My previous post debunked an article that argued scientists have seriously overestimated climate change. Now let’s look at the evidence for a serious underestimation of climate change.

To do that, we must understand the fatal flaw with the IPCC’s over-reliance on the poorly named “equilibrium climate sensitivity” (ECS). 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 in 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 Deniers — 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 — are all 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 [$ub req'd], published in Nature, found Arctic temperatures almost beyond imagination — above 23°C (74°F) — temperatures more than 18°F warmer than current climate models had predicted when applied to this period. The three dozen authors conclude that existing climate models are missing crucial feedbacks that can significantly amplify polar warming.
  • A second study [$ub req'd], published in Geophysical Research Letters, 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 IPCC. 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, also published in Geophysical Research Letters, 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 is poised to release that carbon as we warm the planet.
  • Tropical forests — which store carbon, but are being cut down in places like Brazil and Indonesia. 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 helps 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.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.