Hansen and Sato

[biffvernon]

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.

Rest of Paper: http://pubs.giss.nasa.gov/docs/notyet/i ... n_Sato.pdf

[PS_RalphW]

If ice sheet loss is exponential with a doubling period of 10 years, this should become apparent within 10 years. It would also prove that it is far too late short of nuclear winter to do much about it.

[Blue Peter]

It's worth looking at Stuart Staniford's Blog where this is discussed (3rd Jan 2012, and also see the next entry on hotter summers - very scary indeed),


Peter.

[clv101]

I don't like this paper. The evidence for 5 m this century, for his exponential rise with 10 year doubling time, is pretty much non-existent. He leans on Rahmstorf's semi-empirical model which has it's own problems.

A couple of points I'll add, Greenland loses mass by surface melting, and iceberg calving into the ocean in approximately equal amounts. As these processes continues and glaciers retreat, they pull their feet out of the warming ocean and can no longer lose mass by calving and have to rely solely on surface melt. This is a term acting to slow mass loss some time into the future.

A good proportion of increased ocean mass loss (approximately equal to the loss from Greenland and Antarctica combined) is coming from melting mountain glaciers and small ice caps. When talking about a 5m rise, this current major source is fairly trivial, there just isn't the mass available. This is a problem when projecting large 21st C. sea level increase based on 20th C. observations. The processes are very different.

[UndercoverElephant]

I don't like this paper. The evidence for 5 m this century, for his exponential rise with 10 year doubling time, is pretty much non-existent. He leans on Rahmstorf's semi-empirical model which has it's own problems.

A couple of points I'll add, Greenland loses mass by surface melting, and iceberg calving into the ocean in approximately equal amounts. As these processes continues and glaciers retreat, they pull their feet out of the warming ocean and can no longer lose mass by calving and have to rely solely on surface melt. This is a term acting to slow mass loss some time into the future.

That's a bit misleading. When glaciers "have their feet in the ocean" it is usually because there is a floating ice shelf between the land and the open ocean. This ice shelf backs up the glacier - if it disappears, as has already happened at various locations in the antarctic, the likely result is that the glacier starts moving faster. This process is accelerated even further by melting water getting between the base of the glacier and the underlying rocks. So it's not just surface melt that needs to be accounted for, but also the rate at which the glacier is moving.

I think it is now far too late to stop serious, irreversible climate change. We aren't going to stop burning coal any time soon, further release of methane from both the tundra and the ocean floor are also inevitable. The only scenario I can see which doesn't result in temperature rises on the order of 5 to 10 degrees is if most of the human race gets wiped out in the next few decades by a very nasty disease of some kind.

[clv101]

That's a bit misleading. When glaciers "have their feet in the ocean" it is usually because there is a floating ice shelf between the land and the open ocean. This ice shelf backs up the glacier - if it disappears, as has already happened at various locations in the antarctic, the likely result is that the glacier starts moving faster.

Yes, but the acceleration is a short lived phenomenon until a new new steady state is reached, where the glacier is land terminated and only loses mass by melting rather than calving lumps of ice.

This process is accelerated even further by melting water getting between the base of the glacier and the underlying rocks.

This area of basal hydrology is an interesting topic. Whilst it has been observed that increased surface melt water finds it way down to the bed, lubricates and causes speed up during the melt season that's not where the story ends. In high melt years, the large volume of water getting to the bed erodes larger and efficient drainages channels in the glacial till. This efficient drainage system clears water faster and counter intuitively leads to slower ice flow.

[biffvernon]

Pull their feet out. You're thinking Greenland rather than PIG?

[biffvernon]

Another great piece from James Hansen: “Cowards in Our Democracies”

http://thinkprogress.org/romm/2012/01/2 ... n-cowards/

http://www.columbia.edu/~jeh1/mailings/ ... sPart1.pdf