100 parts per million. That was all the carbon dioxide, a colorless, odorless gas, that was required to end the last ice age. An article in Nature Magazine this month confirms what many scientists have been agreeing upon for years: that CO2, anything but a harmless gas when released into the atmosphere, is a major driver of climate change. Watch out gas-guzzling SUVs and ‘dirty’ fossil fuel industries. I interviewed article author Dr. Jeremy Shakun from Harvard University to learn more about his recent climate study.
The article in the April 2012 issue of Nature describes a study conducted by Jeremy D. Shakun and colleagues to confirm the link between carbon dioxide and climate change during the Pleistocene ice ages. Scientists have known for years that a link existed, according to analysis of air bubbles trapped in ancient ice from the Antarctic. “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”!
“What you see when you put them all together is a pattern of global warming at the end of the ice age that really strongly mirrors the rise in CO2 at the end of the ice age. Even more interesting, you find that the global temperature started warming a bit after the CO2 rose.” This is very different from the view that many people currently hold that temperature changed first during the last glacial melt. “That is true for Antartica, but if you look globally, that’s not the case,” Shakun said. “Global temperatures are following CO2.”
It is hard to ignore the new evidence that Shakun and his colleagues have brought to the table this April. “… Global warming at the end of the ice age, resembling a rise of CO2 and coming after it… It’s pretty hard to look at that and not think that CO2 was a big driver of global warming during the last ice age,” Shakun said.
But what are some of the implications of CO2 being a big driver of global warming during the last glacial cycle? Why should we care that CO2 once ended an ice age?
“[Our findings] are not really going to change scientists’ perspective … I think as far as the actual science on the matter, we have already got a lot of strong pieces of evidence on CO2,” Shakun said. "But obviously when we look out in the public, that’s not quite the case. We meet people all the time who say ‘eh, I don’t believe that. What’s the story with this global warming idea anyway?’ I think this is just one more additional piece that goes on that pile that says ‘yep, like we thought for a bunch of different reasons, CO2 causes global warming.’ "
So what is the take home message? “[Our findings] provide you with a really tangible example of what is means for CO2 to cause warming. You are sitting on the bus and you tell somebody ‘yeah, CO2 is causing global warming,’ and it just sounds like this abstract concept. But you tell somebody that CO2 can end an ice age and they get a feeling for what that means, and how powerful that is,” Shakun said. “Obviously if you have an ice age end, then that means a huge difference for the planet, a huge difference for sea level, for rain fall patterns, for temperature, for all sorts of things that actually matter to us, to society.”
But the punches keep coming. As Shakun points out, the amount that CO2 rose at the end of the ice age was only around 100 parts per million. “That sounds small… but it was apparently enough to really help drive the end of an ice age – that’s a huge effect,” Shakun said.
What is even more sobering is that today, humans have brought CO2 levels up another 100 part per million more. “So we have done just as much,” Shakun said. “And in a century, we are looking to go up, going on as we are, by several hundred more. So 100 parts per million to end an ice age, and we are talking about people bringing it up many times more… this is NOT small potatoes what we are talking about here, what we are doing with CO2. This is big stuff, big changes.”
For more information about the study in Nature, read here.
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Shakun, J., Clark, P., He, F., Marcott, S., Mix, A., Liu, Z., Otto-Bliesner, B., Schmittner, A., & Bard, E. (2012). Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation Nature, 484 (7392), 49-54 DOI: 10.1038/nature10915
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