Review of Chapter 1

Chapter 1 asks the following: Why do temperatures vary vertically (from the surface to the stratosphere) and what do we understand about why they might vary and change over time? The chapter begins with a limited description of the vertical temperature structure of the atmosphere and devotes most of its space to a consideration of forcing mechanisms that affect the local temperature structure. This chapter provides an excellent opportunity to explain the implications of differences in temperature trends between the troposphere and the surface. Such an explanation will be extremely important because, as the first of the Climate Change Science Program (CCSP) synthesis and assessment products, this report will be closely scrutinized with respect to scientific methodology, accuracy, and awareness of current understanding and theory.

The present text is inadequate on several counts, as listed in detail below. Of particular importance, the primary implications of differences or lack thereof in atmospheric and surface trends are never identified. In addition, the chapter pays insufficient attention to the effect of dynamics on temperature structure. The role of dynamics in smoothing temperature both horizontally (over the Rossby radius) and vertically (over the convective depth) is largely ignored. Finally, the chapter would be strengthened if it had further discussion of theory, which would complement the chapter’s current emphasis on bringing models and observations into agreement. It should be recognized that we are discussing temperature changes of tenths of a degree—a challenge not only for instruments, but also for theory and modeling. Dealing with this challenge calls for a more sophisticated conceptual framework.

MAJOR COMMENTS

1. The explanation of the greenhouse effect should more clearly describe its effect on the atmospheric temperature structure. In particular, the chapter should explain how the addition of infrared absorbing gases causes the characteristic emission level to be at a higher altitude, where temperatures are colder and where the reestablishment of radiative balance with space calls for warming at this level and communication of this warming to the surface (Goody and Yung, 1989; Lindzen and Emanuel, 2002). Thus, for example, the absence of any warming within the troposphere might suggest that the greenhouse effect is not responsible for the surface warming. A related topic concerns the question of



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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere Review of Chapter 1 Chapter 1 asks the following: Why do temperatures vary vertically (from the surface to the stratosphere) and what do we understand about why they might vary and change over time? The chapter begins with a limited description of the vertical temperature structure of the atmosphere and devotes most of its space to a consideration of forcing mechanisms that affect the local temperature structure. This chapter provides an excellent opportunity to explain the implications of differences in temperature trends between the troposphere and the surface. Such an explanation will be extremely important because, as the first of the Climate Change Science Program (CCSP) synthesis and assessment products, this report will be closely scrutinized with respect to scientific methodology, accuracy, and awareness of current understanding and theory. The present text is inadequate on several counts, as listed in detail below. Of particular importance, the primary implications of differences or lack thereof in atmospheric and surface trends are never identified. In addition, the chapter pays insufficient attention to the effect of dynamics on temperature structure. The role of dynamics in smoothing temperature both horizontally (over the Rossby radius) and vertically (over the convective depth) is largely ignored. Finally, the chapter would be strengthened if it had further discussion of theory, which would complement the chapter’s current emphasis on bringing models and observations into agreement. It should be recognized that we are discussing temperature changes of tenths of a degree—a challenge not only for instruments, but also for theory and modeling. Dealing with this challenge calls for a more sophisticated conceptual framework. MAJOR COMMENTS 1. The explanation of the greenhouse effect should more clearly describe its effect on the atmospheric temperature structure. In particular, the chapter should explain how the addition of infrared absorbing gases causes the characteristic emission level to be at a higher altitude, where temperatures are colder and where the reestablishment of radiative balance with space calls for warming at this level and communication of this warming to the surface (Goody and Yung, 1989; Lindzen and Emanuel, 2002). Thus, for example, the absence of any warming within the troposphere might suggest that the greenhouse effect is not responsible for the surface warming. A related topic concerns the question of

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere whether temperature changes originating at the surface necessarily lead to temperature changes within the troposphere. 2. Similarly, a discussion of the relation between cumulus convection and the moist adiabat provides an opportunity to use such differential trends to understand the coupling between the surface and the lifting condensation level. Indeed, in the tropics, the temperature structure consists of a surface mixed layer (up to about 500 m) and a trade wind boundary layer (up to about 2 km) above which is the free troposphere. Each of the boundary layers is topped by an inversion which tends to isolate the layer from the region above (Sarachik, 1985). Outside the tropics, the surface communicates with upper levels primarily by quasi-horizontal motions along isentropic surfaces (e.g., Hoskins, 2003). Consequently, the report and the scientific community should move beyond the naïve notion that the lapse rate is a rigid constraint operating from the surface to the tropopause. Instead the observations this report is concerned with should be exploited in order to answer important questions about climate. This objective provides meaningful motivation for ascertaining the accuracy of the temperature measurements and the resulting time series. That said, it should be emphasized that the temperature changes being considered are changes on the order of tenths of a degree (although local changes may be much greater), and current theories may prove inadequate for such small changes. 3. In general, spatial and temporal sampling is not adequately dealt with in the Temperature Trends CCSP report. Given the fact that horizontal temperature variability at the surface tends to get smoothed as one rises to the free troposphere, there may be serious issues of sampling. Horizontal smoothing over large scales occurs above the boundary layer, but that at the surface and within the boundary layer, there can be much more horizontal variation of temperature. Thus, much more data may be required at the surface to get characteristic temperatures. 4. For Chapter 1, explanations of the processes involved in determining vertical profiles of temperature should represent the current state of understanding or lack thereof. The chapter should focus less on details of the vertical profile of temperature that are not resolved by the observations that are the focus of the report. For example, the satellite data are only reported in coarse vertical layers. 5. For discussions that are felt to be too detailed for the body of the text, footnotes are a reasonable device. SPECIFIC COMMENTS 1. The chapter should include more discussion of theories that provide physical constraints on the apparent differences between surface and tropospheric records. 2. The discussion on lines 69-80 should be replaced with a more accurate figure as well as a description of the differences between the tropics, the extratropics, and the polar regions. In the tropics, the temperature is hardly linear with height, given that the lapse rate associated with the moist adiabat goes from about 5 K/km near the surface to almost 9.8 K/km at the tropopause near 16 km. It should also be noted that the tropopause descends sharply to 12 km near 30 degrees latitude and to around 8 km near the poles. The existence of the near surface inversion layer at high latitudes should also be noted as well as its dependence on meteorological conditions.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere 3. Relatedly, lines 100-131 should be replaced by a more complete discussion wherein it is noted that a radiative-convective balance is only likely to be of dominant relevance in the tropics, while in the extratropics, the lapse rate and the tropopause height are mostly determined by the same baroclinic instability that gives rise to weather systems (Schneider, 2004). Planetary-scale forced waves in winter and other circulation features, such as the Hadley and Walker cells, should be mentioned. 4. The authors should provide further discussion of the role played by dynamics. The discussion of dynamics in lines 128-131 should introduce the concept of Rossby radius. This vital concept shows that dynamics tend to homogenize temperatures (above the boundary layer) over horizontal scales that vary from the planetary scale near the equator to a couple of thousand kilometers at midlatitudes and to a few hundred kilometers near the poles. 5. The discussion in lines 133-137 should be strengthened, in particular so that it distinguishes between specific and relative humidity. 6. Remove “especially critical” from line 141. 7. In lines 150-156, the question of internal variability needs to be improved and clarified. For one thing, there can be internal variability without external forcing, and even without air-sea interaction. Further, there are limitations associated with using numerical models to examine the importance of internal variability because such models poorly characterize such things as El Niño/Southern Oscillation (ENSO), the 1976 regime shift, and the quasi-biennial oscillation (QBO) at the levels of tenths of a degree. It should be emphasized that most rules of thumb used for atmospheric structure may not be appropriate at the level of the small temperature changes being considered in this report. 8. It would be worth stressing that the temperature changes that are being discussed are only a few tenths of a degree. Much of our thinking is based on more substantial changes. There is an extensive literature arguing for and against the relevance of the moist adiabat in the tropics (e.g., Xu and Emanuel, 1989). However, even those arguing for its relevance would not argue that it should hold to better than a few tenths of a degree. Similarly, it might be argued that the role of motions should cancel when averaged over the earth. But the above is not strictly true. The existence of radiation leads to irreversibility, and when the strong changes in water vapor with latitude are taken into account, changes in circulation can lead to changes in global mean temperature that might be on the order of a few tenths of a degree. 9. In lines 194-202 and in lines 283-291, the report should be more cautious in arguing that local changes in radiative constituents can lead to local changes in temperature profile in light of such processes as the mean circulation in the tropics, which homogenizes temperature, and quasi-geostrophic dynamics in the extratropics. 10. In line 206, while the radiative impact of clouds is undoubtedly very important, further explanation is needed if one is to attribute to them a role as a “regulator”. 11. In lines 222-224, it should mentioned that greenhouse gas forcing in the tropics is not uniform owing to the current distribution of clouds and water vapor. Thus, greenhouse gas forcing from anthropogenic sources is greatest in dry regions. 12. The claim of local radiative influence in lines 233-234 should either be explained or omitted.

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Review of the U.S. Climate Change Science Program’s Synthesis and Assessment Product on Temperature Trends in the Lower Atmosphere 13. In lines 251-252, caution should be suggested in adding unknown forcings because these can easily become nothing more than adjustable parameters. Of course, care should also be taken to include all forcings that are quantitatively known. 14. In lines 254-255, it should be noted that while the air-sea interaction can play a role in internal variability, such variability can also occur in the atmosphere alone. 15. In lines 258-261, while water vapor and clouds are indeed critical to the high climate sensitivity of many models, the references cited (Stocker et al., 2001; NRC, 2003) carefully note that water vapor and especially clouds are areas of major uncertainty in models, and even in nature. 16. The discussion of volcanic influence on lines 309-312 should be reworked to include additional work that has been done on this subject. For example, there is more on the effects of volcanoes on European temperatures in Jones et al. (2003) and in Robock and Oppenheimer (2003). The most affected region is Northern Europe—not North America and certainly not Siberia. The two studies cited in lines 309-312 are also basically model studies, and evidence from observations is less convincing. 17. The claim on lines 331-336 should note the substantial uncertainty of such factors as solar variability (Frohlich and Lean, 2004), historical volcanic forcing (Bradley, 1988), and aerosols (Charlson et al., 1992; Anderson et al., 2003). 18. The report appropriately notes that the radiosondes show an abrupt increase in temperature in the troposphere around 1976 and the fact that this is missed in the satellite data which starts in 1979. It has been argued that the surface warming is simply the response to this jump with a delay due the heat capacity of the ocean (Lindzen and Giannitsis, 2002). This is distinctly relevant to the present report.