Hansen: paleoclimate shows 2 degrees climate target leads to multiple meters of SLR

Comparing previous interglacial periods, like the Eemian, to the present geological epoch, the Holocene, shows slight average temperature rises are strongly magnified around the poles. Even under the 450 Scenario 21st century sea level rise could reach multiple meters. “Current climate goals are prescriptions for disaster.”

Just recently, James Hansen, the Head of NASA’s Goddard Institute for Space Studies, has written a new (draft) paper on the subject, “Paleoclimate Implications for Human-Made Climate Change” (PDF), currently under review for publication in the Belgrade Milankovitch Symposium.

“It was written in a bit of a rush when the editor told me there was a last chance to submit a paper before the book went to press,” Hansen writes on his blog. Co-author is GISS and Columbia University colleague Makiko Sato.

Slight Milankovitch difference

Due to phase differences in the Milankovitch cycle [both obliquity and eccentricity* were greater than today] the Eemian stage (at its peak 125,000 years ago) was a bit warmer than the Holocene. But probably a lot less warmer than we previously thought, Hansen argues. We have previously compared the two periods on biological fossils, like tree remains in northern Scandinavia, and gas analyses in ice core measurements. It led to believe the Eemian could have been more than 2 degrees Celsius warmer than the Holocene. In fact, it was, around the poles. Greenland even had 5 degrees higher temperatures.

Slight temperature difference

However, comparing ocean sediments – a much better indication of average global temperature – Hansen states the temperature difference between the two periods was much smaller, less than one degree.

Strong feedback response

Hansen then concludes strong polar feedbacks are an inherent part of the present world’s climate, as from a geological perspective the Eemian Earth is almost exactly identical to the world map as we know it today. What was very different though in the Eemian was the amount of ice around the poles – and the mean sea level around the globe, thought to be 4 to 6 meters higher than today.

‘Real’ climate sensitivity

“The fact that both fast and slow climate feedbacks are amplifying feedbacks and substantial in magnitude accounts for the
remarkably high sensitivity of Earth’s climate to the weak Milankovitch perturbations,” Hansen writes. [Want to read more about our flawed definition of climate sensitivity, even excluding fast-acting atmospheric feedbacks, try 'NASA: temperature is all about CO2'.]

Meters of sea level rise

To get to modern-day implications the reasoning is very simple. The amount of climate change we already have in the pipeline (so without future emissions) is more than the Eemian Holocene difference, and will therefore lead to massive melting on Greenland and the West Antarctic ice sheet.

“Deglaciation, disintegration of ice sheets, is nonlinear, spurred by amplifying feedbacks. If warming reaches a level that forces deglaciation, the rate of sea level rise will depend on the doubling time for ice sheet mass loss. Gravity satellite data, although 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.”

From 450 (CO2eq) to 350 ppm (CO2) – or FFPO

Once again Hansen’s point is we’ve lulled ourselves into the idea that setting a politically ambitious climate target, the 450 Scenario and the 2 degrees warming limit, as agreed by the UN and G8 members on many occasions, is doing what we have to do to keep things safe.

It’s not, Hansen’s previous publications taught us. The safe limit according to Hansen and many colleague climatologists is the 350 ppm stabilisation level for atmospheric CO2. This means we actually need to lower carbon concentrations instead of just reducing emissions.

Wartime effort reduction path

Emission reductions can still be the route to doing that, Hansen states. Based on these insights an American Environmental Coalition led by 23 high-ranking officials of American energy, climate and environmental NGOs, recently wrote an open letter to President Barack Obama and President Hu Jintao, calling for wartime-like mobilisation by the governments of the United States and China to cut carbon emissions 80 percent, based on 2006 levels, by 2020, in order to reach the 350 ppm atmospheric CO2 stabilisation level [indeed, that’s just nine years – the price of having done nothing before].

In their letter the group points to the newly established world climate record of 2010. In case living up to long-term interests is not really their thing, politicians can now even base their ambitious climate policy on current affairs.

Scenarios and predictions

Hansen ends his paper with a set of scenario’s and predictions. These are also a worthy read as they help remind that not only the 350 CO2 scenario, but also the 450 CO2eq scenario is still highly theoretical. Despite all the publications and all the talks our current path is simply business as usual:

“Predictions of future sea level change are inherently difficult because, we assert, ice sheet disintegration is fundamentally a non-linear process. However, in addition, the climate forcing scenario is uncertain. When predictions are made, or statements that can be construed as predictions, it is important to be clear what climate forcing scenario is being considered.

IPCC BAU (business-as-usual) scenarios assume that greenhouse gas emissions will continue to increase, with the nations of the world burning most of the fossil fuels including unconventional fossil fuels such as tar sands.

An alternative extreme, one that places a substantial rising price on carbon emissions, would have CO2 emissions beginning to decrease within less than a decade, as the world moves on energy systems beyond fossil fuels, leaving most of the remaining coal and unconventional fossil fuels in the ground. In this extreme scenario, let’s call it fossil fuel phase-out (FFPO), CO2 would rise above 400 ppm but begin a long decline by mid-century (Hansen et al., 2008).

The European Union 2°C scenario, call it EU2C, falls in between these two extremes.

BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain. Such a huge rapidly increasing climate forcing dwarfs anything in the peleoclimate record. Antarctic ice shelves would disappear and the lower reaches of the Antarctic ice sheets would experience summer melt comparable to that on Greenland today.

The other extreme scenario, FFPO, does not eliminate the possibility of multi-meter sea level rise, but it leaves the time scale for ice sheet disintegration very uncertain, possibly very long. If the time scale is several centuries, then it may be possible to avoid large sea level rise by decreasing emissions fast enough to cause atmospheric greenhouse gases to decline in amount.

What about the intermediate scenario, EU2C? We have presented evidence in this paper that prior interglacial periods were less than 1°C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters. It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal.”

It is interesting to see Hansen does not refer to ‘his own’ scenario as the 350 Scenario. Instead he calls it ‘fossil fuel phase-out (FFPO)’, in which not the stabilisation level (350 ppm) is placed central, but the allowed overshoot (400 ppm), the reduction path (CO2 concentration steadily declining from 2050 onwards) and the means (knock it off with the fossils). As you can read above, in this scenario, the process of multi-meter sea level rise would still be triggered, but hopefully slowly and halting once concentrations are effectively brought down and climate forcing is ended – although ‘global cooling’ will present its own climate delays and melting is likely to continue for decades once the atmospheric forcing gets to neutral phase [somehow, it has to go negative anyway].

[* Just one critical note: greater obliquity and greater eccentricity both increase seasonal variations on higher latitudes. Can't these very Milankovitch differences that characterise the Eemian have had something to do with the smaller amounts of ice around the poles, either through increased average temperature, or through decreased snowfall? - If you are educated on the matter and have your own questions: Hansen appreciates substantial feedback, as, once again, we are only discussing a draft paper here - and one that seems to deserve much broader attention.]

© Rolf Schuttenhelm | www.bitsofscience.org

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