Comment (2-19):Carrots to the first to figure out where the 75% of the warming due to CO2 doubling should have already happened comes from
Some commenters write that CO2 is a weak GHG compared to other gases (0425, 0498, 0639.1, 1187.1, 1217.1, 2759, 10595); they note that CH4’s potency is 1000 times greater (0425) or that water is 95% of total greenhouse effect (10158, several others), implying that CO2 emissions can not have a large effect on the earth’s climate.
Other commenters write that CO2 is a weak GHG because it is limited as to how much radiation it can absorb. For example, a commenter asks why Mars is not warm despite a 95% CO2 atmosphere (2895), and another states that doubling CO2 would only have a small (0.4°C) effect (2759). One commenter states that as CO2 concentrations increase, the forcing does not increase—CO2 “has a forcing limit of 325 ppm” (0582). Another cites Plimer, who states that it has a maximum threshold (11454), and another states that CO2 does not absorb infrared (286).
Others point out that CO2 is less than 0.05% of the atmosphere (0153, 0455, 0498, 2885, 3214.1), and therefore presumably has a very small effect. A commenter (3722) claims that because of logarithmic forcing, 75% of the warming due to CO2 doubling should have already happened, therefore future warming due to CO2 will be small. A commenter (1009.1) notes that increased CO2 will not lead to much increase in temperature because of the logarithmic relationship and saturation.
Although it is true that CO2 has a smaller warming effect per kilogram or per molecule than a gas like CH4, it plays a larger role in the warming of the atmosphere. For example, Table 2.14 of Forster et al. (2007) lists radiative effects per ppb, lifetimes, and global warming potentials for a number of gases. CH4 is 73 times as potent as CO2 per kilogram in the atmosphere, 26 times as potent per molecule, or 25 times as potent using the Global Warming Potential metric. However, the concentration by volume of CH4 is 210 times less than that of CO2, and the emissions in kilograms of CH4 are about two orders of magnitude less. Thus, the TSD does not characterize various GHGs as “weak” or “strong,” and we do not find such characterizations useful. Note also that we are unclear the source for the claim that CH4’s potency is 1,000 times greater than CO2’s. We are not aware of such an estimate.
We also find no support for the assertion that water is responsible for 90% or 95% of the greenhouse effect in the scientific literature. Calculations by Kiehl and Trenberth (1997) suggest that water contributes about 60% of the greenhouse effect in clear sky conditions and 75% in cloudy conditions (including the cloud contribution). CO2 contributes about 26% of the greenhouse effect in clear sky 14 conditions, and 15% in cloudy conditions. Because the mass of water in the atmosphere is much larger than the mass of CO2, this implies that per ton or per molecule, CO2 is actually a much more effective GHG than water vapor.
The total effect of increasing CO2 concentrations can be best addressed by actually calculating the radiative forcing resulting from changes in those concentrations. Section 4(a) of the TSD discusses changes in radiative forcing due to increases in CO2 concentrations in the context of other changes in radiative forcing over the last 250 years. This also puts in context how a gas that composes 0.04% of the atmosphere can actually have a large radiative effect.
We disagree with assertions by commenters about a number of the radiative characteristics of CO2. We do agree that the forcing due to increases in CO2 concentrations is roughly logarithmic (Forster et al., 2007). This logarithmic relationship holds over a wide range of concentrations; commenters provided no peerreviewed literature to support the contentions that CO2 has a forcing limit of 325 ppm, a maximum threshold, or no infrared absorption, and we find that these assertions are not consistent with the scientific literature (Forster et al., 2007). Current forcing is almost half (not 75%) of the expected doubling due to the logarithmic relationship cited by one commenter, and because of the inertia of the climate system not all the warming has been realized, so it is not possible to extrapolate future temperature change merely by doubling the past 50 years of change. Comments on future temperature projections are covered in detail in
Regarding Mars, see the response in Section 3.2.3 of Volume 3 of the Response to Comments document.
For these reasons, we have found no support for the commenters’ conclusions that CO2 does not have a large effect on the Earth’s climate. They provided no literature to support their assertions, and we have determined that our discussion of these issues in Section 4(a) of the TSD is reasonable and scientifically sound.