One of the fundamental questions for someone interested in the impact of doubled CO2 is exactly how (1) the greenhouse effect works; and (2) how the “enhanced” greenhouse effect works. AR4 FAQ 3.1 poses the question:
I’m going to show their answer to this question in full because the answer does not rise about a primary school level and can hardly be considered an adequate answer to the question. (And it’s not answered in AR1, AR2 or AR3 either.) While I think that this is the sort of thing that should be laid in detail in one of the reports, I could understand if they chose to refer interested readers to texts containing expositions that met IPCC standards. But no luck there. We simply get a grade school brochure without references.
AR4 FAQ 3.1
Here’s how IPCC “explained” the greenhose effect.
To balance the absorbed incoming energy, the Earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the Sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see Figure 1). Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect. The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth’s greenhouse effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at Earth’s surface would be below the freezing point of water. Thus, Earth’s natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil fuels and clearing of forests, have greatly intensified the natural greenhouse effect, causing global warming.
The two most abundant gases in the atmosphere, nitrogen (comprising 78% of the dry atmosphere) and oxygen (comprising 21%), exert almost no greenhouse effect. Instead, the greenhouse effect comes from molecules that are more complex and much less common. Water vapour is the most important greenhouse gas, and carbon dioxide (CO2) is the second-most important one. Methane, nitrous oxide, ozone and several other gases present in the atmosphere in small amounts also contribute to the greenhouse effect. In the humid equatorial regions, where there is so much water vapour in the air that the greenhouse effect is very large, adding a small additional amount of CO2 or water vapour has only a small direct impact on downward infrared radiation. However, in the cold, dry polar regions, the effect of a small increase in CO2 or water vapour is much greater. The same is true for the cold, dry upper atmosphere where a small increase in water vapour has a greater influence on the greenhouse effect than the same change in water vapour would have near the surface.
Adding more of a greenhouse gas, such as CO2, to the atmosphere intensifies the greenhouse effect, thus warming Earth’s climate. The amount of warming depends on various feedback mechanisms. For example, as the atmosphere warms due to rising levels of greenhouse gases, its concentration of water vapour increases, further intensifying the greenhouse effect. This in turn causes more warming, which causes an additional increase in water vapour, in a self-reinforcing cycle. This water vapour feedback may be strong enough to approximately double the increase in the greenhouse effect due to the added CO2 alone.
Additional important feedback mechanisms involve clouds. Clouds are effective at absorbing infrared radiation and therefore exert a large greenhouse effect, thus warming the Earth. Clouds are also effective at reflecting away incoming solar radiation, thus cooling the Earth. A change in almost any aspect of clouds, such as their type, location, water content, cloud altitude, particle size and shape, or lifetimes, affects the degree to which clouds warm or cool the Earth. Some changes amplify warming while others diminish it. Much research is in progress to better understand how clouds change in response to climate warming, and how these changes affect climate through various feedback mechanisms.
I’m not suggesting that this brochure is wrong, but surely stadiums full of the best climate science minds could have produced something a little more sophisticated than this. And it’s not that the topics are handled in an unsophisticated way in (say) Goody and Yung. The problem is that IPCC chose not to provide an adequate exposition of a fundamental topic under discussion.
So when realclimate scientists get annoyed at the public’s seeming inability to understand some issues, they should probably spend a little less time blaming others and a little more time looking in the mirror and evaluating whether they properly discharged their obligations when this sort of pablum was published in their most influential presentation. How is a scientist from another discipline supposed to come away from this brochure with an understanding of the mechanism or where he can find an approved text exposition, if that’s the alternative.
Ah, you might say that this was already explained in an earlier report. Well, IPCC TAR contained a similar pablum-style exposition, shown below – also without references even to a text book. The TAR paragraph also contains a mention of “the higher the colder” heuristic, also used in Houghton’s text (Which I’ll discuss soon).
The natural greenhouse effect
The atmosphere contains several trace gases which absorb and emit infrared radiation. These so-called greenhouse gases absorb infrared radiation, emitted by the Earth’s surface, the atmosphere and clouds, except in a transparent part of the spectrum called the “atmospheric window’, as shown in Figure 1.2. They emit in turn infrared radiation in all directions including downward to the Earth’s surface. Thus greenhouse gases trap heat within the atmosphere. This mechanism is called the natural greenhouse effect. The net result is an upward transfer of infrared radiation from warmer levels near the Earth’s surface to colder levels at higher altitudes. The infrared radiation is effectively radiated back into space from an altitude with a temperature of, on average, -19’C, in balance with the incoming radiation, whereas the Earth’s surface is kept at a much higher temperature of on average 14’C. This effective emission temperature of -19’C corresponds in mid-latitudes with a height of approximately 5 km. Note that it is essential for the greenhouse effect that the temperature of the lower atmosphere is not constant (isothermal) but decreases with height.
The natural greenhouse effect is part of the energy balance of the Earth, as can be seen schematically in Figure 1.2.
Figure 1.2: The Earth’s annual and global mean energy balance. Of the incoming solar radiation, 49% (168 Wm-2) is absorbed by the surface. That heat is returned to the atmosphere as sensible heat, as evapotranspiration (latent heat) and as thermal infrared radiation. Most of this radiation is absorbed by the atmosphere, which in turn emits radiation both up and down. The radiation lost to space comes from cloud tops and atmospheric regions much colder than the surface. This causes a greenhouse effect. Source: Kiehl and Trenberth, 1997: Earth’s Annual Global Mean Energy Budget, Bull. Am. Met. Soc. 78, 197-208.
TAR included the following “exposition” of the enhanced greenhouse effect also without any references even to a textbook. While it states the “enhanced greenhouse effect” is discussed “in detail” in Chapter 6, this proves not to be the case.
(1.3.1) The enhanced greenhouse effect
The increased concentration of greenhouse gases in the atmosphere enhances the absorption and emission of infrared radiation. The atmosphere’s opacity increases so that the altitude from which the Earth’s radiation is effectively emitted into space becomes higher. Because the temperature is lower at higher altitudes, less energy is emitted, causing a positive radiative forcing. This effect is called the enhanced greenhouse effect, which is discussed in detail in Chapter 6.
If the amount of carbon dioxide were doubled instantaneously, with everything else remaining the same, the outgoing infrared radiation would be reduced by about 4 Wm-2. In other words, the radiative forcing corresponding to a doubling of the CO2 concentration would be 4 Wm-2. To counteract this imbalance, the temperature of the surface-troposphere system would have to increase by 1.2’C (with an accuracy of “10%), in the absence of other changes. In reality, due to feedbacks, the response of the climate system is much more complex. It is believed that the overall effect of the feedbacks amplifies the temperature increase to 1.5 to 4.5’C. A significant part of this uncertainty range arises from our limited knowledge of clouds and their interactions with radiation. To appreciate the magnitude of this temperature increase, it should be compared with the global mean temperature difference of perhaps 5 or 6’C from the middle of the last Ice Age to the present interglacial.
The so-called water vapour feedback, caused by an increase in atmospheric water vapour due to a temperature increase, is the most important feedback responsible for the amplification of the temperature increase. Concern has been expressed about the strength of this feedback, in particular in relation to the role of upper tropospheric humidity. Since the SAR, thinking about this feedback has become increasingly sophisticated thanks both to modelling and to observational studies. Feedbacks are discussed and assessed in Chapter 7. In particular, the present state of knowledge of the water vapour feedback is examined in Section 7.2.1.
It has been suggested that the absorption by CO2 is already saturated so that an increase would have no effect. This, however, is not the case. Carbon dioxide absorbs infrared radiation in the middle of its 15 mm band to the extent that radiation in the middle of this band cannot escape unimpeded: this absorption is saturated. This, however, is not the case for the band’s wings. It is because of these effects of partial saturation that the radiative forcing is not proportional to the increase in the carbon dioxide concentration but shows a logarithmic dependence. Every further doubling adds an additional 4 Wm-2 to the radiative forcing.
I’m not saying that an explanation is impossible, only documenting that IPCC TAR and AR4 failed to provide one. The general public should not be required to wade through Goody and Yung at a university library to get an explanation.