Climate Insensitivity: What the IPCC Knew But Didn’t Tell Us

Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

In a remarkable example of scientific malfeasance, it has become apparent that the IPCC knew a lot more than it revealed in its 2013 climate compendium about how low the earth’s climate sensitivity is likely to be.

The importance of this revelation cannot be overstated. If the UN had played it straight, the “urgency” of global warming would have evaporated, but, recognizing that this might cause problems, they preferred to mislead the world’s policymakers.

Strong words? Judge for yourself.

The report Oversensitive—how the IPCC hid the good news on global warming,” was released today by the Global Warming Policy Foundation (GWPF)—a U.K. think-tank which is “concerned about the costs and other implications of many of the policies currently being advocated” regarding climate change (disclosure: our Dick Lindzen is a member of the GWPF Academic Advisory Council).

The new GWPF report concluded:

We believe that, due largely to the constraints the climate model-orientated IPCC process imposed, the Fifth Assessment Report failed to provide an adequate assessment of climate sensitivity – either ECS [equilibrium climate sensitivity] or TCR [transient climate response] – arguably the most important parameters in the climate discussion. In particular, it did not draw out the divergence that has emerged between ECS and TCR estimates based on the best observational evidence and those embodied in GCMs. Policymakers have thus been inadequately informed about the state of the science.

The study was authored by Nicholas Lewis and Marcel Crok. Crok is a freelance science writer from The Netherlands and Lewis, an independent climate scientist, was an author on two recent important papers regarding the determination of the earth’s equilibrium climate sensitivity (ECS)—that is, how much the earth’s average surface temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide.

The earth’s climate sensitivity is the most important climate factor in determining how much global warming will result from our greenhouse gas emissions (primarily from burning of fossil fuels to produce, reliable, cheap energy). But, the problem is, is that we don’t know what the value of the climate sensitivity is—this makes projections of future climate change–how should we say this?–a bit speculative.

Unsurprisingly, there has been a lot of recent scientific research aimed at gaining a better understanding of what the climate sensitivity may be. We have detailed much of this research in our ongoing series of articles highlighting new findings on the topic. Collectively, the new research indicates an ECS value a bit below 2°C. The latest in our series is here.

But in its Fifth Assessment Report (AR5) finalized this past January, the IPCC gave short shrift to the major implication of this collection of new research results—that the climate sensitivity is much lower than what the IPCC assessed it to be in its collection of previous assessment reports (issued every 6-7 years) and that the rate of climate change is going to be much less.

For example, formerly, in its Fourth Assessment Report (AR4), released in 2007, the IPCC had this to say regarding the equilibrium climate sensitivity:

It [the equilibrium climate sensitivity] is likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantial higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values. [emphasis in original]

In its new AR5, the IPCC wrote this:

Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)16. The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing. [emphasis in original]

And IPCC AR5 footnote 16 states:

No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.

So, facing mounting scientific for a substantially lower climate sensitivity, the best the IPCC could bring itself to do was to reduce the low end of its “likely” range by one-half degree, refuse to put a value on its best guess, and still cling to its high end number. Big deal.

The reason that the IPCC could only make these meager changes was that the collection of climate models that the IPCC employs to make the bulk of its projections of future climate change (and future climate change impacts) has an average ECS value of 3.2°C.  The IPCC couldn’t very well conclude from the scientific evidence that the real value was somewhere south of 2°C—if it were to do so, it would invalidate the climate models and, for that matter the meat of its entire report (that is, its climate change projections).

We described the situation the IPCC faced last summer (prior to releasing the final copy of the AR5) this way:

The IPCC has three options:

  1. Round-file the entire AR5 as it now stands and start again.
  2. Release the current AR5 with a statement that indicates that all the climate change and impacts described within are likely overestimated by around 50 percent, or
  3. Do nothing and mislead policymakers and the rest of the world.

We’re betting on door number 3.

As predicted, the IPCC chose option number 3.

The new GWPF report confirms, in detail, the IPCC’s choice and how it came to make it—by confusing the reader with a collection of evidence that was outdated, already disproven, based upon flimsy assumptions, not directly applicable, or flat-out wrong.

Putting it nicely, Lewis and Crok describe the situation thus:

The AR5 authors might not have wanted to declare that some studies are better than others or to adjudicate between observational and model-based lines of evidence, but we believe that this is exactly what an assessment is all about: using expert knowledge to weigh different sources of evidence. In this section we present reasoned arguments for a different assessment to that in AR5.

Lewis and Crok go, in detail, through each climate sensitivity paper considered (and relied upon) by the IPCC and identify its shortcomings. At the end, they are left with a collection of five papers that, while still containing uncertainties, are built upon the most robust set of assumptions and measurements.

From those papers the Lewis and Crok conclude the following:

A new ‘best observational’ estimate of ECS can now be calculated by taking a simple average of the different observationally-based estimates….This gives a best estimate for ECS of 1.75°C and a likely range of about 1.3–2.4°C. However, recognizing that error and uncertainty may be greater than allowed for in the underlying studies, and will predominantly affect the upper of the range, we conservatively assess the likely range as 1.25–3.0°C.

Now compare these figures with those in AR4 and AR5….Our new ‘best observational’ ECS estimate of 1.75°C is more than 40% lower than both the best estimate in AR4 of 3°C and the 3.2°C average of GCMs used in AR5. At least as importantly, the top of the likely range for ECS of 3.0°C is a third lower than that given in AR5 (4.5°C) – even after making it much more conservative than is implied by averaging the ranges for each of the observational estimates.

And as to what this means about the IPCC global warming projections, Lewis and Crok write:

The [climate models] overestimate future warming by 1.7–2 times relative to an estimate based on the best observational evidence.

This is a powerful and important conclusion.

We recommend that you read the full report. Not only is it a comprehendible and comprehensive description of the current science as it relates to the climate sensitivity, but it is an illumination of how the IPCC process does, or rather doesn’t, work.

The Obama Administration and its EPA will ignore this reality at their peril.