Low-Climate-Sensitivity Theory (Last Climate Skeptic Claim) Gets Debunked… x 3

This is Part 1 of a 2-Part series on climate sensitivity originally posted on Skeptical Science and reposted with permission.

When it comes to global climate change, there are two critical and intertwined, but distinct issues: science, and policy.  We generally focus on the science, because that is what dictates the appropriate policy response, or at least what our climate policy needs to accomplish.

Justin Gillis had an excellent article published in The New York Times this past week, which addresses both science and policy.  The science aspect of the article bears some resemblance to one of our posts from a year ago, Climate Sensitivity: The Skeptic Endgame.  The fundamental premise of both articles involves the fact that, because of the sound basic science supporting the human-caused global warming theory, there only remains one fallback position for the remaining relatively credible climate contrarians.  That fallback position involves climate sensitivity being lower than the body of scientific evidence indicates.

Gillis’ article focuses mainly on Richard Lindzen, who is one of the relatively more credible climate contrarians (although he has a long history of taking contrarian positions on nearly every climate-related issue, and being almost universally wrong on those issues).  Lindzen embodies the low climate sensitivity fallback position perfectly, but as we will see here, the basis of Lindzen’s argument, which itself is the basis of all remaining relatively credible climate contrarianism, is entirely false and undermined by three inescapable flaws.

Lindzen’s Three Sensitive Achilles’ Heels

We know that humans are rapidly increasing the level of CO2 and other greenhouse gases (GHGs) in the atmosphere, we know that this GHG increase is causing some amount of warming, and will continue to cause additional warming as long as GHG levels continue to rise.  The remaining relatively credible climate contrarians like Lindzen acknowledge these realities; where they differ from mainstream climate science is in exactly how much warming the GHG increase will cause.  This is known as the climate sensitivity – how much the planet will warm in response to increasing GHGs, including feedbacks.

For contrarians like Lindzen, climate sensitivity must be low, or they have no case to make.  They have acknowledged that GHGs will cause warming, and their only argument against taking serious action to reduce GHG emissions is this premise that the GHG increase won’t cause very much warming.  That is why we described this argument as the ‘skeptic’ endgame, and Gillis accurately described it as the dissenters’ “last bastion.”

So what is Lindzen’s case for low climate sensitivity?

He summed it up in the recent ABC documentary discussed by John CookI Can Change Your Mind About Climate (see minute 21 in this video).  In response to a comment that the average global surface temperature has warmed about three-quarters of a degree Celsius, Linzen responds:

“Yeah, and we should have seen 3[°C]“

This is a very brief encapsulation of Lindzen’s pet argument, Earth hasn’t warmed as much as expected.  I call it his ‘pet argument’ because he makes it in virtually every talk and presentation he gives, and has been making it since at least 1989, despite the fact that it’s been debunked time and time again (i.e. Skeptical Science alone has debunked it here and here and here and here and here and here).

In short, if climate sensitivity is lower than resulting in climate models, then the climate should have warmed less than climate models have predicted.  In order to argue that this is the case, Lindzen claims that CO2-equivalent (the total radiative forcing for all greenhouse gases in units equivalent to CO2-caused warming) has already doubled from pre-industrial levels; therefore, if climate sensitivity is around 3°C for doubled CO2 (as in climate models), the planet should have warmed 3°C.  It has not warmed nearly so much; therefore, Lindzen asserts, climate sensitivity is low.

The problem with Lindzen’s argument for low sensitivity is that it contains three separate fundamental flaws:

  • Lindzen has completely neglected all non-GHG influences on the climate.  The second-largest influence (behind CO2) is from human aerosol emissions, which have a cooling effect.  Lindzen seizes on the uncertainty associated with aerosols – the strength of their cooling effect is not well-known; however, the scientific evidence does clearly indicate that they have a cooling effect.  In fact, Lindzen’s own sources on the subject conclude that aerosols have a strong cooling effect.  Yet in his argument, he has completely failed to account for this cooling effect.  In short, Lindzen treats the GHG forcing as equivalent to the net radiative forcing (which is what the climate responds to), but the two are not equivalent.
  • 3°C is the equilibrium climate sensitivity – the amount the planet will eventually warm once it reaches a new energy balance.  The planet currently has an energy imbalance (mostly stored as heat in the oceans), so there is still more warming “in the pipeline” from the GHGs we have already emitted.  Lindzen fails to account for this effect.

By themselves, each of these fundamental errors completely invalidates Lindzen’s argument.  Taken together, they form a trio of Achilles’ Heels which leave us puzzled as to how Lindzen has continued to make this obviously and grossly fundamentally flawed argument for over two decades.

Lindzen also Disproven by Reality

In addition to these three glaring errors, we know the Earth has warmed as much as expected because climate scientists compare their model runs to observational data.  We’ve done a whole series of posts looking at the accuracy of past climate model predictions, our most recent entry being Hansen et al. 1981 (Figure 1).

Hansen 1981 projections vs observations

Figure 1: Hansen et al. (1981) global warming projections under a scenario of high energy growth (4% per year from 1980 to 2020) (red) and slow energy growth (2% per year from 1980 to 2020) (blue) vs. observations from GISTEMP with a 2nd-order polynomial fit (black).  Actual energy growth has been between the two Hansen scenarios at approximately 3% per year.  Baseline is 1971-1991.

Thus not only do we know Lindzen’s argument is wrong due to its three Achilles’ Heels, we know it’s wrong just by comparing actual model results to observational data, which show the Earth has warmed consistent with model predictions.

There simply is no question – Lindzen’s claim that the Earth hasn’t warmed as much as expected, which is the basis of his low climate sensitivity argument, which is the basis of all remaining relatively credible climate contrarianism, is entirely false based on three fundamental physical flaws in his argument, as demonstrated by simply comparing the models and observations.

Lindzen’s Cloudy Iris

In Gillis’ Times article, Lindzen brings up his Iris hypothesis.  Back in 2001, Lindzen proposed that in response to global warming, increased sea surface temperature in the tropics would result in reduced cirrus cloud formation and thus more infrared radiation leakage from Earth’s atmosphere.  This radiation leakage in turn would have a cooling effect, dampening global warming as a negative feedback (like the iris in a human eye contracting to allow less light to pass through the pupil in a brightly lit environment – hence the term ‘iris effect’).

However, within a year of the publication of Lindzen’s iris paper, there was one study published concluding that Lindzen had significantly overestimated the iris effect, a second concluding that if the iris effect existed, it would lead to increased warming, and a third and fourth papers finding no evidence for the iris effect.  The vast majority of subsequent research has simply not substantiated Lindzen’s iris hypothesis – it has not withstood the test of time.

All of Lindzen’s Eggs in the Cloud Basket

In another challenge for the low sensitivity crowd, research has shown that the water vapor feedback (which appears to be the largest single feedback) is positive, amplifying global warming.  Therefore, contrarians like Lindzen need a large negative feedback to offset the water vapor effect, and the only credible candidate is cloudcover.  Thus, as Andrew Dessler notes in Gillis’ article,

“If you listen to the credible climate skeptics, they’ve really pushed all their chips onto clouds.”

Not only is climate sensitivity the ‘skeptic’ endgame, but clouds are the low climate sensitivity endgame.  Climate contrarians need a strongly negative cloud feedback to argue that climate sensitivity is low, which they need to be the case in order to argue that global warming is nothing to worry about.  It all boils down to clouds.

Lindzen hasn’t actually published any subsequent research to support his iris hypothesis, but he has attempted to show that climate sensitivity is low, and then proposed his iris hypothesis as the physical explanation for that low sensitivity, even though as noted above, the hypothesis has not withstood the test of time.

The Inconvenient Truth About Clouds and Sensitivity

Unfortunately, most recent climate research has indicated that clouds probably act as yet another positive feedback, amplifying rather than dampening global warming.  The most prominent and recent such paper wasDessler (2010), which found that the short-term cloud feedback is probably positive, although slightly negative values could not be ruled out.

Contrarians like Lindzen still hold out hope because clouds do remain one of the biggest climate uncertainties.  However, it’s not just climate models and studies of climate feedbacks that undermine the low sensitivity argument.  Paleoclimate studies (examining climate data from hundreds to millions of years ago) are also consistent with the climate model sensitivity, and inconsistent with low climate sensitivity arguments (Figure 2).

climate sensitivity estimates

Figure 2: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively.  Adapted from Knutti and Hegerl (2008).

Don’t Bet on the Climate Contrarians

In short, the evidence is heavily stacked against the low climate sensitivity argument.  The premise of the argument – that the Earth should have warmed more if climate models were right about climate sensitivity – is unquestionably wrong on many different levels.  The argument’s last hope lies in a strongly negative cloud feedback, but so far the evidence is pointing in the other direction.  Data from the Earth’s history is also inconsistent with an insensitive climate.

That’s not to say that a low climate sensitivity is an impossibility.  Science is about probabilities, not certainties.  However, as we will explore in Part 2 tomorrow, while there is a very low probability that they are right, the problem is that contrarians like Lindzen refuse to even consider the possibility that they are wrong, and expect us to risk the welfare of future generations on that slim chance that they are right.


In Part 1 of this post, we examined the fundamental flaws in the last hope for climate contrarians – that the planet won’t warm very much in response to rising greenhouse gas (GHG) emissions, because climate sensitivity is low, because clouds will act as a negative feedback and dampen future warming.  While it would be convenient if this picture (best embodied by Richard Lindzen) were accurate, the evidence is stacked heavily against it.

Nevertheless, as Part 1 concluded, there is a slim chance that Lindzen and company are correct, that climate sensitivity is low and global warming is not a major concern.  There is also a high probability that they are wrong, that future global warming will be substantial, and that the consequences will be bad if we don’t do something about it.  How we choose to address these scenarios is a question of risk management, which happens to be a big part of my day job.  Unfortunately, as with his scientific positions, Lindzen’s risk management arguments are ill-conceived.

Irrational Optimism

In the Gillis New York Times article, Lindzen summarizes his approach to this climate risk management question:

“If I’m right, we’ll have saved money…. If I’m wrong, we’ll know it in 50 years and can do something.”

There is a rather obvious flaw in Lindzen’s logic here – it assumes that in 50 years we will be able to simply flip a switch and solve the climate problem.  Unfortunately, that is probably not the reality of the situation.  Figure 1 shows the projected atmospheric CO2 levels under various emissions scenarios.  If we listen to Lindzen, we will follow one of the higher emissions paths (currently we are on track with A2 [yellow]).  Figure 2 shows the resulting global warming based on climate model runs which, as Part 1 showed, have thus far been quite accurate.

Figure 1: Atmospheric CO2 concentrations as observed at Mauna Loa from 1958 to 2008 (black dashed line) and projected under the 6 IPCC emission scenarios (solid colored lines). (IPCC Data Distribution Centre)

Figure 2: Global surface temperature projections for IPCC Scenarios. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is  constant CO2 concentrations at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range.  (Source: IPCC).

The big difference between the higher and lower emissions scenarios is that at mid-century, emissions in the higher scenarios are accelerating upwards, while in the lower scenarios they are flattening out.  It takes time to deploy the necessary infrastructure to reduce GHG emissions – there is no magical switch we can suddenly flip in 2050 when we realize that Lindzen and company were wrong.  As Figure 2 shows, the average global surface temperature difference between Scenario A2 (essentially business-as-usual advocated by Lindzen) and Scenario B1 (which involves serious action to reduce GHG emissions) in 2100 is close to 2°C.

What’s Dangerous?

Internationally, 2°C warming above pre-industrial levels is generally accepted as the “danger limit” we should try to avoid exceeding.  This is not a hard and fast limit – there won’t suddenly be a catastrophe if we reach 2.01°C; in fact, many experts believe even 2°C warming is too risky.  However, some of the impacts listed in the IPCC report for global warming of a Lindzen-level 3-4°C above pre-industrial levels include:

  • hundreds of millions of people exposed to increased water stress
  • 30–40% of species at risk of extinction around the globe
  • about 30% of global coastal wetlands lost
  • increased damage from floods and storms
  • widespread coral mortality
  • the biosphere – soils, plants etc – stops absorbing carbon and starts releasing it
  • reduced cereal production
  • increased death and illness from heat waves, floods and droughts.

In Scenario A2, we reach this level of warming in about 60 to 80 years – within a couple of decades of Lindzen’s 50 year ‘wait and see’ target.  If he is wrong, by the time we’re forced into taking action, it will be too late to avoid some really nasty consequences.

And of course let’s not forget global warming’s evil twin, ocean acidification.  Even if the contrarians are right about low climate sensitivity, the impacts of rising atmospheric CO2 on marine ecosystems will be severe, and a 50-year delay in reducing CO2 emissions could have dire consequences for those ecosystems.

In short, if Lindzen is wrong, future generations are in big, big trouble.  In fact, according to The Critical Decadereport by the Australian Climate Commission, we’ve already burned through 30% of our allotted GHG emissions between 2000 to 2050 if we want to give ourselves a good chance to limit global warming to that 2°C limit.  Not only don’t we have 50 years, we’re already behind where we should be in terms of emissions reductions.

Risk Management 101

Humans are generally very averse to risk.  Everyone who drives a car purchases auto insurance.  Everyone who owns a house purchases homeowners insurance.  Most first world countries have implemented universal health care systems, and in the USA, virtually everyone who can afford it purchases health care insurance.  Personally I’m relatively young and in good health, I’m a safe driver, and I’m not worried that anything will happen to my home; yet I’ve purchased all three types of insurance.  Why?

Because if anything were to do substantial damage to my home, car, or health, it could impose a huge cost which I couldn’t afford without insurance.  The probability of any of these things happening is relatively low, but the potential consequence is so bad that we all mitigate it by purchasing insurance to cover our most valuable assets in case the worst case scenario comes to fruition.

Yet when it comes to the climate, we are behaving in exactly the opposite manner.  It’s hard to imagine a larger potentially diastrous scenario than a major climate change.  It could potentially impact every single living thing on Earth, and impose a huge cost on every person.  This is not just some slim possibility, as shown in Part 1, it’s the most probable outcome!  And yet thus far we have utterly failed to mitigate this massive, highly probable risk.

This is the risk management approach I described in my entry in Why Are We Sure We’re Right, and as Greg Craven has eloquently summarized in this video.  However, to adequately manage the risks posed by climate change, we should perform a cost-benefit analysis.

Mitigation Costs

The costs associated with mitigating climate change are relatively straightforward to evaluate.  Generally, estimates put the cost of reducing GHG emissions 80% by 2050 (which will approximately limit global warming to the 2°C ‘danger limit’) at around 1% of Gross Domestic Product (GDP).  Some estimates put the cost as low as a fraction of a percent of GDP, while others put it as high as 2 to 3%.  While this is a substantial cost, it is not an unmanageable one, and we already have all the technology necessary to accomplish this task.

It’s also worth noting that the longer we wait, the larger the costs become.  Delays force emissions cuts to be steeper, and they also delay the development of cheaper emissions reductions technologies.  A Google.org study found that delaying significant investment in green tech by 5 years could cost the USA $2.3 to $3.2 trillion in GDP (Figure 3).

Google GDP

Figure 3: US GDP gains between 2010 and 2050 in three scenarios.  The green scenario includes a $30 per ton price on utility sector CO2 emissions and strong investment in green tech to develop “breakthroughs” (BTs).  the blue line depicts a scenario in which there is heavy investment in green tech without a carbon price.  The purple line is a 5 year delay before significant investment in green tech.  The red and orange area depicts the difference in GDP growth: $2.3 to 3.2 trillion lost in the 5 year delay scenario. Source: Google.org study

Mitigation Benefits

The benefits of climate mitigation are more difficult to estimate, because it depends who’s right.  If Lindzen and company are right about low climate sensitivity, then reducing GHG emissions has some relatively small benefits.  For example, countries will produce more domestic energy and become more energy-independent.  National security will improve as oil-based wars become less likely.  The transition away from fossil fuels, which are after all non-renewable and limited substances, will be smoother and easier.  Air and water pollution and their associated adverse health impacts will be reduced.

Installing the necessary infrastructure would also create a lot of jobs and help alleviate the current unemployment problems around the world.  Right now, when there is a major surplus in the available work force, is arguably the best time for governments to invest in infrastructure projects like those which will be necessary to reduce GHG emissions.

So there are some definite benefits to reducing emissions even if the Lindzen low sensitivity crowd is right.  If they are wrong, which is the far more likely scenario, the benefits will outweight the costs several times over, by trillions of dollars (Figure 4).

Figure 4:  Approximate costs of climate action (green) and inaction (red) in 2100 and 2200. Sources: German Institute for Economic Research and Watkiss et al. 2005

‘Fingers Crossed’ is Poor Risk Management

Lindzen’s suggested approach of waiting 50 years and hoping the consequences are not disastrous is one of exceptionally poor risk management.  The prudent approach would involve addressing the worst case scenario, particularly when that scenario has a significant probability of coming to fruition if we fail to prevent it.  This is particularly true when the mitigation costs, while not cheap, will certainly not cripple the economy, and when the benefits are also significant in any scenario, and may very well outweigh the costs several times over and by trillions of dollars.

From a risk management standpoint, there is no question that we should be ignoring Lindzen’s ill-conceived approach and instead taking serious action to reduce our GHG emissions.  Unfortunately, for various reasons, we have thus far failed to take the prudent climate risk management approach, and if we continue to fail, future generations will suffer the consequences of Lindzen and his fellow contrarians’ foolhardy approach.

About the Author

Dana earned a Bachelor’s degree in astrophysics from UC Berkeley in 2003 and a Master’s degree in physics from UC Davis in 2005. Through college, he grew increasingly interested in environmental issues, particularly global warming and alternative fuel vehicles.

After earning his Master’s degree, Dana became employed at an environmental consulting firm in the Sacramento, California area. He currently works as an Environmental Scientist, primarily perfoming research and contributing to the cleanup of contaminated former military defense sites.