Greenhouse cooling up high

NEWS AND VIEWS ATMOSPHERIC CHEMISTRY Greenhouse cooling up high Ralph J. Cicerone CHANGES in the atmosphere’s chemical composition are stimulating a lot of care- ful thought: how far will thinning of the ozone layer in the stratosphere proceed?; will radiative forcing, the growing green- house effect, accelerate in the tropo- sphere? The consensus is that it is at least plausible that continued increases, due to human activity, in concentrations of car- bon dioxide, methane and chlorofluoro— carbons, will lead to measurable, if not damaging, environmental disturbances. Roble and Dickinson‘ now widen the debate by considering the effect of emis- sions of carbon dioxide and methane on the upper atmosphere. Dramatic conse- quences could ensue: the upper reaches may cool by tens of degrees and pressures will be reduced; concentrations of oxygen atoms will be reduced and those of hydrogen atoms increased. This is not a wild proposition, but rather a simple and direct one. The increased concentration of CO, in the stratosphere causes cooling because CO, is an efﬁcient radiator to space of energy through infrared emissions (the obverse side of the greenhouse coin in which infrared activity causes tropospheric warming through radiation trapping). This is thought to be the surest consequence‘ of increased C0,; this cooling should extend into the lower mesosphere’. The concentration of hydrogen atoms in the therrnosphere is known to be sensitive to the amounts of water vapour and methane at the top of the stratosphere“. Also, the link between the ground-level concentration of CH, and the stratos- pheric concentration of water vapour has been known for a while. Yet no one had looked at the very high atmosphere to see how its chemistry and physics might be affected by ground-level changes in CO, and CH,. At times like these we see how fragmented are our scientific communi- ties and our knowledge of the chemistry of the Earth’s atmosphere. The upper atmosphere is an arena for a rich variety of phenomena, much studied, such as visible aurorae over the polar regions, airglow optical emissions and electrical current flows due to charged particles. Commercial and military com- munication through radio waves reﬂected by the ionosphere, starting at the meso- sphere, added to the scientiﬁc interest in the upper atmosphere. Intensive scien- tiﬁc research has focused on the atmos- phere above about 100 km altitude. Instruments ﬂown by satellites have probed the ionosphere and the upper atmosphere with remote sensing by ultra- violet and visible light and in situ mass spectrometers, as have ground-based 104 radars and optical airglow sensors. Its gross morphology (temperature, degree of ionization and chemical composition) and variations have been mapped. Atomic and molecular atmospheric processes such as those involved in photoionization of gases and recombination of ions have been studied in the laboratory, and macroscopic collective phenomena have been modelled and observed, such as gravity waves, plasma waves and electri- cal currents. Numerical models of the ionosphere and upper atmosphere have progressed dramatically. Fluid dynamical, thermal balance and chemical kinetic equations are solved for winds and temperatures, / 120 - / / 1°ol_ Thermosphere f Mesopause 80 ,E_ \ _-5 Mesosphere 3 so . 3 g Stratopause 40 - Stratosphere 20 &#39; Tropopause \ Eoposphere o . . . . . -120 -90 -60 -30 0 +30 +60 +90 Temperature (“Cl Variation of temperature with altitude in the atmosphere (solid line) revealing the various strata. Precise details vary with latitude and season. The temperature of the thermo- sphere rises to over 700 °C at 250 km. Above the mesopause, solar radiation ionizes at- mospheric gases to create the ionosphere, divided into sub-layers D, at 90 km, E, at 110 km, F, above 200 km, defined by differing behaviour. Roble and Dickinson are the first to study the effect of CH, and CO, on the upper reaches of the atmosphere. The broken line shows the predicted effect of increased C0,, which extends to cooling by 50°C at the top of the thermosphere. given fairly basic input data for solar input and the major chemical species in air. Roble and Dickinson and their col- league, E.C. Ridley, and others can now calculate reasonably realistic dynamical patterns, ion types and numbers and tem- perature variations, for example, from solar-activity maxima to minima. Although some boundary conditions for the equations are still speciﬁed too rigid- ly, many of the phenomena are simulated amazingly well and the coupling of ﬂuid dynamics, radiation, charged-particle dynamics and chemical reactions in © 1990 Nature Publishing Group modern models is becoming very realistic. Roble and Dickinson have introduced into such a model changes from the reference—state concentrations of CO, and CH, of the 1950s; they doubled and halved the observed values at the model’s lower boundary (62 km). Higher concen- trations for these gases cause the com- puted temperatures to fall by 50 K near 400 km altitude as compared to the refer- ence state, whereas lower concentrations cause temperatures there to rise by 35 K. Concentrations of the dominant gas, oxygen atoms, fall by almost 40 per cent near 400 km when CO, and CH, are doubled and they rise by the same amount when the driving gases are halved. Hydrogen atom concentrations either rise by 50 per cent or fall by 20 per cent for the same changes. The potential consequences of such perturbations are numerous. Roble and Dickinson mention some of them for the case of increasing CO, and CH4: atmos- pheric drag on orbiting satellites will be reduced as the density of the upper atmosphere decreases and noctilucent clouds in the mesosphere will become more common as mesospheric water vapour quantities increase and temper- atures decrease simultaneously. Iono- spheric ion densities would be reduced near the E— and F—region peaks, and top- side plasma scale heights will shrink. Global circulation and spatial morphology of the therrnosphere may also change. All of these and other perturbations must be analysed more thoroughly; Roble and Dickinson have made only the beginning. Various interactions in the system could lead to altered predictions; effects could be larger or smaller. For example, wave propagation and dynamic- al heating through the mesosphere and therrnosphere could be altered as vertical temperature proﬁles change. One notable inconsistency in the paper leads to under- estimated effects. The authors held the water—vapour mixing ratio ﬁxed at 6 parts per million (p.p.m.) at 62 km; it should have risen to 9 ppm. when methane was doubled. Thus, hydrogen-containing spe- cies should increase in concentration even more than the authors predicted and hydrogen escape rates will increase (at least that part of the escape ﬂux that is thermal”) and the ratio of hydrogen] deuterium escape rates will change’. Further, these larger amounts of meso- spheric H20 and simultaneously lower temperatures will increase the regularity of noctilucent clouds“ and possibly alter D-region ion chemistry. It is also conceiv- able that the added water could lead to more cloud formation in the polar strato- sphere, thus priming the system for ozone destruction“, but the size of this effect remains to be determined. In the thermo- sphere, as the H/O ratios must change by perhaps a factor of three, the redox state NATURE - VOL 344 - 8 MARCH 1990