A new, deeply flawed study on the climate’s sensitivity to greenhouse gas emissions reveals just how poorly the media understand key climate science issues. It also reveals how eager some in the media are to push the mistaken message that failure to act quickly and aggressively on GHG emissions would not be catastrophic.
Here’s what you need to know about the study by Schmittner et al in Science (subs. req’d):
- Its key finding is that the so-called “fast-feedbacks sensitivity” of the climate (to a doubling of CO2 levels) is on the low side. This finding is likely wrong, according to many leading climatologists (see below).
- Even if the study’s findings hold up, we are headed toward high warming on our current GHG emissions path. That’s because we are headed toward a tripling or higher of CO2 levels and because the slower feedbacks ain’t so slow (see “NSIDC bombshell: Thawing permafrost feedback will turn Arctic from carbon sink to source in the 2020s, releasing 100 billion tons of carbon by 2100“).
- The study finds that small changes in Earth’s temperature can have huge impacts on the land — that’s why it finds a low sensitivity!
This last, crucial point seems to have escaped the attention of many U.S. reporters on the study — even though it is quite clearly stated in the study’s news release:
“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,” [Schmittner] added.
Note to media: Most Americans live on “land areas at mid- to high-latitudes.”
“Hence, drastic changes over land can be expected,” he said. “However, our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon.”
In short, act quickly and aggressively or suffer drastic impacts.
Now contrast that to Eric Berger, science reporter for the Houston Chronicle, who writes in his mis-summary of the paper:
To me, the real effect of this paper will be to really impair the credibility of the more extreme environmentalists who have been saying the planet faces certain doom from climate change.
I am thinking about such efforts as Bill McKibben’s 350 campaign…. Such environmentalists assert that the planet will warm as much as 6 Celsius degrees with a doubling of atmospheric carbon dioxide levels.
That’s a big number and doubtless would have catastrophic consequences for the planet. This is not in dispute. But scientists are now telling us this is not going to happen.
Actually, it is Berger’s credibility that has taken a hit, and he owes McKibben an apology. For the record, McKibben doesn’t say we are facing “certain doom” from climate change. He — and NASA’s James Hansen, the scientist whose work inspired McKibben — typically say we are headed for multiple catastrophes if we stay anywhere near our current emissions path.
This new study does not contradict that view. Indeed, it supports it. As Skeptical Science (SkS) concluded its summary:
In fact if Schmittner et al. are totally correct, we may be in for some rapid climate changes in the relatively near future.
Moreover, many major, independent studies suggest that, yes, we are risking 6°C (11°F) warming on our current emissions path (see Hadley Center: “Catastrophic” 5-7°C warming by 2100 on current emissions path and M.I.T. doubles its 2095 warming projection to 10°F — with 866 ppm and Arctic warming of 20°F.
And don’t forget those darn extreme environmentalists at the International Energy Agency this month — see IEA’s Bombshell Warning: We’re Headed Toward 11°F Global Warming and “Delaying Action Is a False Economy”.
Berger actually cites Hansen’s main finding but then trashes it because he doesn’t appear to understand what Hansen (and many others) have found. As Skeptical Science explained (see this post):
A 2008 study led by James Hansen found that climate sensitivity to “fast feedback processes” is 3°C, but when accounting for longer-term feedbacks (such as ice sheet disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimatic (historical climate) data.
The fact is we’re already seeing the acceleration of the “slower” amplifying feedbacks (see Stunning Peatlands Amplifying Feedback: Drying Wetlands and Intensifying Wildfires Boost Carbon Release Ninefold).
To be crystal clear, since it isn’t just Berger but also the New York Times and others who seem confused about this, the amount of warming we are going to subject our children and countless future generations to depends primarily on three factors:
- The sensitivity of the climate to fast feedbacks like sea ice and water vapor (how much warming you get if we only double CO2 emissions to 560 ppm and there are no major “slow” feedbacks). We know the fast feedbacks are strong by themselves (see Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius”). Indeed, the results of a major recent Journal of Climate study on cloud feedback “provide support for the high end of current estimates of global climate sensitivity.”
- The real-world slower (decadal) feedbacks, such as tundra melt and peatlands and drought-driven emissions (see Science: Second ’100-year’ Amazon drought in 5 years caused huge CO2 emissions. If this pattern continues, the forest would become a warming source).
- The actual CO2 concentration level we are likely to hit, which is far beyond 550 ppm (see Biggest Jump Ever in Global Warming Pollution in 2010).
Indeed, it was the IPCC itself that warned we are headed toward 1000 ppm, particularly if the decadal carbon-cycle feedbacks do kick in — see “Hidden Bombshell in the IPCC Fourth Assessment” and my discussion in Nature online.
Schmittner et al only deals with #1. Sadly, #2 and #3 are more than enough to ensure humanity’s destruction even if Schmittner et al is right, indeed, especially if it is right. But it probably isn’t.
Why Schmittner et al is likely wrong about the fast-feedbacks sensitivity
There are a few big flaws in the paper. First, a potentially big issue:
Dr. Schmittner acknowledged another potential problem: that his computer analysis did not take good account of potential changes in clouds. Because clouds affect sunlight absorbed at the Earth’s surface, excluding them means “the range that we estimate for climate sensitivity may be too narrow,” he said.
Well, if the range is too narrow, that means the whole paper may be consistent with a high sensitivity — a point climatologist Richard Alley makes in detail below. And that would mean the entire paper is much ado about nothing.
Consider the November 2010 news release from the Journal of Climate study on potential changes in clouds:
Co-author Kevin Hamilton concludes, “If our model results prove to be representative of the real global climate, then climate is actually more sensitive to perturbations by greenhouse gases than current global models predict, and even the highest warming predictions would underestimate the real change we could see.“
But the key reason Schmittner et al is probably wrong is the same reason it is still worrisome if it is true — the entire analysis is based on a low-ball estimate of the warming since the Last Glacial Maximum (LGM) approximately 20,000 years ago:
“combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations we estimate a lower median (2.3 K) and reduced uncertainty (1.7–2.6 K 66% probability).”
As SkS notes, “This estimate is significantly narrower and a bit lower than the IPCC-estimated 66% probability range for equilibrium climate sensitivity of 2 to 4.5°C for doubled atmospheric CO2.” Skeptical Science explains the issue:
There are some unusual aspects about this study which require further investigation before the conclusions of the study can be accepted, as the authors themselves point out. For example, the study uses a relatively new global mean surface temperature reconstruction for the LGM of just 2.2°C cooler than interglacial temperatures in the locations where they have proxy data, or 2.6°C when averaging globally. This is significantly lower than most paleoclimate estimates, which generally put the LGM in the range of 4 to 7°C cooler than current temperatures. For comparison, in their study also using the LGM to constrain climate sensitivity, Hansen and Sato (2011) used a mean surface temperature change of 5°C, consistent with the body of literature (Figure 2).
Figure: Climate forcings during the ice age 20 ky ago relative to the pre-industrial Holocene from Hansen and Sato (2011)
Since the radiative forcing associated with doubled CO2 is 3.7 Watts per square meter (W/m2), Hansen and Sato’s result implies a fast-feedback climate sensitivity of 2.8°C, which is slightly outside the Schmittner et al. 66% probability range (at the upper end of their 90% probability range). In fact, as Urban explains, the main reason Schmittner et al. arrive at a lower climate sensitivity estimate than previous studies is due to their lower LGM temperature reconstruction:
“our LGM temperature reconstruction is quite different from what has been commonly assumed, and our study may prove inconsistent with other evidence that we have not yet considered. This is something that will have to be sorted out by further debate and research … our new temperature reconstruction explains a lot of the difference between our climate sensitivity estimate and previous estimates.”
In an interview with New Scientist on this paper, Gavin Schmidt said:
“The model estimate of the cooling during the Last Glacial Maximum is a clear underestimate…. A different model would give a cooler Last Glacial Maximum, and thus a larger sensitivity.”
Climatologist Richard Alley made the same point in great detail in an email. As the NY Times cited him:
Richard Alley, a geoscientist at Pennsylvania State University, said some scientists have questioned the data set’s accuracy and prefer using other ocean temperature estimates than the ones the researchers employed. This points to a flaw in the data set rather than the research design, he said in an email, and he thinks the authors might have found similar results to previous work if they had used a more “fully consistent and accurate” database.
“I would not conclude from this study that the estimates of future global warming are overblown,” Dr. Alley said. Given the vast array of data from other studies, Dr. Alley thinks that it is “much too early to assume that the climate sensitivity is low.”
The new study uses “global sea surface temperatures (SSTs) from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) project.” In his email to me, Alley explained that our understanding of sensitivity is pretty solid:
Very many estimates are included in our understanding that the climate sensitivity is near 3 C, with some chance of a value a little higher or lower, and very slight chance of a value much different from 3 C but with somewhat more chance on the high end than the low end.
He directs people who want to understand the issue to one of the most comprehensive review articles on the subject, Knutti and Hegerl. Alley points out that
Notice, however, in Figure 4 in the new paper, that the most-likely answer based on the land data is very different from the answer based on the ocean data (the global combined result is almost identical to the ocean result, not surprising given that the world is mostly ocean). Furthermore, the land result is fairly consistent with the results in the Knutti and Hegerl paper from many different techniques and times and groups and models, whereas the ocean result looks rather different.
Alley believes “there is a reasonably high probability that either the land data, or the ocean data, or both, are in error.” As he explains, it’s probably the ocean data:
MARGO made a solid effort, which indicates very small temperature changes. But, there are other ways to do it, and indeed, coauthor Alan Mix has published independent papers indicating that the temperature changes were larger in some regions than indicated by MARGO. David Lea and others have also obtained larger temperature shifts….
In short, the MARGO data for the ocean show very small temperature change from the ice age to today, and thus lead to the low climate sensitivity, but they disagree with some independent estimates showing larger temperature change. They also lead to disagreement with the pollen-based land temperature data. Furthermore, they lead to an answer that disagrees with many other lines of evidence for climate sensitivity.
It remains possible that the new MARGO-based ocean-based estimate is correct, but with so many disagreements with other data, other studies of the last glacial maximum, and other studies of climate sensitivity not using the last glacial maximum, it is in my opinion much too early to assume that the climate sensitivity is low. It would be much less surprising to me to learn that the MARGO data have underestimated the warming from the ice age, that the pollen data for the land are more reliable, and that the climate sensitivity from this new study, if done with a fully consistent and accurate database, would agree closely with the many other lines of evidence. Indeed, given the comparison between MARGO and other ocean temperature data sets, including work by Alan Mix, indicating that the temperature changes were larger than reconstructed by MARGO and thus that the climate sensitivity is higher than in the ocean-only and combined ocean-land shown here, I believe that this paper is consistent with the great mass of other data indicating a sensitivity near 3 C.
That conclusions seems pretty solid.
A number of other climate scientists interviewed for a BBC article also expressed reservations about the study’s assumptions and results. For example, the climate sensitivity in transitioning from a cold to warm period may be different than that in transitioning from a warm to a hot period, as Andrey Ganopolski noted:
“There is evidence the relationship between CO2 and surface temperatures is likely to be different [during] very cold periods than warmer.”
… “One should be very careful about using cold climates to [construct] the future,” he added.
This is particularly true since the LGM only experienced fast feedbacks, whereas due to the rapid rate of the current climate change, slower feedbacks may be triggered on century timescales.
This is a key point. Indeed, Alley makes this point in his email:
There is one other point possibly worth making. The authors set the ice sheets and other features of the ice age to their full ice-age values. The extended ice, lowered sea level, raised dust, and other features of the ice age, including features of the orbit, cause cooling that is not attributed to CO2 or other greenhouse gases. But, we have fairly high confidence that the cooling from the drop in CO2 contributed to the ice growth and other changes leading up to the full ice age. Thus, by initially specifying the full ice-age conditions, some of the cooling effect of the CO2 is omitted, lowering the climate sensitivity. I believe that most commonly you will find the full effect of the CO2 discussed as a long-term climate sensitivity, as opposed to the short-term one calculated as these authors have done. What these authors did is NOT wrong, and indeed is consistent with most of the literature in the field, but some authors have argued that it underestimates the climate sensitivity.
The key point is that if Schmittner et al were right, the climate is incredibly sensitive to small changes. As SkS notes, “Schmittner et al. have assumed that the difference between a glacial maximum and interglacial temperature is a mere 2.6°C. The global average surface temperature has already warmed 0.8°C over the past century. During the LGM, the surface was covered with huge ice sheets, plant life was different, and sea levels were 120 meters lower.” Schmittner himself notes the key conclusion:
“Very small changes in temperature cause huge changes in certain regions, so even if we get a smaller temperature rise than we expected, the knock-on effects would still be severe.”
But remember, we’re headed toward a tripling or more of CO2 levels (from preindustrial values). And the decadal feedbacks are already starting to kick. Thus as Skeptical Science puts it:
If Schmittner et al. are correct and such a small temperature change can cause such a drastic climate change, then we may be in for a rude awakening in the very near future, because their smaller glacial-interglacial difference would imply a quicker climate response a global temperature change…. although the Schmittner et al. best estimate for climate sensitivity results in approximately 20% less warming than the IPCC best estimate, we also achieve their estimated temperature change between glacial and interglacial periods (the dashed lines) much sooner….
If Schmittner et al. are totally correct, we may be in for some rapid climate changes in the relatively near future, as we approach the amount of warming that separates a glacial from an interglacial period.
The bottom line is that this new study will very likely not result in any major revisions of the climate sensitivity range of the IPCC, and, if it were true, it is cause to be more concerned about our current do-nothing approach to emissions.
This post has been updated (because of an error in the Skeptical Science post).
- Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher — “We have shown that this dramatic rise in sea level is associated with an increase in CO2 levels of about 100 ppm”).
- A detailed look at climate sensitivity