Thursday, June 28, 2007

100 Years of Science


Now that the science of climate change is becoming more and more understood, and those who choose to ignore it for political and perhaps psychological reasons become more and more angry, all the while more convinced that it is a hoax perpetrated by their imagined enemies, it is useful to look back and see why science didn't see this coming.

In fact, some scientists did see it coming over 100 years ago.

Here is a part of story by Spencer Weart from Real Climate. I've edited it quite a lot.

"Some people have been arguing that simple physics shows there is already so much CO2 in the air that its effect on infrared radiation is "saturated"— meaning that adding more gas can make scarcely any difference in how much radiation gets through the atmosphere, since all the radiation is already blocked.

And besides, isn't water vapor already blocking all the infrared rays that CO2 ever would?

The arguments do sound good, so good that in fact they helped to suppress research on the greenhouse effect for half a century.

In 1900, shortly after Svante Arrhenius published his pathbreaking argument that our use of fossil fuels will eventually warm the planet, another scientist, Knut Ångström, asked an assistant, Herr J. Koch, to do a simple experiment.

He sent infrared radiation through a tube filled with carbon dioxide, containing somewhat less gas in total then would be found in a column of air reaching to the top of the atmosphere. That's not much, since the concentration in air is only a few hundred parts per million(390). Herr Koch did his experiments in a 30cm long tube, though 250cm would have been closer to the right length to use to represent the amount of CO2 in the atmosphere.

Herr Koch reported that when he cut the amount of gas in the tube by one-third, the amount of radiation that got through scarcely changed.

The American meteorological community was alerted to Ångström's result in a commentary appearing in the June, 1901 issue of Monthly Weather Review, which used the result to caution "geologists" against adhering to Arrhenius' wild ideas.

Still more persuasive to scientists of the day was the fact that water vapor, which is far more abundant in the air than carbon dioxide, also intercepts infrared radiation.

In the infrared spectrum, the main bands where each gas blocked radiation overlapped one another. How could adding CO2 affect radiation in bands of the spectrum that H2O (not to mention CO2 itself) already made opaque?

As these ideas spread, even scientists who had been enthusiastic about Arrhenius's work decided it was in error. Work on the question stagnated. If there was ever an "establishment" view about the greenhouse effect, it was confidence that the CO2 emitted by humans could not affect anything so grand as the Earth's climate.

Nobody was interested in thinking about the matter deeply enough to notice the flaw in the argument. The scientists were looking at warming from ground level, so to speak, asking about the radiation that reaches and leaves the surface of the Earth.

Like Ångström, they tended to treat the atmosphere overhead as a unit, as if it were a single sheet of glass. (Thus the "greenhouse" analogy.) But this is not how global warming actually works. (clip)

The breakthroughs that finally set the field back on the right track came from research during the 1940s. Military officers lavishly funded research on the high layers of the air where their bombers operated, layers traversed by the infrared radiation they might use to detect enemies.

Theoretical analysis of absorption leaped forward, with results confirmed by laboratory studies using techniques orders of magnitude better than Ångström could deploy. The resulting developments stimulated new and clearer thinking about atmospheric radiation.

Among other things, the new studies showed that in the frigid and rarified upper atmosphere where the crucial infrared absorption takes place, the nature of the absorption is different from what scientists had assumed from the old sea-level measurements.

In sum, the way radiation is absorbed only matters if you want to calculate the exact degree of warming — adding carbon dioxide will make the greenhouse effect stronger regardless of saturation in the lower atmosphere. But in fact, the Earth's atmosphere is not even close to being in a state of saturation.

With the primitive techniques of his day, Ångström got a bad result, as explained in the Part II . Actually, it's not clear that he would have appreciated the significance of his result even if he had gotten the correct answer for the way absorption varies with CO2 amount.

From his writing, it's a pretty good guess that he'd think a change of absorption of a percent or so upon doubling CO2 would be insignificant.

In reality, that mere percent increase, when combined properly with the "thinning and cooling" argument, adds 4 Watts per square meter to the planets radiation balance for doubled CO2.

That's only about a percent of the solar energy absorbed by the Earth, but it's a highly important percent to us! After all, a mere one percent change in the 280 Kelvin surface temperature of the Earth is 2.8 Kelvin (which is also 2.8 Celsius).

And that's without even taking into account the radiative forcing from all those amplifying feedbacks, like those due to water vapor and ice-albedo." more

For his work, Angstrom was vicariously honored with his name becoming a unit of measurement. The angstrum was actually named after Anders Jonas Ångström.

But even with his 1903 Nobel Prize in chemistry, Svante Arrhenius' name sunk into the dust bin of the halls of science.

Until now.

Way back in 1896, he published "The Influence of Carbonic Acid in the Air upon the Temperature of the Ground" in the Philosophical Magazine.

In that paper, Arrhenius estimated that halving of CO2 would decrease temperatures by 4 - 5 oC and a doubling of CO2 would cause a temperature rise of 5 - 6 degrees Celsius or 7 - 11 degrees Fahrenheit.

What is remarkable is that Arrhenius came so close to the most recent IPCC estimate.

Arrhenius expected CO2 levels to rise at a rate given by emissions in his time. Since then, industrial carbon dioxide levels have risen at a much faster rate: Arrhenius expected CO2 doubling to take about 3000 years.

It may take a few decades more than a century.



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2 Comments:

Blogger OZ said...

I made a few small changes on the post in the evening for readability.

6:20 PM  
Anonymous Anonymous said...

this is a great tale. thanks SP

10:03 PM  

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