James Hansen and company have a new paper attempting to define “dangerous climate interference” and suggest where we currently stand relative to their chosen standard. Their answer? Close:
We examine potential criteria for dangerous climate change assuming that humanity wants to preserve planetary conditions similar to those in the period of civilization, thus near the range of climate in the present interglacial period, the Holocene, which has existed almost 12 000 years. Even with such an assumption, various uncertainties make definition of the dangerous level imprecise and imply that estimates must be continually refined as knowledge improves.
Nevertheless, we can conclude that the world is already close to the dangerous level. The sharpest criterion is probably maintenance of long-term sea level close to the present level, as about one billion people live within 25m elevation of today’s sea level.
There’s also a news release.