Lindzen Illusion #1: We Should Have Seen More Warming

Australia has begun to discuss the possibility of implementing a carbon tax, and this seems to have lit a fire under the purportedly non-political global warming "skeptic" movement.  David Evans and Jo Nova have spoken at anti-carbon tax rallies, Chris Monckton wants to join them in an Australian speaking tour, and Aussie radio talk shows have even interviewed some prominent American climate "skeptics," including John Christy and Richard Lindzen.

Disappointingly, but perhaps predictably, both climate scientist "skeptics" used the opportunity for some serious Gish Galloping, as though they were in competition to see who could regurgitate more climate myths in his Australian radio interview.  As a consequence, we'll now be launching Lindzen Illusions to refute the myths of the latter, and adding to Christy Crocks to respond to those of the former.

In this first edition of Lindzen Illusions, we examine a rather old and stale myth; one which Lindzen has been making at minimum on an annual basis since 2002, and was making as early as 1989, despite the fact that it is flat-out rubbish.  Skeptical Science readers may recall that we have addressed this one before.  I am of course referring to "Earth hasn't warmed as much as expected."

"The models do say you should have seen 2-5 times more [warming] than you've already seen, you know, you have to then accept, if you believe the models, that you actually should have gotten far more warming than you've seen, but some mysterious process has cancelled part of it...if nothing else changed, adding the amount of CO2 that we have added thus far should account for maybe a quarter of what we have seen, we have added some other greenhouse gases, methane, fluorocarbons, freons, this sort of thing, and that should bring one to perhaps 0.5 C."

As we have already addressed this myth, the remainder of this post will be an updated version of the previous rebuttal (utilizing a slightly different approach to account for ocean thermal inertia, and looking at some more up-to-date numbers).  If Lindzen is going to regurgitate the same old long-debunked nonsense, we may as well replay the same old scientific piledriver of that myth!

Lindzen's argument hinges on ignoring two critical effects on the global surface temperature: the thermal inertia of the oceans, and the cooling effects of aerosols.

Thermal Inertia and Climate Sensitivity

Due to the fact that much of the Earth is covered in oceans, and it takes a long time to heat water, there is a lag before we see the full warming effects of an increase in atmospheric greenhouse gases (this is also known as "thermal inertia").  In fact, we know there remains unrealized warming from the greenhouse gases we've already emitted because there is a global energy imbalance.  The amount of unrealized warming is dependent upon the amount of CO2 in the atmosphere (or other radiative forcing causing the energy imbalance) and the thermal inertia of the oceans (which causes a lag before the warming is realized).  Lindzen does briefly acknowledge thermal inertia in a previous version of this myth, in testimony to the British Parliament:

"the observed warming is too small compared to what models suggest. Even the fact that the oceans' heat capacity leads to a delay in the response of the surface does not alter this conclusion."

Unfortunately, Lindzen does not substantiate this claim, or provide any references to support it.  However, the easiest way to incorporate this thermal lag is, rather than using the equilibrium climate sensitivity (ECS) to calculate the amount of global warming once the planet has reached equilibrium, using the transient climate sensitivity (TCS) to calcuate the transient climate response: how much the planet should have warmed right now in response to the CO2 we have emitted thus far.  The IPCC puts TCS between 1 and 3°C for a doubling of CO2, with a most likely value of 2°C.

Aerosols and Other Cooling Effects

Lindzen briefly addresses aerosols in another previous version of this argument:

"Modelers defend this arguing that aerosols have cancelled [sic] much of the warming (viz Schwartz et al, 2010)...However, a recent paper (Ramanathan, 2007) points out that aerosols can warm as well as cool"

In short, Lindzen's argument is that the radiative forcing from aerosols is highly uncertain with large error bars, and that they have both cooling (mainly by scattering sunlight and seeding clouds) and warming (mainly by black carbon darkening the Earth's surface and reducing its reflectivity) effects.  These points are both accurate. 

However, neglecting aerosols in calculating how much the planet should have warmed does not account for their uncertainty.  On the contrary, this is treating aerosols as if they have zero forcing with zero uncertainty.  It's true that aerosols have both cooling and warming effects, but which is larger?

In some of his many previous instances deploying this argument, Lindzen referred us to Ramanathan et al. (2007).  This study examined the warming effects of the Asian Brown Cloud and concluded that "atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent."  The study also noted that, consistent with Lindzen's claims about the aerosol forcing uncertainty, there is "at least a fourfold uncertainty in the aerosol forcing effect."  However, this study focused on the warming effects of black carbon, and did not compare them to the cooling effects of atmospheric aerosols.

Ramanathan and Carmichael (2008), on the other hand, examined both the warming and cooling effects of aerosols.   This study found that black carbon has a warming effect of approximately 0.9 W/m2, while aerosol cooling effects account for approximately -2.3 W/m2.  Thus Ramanathan and Carmichael find that the net radiative forcing from aerosols + black carbon is approximately -1.4 W/m2.  This is similar to the IPCC net aerosol  + black carbon forcing most likely value of -1.1 W/m2 (Figure 1). 

Figure 1:  Global average radiative forcing in 2005 (best estimates and 5 to 95% uncertainty ranges) with respect to 1750.  Source (IPCC AR4).

Note that Lindzen's assumed zero net aerosol + black carbon forcing is outside of this confidence range; therefore, neglecting its effect cannot be justified.  However, since the IPCC provides us with the 95% confidence range of the total net anthropogenic forcing in Figure 1, we can account for the uncertainties which concern Lindzen, and evaluate how much warming we "should have seen" by now.

Expected Forcing Effects on Temperature Thus Far

In fact, this is a simple calculation.  The IPCC 95% confidence range puts the total net anthropogenic forcing at 0.6 to 2.4 W/m2 (Figure 1).  A doubling of atmospheric CO2 corresponds to a radiative forcing of 3.7 W/m2, according to the IPCC.  Therefore, the net anthropogenic radiative forcing thus far is between approximately 16% and 65% of the forcing associated with a doubling of atmospheric CO2, with a most likely value of 45%. 

In order to be thorough, we can also include the natural radiative forcings.  Most have had approximately zero net effect since 1750, with the exception of the Sun, which has had a forcing of 0.06 to 0.30 W/m2 with a most likely value of 0.12 W/m2 over this period (Figure 1).  Therefore, net forcing since 1750 is approximately 0.66 to 2.7 W/m2, with a most likely value of 1.78 W/m2.  Thus the total net forcing thus far is between 18% and 73% of the forcing associated with a doubling of atmospheric CO2, with a most likely value of 48%.

What Does This Tell Us About Climate Sensitivity?

So far, global surface air temperatures have increased approximately 0.8°C  in response to these radiative forcings.  Since we're 18% to 73% of the way to the radiative forcing associated with a doubling of atmospheric CO2 (most likely value of 48%), the amount we should expect the planet to immediately warm once CO2 doubles (TCS) has a most likely value of 1.9°C, with a range of 1.1 to 4.4°C.  Although the upper bound is a bit high, this is very consistent with thr IPCC TCS of 1 to 3°C with a most likely value of 2°C.

The TCS is also approximately two-thirds of the ECS, which tells us that the warming we have seen so far is consistent with an equilibrium sensitivity of 1.6 to 6.6°C for a doubling of atmospheric CO2, with a most likely value of 2.9°C.  This is also broadly consistent with the IPCC ECS range of 1.5 to 4.5°C with a most likely value of 3°C.

How Much Warming Should We Have Seen?

We can also flip the calculation backwards and address Lindzen's central claim (how much warming should we have seen so far?), assuming the IPCC most likely TCS of 2°C for a doubling of atmospheric CO2 and using the numbers above.  In this case, we should have seen from 18% to 73% of 2°C, or about 0.36 to 1.46°C.   Clearly the amount of warming we have seen so far (0.8°C) is well within this range.  Additionally, the most likely amount of warming is 48% of 2°C, which is 0.96°C.  In other words, we have seen very close to the amount of warming that we "should have" seen, according to the IPCC.

We can also update the calculation with some more recent numbers.  For example, Hansen et al. have a new draft paper out which puts the aerosol forcing at -1.6 W/m2.  CO2 levels have continued to rise since the IPCC report, and the CO2 forcing is now 1.77 W/m2.  If we incorporate these figures, the most likely net forcing value becomes 1.5 W/m2, or 40% of the way to the doubled CO2 forcing.  Using these values, we would expect to have seen 0.8°C warming of surface temperatures to this point - precisely what has been observed.

Warming is Consistent with What We Expect

In short, contrary to Lindzen's claims, the amount of surface warming thus far (0.8°C) is consistent with what we "should have seen."  Moreover, this calculation puts the most likely climate sensitivity parameter value within the IPCC's stated range, whereas the much lower value claimed in Lindzen and Choi (2009) (less than 1°C for CO2 doubling) is very inconsistent even with our calculated ECS lower bound (1.6°C).  For additional discussion of the errors with Lindzen and Choi (2009), see here

When we actually account for thermal inertia and negative forcings, we find that the amount of warming we have seen is consistent with what the IPCC would expect, but inconsistent with Lindzen and Choi 2009.  Thus the correct conclusion is that if Lindzen is correct about low climate sensitivity, we should already have seen much less warming than we have seen thus far.

However, I'm not going to hold my breath waiting for Lindzen to retire this old jalopy of a myth.

Posted by dana1981 on Friday, 22 April, 2011

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