Are Scientists Overestimating — or Underestimating — Climate Change, Part II

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"Are Scientists Overestimating — or Underestimating — Climate Change, Part II"

The previous post debunked an article that argued scientists have seriously overestimated climate change. Now let’s look at the evidence they are seriously underestimating climate change.

To do that, the fatal flaw with the IPCC’s over-reliance on the poorly named “equilibrium climate sensitivity” (ECS) must be understood. Recall that the ECS is the “equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration,” which the IPCC’s 2007 Fourth Assessment Report concluded was 2 to 4.5°C.

You might think that the ECS tells you how much the planet’s temperature will rise if humans emit enough CO2 to double its atmospheric concentration. But it doesn’t. It is just a theoretical construct. It tells you only how much the planet’s temperature will rise if CO2 concentrations double and then are magically frozen.

That’s because the ECS omits key carbon cycle feedbacks that a rise in the planet’s temperature will likely trigger. For instance, a doubling of CO2 to 550 ppm will lead to the melting of the permafrost and the release of huge amounts of carbon currently frozen it it. These amplifying (or positive) feedbacks are the main subject of this post.

The ECS includes only “fast feedbacks” which NASA’s James Hansen defines as follows:

For example, the air holds more water vapor as temperature rises, which is a positive feedback magnifying the climate response, because water vapor is a greenhouse gas. Other fast feedbacks include changes of clouds, snow cover, and sea ice. It is uncertain whether the cloud feedback is positive or negative, because clouds can increase or decrease in response to climate change. Snow and ice are positive feedbacks because, as they melt, the darker ocean and land absorb more sunlight.

While some Denyers, like MIT’s Richard Lindzen, have argued that negative feedbacks dominate the climate — all of the evidence points to amplifying feedbacks dominating. That was a key point of Part I of this post, that in the real world, key climate change impacts — sea ice loss, ice sheet melting, temperature, and sea level rise — all are either near the top or actually in excess of their values as predicted by the IPCC’s climate models. The models are missing key amplifying feedbacks.

A number of major studies looking at paleoclimate data come to the same conclusion. Here are three:

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 Artic 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 second 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.

The third study, published in Geophysical Research Letters (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.

What are these “missing feedbacks” in the global carbon cycle? I devote a chapter in my book to this question (where you can find all the source material). They include four key carbon sinks:

  • The oceans — which likely become less able to take up carbon dioxide as they heat up and become more acidic.
  • The soil — which also takes up less CO2 and starts emitting CO2 as it heats up.
  • The tundra — which contains more carbon than the atmosphere does (much of it in the form of methane, a much more potent greenhouse gas than CO2) and which is poised to release that carbon as we warm the planet.
  • Tropical forests — which store carbon but in places like Brazil and Indonesia are being cut down. Deforestation coupled with warming-induced drought could lead to the complete collapse of the Amazon rain forest.

Some combination of these carbon sinks saturating — or turning into carbon sources — probably help drive the amplifying feedbacks that the paleoclimate studies show make the planet’s true climate sensitivity far greater than the equilibrium climate sensitivity in the IPCC models.

In Part III, I will look at how these feedbacks may create a climate “point of no return” and constrain greenhouse gas targets needed to avoid climate catastrophe.

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2 Responses to Are Scientists Overestimating — or Underestimating — Climate Change, Part II

  1. Rob says:

    Hi Joe,

    This is a great series of posts on how climate models can underestimate global warming because of what they’re missing.

    But I thought the ECS experiment didn’t specify how the atmosphere gets to 2xCO2. I think the experiment is posed as “If the atmosphere reaches 2xC02 levels, after all anthropogenic emissions and feedbacks have occurred, what will be the equilibrium temperature?”. Your overall point is still valid: given that we don’t know how the feedbacks will play out and we haven’t yet halted our additions of CO2, stopping at twice pre-industrial levels, about 560 ppm, may already be out of the question.

    Rob

  2. Timothy Chase says:

    Rob wrote:

    But I thought the ECS experiment didn’t specify how the atmosphere gets to 2xCO2. I think the experiment is posed as “If the atmosphere reaches 2xCO2 levels, after all anthropogenic emissions and feedbacks have occurred, what will be the equilibrium temperature?”

    This is probably the most accurate way of stating the same idea.

    The “held constant” expression would amount to the same thing if one assumes that there is a one-to-one correspondence between the equilibrium CO2 level and the equilibrium temperature such that “holding CO2 constant” (whatever that would actually mean, or as he says “magically frozen”) would force the temperature to rise to the corresponding equilibrium temperature. This isn’t such a problem if one is dealing with a small initial pulse of carbon dioxide as this will involve very little carbon cycle feedback, but the larger the pulse, the more the feedback, and thus the greater the difference between the initial pulse and the equilibrium level of CO2.

    Anyway, for those who are interested, a good paper to look up might be:

    Climate –carbon cycle feedback analysis, results from the C4MIP model intercomparison
    Friedlingstein, et al
    15 Jul 2006
    Journal of Climate, Vol 19, pp. 3337-53

    It mentions the figures of between 0.1 and 1.5 degrees Celsius additional rise in temperature for BAU due to carbon cycle feedback.