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Just out in Nature today, scientists have assembled an 800,000 year record of CO2, methane and temperature. It looks like we need to revise our talking points a bit about greenhouse gases...
The change: Today's greenhouse gas levels are even more anomalous than we thought: CO2 and methane levels are higher today than they have been in at least the past 800,000 years. (It used to be 440,000 years, then it was 650,000 years...)
As an accompanying news article put it:
The fundamental conclusion that today's concentrations of these greenhouse gases have no past analogue in the ice-core record remains firm. The general long-term behaviour of methane and carbon dioxide, following patterns driven ultimately by slow changes in Earth's orbit, continues throughout the older sections of the records. The remarkably strong correlations of methane and carbon dioxide with temperature reconstructions also stand.
Check out the news article's pretty picture:
CO2 (red) is in parts-per-million (ppm); methane (green) is in parts-per-billion (ppb); temperature (black) is relative to average of past millennium. Reprinted by permission from Macmillan Publishers Ltd: Nature. "Windows on the greenhouse" by Ed Brook, Nature 453: 291-292, copyright 2008
What's next? The ice coring community is now aiming for a 1.5 million year record. We've got proxies for CO2 going back much further than that, but such a long record of direct measurements would be remarkable. It will take a while to assemble such a record (they've nearly reached bedrock for this particular core, so they have to set up shop somewhere with even older ice). In the meantime, scientists have plenty to ponder. For example, why does the temperature record seem to change around 450,000 years ago? Cold phases have remained roughly similar throughout the 800,000 year record, but "recent" warm phases have been warmer than in the deeper past. And what accounts for the "lowest value ever found in ice cores" -- 172 ppm about 667,000 years ago?
This is an exciting, thought-provoking record that will be fun to follow as scientists drill down into these and other details (sorry, I couldn't help myself). An immediate take-home message, though, is just how much humans have affected Earth's carbon cycle. Our influence is undeniable - it's time to take responsibility for it and cap emissions.