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Emerging Pattern of Global Change in the Upper Atmosphere and Ionosphere

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Emerging Pattern of Global Change in the Upper Atmosphere and Ionosphere View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Publications of the IAS Fellows Ann. Geophys., 26, 1255–1268, 2008 www.ann-geophys.net/26/1255/2008/ Annales © European Geosciences Union 2008 Geophysicae Emerging pattern of global change in the upper atmosphere and ionosphere J. Lastoviˇ ckaˇ 1, R. A. Akmaev2, G. Beig3, J. Bremer4, J. T. Emmert5, C. Jacobi6, M. J. Jarvis7, G. Nedoluha5, Yu. I. Portnyagin8, and T. Ulich9 1Institute of Atmospheric Physics, Bocni II, 14131 Prague, Czech Republic 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA 3Indian Institute of Tropical Meteorology, Pune-411 008, India 4Leibnitz-Institute of Atmospheric Physics, Schloss-Street 6, 18225, Kuhlungsborn,¨ Germany 5Naval Research Laboratory, Washington, D.C. 20375, USA 6Institute for Meteorology, University of Leipzig, Stephanstr. 3, 04103 Leipzig, Germany 7British Antarctic Survey, Cambridge, CB3 0ET, UK 8Institute for Experimental Meteorology, Lenin Str. 82, Obninsk 249038, Russia 9Sodankyla¨ Geophysical Observatory, Tahtel¨ antie¨ 62, 99600 Sodankyla,¨ Finland Received: 11 May 2007 – Revised: 4 October 2007 – Accepted: 4 October 2007 – Published: 28 May 2008 Abstract. In the upper atmosphere, greenhouse gases pro- at higher levels of the atmosphere may be important for life duce a cooling effect, instead of a warming effect. Increases on Earth, as well. Moreover, with society becoming more in greenhouse gas concentrations are expected to induce sub- dependent on space-based technologies, it is important to un- stantial changes in the mesosphere, thermosphere, and iono- derstand the long-term evolution of the upper atmosphere. sphere, including a thermal contraction of these layers. In The upper atmosphere consists of the mesosphere (∼50– this article we construct for the first time a pattern of the 90 km), thermosphere (∼90–800 km), and its ionised part, observed long-term global change in the upper atmosphere, the ionosphere, which is embedded within these regions. The based on trend studies of various parameters. The picture thermosphere is the operating environment of many commu- we obtain is qualitative, and contains several gaps and a few nication and weather satellites, as well as the International discrepancies, but the overall pattern of observed long-term Space Station, and knowledge of the atmospheric drag ex- changes throughout the upper atmosphere is consistent with erted on these objects is necessary for accurate prediction of model predictions of the effect of greenhouse gas increases. their location. The structure of the ionosphere directly affects Together with the large body of lower atmospheric trend re- the performance of the Global Positioning System (GPS) and search, our synthesis indicates that anthropogenic emissions other satellite navigational systems, whose signals are phase- of greenhouse gases are affecting the atmosphere at nearly shifted by ionospheric plasma. The mesosphere critically all altitudes between ground and space. affects lower atmosphere-thermosphere coupling via atmo- Keywords. Atmospheric composition and structure (Ther- spheric waves and electromagnetic phenomena; this complex region is less understood than the regions above and below. mosphere – composition and chemistry; Evolution of the ◦ atmosphere) – Ionosphere (Ionosphere-atmosphere interac- The 0.6 C increase in global surface air temperature dur- tions) ing the twentieth century (e.g. IPCC, 2001) has been at- tributed predominantly to the increasing atmospheric con- centration of greenhouse gases. In the upper atmosphere, the radiative effects of greenhouse gases, particularly CO2, 1 Introduction become more pronounced and produce a cooling, rather than a warming effect. At these altitudes, CO is optically thin Life on Earth is more directly affected by climate change 2 and is unable to contain outgoing infrared radiation; thermal near the surface than in the middle and upper atmosphere, energy is transferred by collisions with ambient gas to the but as the story of the Earth’s ozone layer illustrates, changes excited states of CO2 molecules and then lost to space via Correspondence to: J. Lastoviˇ ckaˇ its infrared radiation. A much stronger grenhouse effect is ([email protected]) observed on Venus where the 96% concentration of carbon Published by Copernicus Publications on behalf of the European Geosciences Union. 1256 J. Lastoviˇ ckaˇ et al.: Emerging pattern of global change dioxide in the atmosphere results in a troposphere that is 2 Difficulties – methodology, data uncertainties, natural more than twice as warm as Earth’s and a thermosphere that variability is 4–5 times cooler (e.g. Bougher and Roble, 1991). The same relative increases in greenhouse gas concentra- Important points to be discussed in relation to long-term tions observed near the surface are expected to occur in the trend studies include various uncertainties in data, the long- upper atmosphere. Consequently, the chemical composition term consistency of measurements, and effects of applica- and thermal structure of the upper atmosphere may be altered tions of different methods of trend determination. These is- substantially by human activities, which, through feedbacks, sues are particularly important for detecting weak trends in could potentially affect the lower atmosphere. As with global noisy time series. Different methods have been developed for change near the Earth’s surface, the challenge facing upper extracting long-term trends from particular data sets. These atmospheric climate scientists is to detect long-term trends methods often include averaging and interpolation proce- and attribute them to their primary causes, so that society dures, and thus it is very difficult to compare trend estimates can mitigate against harmful change scenarios. obtained by different authors. Ulich et al. (2003) discussed Roble and Dickinson (1989) and Rishbeth and Roble some practical problems of determining long-term change. (1992) presented the first theoretical simulations of the ef- Long time series of measurements need to be consistent. fects of hypothetical future increases in greenhouse gas con- Instrumental changes and malfunctions need to be well doc- centrations on the upper atmosphere and ionosphere. These umented and corrected for, and subsequent instruments must studies predicted substantial temperature reductions in the be well inter-calibrated with previous ones. Before attempt- upper atmosphere with doubled CO2, an effect that was ing to compute any trend, one has to verify whether or not the termed “greenhouse cooling” (Cicerone, 1990). The global time series in question includes instrumental effects. Further- atmosphere is very nearly in a hydrostatic equilibrium, which more, historic time series usually contain data gaps, which means that the height of a given pressure surface is deter- have to be treated with care. Some analysis methods re- mined by the average atmospheric temperature below. The quire continuous data sets with constant sampling frequency, cooling is therefore expected to result in thermal contrac- but interpolation can introduce incorrect data points, which tion of the upper atmosphere and we may expect a signif- might skew the results of the analysis. icant decline in thermospheric density at fixed heights (not Studies of the F2-layer peak height, hmF2, suffer from necessarily at fixed pressure levels), as well as a downward an additional complication. The height cannot be directly displacement of ionospheric layers (Rishbeth, 1990). Meso- scaled from ionograms; it is computed from the critical fre- spheric behaviour might be affected also by the stratospheric quencies of the E and F2 layer as well as the propagation ozone depletion (e.g. Bremer and Berger, 2002; Akmaev et parameter M(3000)F2 using empirical formulae. These for- al., 2006) observed over the last three decades (e.g. Staehelin mulae were derived in the 1970s based on various regional et al., 2001). data sets and to be simple and straightforward enough to be Information about long-term trends in various upper at- used by the computers of the time, but these formulae are mospheric parameters has accumulated to a level such that by no means globally valid, and their use results in different a coherent pattern of upper atmospheric climate change is trends. One has to check carefully which one is applicable to beginning to emerge. In this article, we present the first the ionosonde in question. comprehensive assessment of observed long-term changes Atmospheric time series usually vary on diurnal and sea- and trends in the mesosphere, thermosphere, and ionosphere. sonal time scales. In order to see beyond this variability, We identify gaps in our knowledge and understanding of cyclic influences on the measurements need to be properly long-term trends in various parameters, as well as possi- removed and/or considered in interpretation of results. This ble inconsistencies among trends or their substantial changes is usually done by fitting multi-parameter models to the data. with time, and we investigate whether the observed trends Care must be taken that these cyclic parameters are ade- are consistent with the theoretically predicted effects of in- quately removed and/or considered, especially if the diurnal creasing greenhouse gas concentrations. International col- and seasonal sampling is not uniform. laboration on upper atmospheric trend studies is realized Solar and geomagnetic activity
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