When James Hansen talks climate change, people listen. The head of climate studies at NASA, Hansen first gave evidence on the issue to the US Congress in 1988, and is now an eminent scientist and a prominent public advocate. In new research just out, Hansen concludes that at the current temperature, no "cushion" is left to avoid dangerous climate change, and that the Australian government target goals  "... of limiting human-made warming to 2° and CO2 to 450 ppm are prescriptions for disaster". The question Hansen raises is direct and brutal in its implications: is the planet already entering a zone of dangerous climate change? With Arctic sea-ice in a "death spiral", Greenland in 2010 melting at an unprecedented rate, a seemingly extraordinary number of extreme climate events in the past year from the Russian fires to the Pakistan floods, and 18 countries setting temperature records, have we already gone too far for a safe climate? In a draft of a new research paper, Hansen and his collaborator Makiko Sato has opened a new debate about what might be the conditions for a safe climate; that is, one in which  people and nations can continue to live where and as they have been, with secure food production, and in a bio-diverse environment.

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The period of human settlement over the past 10,000 years is known as the Holocene, during which time temperatures and hence sea levels (the two having  a close correspondence) have been remarkable stable.  Temperatures over the period have not been more than 0.5C warmer or cooler than the mid-line (see chart). The warmest part of the Holocene (the "Holocene maximum") was about 8000 years ago, and according to Hansen, today's temperature is about, or slightly above, the Holocene maximum:
"... we conclude that, with the global surface warming of 0.7C between 1880 and 2000, global temperature in year 2000 had returned, at least, to approximately the Holocene maximum."
Note, this is to the year 2000, and temperatures have increased ~0.15C in the last decade, so:
"Global temperature increased 0.5C in the past three decades to a level comparable to the prior Holocene maximum, or a few tenths of a degree higher."

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