According to Lahouari Bounoua of NASA's Goddard Space Flight Center, and other scientists from NASA and the US National Oceanic and Atmospheric Administration (NOAA), existing models fail to accurately include the effects of rising CO2 levels on green plants. As green plants breathe in CO2 in the process of photosynthesis - they also release oxygen, the only reason that there is any in the air for us to breathe - more carbon dioxide has important effects on them.
In particular, green plants can be expected to grow as they find it easier to harvest carbon from the air around them using energy from the sun: thus introducing a negative feedback into the warming/carbon process. Most current climate models don't account for this at all, according to Bounoua. Some do, but they fail to accurately simulate the effects - they don't allow for the fact that plants in a high-CO2 atmosphere will "down-regulate" and so use water more efficiently.
Bounoua and her colleagues write:
"Increase in precipitation contributes primarily to increase evapotranspiration rather than surface runoff, consistent with observations, and results in an additional cooling effect not fully accounted for in previous simulations with elevated CO2."The NASA and NOAA boffins used their more accurate science to model a world where CO2 levels have doubled to 780 parts per million (ppm) compared to today's 390-odd. They say that world would actually warm up by just 1.64°C overall, and the vegetation-cooling effect would be stronger over land to boot - thus temperatures on land would would be a further 0.3°C cooler compared to the present sims.
International diplomatic efforts under UN auspices are currently devoted to keeping global warming limited to 2°C or less, which under current climate models calls for holding CO2 to 450 ppm - or less in many analyses - a target widely regarded as unachievable. Doubled carbon levels are normally viewed in the current state of enviro play as a scenario that would lead to catastrophe; that is, to warming well beyond 2°C.
It now appears, however, that the previous/current state of climate science may simply have been wrong and that there's really no need to get in an immediate flap. If Bounoua and her colleagues are right, and CO2 levels keep on rising the way they have been lately (about 2 ppm each year), we can go a couple of centuries without any dangerous warming. There are lots of other factors in play, of course, but nonetheless the new analysis is very reassuring.
"As we learn more about how these systems react, we can learn more about how the climate will change," says Bounoua's colleague Forrest Hall, in a NASA statement accompanying the team's scholarly paper. "Each year we get better and better. It's important to get these things right."
The NASA/NOAA boffins' paper Quantifying the negative feedback of vegetation to greenhouse warming: A modeling approach is published in the journal Geophysical Research Letters (subscription required). ®