, and with more than 15,000 temperature records for warmth broken in March, 2012, for example). It is also what scientists now say happened to bring the last Ice Age to an end. These scientists used a supercomputer and a global dataset of paleoclimate records to analyze 15,000 years of climate history. Their results are published in the April 5, 2012 issue of the peer-reviewed journal Nature.
Why is this study important? It deepens scientists’ understanding of climate history, for one thing. More critically, the findings contrast with earlier studies, which skeptics of human-triggered global warming said showed that carbon dioxide wasn’t important in bringing the last Ice Age to an end. According to an April 5 article in the Christian Science Monitor:
The result stands in contrast to previous studies that showed temperatures rising ahead of increases in atmospheric CO2 levels. This has led some skeptics of human-triggered global warming to argue that if warming temperatures came first, CO2 [carbon dioxide] wasn’t an important factor then and so can’t be as significant a factor today as most climate scientists calculate it to be.
This multi-institutional study was led by climate researchers at Harvard, Oregon State University and the University of Wisconsin. They used the Jaguar supercomputer at Oak Ridge National Laboratory to answer the question: Which came first, greenhouse gases or global warming? The answer provided by their study is greenhouse gases.
Jeremy Shakun is a National Oceanic and Atmospheric Administration (NOAA) Climate and Global Change postdoctoral fellow at Harvard and Columbia Universities and first author of the paper. He said in a press release:
We constructed the first-ever record of global temperature spanning the end of the last ice age based on 80 proxy temperature records from around the world. It’s no small task to get at global mean temperature. Even for studies of the present day you need lots of locations, quality-controlled data, careful statistics.
The proxy temperature records he speak of are from ice cores and ocean and lake sediments, collected by scientists in locations around the world. Carbon-14 dating helps show what temperatures were occurring at what times in the past. Shakun said:
We found that global temperature mirrored and generally lagged behind rising carbon dioxide during the last deglaciation, which points to carbon dioxide as the major driver of global warming.
After examining the evidence in the climate record, the researchers turned to a supercomputer, running simulations that used 4.7 million processor hours in 2009, 6.6. million in 2010, and 2.5 million in 2011 – coupled with a climate model called the Community Climate System Model version 3. In other words, they used the climate model to look at multiple possible interactions between Earth’s atmosphere, oceans, lands, and sea ice, seeking the right combination of inputs that would match the temperature record as observed in lakes sediments and ice cores.
As a result of these simulations, these scientists are now convinced that increased carbon dioxide in the air drove the global warming that ended the last ice age. This result is in contrast to results from an earlier study, based on Antarctic ice cores, which had indicated that local temperatures in Antarctica started warming before carbon dioxide began rising. That earlier result implied that carbon dioxide was merely a feedback, or result of warming, not a main driver of warming. This study found the opposite – that carbon dioxide was the primary driver of worldwide warming.
Where did the excess carbon dioxide come from, and why is this result opposite the earlier result from Antactica? According to the press release:
Geologic data show that about 19,000 years ago, Northern Hemisphere glaciers began to melt, and sea levels rose. Melting glaciers dumped so much freshwater into the ocean that it slowed a system of currents that transports heat throughout the world. Called the Atlantic meridional overturning circulation (AMOC), this ocean conveyor belt is particularly important in the Atlantic where it flows northward across the equator, stealing Southern Hemisphere heat and exporting it to the Northern Hemisphere. The AMOC then sinks in the North Atlantic and returns southward in the deep ocean. A large pulse of glacial meltwater, however, can place a freshwater lid over the North Atlantic and halt this sinking, backing up the entire conveyor belt.
The simulation showed weakening of the AMOC due to the increase in glacial melt beginning about 19,000 years ago, which decreased ocean heat transport, keeping heat in the Southern Hemisphere and cooling the Northern Hemisphere. Other studies suggest this southern warming caused sea ice to retreat and shifted winds around the Southern Ocean, uncorking carbon dioxide that had previously been stored in the deep ocean and venting it to the atmosphere around 17,500 years ago. This rise in carbon dioxide then initiated worldwide warming.
Bottom line: Increased carbon dioxide in the atmosphere drove global warming to bring the last Ice Age to an end, say scientists who used a supercomputer and a global dataset of paleoclimate records to analyze 15,000 years of climate history. Their results are published in the April 5, 2012 issue of the peer-reviewed journal Nature.