Jeremy Shakun is lead author of the new study in Nature this week, confirming from multiple proxy records that “temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation.”
He was interviewed for Nature.com by Paige Brown. (for voice interview, and “plain speech”, see the podcast audio above)
“People drilled down through the Antarctic ice sheets, and we actually have a record of [the link between CO2 and temperature] that goes back to almost a million years ago,” Shakun told me in a recent interview. From these air bubbles, scientists could figure that carbon dioxide rose and fell over our planet’s most recent ice age, suggesting that carbon dioxide had something to do with rising temperatures that ended that same ice age. “…if you look at these two [CO2 and temperature] together, you see that they have this amazing correlation. It’s a better correlation than you almost ever get from nature – the two just go lockstep up and down together over the ice ages for the last 1 million years almost,” Shakun said. But just what exactly was that relationship? This is where strong debate has plagued many scientists’ efforts to pin the blame on carbon dioxide.
“People have realized that there is clearly some link between CO2 and temperature in the past, but the question you get to is, well, how does it work? Which one is cause and which one is effect? How do the two interplay off of each other?” Shakun said.
The curveball, as Shakun puts it, is that when scientists looked more closely at the ice-core records they had from Antartica, they found that the temperature in Antartica actually started changing a bit before the CO2 did. Not exactly the best of news for scientists and climate change communicators trying to stave off arguments from climate deniers that there is no ‘CO2 problem’ today.
“This is something that [current] global warming skeptics have jumped on, to say ‘ah jeez, obviously CO2 must not cause warming because if we look in the past, in these ice cores, the CO2 comes after the warming… so we are in the clear today’,” Shakun said. Climate deniers have pointed to the fact that CO2 might be an effect of global warming, but not a cause. They argue, based on these important old records, that carbon emissions don’t really matter for climate.
They couldn’t be more wrong.
“Scientists don’t really buy that logic for a lot of good reasons,” Shakun said. “Most climate scientists have seen that timing difference to mean that CO2 wasn’t the trigger for the past climate changes over the last ice age, but that it was an amplifier.”
Shakun’s study with colleagues affiliated with Harvard University, Columbia University, and other major research universities in the U.S. and abroad sought to fill the gap that currently exists in the relationship record between CO2 and climate change in the last ice age. “These ice cores tell you about the global level of CO2, but they only tell you about temperatures just in Antartica, and that’s it. That is just one dot on the map,” Shakun said.
Shakun describes how, for an analysis today, one can’t just go look at one place in the world to demonstrate a global phenomenon. “You go find some place in the last 100 years that got colder, and that doesn’t disprove global warming in the last 100 years – it’s just that one spot happened to get colder,” he said. “It’s global climate change we are talking about. It’s about the whole planet.”
Shakun and his colleagues set out to gain insight on global temperatures during the last ice age.
“People have records of temperature from ice cores in Greenland, we have lots of ocean cores that people pull up from the sea floor, we have lake cores on land… people have used all these different kinds of ways to construct what temperature was in the past,” Shakun said. This data is especially rich from around the last ice age, as a point in the not-too-distant past of vast importance for past climate research. Samples can also be dated reliably using carbon-dating, ensuring an excellent picture of past climate conditions. Shakun and colleagues went to this data to solve the ‘mystery’ of CO2 and the last ice age. Sort of a “Who dun’ it?” for the last major glacial melt.
“We went to the literature, and we just dug up as many of these good temperature records as we could find. We got a total of 80 of them,” Shakun said. “They come from pretty much all over the world.”
“It was really simple science,” he said. “We said, we’ve got 80 records from around the world, let’s just slap them together, average them into a reconstruction of global temperature.” What a fabulous idea, for such “simple science”!
For those who may be wondering, the “Holy s**t” comes at the very end of the 3-minute video (actually an audio podcast) at the bottom of the Nature blog post.
or, at the top of this page…
Hmm, good point, I’m not much of a video guy (partially because I don’t have broadband), so skipped over that and went on to the blog. Apologies.