Lindzen Illusion #6: Importance of Greenhouse Gases

Throughout the Lindzen Illusions series, we've gone back and scrutinzed his 1989 MIT Tech Talk to evaluate how Lindzen's climate positions in 1989 compare to his beliefs today, and to observational reality.  In the talk, Lindzen made a statement about greenhouse gases (GHGs), downplaying the importance of carbon dioxide (CO2):

"Water vapor is far and away the most important greenhouse gas, except for one form which isn't a greenhouse gas: clouds.  Clouds themselves as liquid water are as important to the infrared budget as water vapor.  Both swamp by orders of magnitude all the others.  With CO2 one is talking about three watts per square meter at most, compared to a hundred or more watts per square meter for water."

Similarly, in a 1992 article, Lindzen said:

"Even if all other greenhouse gases (such as carbon dioxide and methane) were to disappear, we would still be left with over 98 percent of the current greenhouse effect."

It's worth noting that this is one of Lindzen's previous positions which has gone by the wayside.  Lindzen still believes that water vapor and clouds play key roles as feedbacks (which is true, although he has the magnitude and sign of those feedbacks wrong, as we saw in Lindzen Illusion #4), but he no longer downplays the significance of CO2 in the greenhouse effect.  However, "water vapor is the most powerful greenhouse gas" has become one of the more popular climate myths, and remains pervasive among "skeptics", so it's worth once again examining the validity of this argument.

Schmidt et al. (2010)

This issue was addressed by two recent papers from NASA GISS, Lacis et al. (2010) and Schmidt et al. (2010).  Schmidt et al. examined the contributions of various GHGs to the Earth's greenhouse effect.  Schmidt et al. concluded as follows:

"we find that water vapor is the dominant contributor (∼50% of the effect), followed by clouds (∼25%) and then CO2 with ∼20%."

Schmidt et al. estimate the global mean longwave radiative flux from CO2 at  approximately 30 Watts per square meter (W/m2), clouds at 40 W/m2, and water vapor at 80 W/m2. This is a stark contrast to Lindzen's 1989 claims:

"With CO2 one is talking about three watts per square meter at most, compared to a hundred or more watts per square meter for water."

Lindzen claimed that water vapor and clouds contribute at least 30 times more to the greenhouse effect than CO2, but as Schmidt et el. show, the ratio is closer to a factor of four.  On this issue, Lindzen was off by an order of magnitude because he underestimated the CO2 radiative flux by a factor of ten.

Similarly, Lindzen claimed without GHGs other than water vapor and clouds, "we would still be left with over 98 percent of the current greenhouse effect."  In reality, water vapor and clouds account for approximately 75% of the greenhouse effect.

Lacis et al. (2010)

Lacis et al. answer the question of "the most important GHG" even more directly.  In fact, the first sentence in the abstract of their paper reads:

"Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere."

The authors note that unlike water vapor, CO2 does not condense and precipitate from the atmosphere at current climate temperatures.  Unlike many other greenhouse gases such as CO2 which can be added to the atmosphere, the level of water vapor in the atmosphere is a function of temperature.  If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two.  In short, water vapor is a feedback, not a forcing.  NASA GISS summarizes the results of Lacis et al. (2010) as follows (emphasis added):

"Because carbon dioxide accounts for 80% of the non-condensing GHG forcing in the current climate atmosphere, atmospheric carbon dioxide therefore qualifies as the principal control knob that governs the temperature of Earth."

So when it comes to governing global temperature changes, Lindzen's statement is backwards.  CO2 plays a much larger role than water vapor and clouds, which act to amplify the CO2-caused warming, but don't remain in the atmosphere long enough to drive global temperature changes themselves.

A Lindzen History

As noted above, this is one of Lindzen's 1989 climate beliefs which has gone by the wayside, but unfortunately, remains a fairly widespread belief amongst global warming "skeptics".  Once a climate myth is born, it's very difficult to kill, as the Skeptical Science Arguments database shows.

In the next Lindzen Illusions installment, we'll examine the history of Lindzen's climate arguments - what he argued in 1989, what he argues today, what is necessary for his arguments to be correct, and how those requirements stack up against observational reality.

Posted by dana1981 on Tuesday, 10 May, 2011

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