“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 suggest that CO2 will need to be reduced from [current levels] to at most 350 ppm.”
Dr. James Hansen
"Since the beginning of human civilization, our atmosphere contained about 275 ppm of carbon dioxide. That is the planet “on which civilization developed and to which life on earth is adapted.” 350.org website
I've been wondering of late just how -- and when -- Earth's climate system might stabilize at another global average temperature ... or maybe begin to cool?
Most climatologists and climate activists seem to be saying that if we were to radically reduce CO2 and other anthropogenic greenhouse gas emissions, thus reaching a plateau in the graph line of atmospheric GHG emissions -- and declining from there -- we should expect to avoid worst case scenarios.
Trouble is, there are some positive feedbacks already at work. For example, the Arctic sea ice albedo - http://en.wikipedia.org/wiki/Ice-albedo_feedback
"Dissolution of CO2 into the oceans is fast but the problem is that the top of the ocean is “getting full” and the bottleneck is thus the transfer of carbon from surface waters to the deep ocean. This transfer largely occurs by the slow ocean basin circulation and turn over (*3). This turnover takes 500-1000ish years. Therefore a time scale for CO2 warming potential out as far as 500 years is entirely reasonable (See IPCC 4th Assessment Report Section 2.10)."
~ from http://www.skepticalscience.com/co2-residence-time.htm
I'm wondering how long it should be expected to take -- under the ideal hypothetical condition of a complete and sudden cessation of all anthropogenic GHG -- for, say, the current (e.g.) atmospheric CO2 concentration of roughly 400 ppm to lower to the ostensibly safe target of 350 ppm?
Should this be expected to require centuries? And if so, what prevents global average temperatures from going up and up and up ... far beyond anything life as we know it can tolerate?
There appear to be more known positive feedback mechanisms than negative ones, with the known negatives being, apparently, much too weak to counteract the positives.
So I'm perplexed. Scientists and climate activists say there is urgent work to be done to avert the worst. But I'm not sure, as a purely scientific question, how even the best possible emissions scenario (zero anthropogenic emissions) could prevent an ever heating planet.
(Folks in here may or may not know enough to provide the help I'm asking for. But maybe you know of folks who could?)
Some recent news on Arctic sea ice and albedo:Arctic thaw significantly worsens global warming riskExcerpt:
"Melting ice is cooking the planet. Shrinking Arctic sea ice means the ocean is absorbing more energy from the sun, and it's now clear the effect is twice as big as thought – adding significantly to heating from greenhouse gases.http://www.newscientist.com/article/dn25072-arctic-thaw-significantly-worsens-global-warming-risk.html#.UwU5VH3A-XM
Arctic temperatures have risen 2 °C since the 1970s, leading to a 40 per cent dip in the minimum summer ice coverage in the Arctic Ocean. Open water soaks up more sunlight than ice, so as the ice retreats the ocean absorbs more energy, warming it and causing even more melting."