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Study using paleoclimate data suggests climate sensitivity to CO2 doubling may be less severe than projected

A new study, funded by the National Science Foundation’s Paleoclimate Program and published online this week in the journal Science, suggests that the rate of global warming from doubling of atmospheric carbon dioxide from pre-industrial times may be less than the most extreme estimates of some previous studies and may be less severe than that projected by the Intergovernmental Panel on Climate Change (IPCC) report in 2007.

The team combined extensive sea and land surface temperature reconstructions from the Last Glacial Maximum (LGM) with a climate model of intermediate complexity to estimate the equilibrium climate sensitivity for a doubling of atmospheric carbon dioxide concentrations (ECS2xC) from preindustrial times. (Climate sensitivity is the change in global mean surface air temperature caused by radiative forcing of Earth’s radiative balance at the top of the atmosphere with respect to a given reference state.)

Their results showed an estimated lower median temperature increase (2.3 K) and reduced uncertainty (1.7 to 2.6 K 66% probability). Assuming paleoclimatic constraints apply to the future as predicted by their model, the researchers concluded, their results imply lower probability of imminent extreme climatic change than previously thought.

The experiments collectively favor sensitivities between 1 and 3 K. However, we cannot exclude the possibility that the analysis is sensitive to uncertainties or statistical assumptions not considered here, and the underestimated land/sea contrast in the model, which leads to the difference between land and ocean based estimates of ECS2xC, remains an important caveat.

Our uncertainty analysis is not complete and does not explicitly consider uncertainties in radiative forcing due to ice sheet extent or different vegetation distributions. Our limited model ensemble does not scan the full parameter range, neglecting, for example, possible variations in shortwave radiation due to clouds. Non-linear cloud feedbacks in different complex models make the relation between LGM and 2×CO2 derived climate sensitivity more ambiguous than apparent in our simplified model ensemble. More work, in which these and other uncertainties are considered, will be required for a more complete assessment.

In summary, using a spatially extensive network of paleoclimate observations in combination with a climate model we find that climate sensitivities larger than 6 K are implausible, and that both the most likely value and the uncertainty range are smaller than previously thought. This demonstrates that paleoclimate data provide efficient constraints to reduce the uncertainty of future climate projections.

—Schmittner et al.

The authors emphasize that global warming is real and that increases in atmospheric CO2 will have multiple serious impacts. However, the most extreme projections of temperature increases from the doubling of CO2 are unlikely, according to their work.

Many previous climate sensitivity studies have looked at the past only from 1850 through today, and not fully integrated paleoclimate data, especially on a global scale. When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago—which is referred to as the Last Glacial Maximum—and compare it with climate model simulations of that period, you get a much different picture.

If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought.

—Andreas Schmittner, Oregon State University researcher and lead author

Scientists have struggled for years trying to quantify climate sensitivity—how the Earth will respond to projected increases of atmospheric carbon dioxide. The 2007 IPCC report estimated that the air near the surface of the Earth would warm on average by 2 to 4.5 °C with a doubling of atmospheric CO2 from pre-industrial standards. The mean, or expected value increase in the IPCC estimates was 3.0 degrees; most climate model studies use the doubling of CO2 as a basic index.

Some previous studies have calculated the impacts could be much more severe—as much as 10 degrees or higher with a doubling of CO2. Studies based on data going back only to 1850 are affected by large uncertainties in the effects of dust and other small particles in the air that reflect sunlight and can influence clouds, known as “aerosol forcing,” or by the absorption of heat by the oceans, the researchers say.

To lower the degree of uncertainty, Schmittner and his colleagues used a climate model with more data and found that there are constraints that preclude very high levels of climate sensitivity.

The researchers compiled land and ocean surface temperature reconstructions from the Last Glacial Maximum and created a global map of those temperatures. During this time, atmospheric CO2 was about a third less than before the Industrial Revolution, and levels of methane and nitrous oxide were much lower. Because much of the northern latitudes were covered in ice and snow, sea levels were lower, the climate was drier (less precipitation), and there was more dust in the air. All these factors, which contributed to cooling the Earth’s surface, were included in their climate model simulations.

The new data changed the assessment of climate models in many ways, said Schmittner. The researchers’ reconstruction of temperatures has greater spatial coverage and showed less cooling during the Ice Age than most previous studies.

High sensitivity climate models—more than 6 degrees—suggest that the low levels of atmospheric CO2 during the Last Glacial Maximum would result in a runaway effect that would have left the Earth completely ice-covered.

Clearly, that didn’t happen. Though the Earth then was covered by much more ice and snow than it is today, the ice sheets didn’t extend beyond latitudes of about 40 degrees, and the tropics and subtropics were largely ice-free—except at high altitudes. These high-sensitivity models overestimate cooling.

—Andreas Schmittner

On the other hand, models with low climate sensitivity—less than 1.3 degrees—underestimate the cooling almost everywhere at the Last Glacial Maximum, the researchers say. The closest match, with a much lower degree of uncertainty than most other studies, suggests climate sensitivity is about 2.4 degrees.

Reconstructing sea and land surface temperatures from 21,000 years ago is a complex task involving the examination of ices cores, bore holes, fossils of marine and terrestrial organisms, seafloor sediments and other factors. Sediment cores, for example, contain different biological assemblages found in different temperature regimes and can be used to infer past temperatures based on analogs in modern ocean conditions.

When we first looked at the paleoclimatic data, I was struck by the small cooling of the ocean. On average, the ocean was only about two degrees (Celsius) cooler than it is today, yet the planet was completely different—huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air. It shows that even very small changes in the ocean’s surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes.

—Andreas Schmittner

Schmittner said continued unabated fossil fuel use could lead to similar warming of the sea surface as reconstruction shows happened between the Last Glacial Maximum and today.

Hence, drastic changes over land can be expected. However, our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon.

—Andreas Schmittner

Other authors on the study include Peter Clark and Alan Mix of OSU; Nathan Urban, Princeton University; Jeremy Shakun, Harvard University; Natalie Mahowald, Cornell University; Patrick Bartlein, University of Oregon; and Antoni Rosell-Mele, University of Barcelona.

Resources

  • Andreas Schmittner, Nathan M. Urban, Jeremy D. Shakun, Natalie M. Mahowald, Peter U. Clark, Patrick J. Bartlein, Alan C. Mix, and Antoni Rosell-Melé (2011) Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum. Science doi: 10.1126/science.1203513

Comments

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Reel$$

Indeed education derives from peer-reviewed papers, and studies by qualified scientists and experts. Weighing AGW skeptics vs alarmists proved Michael Crichton to have been correct all along. Mann-made Global Warming (not metaphorical) is an unproven theory.

The fact the "climate campaign" has collapsed (even Socialist Canada will pull out of Kyoto!) is because it refused to listen to or allow opposing points of view. Arrogance and hubris is no substitute for tolerance and impartiality.

Lesson: no matter how big the propaganda budget (AGW = trillion$), human beings will make up their own minds. Fair warning for future propagandists. And Energy Independence is arriving via radical new technology like LENR being introduced to North Americans at events like this:

http://citi5.org/launch/?p=1826

And Emeritus Prof. Focardi's TED lecture in Bolgona:

http://bit.ly/u6g04z

ai_vin

ROTFLMAO

Roger Pham

Let's make it simpler, Reel$$. Let's look at the facts ourselves and make our own conclusion!

Fact 1: Overwhelming number of independent sources confirmed the escalation in global warming in the last several decades.
Fact 2: Human have been escalating the release of GHG's into the atmosphere.
Fact 3: The effects of GHG's in keeping out the heat rays are well quantitated by experimentation and confirmed by mathematical calculations.

Conclusion: We should slow down and stop the release of GHG's to affect the devastating trend in global warming in the best way we can.

The exact cause of Global Warming is quite irrelevant! The exact computer modeling and prediction of the degree of warming are quite irrelevant! No one can 100% predict the future!

What the climate scientists are communicating among themselves are quite irrelevant.

Reel$$

"The exact cause of Global Warming is quite irrelevant!"

Huh?? Roger, that does not sound like science. If, e.g., 98% of any warming is caused by natural causes... why let scammers set up Cap N' Trade schemes? Why let a supposed "Climate Research Unit" get in bed with Wall Street Goldman Sachs?? CRU = 1% against 99% regular "folks."

OCCUPY CRU!!

Roger Pham

When your house feels warm, what do you do, Reel$$? You would reach the thermostat and turn on the AC, won't you? You don't care about what causes you house to get warm, would you? Would you tell your wife to quit cooking to cool down the house? What if you are running a restaurant that require a lot of cooking? Just turn on the AC!!!

Human's thermostat for the earth is the control of GHG level. The sun's excessive activity may be the cause of increase in GW, but our recourse will still be the same: REduce GHG level! No matter what is or will be the cause of GW, our action will still be the same: REDUCE GHG LEVEL. Simple.

No matter what the "Climate Research Unit" or GS are doing, we will need to REDUCE GHG LEVEL. Just use the KISS principle. (Keep It Simple, Sir!)

Reel$$

Climate is a failed campaign - even Canada has pulled out of Kyoto... But we ARE making changes to our use and distribution of combustion-based ENERGY:

Here is NASA Langley Chief Scientist Dr. Dennis Bushnell on the impact of LENR:

"In Short, LENR , depending upon the TBD performance, appears to be capable of Revolutionizing Aerospace across the board. No other single technology even comes close to the potential impacts of LENR upon Agency Missions."

http://www.ecatplanet.net/content.php?123-LENR-Presentation-by-Dennis-Bushnell-Chief-Scientist-NASA-Langley

Bushnell is just one of two dozen highly qualified scientists supporting the commercialization of LENR technology. This, along with moderate development of solar, and alternatives will dramatically reduce GHGs.

ai_vin

It's Stephen Harper that has pulled out of Kyoto. Ordinary Canadians OTOH support it with polling numbers of between 67% to 75%.

What Canadians don't support is Stephen Harper. He may have won a majority in the last election but he did it with only 39.62% of the popular vote. That's up from the 37.65% that voted for him in 2008.

How can someone so unpopular win like that? Simple: In Canada we have 1 rightwing party collecting all conservative votes and 4 leftwing parties dividing up the progressive votes.

Reel$$

ai_vin, sorry you can't sell AGW to Canada.

Anyone who REALLY cares about Earth's environment, would be celebrating the momentous announcement from Dr. Bushnell - we have a carbonless, non-radiative, low, low cost of energy coming online. Replacing billions of gallons of oil and billions of tons of coal. Freeing millions from energy poverty and bringing light, heat and clean water to multitudes in need.

Why is everyone so terrified? Do you not REELLY care for the welfare of human beings after all????

ai_vin

Don't equate just our PM with "Canada," Canada is its people - all of its people.

Reel$$

Avoiding the questions provides no absolution.

ai_vin

"Avoidance" be your middle name, Reel.

Reel$$

As pointed out, those who do not applaud or publicize the recent (and continuing) disclosures of positive developments in LENR - Low Energy Nuclear Reactions science - do not give a shite about the planet, humanity or its "grandchildren." Your silence and refusal to support disclosure, are your swastikas of denial. Real denial. Pathetic. Misanthropic. Enemies of EARTH and the human race.

Energy Independence. Your resistance is futile.

ai_vin

First off, my silence(?) comes only from the fact you haven't added anything new to this thread. Any questions/statements you've made HAVE been answer elsewhere and they had no effect because your the one who's in real denial. You may like repeating yourself, I do not.

And second, "swastikas of denial" ???

I declare Godwin's Law, you lose.

Reel$$

Who said YOU are the subject of discussion ai_vin??

"Hoy-day, what a sweep of vanity comes this way!" Shakespeare

Or must you remain secreted behind your soured mirror - casting aspersion upon the plaintiff?

HarveyD

I'm sorry to hear that. We were just getting use to +2.5C and another +3C tp +4C or so would have been even better.

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