NASA climatologist James Hansen at Sydney Uni: “Australia doesn’t agree now that they got to stop their coal, but they are going to agree. I can guarantee you that within a decade or so because the climate change will become so strongly apparent that’s going to become imperative”
Paleoclimate Implications for Human-Made Climate Change
James E. Hansen and Makiko Sato
NASA Goddard Institute for Space Studies and Columbia University Earth Institute, New York
Paleoclimate data help us assess climate sensitivity and potential human-made climate effects. We conclude that Earth in the warmest interglacial periods of the past million years was less than 1°C warmer than in the Holocene. Polar warmth in these interglacials and in the Pliocene does not imply that a substantial cushion remains between today’s climate and dangerous warming, but rather that Earth is poised to experience strong amplifying polar feedbacks in response to moderate global warming. Thus goals to limit human-made warming to 2°C are not sufficient – they are prescriptions for disaster. Ice sheet disintegration is nonlinear, spurred by amplifying feedbacks. We suggest that ice sheet mass loss, if warming continues unabated, will be characterized better by a doubling time for mass loss rate than by a linear trend. Satellite gravity data, though too brief to be conclusive, are consistent with a doubling time of 10 years or less, implying the possibility of multi-meter sea level rise this century. Observed accelerating ice sheet mass loss supports our conclusion that Earth’s temperature now exceeds the mean Holocene value. Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.
KERRY O’BRIEN: What are your particular fears with regard to the melting of the polar ice caps?
JAMES HANSEN: Well, the problem is that the climate system in general has a lot of inertia and that means that it takes time for the changes to begin to occur but then, once they do get under way, it becomes very difficult to stop them and that is true in spades for the ice sheets. If we once begin to disintegrate it will become very difficult, if not impossible, to stop them and we are beginning to see now on both Greenland and west Antarctica disintegration of those ice sheets. They’re both losing ice at a rate of about 150 cubic kilometres per year and that’s still not a huge sea level rise. Sea level rise is now going up about 3.5 centimetres per decade. So that’s more than double what it was 50 years ago. But it’s still not disastrous; it’s a problem, but it’s not disastrous. But the potential is for a much larger sea level rise. If we get warming of two or three degrees Celsius, then I would expect that both West Antarctica and parts of Greenland would end up in the ocean, and the last time we had an ice sheet disintegrate, sea level went up at a rate of 5 metres in a century, or one metre every 20 years. That is a real disaster, and that’s what we have to avoid.
KERRY O’BRIEN: What is the most recent evidence of what’s really going on with the ice caps, the Arctic and the Antarctic?
JAMES HANSEN: There are two things that are cause of concern. First of all, if we look at the history of the Earth, we know that at the warmest interglacial periods, which were probably less than 1 degree Celsius warmer than today, it was still basically the same planet. Sea level was perhaps a few metres higher. But if we go back to the time when the Earth was two or three degrees Celsius warmer, that’s about three million years ago, sea level was about 25 metres higher, so that tells us we had better keep additional warming less than about one degree. And the other piece of evidence is not from the history of the Earth but from looking at the ice sheets themselves, and what we see is that the disintegration of ice sheets is a wet process and it can proceed quite rapidly. We see that the ice streams have doubled in their speed on Greenland in the last few years and even more concern is west Antarctica because it’s now losing mass at about the same rate as Greenland, and west Antarctica, the ice sheet is sitting on rock that is below sea level. So it is potentially much more in danger of collapsing and so we have both the evidence on the ice sheets and from the history of the Earth and it tells us that we’re pretty close to a tipping point, so we’ve got to be very concerned about the ice sheets.
JAMES HANSEN: Yeah, that’s the screwiest argument which keeps being made again and again and again. The amount of water vapour in the atmosphere is determined by the atmosphere’s temperature, everyone should know that. Look at the difference between winter and summer.
As you go to a warmer climate the atmosphere holds more water vapour because at the places where the humidity reaches 100 per cent the water vapour falls out as water or snow. And therefore, as the planet becomes warmer, the atmosphere holds more water vapour.
That’s why we get heavier rain falls as the planet gets warmer. So this water vapour is an amplifying feedback. It makes the greenhouse effect much stronger. But it’s not something that just changes on its own accord; it changes in response to the temperature changes.