That atmospheric gases transmit short-wavelength visible solar radiations and absorb long-wavelength infrared Earth radiations, and thereby produce a “greenhouse” effect, has been known at least as far back as the time of Fourier [Fourier, 1824]. Tyndall  realized that the asymmetric water vapor molecule is orders of magnitude more efficient at absorbing these Earth radiations than the dominant symmetric molecules N2 and O2. Arrhenius [1896, p. 267] undertook a numerical calculation of the effect of CO2 on atmospheric temperature owing to the extraordinary interest of the times in the question of the cause of ice ages, when “the countries that now enjoy the highest civilization were covered with ice.” Chamberlin [1899, p. 528], editor in general charge of the Journal of Geology, noted that further “abstraction of carbon dioxide from the atmosphere…would insure…the permanent and final winter of the Earth.” Callendar [1938, p. 236] claimed that fuel combustion in the previous 50 years was then raising atmospheric temperature; this result, he concluded, would be beneficial to man in the growth of plants and in the expansion of land cultivation poleward, and, “in any case the return of the deadly glaciers should be delayed indefinitely.” It was not until the 1950s that concern was voiced that such warming might damage our environment (see, e.g., Plass [1956, p. 387]: “The accumulation of carbon dioxide in the atmosphere from continually expanding industrial activity may become a real problem in several generations.”)
 Scant attention to the change in temperature aboveground level was given in these early considerations except insofar as conditions there may affect ground-level temperature. Manabe and Wetherald  showed that raising CO2 concentration would raise temperatures in the troposphere but lower them at higher altitudes. While at low altitudes essentially all emissions from CO2 are reabsorbed by surrounding CO2 (or other greenhouse gases), in the higher thinner atmosphere upward emissions are more likely to be lost to space. Roble and Dickinson  first investigated the cooling effects of increased CO2 and CH4 on the mesosphere and thermosphere, finding changes of some 50 K in the high thermosphere and resultant large compositional redistribution for a doubling of these gases. Rishbeth  estimated from theoretical considerations that such a change in the thermosphere would lower the ionospheric E region by 2 km and F region by 15–20 km with small change in layer densities. Rishbeth and Roble  performed model calculations to confirm these ionospheric predictions. Bremer  detected an 8 km decrease in the height of the F region peak during the period 1957–1990. CO2 increased by about 12% over this period [see, e.g., Keeling et al., 1995], and Rishbeth's estimate of 15–20 km change in F layer height for a doubling of CO2 would scale to only 2 km for a 12% change. This result, that the measured change greatly exceeds that estimated on the basis of a change in greenhouse gases, anticipates our own results.
 Since that time, investigators have scoured upper atmosphere data for evidence of long-term trends. The reports of many of these investigations may be found in the proceedings of conference sessions held on the topic of “Long-term changes and trends in the atmosphere” (or similar names) in Moscow, Russia, in 1998 [Golitsyn, 1998], Pune, India, in 1999 [Beig, 2000], Prague, Czech Republic, in 2001 (Second IAGA-ICMA-PSMOS Workshop, “Long-Term Changes and Trends in the Atmosphere,” Phys. Chem. Earth, 27(6–8), 397–615, 2002), Sozopol, Bulgaria, in 2004 (Long-Term Changes and Trends in the Atmosphere, Phys. Chem. Earth, 31(1–3), 1–128, 2006), Toulouse, France, in 2005 (Long-Term Trends and Short-Term Variability in the Upper, Middle and Lower Atmosphere, J. Atmos. Sol. Terr. Phys., 68(17), 1853–2052, 2006), Sodankyla, Finland, in 2006 (Fourth IAGA-ICMA-CAWSES Workshop, “Long-Term Changes and Trends in the Atmosphere,” Ann. Geophys., 26, 1171–1326, 2008), and Perugia, Italy, in 2007 (Long-Term Changes and Solar Impacts in the Atmosphere-Ionosphere System, J. Atmos. Sol. Terr. Phys., 71(13), 1413–1510, 2009); conferences on this topic are planned for Boulder, Colorado, and Berlin, Germany, in 2010. Recent review articles on this topic include those of Beig et al. , Beig , Lastovicka et al. , and Lastovicka . While these works include a number of reports related to ionospheric density, works concerning direct measurements of temperature above the mesopause are scarce. Holt and Zhang  have reported the long-term trend in noontime temperature at 375 km altitude above Millstone Hill, apparently the only direct measurement of temperature above 110 km. The consensus of these measurements gives the consistent picture of slow cooling with time at all altitudes above the troposphere, but with little trend in the mesopause region, and very strong cooling in the high thermosphere.