Making Sense of Climate Sensitivity: How The Economist And MSM Keep Getting It Wrong

Memo to media: “Climate sensitivity” is NOT the same as projected future warming!

Projected warming even with (an unlikely) low climate sensitivity of between 1.5°C and 2.0°C from Michael Schlesinger et al 2012.

The Economist has joined the ranks of the major media who continue to sow confusion on one of the key questions of our time: How much warming will we subject our children and the next 50 generations to?

I addressed this two months ago in the post “Memo To Media: Climate Sensitivity Is NOT The Same As Projected Future Warming, World Faces 10°F Rise.” And that was two months after I discussed it in debunking an error-riddled Matt Ridley piece in the Wall Street Journal. But as long as the MSM keeps getting it wrong, I’ll keep updating my correction.

The answer to the question of how much warming we face depends primarily on four factors:

  1. The so-called “equilibrium climate sensitivity” — the sensitivity of the climate to fast feedbacks like sea ice and water vapor. The ECS is how much warming you get if we suddenly adopt a super-aggressive effort to cut carbon pollution and only double CO2 emissions to 560 ppm — and there are no major “slow” feedbacks. We know the fast feedbacks, like water vapor, are strong by themselves (see Study: Water-vapor feedback is “strong and positive,” so we face “warming of several degrees Celsius” and Skeptical Science piece here).
  2. The actual CO2 concentration level we hit, which on our current emissions path is far, far beyond 550 ppm (see U.S. media largely ignores latest warning from climate scientists: “Recent observations confirm … the worst-case IPCC scenario trajectories are being realised” — 1000 ppm).
  3. The real-world slower (decade-scale) feedbacks, such as tundra melt (see “Carbon Feedback From Thawing Permafrost Will Likely Add 0.4°F — 1.5°F To Total Global Warming By 2100“).
  4. Where they live — since people who live in the mid-latitudes (like most Americans) are projected to warm considerably more than the global average.

The media, perhaps aided by some scientists who aren’t great at communications, tend to focus on just #1, a number the IPCC Fourth Assessment Report pegged as “likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.” While the majority of studies tend to be in the middle of the range, a couple have been near the low end, though some have been at the higher end — see for instance “Science Stunner (11/12): Observations Support Predictions Of Extreme Warming And Worse Droughts This Century.”

Dana Nuccitelli makes some good points on the Economist piece at Skeptical Science:

… the article focused heavily on the slowed global surface warming over the past decade, and a few studies which, based on that slowed surface warming, have concluded that climate sensitivity is relatively low. However, as we have discussed on Skeptical Science, those estimates do not include the accelerated warming of the deeper oceans over the past decade, and they appear to be overly sensitive to short-term natural variability. The Economist article touched only briefly on the accelerated deep ocean warming, and oddly seemed to dismiss this data as “obscure.”

The Economist article also referenced the circular Tung and Zhou (2013) paper we addressed here, and suggested that if equilibrium climate sensitivity is 2°C to a doubling of CO2, we might be better off adapting to rather than trying to mitigate climate change. Unfortunately, as we discussed here, even a 2°C sensitivity would set us on a path for very dangerous climate change unless we take serious steps to reduce our greenhouse gas emissions.

Ultimately it was rather strange to see such a complex technical subject as climate sensitivity tackled in a business-related publication. While The Economist made a good effort at the topic, their lack of expertise showed.

Then Nuccitelli reposted “an article published by Zeke Hausfather at the Yale Forum on Climate Change & the Media” so readers could get “a more expert take on climate sensitivity.” I recommend the whole piece. Here’s the key part:

There are several different ways to estimate climate sensitivity:

  • Examining Earth’s temperature response during the last millennium, glacial periods in the past, or periods even further back in geological time, such as the Paleocene Eocene Thermal Maximum;
  • Looking at recent temperature measurements and data from satellites;
  • Examining the response of Earth’s climate to major volcanic eruptions; and
  • Using global climate models to test the response of a doubling of CO2 concentrations.

These methods produce generally comparable results, as shown in the figure below.

Figure from Knutti and Hegerl 2008.

… So what about climate sensitivity? We are left going back to the IPCC synthesis, that it is “likely” between 2 C and 4.5 C per doubling of CO2 concentrations, and “very likely” more than 1.5 C. While different researchers have different best estimates (James Annan, for example, says his best estimate is 2.5 C), uncertainties still mean that estimates cannot be narrowed down to a far narrower and more precise range.

And, again, the ECS is not the same as our projected future warming.

What follows is an updated excerpt from my February post. Regular readers don’t need to read it again.

Focusing on the fast-feedback sensitivity perhaps made sense in the distant past when we thought the world would actually listen to climate scientists and so there was some reasonable chance of stabilizing at 560 parts per million atmospheric concentrations of carbon dioxide (double the preindustrial level) — and some hope the slow feedbacks might not matter.

As I explained in Nature online back in 2008 (here), once you factor in carbon-cycle feedbacks, even the uber-cautious Fourth Assessment report (AR4) of the IPCC makes clear we are headed toward 1000 ppm (the A1FI scenario). That conclusion has been supported by just about every major independent analysis, including a recent report by PricewaterhouseCoopers (see Study: We’re Headed To 11°F Warming And Even 7°F Requires “Nearly Quadrupling The Current Rate Of Decarbonisation“). That means it simply doesn’t matter terribly much whether the ECS is 3C, or, say, only 2.5C.

But The Economist concludes:

a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly.

Even if one agrees with this, it doesn’t merit any celebration, let alone a nearly 3,000-word article.

It is worth noting that while the Thawing Permafrost Could Cause 2.5 Times the Warming of Deforestation (!) and add up to 1.5°F to warming in 2100 by itself, “Participating modeling teams have completed their climate projections in support of the [IPCC’s] Fifth Assessment Report, but these projections do not include the permafrost carbon feedback.” D’oh!

Given that the Arctic is already losing ice decades faster than any AR4 model had projected, we should expect that the permafrost will go faster than the models suggest. Indeed a 2008 study by leading tundra experts found “Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss.” The study’s ominous conclusion:

We find that simulated western Arctic land warming trends during rapid sea ice loss are 3.5 times greater than secular 21st century climate-change trends. The accelerated warming signal penetrates up to 1500 km inland….

Anyone who tells you the recent literature suggests things will be better than we thought, hasn’t read the recent literature. In a 2010 AAAS presentation, the late William R. Freudenburg of UC Santa Barbara discussed his research on “the Asymmetry of Scientific Challenge“: New scientific findings since the 2007 IPCC report are found to be more than twenty times as likely to indicate that global climate disruption is “worse than previously expected,” rather than “not as bad as previously expected.”

“Projections of global warming relative to pre-industrial for the A1FI emissions scenario” — the one we’re currently on. “Dark shading shows the mean ±1 s.d. [standard deviation] for the tunings to 19 AR4 GCMs [IPCC Fourth Assessment General Circulation Models] and the light shading shows the change in the uncertainty range when … climate-carbon-cycle feedbacks … are included.

Again, we are headed to 11F and just keeping to 7F will take a major effort. But warming beyond 7F is “incompatible with organized global community, is likely to be beyond ‘adaptation’, is devastating to the majority of ecosystems & has a high probability of not being stable (i.e. 4°C [7F] would be an interim temperature on the way to a much higher equilibrium level,” as climate expert Kevin Anderson explains here.

Everyone interested in what we face should should read the recent World Bank Climate Report, which concluded, “A 4°C [7°F] world can, and must, be avoided” to avert “devastating” impacts. Also worth reading is the Royal Society Special Issue on Global Warming, which details the “hellish vision” of 7°F (4°C) world (and is the source of the figure above). The concluding piece in the issue notes soberly:

… a 4°C world would be facing enormous adaptation challenges in the agricultural sector, with large areas of cropland becoming unsuitable for cultivation, and declining agricultural yields. This world would also rapidly be losing its ecosystem services, owing to large losses in biodiversity, forests, coastal wetlands, mangroves and saltmarshes, and terrestrial carbon stores, supported by an acidified and potentially dysfunctional marine ecosystem. Drought and desertification would be widespread….

In such a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world.”

I’ll end by noting once more that the paleoclimate record suggests the ultimate warming we are going to see is likely to be considerably higher than the fast-feedbacks sensitivity suggests:

  • Science (2009): 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
  • Scientists analyzed data from a major expedition to retrieve deep marine sediments beneath the Arctic to understand the Paleocene Eocene thermal maximum, a brief period some 55 million years ago of “widespread, extreme climatic warming that was associated with massive atmospheric greenhouse gas input.” This 2006 study, published in Nature (subs. req’d), found Arctic temperatures almost beyond imagination–above 23°C (74°F)–temperatures more than 18°F warmer than current climate models had predicted when applied to this period. The three dozen authors conclude that existing climate models are missing crucial feedbacks that can significantly amplify polar warming.
  • A study published in Geophysical Research Letters (subs. req’d) looked at temperature and atmospheric changes during the Middle Ages. This 2006 study found that the effect of amplifying feedbacks in the climate system — where global warming boosts atmospheric CO2 levels — ”will promote warming by an extra 15 percent to 78 percent on a century-scale” compared to typical estimates by the U.N.’s Intergovernmental Panel on Climate Change. The study notes these results may even be “conservative” because they ignore other greenhouse gases such as methane, whose levels will likely be boosted as temperatures warm.
  • Another study published in Geophysical Research Letters, “Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming” (subs. req’d), looked at temperature and atmospheric changes during the past 400,000 years. This study found evidence for significant increases in both CO2 and methane (CH4) levels as temperatures rise. The conclusion: If our current climate models correctly accounted for such “missing feedbacks,” then “we would be predicting a significantly greater increase in global warming than is currently forecast over the next century and beyond”–as much as 1.5°C warmer this century alone.
  • Science stunner (2011): On our current emissions path, CO2 levels in 2100 will hit levels last seen when the Earth was 29°F (16°C) hotter. Paleoclimate data suggests CO2 “may have at least twice the effect on global temperatures than currently projected by computer models.”