Earth Protect Blog

One of the more common arguments from skeptics of anthropogenic climate change is that the Earth has experienced periods during which atmospheric carbon dioxide levels were much much higher than they are today -- as much as 10 times higher. Why worry about a mere 30% increase over pre-industrial levels?

Category: climate Posted on: January 7, 2010 11:13 AM, by James Hrynyshyn

There are several answers to that challenge. The most obvious is that while it may be true that CO2 levels have been several times higher that today's 387 parts per million, the Earth was also a very different place back then. The sea level was much higher, the temperature was much warmer and it bears noting that the biodiversity at such times was quite different. But the assumption that the atmospheric CO2 levels might have been 3,000 ppm did pose some challenges to climatologists trying to figure out how the rest of the planetary ecosystem responded to dramatic changes in CO2. Some of the things that were apparently happening during high CO2 periods didn't quite make sense.

Now, however, comes a paper in PNAS (access priviledges req'd) that reports a strong possibility that CO2 levels during one of those curious periods -- the Mesozoic, which was between 251 and 65 million years ago -- were much lower that previous estimates. If the authors are right, we may have to reconsider our predictions of what might happen over the next century if CO2 levels don't stabilize soon.

"Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100" by soil scientist D. O. Breecker, Z. D. Sharp, and L. D. McFadden at the University of New Mexico, posits that soil carbonate "paleobarometers" used in the past to figure out how much CO2 was in the air millions of years ago involved some inappropriate assumptions. Their new analyses, which measures the amount of calcite found in North American soils, comes up with much lower estimates. Instead of upwards of 3,000 ppm, the most often quoted CO2 atm values, those previously determined from pedogenic carbonate, are too high,and that paleo CO2 atm values did not persist above 1,500 ppmV during the past 400 million years.

These numbers are easier to reconcile with models of carbon cycles and other CO2 proxies, the authors write. And so:
the agreement between multiple proxies strongly supports the conclusion that the warmest episodes of the Mesozoic were associated with CO2 atm equal to ∼1; 000 ppmV rather than 2,000-3,000 ppmV. The relatively low CO2 atm of 1,000 ppmV during greenhouse episodes suggest that either Mesozoic warmth was partially caused by a factor unrelated to CO2 or that the Earth's climate is much more sensitive to atmospheric CO2 than previously thought.
It is entirely possible that Breecker's approach is flawed, or that the unrelated and unknown factor is responsible for the warmth that accompanied the 1,000 ppm periods. This is, after all, just one study, and who's to say that the next one won't find that the real CO2 levels during ancient greenhouse episodes were actually higher than we thought, rather than lower? To insist this one paper is proof that we're much closer to catastrophic tipping points than generally assumed would be just as foolish as changing your diet according to the latest findings on the metabolic effects of caffeine or fiber. If you did that every time a contrary study was published you'd never be able to assemble a grocery list.

The way science works -- most of the time -- is that nailing down what really happened eons ago is a kind of zig-zaggy affair. One approach might give us a lower value, in this case for Mesozoic CO2 levels, and then the next might pump that number up a bit, and the next back down a bit. As long as the peer-review process is operating as it should, however, each subsequent estimate will be closer to the actual figure than the previous one.

So while it is possible that Breecker's conclusions might have gone too far in one direction, the next analysis is likely to generate estimates that are closer to Breecker's than they are to the previous, much higher values. Eventually, we'll settle on a number that doesn't change significantly with easy subsequent attempt to nail it down.

As one scientist told Nature's Richard A. Lovett last month, the likelihood that Breecker et al are way off the marks seems unlikely.

"It makes a major revision to one of the most popular methods for reconstructing palaeo-CO2," says Dana Royer, a palaeobotanist at Wesleyan University in Middletown, Connecticut, who was not involved in the work. "This increases our confidence that we have a decent understanding of palaeo-CO2 patterns."

Lending more weight to this new approach are the findings of James Hansen and his colleagues, who last year used entirely different methods to estimate a maximum CO2 concentration 50 million years ago of just 1,400 ppm. (See also his new book, Storms of My Grandchildren.)

This is not the only new paper to conclude that that the Earth is more sensitive to CO2 levels than previously assumed. Late last year also saw the publication, in Nature Geoscience of "Earth system sensitivity inferred from Pliocene modelling and data" in which Daniel Lunt of the University of Bristol and colleagues estimate that the response of the Earth system to elevated atmospheric carbon dioxide concentrations is 30-50% greater than the response based on those fast-adjusting components of the climate system that are used traditionally to estimate climate sensitivity.

What does all this mean for the future: Breecker and colleagues wrap up their findings with this typically understated warning:
Comparison of projected future CO2 atm with results from the recalibrated CO2 paleobarometer (Fig. 2B) indicate atmospheric CO2 may reach levels similar to those prevailing during the vegetated Earth's hottest greenhouse episodes by A.D. 2100..... Given that the Early Permian CO2 increase may have caused the termination of the Late Paleozoic Ice Age (7, 8, Fig. 2D), the only known icehouse-greenhouse transition on a vegetated Earth, the effects that unmitigated CO2 increases may have on future climate warrant careful consideration.

It is entirely possible then, that keeping CO2 levels to no more than 450 ppm -- or keeping our cumulative carbon emissions to no more than a trillion tonnes -- will be insufficient to ensure the temperature increase over pre-industrial times stays below 2°C. Which only highlights the need to begin the transition to a low- or zero-carbon economy sooner rather than later.
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Breecker, D., Sharp, Z., & McFadden, L. (2009). Atmospheric CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100 Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0902323106

http://scienceblogs.com/islandofdoubt/2010/01/is_the_earth_even_more_sensiti.php