This is is an important paper as it discusses the impact of feedback mechanisms on climate change. The key recommendation is that if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggests that CO2 levels will need to be reduced from the current levels of around 385 ppm to at most 350 ppm.
Hansen says that once we understand that our climate is now far out of equilibrium because of current climate ‘forcings’ this “raises the spectre of ‘tipping points’; the concept that climate can reach a point where, without additional forcing, rapid changes proceed practically out of our control.”
He gives the dramatic loss of Arctic sea ice and the changes in the West Antarctic Ice Sheet as examples of potential tipping points. He says that Arctic sea ice loss is magnified by the positive feedback of increased absorption of sunlight as global warming initiates sea ice retreat. In addition he notes that West Antarctic ice loss can be accelerated by several feedbacks, once ice loss is substantial.
In the article Hansen says that alpine glaciers are in near-global retreat, and that , glacier loss will mean long summers of frequently dry rivers. The areas affected will include those areas watered by rivers originating in the Himalayas, the Andes and the Rocky Mountains that currently supply water to hundreds of millions of people. He says that, “Present glacier retreat, and warming in the pipeline, indicate that 385 ppm CO2 is already a threat.”
Hansen also says that today’s 385 ppm CO2 means that we are already committed to sea level rise of at least several meters, judging from paleoclimate history. There is also heightened concerns about ice sheet stability, losses from Greenland and West Antarctica are growing. Therefore an initial CO2 target of 350 ppm, will need to be reassessed in the light of observations, the critical need is to restore the planetary energy balance.
This is an important paper and will provide information to the debate about the need to drop ZERO CO2 targets in favour of NEGATIVE CO2 targets.
Hansen’s work on tipping points is also important and needs to be understood in the light of other work on abrupt climate change.
Download the paper: http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf