Scientists agree that an increase in atmospheric greenhouse gases causes the Earth’s temperature to rise, but they’ve also noticed that relationship seems to swing both ways: warmer temperatures also seem to correspond with an increase in greenhouse gases. But drawing conclusions about the nature of the relationship is tricky, because though scientists have seen a correlation, they haven’t been able to show causation.
Now, scientists believe they’ve untangled the relationship. In a paper published Monday in Nature Climate Change, researchers from the University of Exeter claim to have found direct evidence that as global temperatures rise, so does the atmospheric concentration of greenhouse gases, creating a positive feedback that in turn warms the Earth even more — basically, global warming creates more global warming.
“We discovered that not only does thickening the blanket of heat-trapping gases around our planet cause it to get warmer, but also, crucially, when it gets warmer this increases thickens the blanket of heat-trapping gases,” Tim Lenton, the paper’s author, told ThinkProgress, “so we have a process called a ‘positive feedback’ that amplifies changes in the Earth’s temperature.”
This isn’t the first time this relationship has been suggested. Scientists have previously used data from Antarctic ice cores to show that historic temperature rises were accompanied by spikes in global carbon dioxide levels, but other studies cast doubt on that timing, showing a lag of some thousand years.
While several models suggest a correlation between warming temperatures and an increase in greenhouse gas, Lenton’s team is the first to prove the relationship using direct evidence, taken from ice cores nearly one million years old.
The team — comprised of scientists from the University of Exeter, the Scripps Institution of Oceanography, and Wageningen University in the Netherlands — analyzed Antarctic ice core data from the end of ice age cycles 400,000 and 800,000 years ago. That ancient ice is important, because it offers an extremely large amount of historical global temperature and greenhouse gas concentration data, which the scientists were able to analyze to figure out how the two interact.
Combining historical data about temperature and greenhouse gas composition, the scientists used a mathematical approach known as Takens’ theorem to look at the relationship between the two. The approach, Lenton explained, is based on the idea that if one variable causes even a small change in the other, the more information you have about the first variable. The more information you have about the first variable, the better you should be able to predict the change in the second. Eventually the variables will converge, giving researchers an idea of how strong the first is in predicting change in the second.
“We find that if A and B are temperature and CO2 (or temperature and methane) we get strong reciprocal causality,” Lenton said, proving that warmer temperatures cause an increase in atmospheric greenhouse gases.
The findings provide even more support to the overwhelming evidence that humans are causing global warming by pumping greenhouse gases into the atmosphere. The surprise, Lenton explained, is that the findings also show that increasing temperature eventually increases greenhouse gases.
“It implies that we should expect the ‘Earth system’ to respond to anthropogenic global warming by amplifying it with the release of additional greenhouse gases,” Lenton said.
Though the study looks at historical data, Lenton acknowledges that current implications can’t be overlooked. “The Earth is a complex system containing many feedbacks, and they are strong enough to swing the planet between the depths of an ice age and a warm ‘interglacial’,” Lenton said.
The Earth is currently warming at a much faster rate than previous warming events, roughly ten times faster than ice-age-recovery warming, according to NASA. In 2013, atmospheric greenhouse gas hit a record high, and scientists warned that the Earth’s ability to store and mediate gas, through plants and oceans, might been approaching its saturation point.
We’re already seeing unexpected changes in the climate: the West Antarctic ice shelves, for instance, is melting at a much faster rate than scientists predicted. “As we meddle with the climate system now, driving it to hotter temperatures, we should expect the Earth to reply by amplifying the changes we are causing,” Lenton said.