A footnote to Mark’s post on uncertainty and the risks of global warming.”Hear, hear!” to his argument, but he understates the problem. This makes no difference to the logic, but a lot to the urgency. Mark (my italics):
The world – especially the much richer world of our great-great-grandchildren in 2100 – could adjust to a 3°, or even a 4°, increase in global average temperature, though at great cost in species extinctions, land area lost to rising sea levels (and therefore the forced migration of some large populations), and more extreme weather. That hotter planet would be, on average, a less pleasant place to live. But it would still be habitable.
But the climate scientists – the blue-ribbon Copenhagen Diagnosis group that issued an update to IPPC4 last month – don’t quite say this (again my italics):
Global mean warming of even just 1.5-2.0°C still carries a significant risk of adverse impacts on ecosystems and human society. For example, 2°C global temperature rise could lead to sufficient warming over Greenland to eventually melt much of its ice sheet (Oppenheimer and Alley 2005), raising sea level by over six meters and displacing hundreds of millions of people worldwide.
The difference between 2° and 3° C is important, for we have already used up half of the former and gone up 1 degree, and keeping to 2° implies peaking emissions in ten years or less (report page 51). The 2°C limit isn’t a scientifically based assurance of safety, but a horse-traded EU political target that may already be too generous. From the Copenhagen Diagnosis scientists again, page 50 (links and italics added):
…the group of Least Developed Countries as well as the 43 small island states (AOSIS) are calling for limiting global warming to only 1.5°C. The Synthesis Report of the Copenhagen climate congress (Richardson et al. 2009), the largest climate science conference of 2009, concluded that
Temperature rises above 2°C will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond.
The only world we know to be safe is the one we evolved in, with zero anthropogenic emissions; heating it up beyond the small historical variation (well under ± 1°C, judging by the chart on the same page) is a risky venture into unknown feedback loops. Zero net emissions are what we have to go back to, with high technology replacing hunting and gathering. And there is no margin of safety for more dithering.
PS Trial comments policy
I’m not a climate scientist and I’m not running Climate Science 101 for denialists and amateurs here; this is a policy blog. Commenters are firmly invited to stick to the policy point. Comments that just try to challenge established climate science will be treated as trolling and gutted: please go to other fora to jack off. Corrections from qualified climate scientists, citing references in peer-reviewed journals, will of course always be welcome.