"New mesuring technique hints at higher CO2 sensitivity " Topic

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When it comes to carbon dioxide (CO2) and climate, the past is prologue. Barring radical change to humanity's voracious consumption of fossil fuels, atmospheric CO2 is bound to go up and up, driving global warming. But it won't be the first time that CO2 has surged. In Earth's ancient atmosphere, scientists see the faint outlines of a CO2 roller coaster, climbing and dipping across deep time in repeated bouts of climate change. "Each little slice in Earth's past is a replicated experiment," says Dana Royer, a paleoclimatologist at Wesleyan University in Middletown, Connecticut. "It helps us think about where we may be headed in the near future."

If only the past could be seen more clearly. Models of ancient atmospheres and tools for teasing out past CO2 levels from fossils and rocks all have limitations. Now, scientists have developed a new method for wringing CO2 estimates from fossilized leaves—one that can go deeper into the past, and with more certainty. "At the moment, it's very promising and it's probably the best tool that we've got," says David Beerling, a biogeochemist at the University of Sheffield in the United Kingdom who helped develop the so-called fossil leaf gas exchange technique. Already, it is solving ancient climate puzzles and delivering some unsettling news about the future.

Last month, at a meeting of the American Geophysical Union in San Francisco, California, another pioneer of the technique, plant physiologist Peter Franks of the University of Sydney in Australia, trained it on one of those puzzles: the time shortly after an asteroid impact killed off the dinosaurs 66 million years ago. Tropical forests were growing at temperate latitudes, yet earlier studies suggested CO2 levels of about 350 parts per million (ppm)—less than levels today and seemingly too low to create a global hothouse. Based on a gas exchange analysis of fossil leaves in what was once a tropical forest at Castle Rock, near Denver, Franks and his colleagues now conclude that the atmosphere 1.5 million years after the impact contained CO2 at about 650 ppm—a far more plausible level.[…]

If this holds, we're looking at a climate sensitivity of 4 degrees per doubling of pre-industrial CO2 rather than the consensus 3 from the latest IPCC report. That's still within the error bars, but looking a bit more sensitive.