GWPF has released a biased report on the subject of climate sensitivity, but it acknowledges the dangerous path we're on

The UK anti-climate policy advocacy group Global Warming Policy Foundation (GWPF) has published a report written by Nic Lewis and Marcel Crok claiming "the IPCC hid the good news" regarding climate sensitivity (how much the climate will warm in response to the increased greenhouse effect). Lewis is an amateur researcher and retired financier who has published a few papers estimating climate sensitivity, and Crok is a freelance science writer.

The report itself is essentially a commentary and includes no new information. It boils down to Lewis and Crok trying to make the case that climate sensitivity is on the lower end of the IPCC estimated range. The report represents a very selective and biased review of the scientific literature on the subject. Recent papers by Gavin Schmidt and Drew Shindell at NASA, not considered in the GWPF report, entirely contradict its conclusions, for example. As climate scientist Steven Sherwood described it,

"The report is standard cherry-picking. It offers no new evidence not already considered by the IPCC, relying very heavily on a few strands of evidence that seem to point toward lower sensitivity while ignoring all the evidence pointing to higher sensitivity."

However, the good news is that the report is consistent with the 97 percent expert consensus on human-caused global warming. It acknowledges that global warming will continue as long as humans continue increasing the greenhouse effect, and merely suggests that future warming will be toward the lower, slower end of the IPCC estimates. As climate scientist Ed Hawkins at the University of Reading also noted,

"Remarkably for a report published by the GWPF, the authors agree with mainstream climate scientists that significant further warming is expected ... It is great to see the GWPF accepting that business-as-usual means significant further warming is expected. Now we can move the debate to what to do about it."

That being said, the conclusions of the report are poorly justified. There are several approaches used to estimate the sensitivity of the global climate to the increased greenhouse effect. Lewis and Crok make their case for why only the methods that yield lower estimates are valid, but when considering the full body of available evidence, their arguments don't hold water.

Paleoclimate Studies

Paleoclimate studies attempt to estimate climate sensitivity based on the global energy imbalances and associated temperature responses from climate change events in the geologic record. The most robust study of this type was done by the PALEOSENS team, published in Nature in 2012. This study evaluated past climate changes over the previous 65 million years, considering nearly two dozen investigations of many different geological time periods.

The study estimated that during past large climate events, global surface temperatures warmed between 2.2 and 4.8°C in response to an energy imbalance equivalent to a doubling of atmospheric carbon dioxide. This range is consistent with IPCC estimates (1.5 to 4.5°C), and mostly inconsistent with the lower estimates preferred by GWPF (1.25 to 3°C).

The GWPF report has very little discussion of paleoclimate sensitivity estimates. They just say that these studies don't tightly constrain the possible climate sensitivity range, and past climate states are different than current and future climate states, so "little weight can be put on the palaeoclimate estimates." While there is some truth to these critiques, entirely disregarding the results of these studies is simply not justifiable.

In summary, paleoclimate studies provide one line of evidence that supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is weak.

General Circulation Models

Climate models (general circulation models or GCMs) provide another method by which to estimate climate sensitivity. The physics of the climate system are input into very detailed climate models, which can then estimate how the global temperature will respond to various influences. The results can give us projections of future global warming under a variety of scenarios, and also give us an estimate of the global climate sensitivity. Most GCM equilibrium climate sensitivities range between 2 and 4.5°C, consistent with paleoclimate estimates.

Lewis and Crok argue that the GCMs used in the 2014 IPCC report don't include the latest estimates of the cooling effects from aerosol pollution, which have been revised downwards, and argue this means the models are overly sensitive. However, according to climate modeler Gavin Schmidt of NASA, this is incorrect.

"Their logic is completely backwards. Climate model sensitivity to a doubling of atmospheric CO2 is intrinsic to the model itself and has nothing to do with what aerosol forcings are. In CMIP5 there is no correlation between aerosol forcing and sensitivity across the ensemble, so the implication that aerosol forcing affects the climate sensitivity in such 'forward' calculations is false ... The spread of model climate sensitivities is completely independent of historical simulations."

In fact, Schmidt recently published a study noting that about half of the models used in the latest IPCC report don't include indirect aerosol effects (for example via their influence on cloud formation), and those indirect effects accounted the main change in the IPCC aerosol influence estimates. Schmidt's paper also showed that when accounting for the most recent estimates of solar, aerosol pollution, and El Niño influences on the climate, models actually do a better job estimating temperature changes, contrary to the GWPF report argument.

Climate model mean (dark blue #1) and envelope (lighter blue) range of global surface temperature projections vs. HadCRUT4 (red #1) and Cowtan & Way (red #2) global surface temperature instrumental estimates. The model mean result incorporating changes in El Niño cycles and updated solar and aerosol forcings (blue #2 and #3) are also shown. Adapted from Schmidt et al. (2014) by Kevin Cowtan.

In summary, GCMs provide another line of evidence that generally supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is incorrect.

'Instrumental' Estimates

The method preferred by the GWPF report, and that which Lewis has used in his own papers, involves estimating climate sensitivity using a combination of recent instrumental temperature data, less complex climate models, and statistics. A few studies using this approach since about 2012 have begun yielding climate sensitivity estimates toward the lower end of the IPCC range.

The challenge with this 'instrumental' method of estimating equilibrium sensitivity is that it's based on 'transient' measurements of an unbalanced climate. There is currently a global energy imbalance, and reaching a new equilibrium state will take over a century. Therefore, estimating equilibrium climate sensitivity based on measurements of a climate that's out of equilibrium requires making some significant assumptions. However, several recent studies have suggested that these assumptions may not be correct, and that the conclusions of these 'instrumental' studies therefore may not be reliable.

None of these papers or concerns with 'instrumental' climate sensitivity estimation methods are mentioned in the GWPF report. Instead, the report argues that this approach provides the only reliable method for estimating climate sensitivity, and that all other methods that produce higher estimates (e.g. paleoclimate and GCMs) are wrong.

However, an important new paper just published by Drew Shindell at NASA reconciles the difference between the climate sensitivity estimates in these varying approaches, but not in the direction advocated by the GWPF report. Shindell, who along with Nic Lewis was a co-author on one of the 'instrumental' method papers cited heavily in the GWPF report, notes that studies using this approach have assumed that the global mean temperature response to all influences is equal. His study investigates this assumption by comparing GCM temperature responses to greenhouse gases with their responses to aerosols and ozone.

As it turns out, the climate is most sensitive to influences in the northern hemisphere at high latitudes, in large part because these are amplified by changes in Arctic sea ice. The forcing from aerosols and ozone isn't globally uniform, but instead focused more in the northern hemisphere high latitudes. Hence it results in a relatively larger temperature response than an equally strong influence from greenhouse gases, which are well mixed throughout the atmosphere.

When assuming equal sensitivity to all forcings, Shindell estimates a climate sensitivity almost identical to the GWPF estimates. However, when Shindell accounts for the more sensitive temperature response to aerosols and ozone, the new results are consistent with the higher end of the IPCC climate sensitivity range, and inconsistent with the lower GWPF estimates.

Accounting for Cloud and Water Vapor Observations

The GWPF report also notes that changes in cloud cover in a warming world are a key to determining climate sensitivity. On this topic the report merely claims, "Observational evidence for a positive cloud feedback is weak, at best." However, there have been several studies comparing observed changes in cloud cover to cloud simulations in climate models. These have shown that GCMs with lower climate sensitivity tend to inaccurately represent observed cloud changes, which are more accurately captured by higher sensitivity models.

These studies were also omitted from the GWPF report, but they provide yet another line of evidence for high and against low climate sensitivity.

Climate Policies are Insufficient in Any Case

The GWPF report concludes by complaining that by not emphasizing the lower climate sensitivity estimates, the IPCC has "inadequately informed" policymakers about the state of the science. However, from a policy standpoint, we're not doing nearly enough to reduce emissions even in the best case scenario advocated by the GWPF report.

Moreover, as detailed above, the full body of scientific evidence suggests that climate sensitivity is relatively high. Even if you believe the GWPF report is right, there's a good chance it's not. Proper risk management therefore mandates that we must take action to mitigate the threat of dangerous climate change.

In any case, the full body of evidence is firmly against the conclusions of the report. The authors merely dismiss or ignore the research that doesn't support their desired conclusion, and overlook the shortcomings of the research that does.