Climate sensitivity - paleoclimate record points towards potential rapid climate change

In a press conference at the American Geophysical Union Fall meeting 2011 in San Fransisco climate scientists James Hansen, Ken Caldeira and Eelco Rohling explained that the climate sensitivity may be greater than previously thought. This has implications particularly for action on climate change mitigation and adaptation with major impacts on sea level rise, ocean acidification and many other areas. The latest proposals and pledges from Durban put the world on the path of 4.3°C of warming by the end of the century.

Detailed examination of the paleoclimate record indicates that for every degree Celsius of global temperature rise will ultimately equate to 20 meters of sea level rise. However, that sea level increase due to ice sheet loss would be expected to occur over centuries, and large uncertainties remain in predicting how that ice loss would unfold.

James Hansen on Paleoclimate record and climate sensitivity

James Hansen indicated that disintegration of ice sheets is a non-linear process that is already being seen in Greenland, and the Pine Island Glacier in West Antarctica, where the rate of ice mass loss has continued accelerating over the past decade. Indications from measurements collected from the GRACE satellites indicates that the rate of ice sheet mass loss in Greenland and West Antarctica doubles every ten years. Although the data record trend from GRACE measurements is short which entails significant uncertainty with regard to predictions, Hansen argues the continued rate of ice loss has the potential to cause multiple meters of sea level rise by 2100.

According to ice and sediment cores, we are approaching temperatures in polar regions similar to epochs when sea level was tens of metres higher. "We don't have a substantial cushion between today's climate and dangerous warming," Hansen said. "Earth is poised to experience strong amplifying feedbacks in response to moderate additional global warming."

The difference between climate change in past epochs and today is the rapid rate of change. Where in the past atmospheric CO2 content would change over thousands of years in a slow process allowing the biosphere and ecosystems to adjust, we have changed the rate of warmer at a significantly faster rate. "Humans have overwhelmed the natural, slow changes that occur on geologic timescales," Hansen said.

Hansen has been emphasising the importance of a moratorium on new coal fired power stations and phasing out dirty coal use. The IEA has estimated recently that existing and proposed power stations lock in 80 per cent of the total energy-related CO2 emissions permitted to 2035 in the 450ppm Scenario.

"We cannot burn all the fossil fuels." said Hansen at the AGU press conference, "If we burnt all of the fossil fuels we would send the planet back to the ice free state. We can't say how long it would take to get there but we know that the rate of change would be accelerating rapidly, and yet the governments and fossil fuel industry assume that we can go right ahead and go after all the coal and even begin to develop these unconventional fossil fuels like tar sands and tar shale and hydro fracking. That is a message which somehow we do not seem to have communicated as well as we need to."

Hansen also emphasised the importance of improved agricultural and forestry practices to absorb more atmospheric carbon in land carbin sinks.

In answer to a journalist question on the accelerating CO2 emissions and how long we can go on before it is too late to do anything, Hansen replied:

"What the paleo climate record tells us is that the dangerous level of global warming is less than what we thought a few years ago. It was natural to think that warming of a few degrees didn't sound so bad, but when we look at the paleo-climate record we see that, just going back to the Pliocene, although the poles were alot warmer then, the global mean temperatures wasn't that mush warmer." "The target that has been talked about in international negotiations of two degrees celsius for global warming is actually a prescription for long-term disaster. We can't say exactly what long-term is, but we are beginning to see signs of slow feedbacks beginning to come into play already... ice sheets are beginning to lose mass at a significant rate, even methane hydrates are to some degree beginning to bubble out of melting Permafrost....We really should be aiming to keep CO2 no higher than about 350 parts per million and possibly some want less than that if we want to keep the climate similar, not too different than the Holocene, and that is probably necessary if we want to maintain stable ice sheets and stable shorelines and avoid many other issues that we did not try to go into today." "If that is the goal, to get the CO2 down to 350ppm within a century or so, that would require we started today and if we assume that reforestation could take up 100 gigatonnes of CO2, that would essentially be going back to the pre-deforestation levels, then we would have to reduce CO2 emissions at 6% per year if we began next year. If we began 5 years ago, then it would have been 3% per year. If we wait for 2020 it becomes 15% per year. So, if we are hoping to maintain the climate, a planet that looks like the one that humanity has known, then we are basically out of time. Right now. We've got to start to reduce emissions and so this continued rapid growth makes it exceedingly difficult. We've got to turn that around. It's scientifically clear."

Eelco Rohling on Ice Sheet and sea level rise

Eelco Rohling, Professor of Ocean and Climate Change at Southampton University, Southampton, United Kingdom, presented on Ice Sheet and sea level response at the AGU press conference.

Rohling spoke on the sea level disequilibrium and ice volume adjustment, and how fast we can expect some of these adjustments to take place.

"We have this relationship, what is the basic underlying natural relationship between sea level and climate forcing and if we use that, then at 1.6 watts per square metre forcing where we are at the moment, then we would expect the equilibrium sea level in the natural state to be 25 metres (±3-5 metres) above the present. So this is essentially the elastic band of climate that we are stretching. We are creating a disequilibrium. Now the problem is we are not only stretching but we're stretching it really quickly so the system cannot keep up at all. ...The adjustment is similar to yanking an elastic band really quickly - there could be snaps in there - and we could have very abrupt adjustments happening. And these abrupt adjustments we can say something about if we go to the last interglacial."

"In the last interglacial sea level went to a mean position between 4 and 6 metres above the present, so it's higher than today. Part of that is because of the reduction of the Greenland icesheet... about half the icesheet disappears. That gives you about 3 metres of sea level rise. So these numbers suggest that Antarctica was in force. We don't have to guess, we know Antarctica was involved."

"what we have found is that there were a couple of fast adjustments in there. These are all natural adjustments so not anything to do with anthropogenic change but nature shows us that the adjustment rates that you can get at sea level above the present - so ice sheet reducing further than today - at 1 to 2.5 metres per century. So Nature knows how to do this. ...These rates of rise can become of the order of metres per century. That's what nature is telling us here. It knows how to do this and that is the sort of rapid adjustment rates that we are potentially toying with."

A recent study published in the Journal of Climate (Sea surface and high-latitude temperature sensitivity to radiative forcing of climate over several glacial cycles) which Rohling was a lead researcher, infered that Earth's climate sensitivity over the last half million years most likely amounted to a 3.1 to 3.9 °C temperature increase for the radiative equivalent of a modern doubling of atmospheric carbon-dioxide concentrations (with a total range of 1.7 to 5.7 °C).

"We use long time-series of data that are each obtained using a single method. This reduces uncertainty in the estimates of temperature change, relative to previous work that contrasts reconstructions of a single past climate state with modern instrumental data. Our method can be especially improved by extending the global network of long records." said Eelco Rohling in a media release.

He continues: "Because our climate sensitivity values are based on real-world data from a substantial interval of time in the recent geological past, our results provide strong observational support to the climate sensitivities used in IPCC-class climate models. If anything, our results suggest slightly stronger sensitivity."

Ken Caldeira on radiative forcing and climate response from paleoclimate to future

The third person on the panel was Ken Caldeira, Senior Scientist with the Department of Global Ecology, Carnegie Institute of Washington at Stanford University. Ken spoke on radiative forcing and climate response from paleoclimate to future. He explained that climate sensitivity is not one number, but depends on what factors you include and which are allowed to stay constant and which are allowed to vary.

The recent study by Schmittner et al (Abstract) published in Science on November 24 2011, that said climate sensitivity may be low - "results imply a lower probability of imminent extreme climatic change than previously thought" - was at the bottom of the range of studies assessing climate sensitivity, and its results would be highly dependent on the assumptions used, said Caldeira.

Caldeira discussed the paleoclimate during the PETM event some 50 million years ago:

"Looking at an event for fifty million years ago suggested that a lot of methane was released into the environment and that the earth heated up something like 5.5 to 8 degrees C for each CO2 doubling which is 10 to 14 degrees fahrenheit. It is still not understood why the earth heated up so much. It might be that wetlands formed at high latitudes and there was a methane feedback. This was a time without ice sheets, so it can't be the ice sheet feedback, so there is paleo evidence that climate sensitivity might be dramatically higher than what a lot of the current climate models are suggesting." he said.

Ken Caldeira explained that there is evidence for huge climate sensitivity in the past when correlating temperature and sea level amounting to tens of metres of sea level change with a doubling of CO2. Even with changes occurring over thousands of years due to slow changes in orbital parameters, there were short pulse events. "When we hit the system with a hammer like a huge pulse of CO2, well, will we get this flow kind of response or will we see faster response? We don't really know." he said.

While in Durban there was more talk and little action on deep cuts to emissions

The press conference ocurred while the UN Framework Convention on Climate Change (UNFCCC) was meeting in Durban South Africa (COP17). The Copenhagen and Cancun meetings have both affirmed a 1.5 or 2 degree limit should be the maximum global average temperature increase, even though countries have not committed to deep cuts in emissions that would be required to avoid dangerous climate change and stay under this limit. Voluntary emission cut pledges has left a sizeable Global emissions Gap. At the plenary in the morning on Friday - the last scheduled day - a youth speaker representing civil society, Anjali Appadurai, challenged the conference by calling for deep emissions cuts from developed countries to meet their commitments under the Bali roadmap of the Kyoto Protocol. While a framework for a new legally binding agreement to start in 2020 was established in the final hours, the delays in commencement means disaster for climate mitigation action. Many NGOs condemned the lack of action, even before the negotiations that continued throughout the weekend, long past the closing time.

The latest proposals and pledges from Durban put the world on the path of 4.3°C of warming by the end of the century. A statement released by Climate Action Tracker on December 11, 2011 stated:

Without new pledges for emissions reduction on the table, our Climate Scoreboard analysis projects future global temperature increases far above the global goal of 2°C (3.6 °F) , pointing towards temperature increase of 4.3°C (2.6 - 6.9°C) or 7.7°F (4.6 - 12.3°F) by the end of the century.

A good summary of the COP17 Durban conference outcome was written by Mark Lynas, who is on the Maldives delegation and advises President Nasheed on climate issues : The verdict on Durban - a major step forward, but not for ten years.

Sources:

• NASA December 8 2011 - Paleoclimate Record Points Toward Potential Rapid Climate Changes
• AGU Press conference, December 6, 2011 - AGU FM11 - Paleoclimate record points toward potential rapid climate changes (Youtube video as embedded above)
• Southampton University Media release December 6, 2011 - Global sea surface temperature data provides new measure of climate sensitivity over the last half million years
• Journal of Climate - Sea surface and high-latitude temperature sensitivity to radiative forcing of climate over several glacial cycles (Abstract) by Rohling, E. J., Medina-Elizalde, M., Shepherd, J. G., Siddall, M., Stanford, J. D, doi: http://dx.doi.org/10.1175/2011JCLI4078.1
• Climate Interactive Statement December 11, 2011 - Durban Talks Open the Door to a Future Global Legal Agreement, But Produce No Immediate Strengthening of Pledges