Long term trends in the temperature of the mesosphere/lower thermosphere region: 1. Anthropogenic influences

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011ja016646, 2011 Long term trends in the temperature of the mesosphere/lower thermosphere region: 1. Anthropogenic influences Gufran Beig 1 Received 11 March 2011; revised 30 June 2011; accepted 11 July 2011; published 15 October [1] A detailed overview of long term secular trends in temperature of the mesosphere and lower thermosphere considered to be induced by increase in greenhouse gases has been provided by Beig et al. (2003). Since then, quite a few new results have been emerged as some of the data series have become sufficiently large enough to provide results with improved confidence. Our understanding on the nature of temperature trends in the mesosphere/lower thermosphere (MLT) region is relatively better now. In the mesosphere, some of the results confirmed the earlier findings, and some new results obtained by satellite and lidar data over the tropical region have indicated a relatively weaker cooling trend as compared to the past but nevertheless strengthened the conclusion about the cooling trends. However, in the mesopause region, some of the new results now indicate a break in trend and tendency of negative signal where earlier no trend feature was noticed. This slice of no trend feature in between two cooling regimes was puzzling the modeling community, who were in search of a convincing explanation. This paper briefly outlines the progress made over the recent past in the field of MLT region secular temperature trends attributed mainly to growth of greenhouse gases near the Earth s surface. Citation: Beig, G. (2011), Long term trends in the temperature of the mesosphere/lower thermosphere region: 1. Anthropogenic influences, J. Geophys. Res., 116,, doi: /2011ja Introduction [2] The composition and thermal structure of the Earth s atmosphere has changed considerably over geological time. Recently, several measurements and model calculations have made it increasingly clear that releases of trace gases from human activity have a potential for causing a change in the present day climate of the Earth. Most discussions of the projected change during the past decade have dealt with changes in the Earth s troposphere and stratosphere. There have been many papers written on the subject, and several assessment panels have issued reports that indicate that the troposphere will warm and the stratosphere will cool in response to a doubling of the present day composition of several greenhouse trace gases such as carbon dioxide and methane. It is around 2 decades since climate scientists began to calculate how the greenhouse effect might influence the region above the stratosphere in the mesosphere/ ionosphere. The mesosphere region gradually became an important region to the climate change debate because the magnitude of change predicted for this region is expected to be larger than at lower altitudes and poses a significant relevance to weather and climate. However, relatively little was known about how the upper atmosphere will respond to these 1 Indian Institute of Tropical Meteorology, Pune, India. Copyright 2011 by the American Geophysical Union /11/2011JA climate forcing parameters. Recently, the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission, and the significant ground based program that accompanies it, has provided a much better understanding about the energy budget of this critical region of Earth s atmosphere [Mlynczak et al., 2010]. [3] The measurements reported during the past few decades suggest that rapid changes in upper atmosphere structure are occurring much faster than model predictions and have latitudinal as well as longitudinal variations, indicating that there was an important need to understand their longterm variability. To investigate the above issues in detail, a joint working group of the International Association of Geomagnetism and Aeronomy (IAGA) and International Commission for Middle Atmosphere (ICMA) on trends was established which constituted the Mesospheric Temperature Trend Assessment (MTTA) panel. This panel provided a comprehensive review about long term changes and trends in the most vital meteorological parameter, namely, temperature of the region from 50 to 100 km, which is attributed to anthropogenic activities at the ground [Beig et al., 2003]. This review paper (and references therein) concluded significant cooling in most part of the mesosphere/lower thermosphere (MLT) region, with the exception of the mesopause region. Thereafter, a few new results and updates were also reported [Remsberg, 2007, 2008, 2009; Laštovička et al., 2006, 2008; Beig, 2006]. 1of9

2 Figure 1. Annual mean long term temperature trends (K/decade) in the mesosphere over the tropical latitudes. The rocketsonde trends of the 1970s and 1980s are compared with the trend obtained during the past 2 decades using satellite and lidar data. The horizontal line shading represents roughly the range of trends as revealed during the past 2 decades. [4] There has been an ever growing impetus for temperature trend investigations in the MLT region during the last few years, now that it has been established that the secular increases in greenhouse gases at the ground should have a substantial impact on the radiative chemical dynamical equilibrium. This article present an update to the above mentioned baseline review papers on the MLT region temperature trends. It also highlights new emerging features and findings as revealed in recent years. 2. Results [5] As per the terminology adopted in the earlier review paper [Beig et al., 2003], a region from 50 to 79 km is referred as the mesosphere, whereas the region between 80 and 100 km is referred as the mesopause region in the rest of this paper. We discuss here in detail only those results which were either not included or could not be reported in the earlier baseline review paper [Beig et al., 2003]. Hence, for other results which are reported in subsequent figures but not discussed here in detail, the reader is referred to Beig et al. [2003] Mesospheric Trends [6] Recently, Halogen Occultation Experiment (HALOE) measurements from the UARS satellite have provided a valuable data set of temperature in the middle atmosphere for the period of which has been rigorously analyzed. At the time of Beig et al. [2003], HALOE data, which included the analysis of only 9.5 year time series [Remsberg et al., 2002], could not reveal any significant trend, in part because of their incorrect treatment of the effects of the autoregression. Remsberg and Deaver [2005] reported on multiple linear regression analyses of time series of middle atmosphere temperature of 12.5 years versus pressure profiles from the HALOE data for its seasonal and longer period cycles. Remsberg [2007, 2008] have noted that a separate analysis is necessary for each level and latitude zone, rather than assuming that the same terms are present and significant at all locations. Hence, in their new analysis, proper accounting for the effects of autoregression for the adjacent points of the time series has been taken which was missing in their earlier analyses and publications. The new analysis accounts for the first order autoregression term properly, leading to seasonal and longer period terms of larger amplitude. Remsberg [2007, 2008, 2009] analyzed the time series of zonally averaged temperature versus pressure profile (or fixed altitudes) data of complete 14+ years of HALOE measurements which account for the effects of autocorrelation as mentioned above and discussed in detail elsewhere [Remsberg, 2007]. The updated seasonal and annual average terms were provided, and they were used to generate near global temperature distributions from 2 to hpa that are representative of the period Remsberg [2007, 2008] considered the temperature versus pressure or T (p) time series from the HALOE data of for the mesosphere in 10 wide latitude zones from 60 S to 60 N. Even though sampling from a solar occultation experiment is somewhat limited, it is shown to be quite adequate for developing both the seasonal and longer term variations in temperature. Remsberg [2009] binned the data according to 10 wide latitude zones from 40 S to 40 N and at 10 altitudes from 43 to 80 km, and multiple linear regression analysis techniques have been applied to those time series. This analysis includes several terms, including the solar cycle like term. The findings from the reanalyses are now more definitive because of a proper accounting for the effects of autoregression for the adjacent points of the time series and because of the addition of several more years of data. Remsberg [2009] has stated that it is the first time that interannual terms have been obtained from a single satellite data set covering such a large region of the mesosphere and for a time span of more than a decade. Highly significant, linear cooling trends were found in the above HALOE publications. The cooling trends are found to be significant at most latitudes of the middle and lower mesosphere. They range from 1 K/decade at low latitudes to about 3 K/decade at the middle latitudes. It is reported that at middle latitudes of the middle to upper mesosphere, a trend of the order of 1.5 to 2.0 K/decade is found. The diagnosed trends from HALOE for the middle to upper mesosphere at middle latitudes are larger than those simulated with most models, perhaps an indication of decadalscale dynamical forcings that are not being simulated so well [Remsberg, 2008, 2009]. [7] Figure 1 shows the summary of the annual mean longterm temperature trends (K/decade) as reported in the recent literature for the mesospheric region for the tropics. Figure 1 indicates a horizontal bar area which indicates an approxi- 2of9

3 Figure 2. Annual mean long term temperature trends (K/decade) as reported in the recent literature for the mesopause region in the Northern Hemisphere after Beig et al. [2003]. Here w/o Vol Treat indicates without volcanic term and with Vol Treat means with volcanic treat considered in the analysis. mate range of trends, including the statistical uncertainty as obtained by different workers representative of the trends by analyzing the data obtained during the immediate past 2 decades [Remsberg, 2007, 2008, 2009; Sridharan et al., 2009; Batista et al., 2009]. The range of trends in recent time is found to be around 1to 3 K/decade which appears to be relatively weaker cooling as compared to the earlier data obtained from rocketsondes. HALOE data reveals a trend at tropical latitudes (10 N ± 5 N) of the lower mesosphere of about 0.8 to 1.4 K/decade, which becomes negligible at 70 km. It again becomes around 1 K/decade at 75 km and 1.5 K/decade at 80 km (later value not shown in the figure). All these recent HALOE trends are reported to be within high probability of significance, in contrast to earlier results included in the work by Beig et al. [2003] but with about a 5 year shorter series of data. Trends obtained by HALOE T(z) are compared with the trend profiles from two low latitude lidar stations, namely, São José dos Campos, Brazil (23 S) [Batista et al., 2009], and Gadanki, India (13.5 N) [Sridharan et al., 2009]. The comparison provides a good agreement within the 2 sigma error limit in most altitudes. Cooling trends were found over the altitudes of the upper stratosphere and lower mesosphere from all the data sets. For the lidar comparison in Brazil [Batista et al., 2009], the difference for the trends at 50 km is outside the combined error estimates from the two data sets. It is important to remember that the HALOE results are zonally averaged and hence not specific to the longitude of the lidar measurements reported by Batista et al. [2009]. The trends from HALOE at three altitudes of the mesosphere with the values for previous decades from U.S. rocketsondes at 20 N and 30 N [Keckhut et al., 1999] indicate reasonable agreement, but these trends are relatively smaller when compared with Indian rocketsonde data [Beig and Fadnavis, 2001] of Thumba (8 N). Cooling trends of the lower mesosphere from HALOE are not as large as those from rocketsondes and from the lidar station time series (midlatitude) of the previous 2 decades, presumably because the changes in the upper stratospheric ozone are near zero during the HALOE time period and do not contribute to its trends. In addition, rocketsonde data are always marked with some uncertainty resulting from either the improvement in sensor with time due to technological advancement (if not accounted for properly in regression analysis) or aerodynamic flow, which is difficult to account for, especially in the upper mesosphere Mesopause Region Trends Northern Hemisphere [8] Figure 2 shows the annual mean long term temperature trends (K/decade) as reported in the recent literature for the mesopause region in recent time after Beig et al. [2003] for the Northern Hemisphere. Quite a few new results have been reported recently with an extended series of temperature data. Almost all these new data sets indicate that the trend in this region is showing the tendency toward cooling, which is a drift from the earlier perception where the majority of mesopause region trends were centered around zero. [9] One of the noticeable results in this direction is reported by Offermann et al. [2010] using one of the most reliable data sets obtained by OH airglow measurements during the period over Wuppertal (51 N, 7 E) that agree well with satellite borne observations from Sounding of the Atmosphere Using Broadband Emission Radiometry. A significant cooling trend of 2.3 K/decade with an uncertainty of about 0.6 K is reported. Trend analysis of monthly mean temperatures yields substantial variations from one month to another, between 0 and 0.6 K/yr, hence questioning the value of seasonal mean trends. Offermann et al. found a well known characteristic form of seasonal variation in OH temperature. 3of9

4 Figure 3. The temperature time series during the period from 1986 to 2008 showing a break in secular trend around 1997 as obtained by OH airglow measurements over Wuppertal (51 N, 7 E) (adapted from Offermann et al. [2010]). T cor refers to corrected temperatures which is results from the second iteration step in the analysis. Figure 3 shows the plot of absolute temperature with time (years) from 1988 to 2008 at the OH airglow height [Offermann et al., 2010]. An important finding of this study is a tendency of a break in trend around 1996 as shown in Figure 3. As evident from Figure 3, the temperature indicates a faster fall off with year after 1997 where a breaking kind of feature is noticed around 1996 when a linear fit is made in the raw data series. As a result of which the cooling trend is found to be faster in recent time as compared to the period before The interpretation of this break in trend is not yet clear, but it may be related to the variability in ozone in the mesospheric region. This is an area where detailed modeling study may reveal some interesting results. Earlier, Offermann et al. [2004] and Bittner et al. [2002] have reported near zero trends using the same set of data with shorter time series. [10] Another recent trend analysis for the mesopause and lower thermospheric region is reported by She et al. [2009]. They have carried out an assessment of temperature trends between 85 and 105 km based on 960 nights of Na lidar observation over a period of 20 years over Fort Collins (41 N, 105 W), where all the data until the publication of their results are included. Figure 2 includes two profiles of She et al. [2009], where w/o Vol Treat indicates results obtained without accounting for volcanic term and with Vol Treat represents results obtained by including volcanic terms in the analysis. This study suggests that the impact of volcanic eruption should be accounted for as a volcanic free period much longer than a solar cycle is rare. The temperature trend using lidar data by ignoring the volcanic response yields a cooling trend of as much as 6.5 K/decade. With a Pinatubo term included, the observed negative linear trend between 85 and 100 km peaks at 91 km with a value of 1.3 K/decade, and it turns positive at 102 km. She et al. observe an insignificant cooling trend of 0.1 K/decade at 87 km. Earlier, She and Krueger [2004], using the same data set but with shorter time series, reported a much stronger cooling trend of the order of 2.9 ± 1.5 K/decade, whereas She et al. [2002] have found no trend at 92 km. Remsberg [2007, 2009] has recently reported a detailed analysis of long term changes in temperature based on a 14+ year time series ( ) of zonal average temperatures at pressure level of hpa for latitude zones covering the tropical regions derived from the HALOE version 19 (or V19), level 2 data set of the Upper Atmosphere Research Satellite (UARS). A cooling trend of the order of about K/decade has been reported with a probability of occurrence between 76% and 96%. Remsberg et al. [2002] have obtained no significant trend at about 85 km in their global data analysis using the HALOE data series but with only 9.5 years of data. Lowe [2004] used the full time period of his measurement of OH airglow temperature before measurements got discontinued and found a weak cooling trend of the order of 0.56 ± 0.23 K/decade, which is in contrast to the earlier no significant trend reported by Lowe [2002] using the same data set with slightly shorter time series. In addition, numbers of other measurements near the mesopause region were reported during , but all of them have shown near zero or insignificant trends [Semenov, 2004; Sigernes et al., 2003; Espy and Stegman, 2002]. [11] The advent of regular satellite observations of polar mesospheric clouds (PMC) during the past few decades has led to maturation of PMC studies as discussed in detail by DeLand et al. [2006]. PMCs are normally observed at altitudes of km, with higher altitudes at the start and end of each season. Hemispheric differences in behavior are also observed. Northern Hemisphere PMCs are consistently both more frequent and brighter than Southern Hemisphere clouds. DeLand et al. [2006] did find it significant that all 4of9

5 Figure 4. Annual mean long term temperature trends (K/decade) as reported in the recent literature for the mesopause region during the past 2 3 decades for the Southern Hemisphere. trends for both hemispheres are positive because this implies a corresponding trend in some aspect of the PMC formation process. The most plausible candidates are mesospheric water vapor and temperature. Mesospheric temperatures and water vapor abundances are expected to respond to lower atmosphere changes. Since observations of noctilucent clouds extend for more than 120 years, they represent an excellent candidate for studies of long term terrestrial climate change. A correlation between cloud occurrence frequency and solar activity is quite obvious in modern time series of the number of NLC nights per season in northwest Europe [Gadsden, 1998], although it is not statistically significant in an independent 40 year time series at Moscow [Romejko et al., 2003]. Gadsden s data also identified a secular increase, which has later been shown to be mostly an artifact of uneven collection statistics and due to addition of inhomogeneous data from Finland [Kirkwood and Stebel, 2003]. Kirkwood [2004] has recently confirmed this finding. Gadsden [2002] has earlier shown that there is still a weak secular increase in the northwest Europe time series, which is primarily due to fewer nights with NLCs in the 1960s compared with the later years. The recently analyzed Moscow data show no significant long term trend. Although the long data record of NLC observations is extremely valuable, several factors limit the usefulness of compilations based on visual reports of NLC sightings for long term studies. Quantitative discussions of long term trends in satellite PMC data are fairly recent simply because data sets of sufficient length are only now becoming available. DeLand et al. [2006] have concluded that there is no clear evidence that long term mesospheric temperature variations are a major component of observed PMC trends Southern Hemisphere [12] The observations of mesopause region temperature in the Southern Hemisphere are not many, and relatively very few trend analysis results are reported as compared to the Northern Hemisphere. Nevertheless, whatever the results, they are reported and periodically updated and provide a general perspective of the tendency of trend in this region. Figure 4 shows the updated trends in the mesopause region temperature in the Southern Hemisphere. The most significant result in the Southern Hemisphere is reported by French [2010], who carried out a detailed analysis of sufficiently longer series of 15 years ( ) of observations of the hydroxyl nightglow emission with a scanning spectrometer at Davis station, Antarctica (68 S, 78 E), over each winter season. This is the only analysis for the Southern Hemisphere where data of 15 years are used. Rotational temperatures are derived from the P branch lines of the OH (6 2) band around l840 nm and are a layerweighted proxy for kinetic temperatures near 87 km altitude. When compared with the results of earlier publications of thesamegroup[french et al., 2005; French and Burns, 2004] where a relatively shorter time series is used, we find the tendency of a gradual shift from an earlier nonexistent trend to a weaker but significant cooling trend. French [2010] reported the tentative estimation of linear cooling coefficient of 1.3 ± 0.9 K/decade for the winter mean temperature calculated from nightly averages between day 108 to day 258 each year in these data of 15 years ( ). A seasonal dependence is found in trend coefficients with a maximum long term cooling rate of 2 3 K/decade in the months of September and October. This is interestingly found to be coincident with the Antarctic ozone depletion. Earlier, French et al. [2005] reported a statistically nondiscernable trend of 0.23 ± 0.9 K/decade for the winter in a relatively shorter series of data. They have investigated the impact of seasonal variability in large scale temperature oscillations on long term trend assessments. French and Burns [2004] have also found a dramatic seasonality in the trend coefficient. French and Mulligan [2010] have compared the above mentioned temperature profiles from two satellite instruments, TIMED/SABER and Aura/MLS. They have been used hydroxyl layer equivalent temperatures for comparison with values measured from OH(6 2) emission lines observed by a ground based spectrometer located at Davis station. The maximum difference between all derived hydroxyl layer equivalent temperatures was less 5of9

6 than 3 K. A significant trend was found in the bias between SABER and Davis OH of 0.7 K/yr over the 8 year period with SABER becoming warmer compared with the Davis OH temperatures. In contrast, Aura/MLS exhibited a cold bias of 9.9 ± 0.4 K compared with Davis OH, but, importantly, the bias remained constant over the period. The differences in bias behavior of the two satellites can have significant implications for long term studies using their data. [13] Azeem et al. [2007] have reported the analysis of Michelson interferometer OH airglow temperature data to investigate long term variations of mesospheric temperatures at South Pole station, Antarctica (90 S). The data set used by them was continuous (24 hours a day) and was taken during austral winters mainly during mid April to late August. The linear trend component seen by them in the OH rotational temperature time series is statistically not significant and is about 1 ± 2 K/decade. As stated in section 2.2.1, Remsberg [2007, 2008] has also analyzed long term changes in temperature on the basis of a 14+ year time series ( ) of HALOE sunrise and sunset T (p) data of temperatures at pressure level of hpa for latitude zones covering the tropical Southern Hemisphere. This satellite data revealed a cooling trend ranging from 0.4 to 1.0 K/decade with good probability of occurrence. [14] Airglow intensities and rotational temperatures of the OH(6 2) and O2b(0 1) bands acquired at El Leoncito (32 S, 69 W) provided good annual coverage in 1998 to 2002, 2006, and 2007, with between 192 and 311 nights of observation per year [Reisin and Scheer, 2009]. Reisin and Scheer [2009] derived the seasonal variations during each of these 7 years in airglow brightness and temperatures at altitudes of 87 and 95 km. From 1998 to 2001, seasonal variations are similar enough so that they can be well represented by a mean climatology for each parameter. On the other hand, this climatology does not agree with what is usually observed at other sites, maybe due to the particular orographic conditions at El Leoncito. With respect to the last three fully documented years (2002, 2006, and 2007), the similarity from year to year deteriorates, and there are greater differences in the seasonal behavior which suggest the buildup of a new regime of intraseasonal variability, with a possible relationship to corresponding changes in wave activity. Reisin and Scheer [2002] have reported a negligible linear trend at about 95 km altitude. Preliminary trend results for the most recent data from El Leoncito based on the self consistent data subset from 2006 to early 2011 are approximately consistent with earlier short term trend results as reported by Reisin and Scheer [2002]. The analysis of a much longer combined data set from 1998 to 2011 from this station is still under way. [15] Clemesha et al. [2004, 2005] analyzed the data obtained by lidar measurements of the vertical distribution of atmospheric sodium covering a period of around 30 years ( ) and reported the negligible net long term trend in the centroid height of the sodium layer in Brazil (23 S, 46 W) for the entire period which are representative of the height range km. How the sodium layer is representative of the height range km is explained elsewhere [Clemesha et al., 2004, 2005]. This result does not confirm the significant negative trend reported earlier by Clemesha et al. [1997] on the basis of the same observations made over for a slightly shorter period of time which were cited in the review paper by Beig et al. [2003]. This discrepancy was mainly attributed to a positive trend in recent years which has compensated the earlier period negative trend and hence made trends over the 30 years negligible. However, the indirectly derived temperature trend estimate of Clemesha et al. [2004, 2005] is given for a broad height range. Temperature trends have significant vertical structure in the middle atmosphere as noticed from the discussion in section Hence, such results should be visualized as an indicator of tendency of trends rather than considering the absolute magnitude. [16] The SABER data set offers the opportunity to examine trends of temperature in the MLT region. SABER observes emission from the 15 and 4.3 mm bands of carbon dioxide and derives temperature (and carbon dioxide) from these. A key factor in the SABER temperature retrievals is the ability to derive temperature in the MLT despite the departure from local thermodynamic equilibrium (LTE) in the vibration rotation bands of the carbon dioxide molecule [Mertens et al., 2002, 2004; Kutepov et al., 2006]. The retrieval of temperatures in the stratosphere is essentially unaffected by the departure from LTE in the mesosphere. The MLT temperatures derived from the above mission is found to be up to 4 K cooler in 2006 than in These results are coincident with the declining phase of the present solar cycle and are suggestive of MLT cooling associated with the solar cycle. Mlynczak et al. [2010] have presented 7 years of observations of the radiative cooling in the thermosphere as observed by the SABER instrument on the TIMED mission. Decreases in atomic oxygen between 2002 and 2008 likely contribute to the observed decrease in both NO and CO 2 cooling. The variability of the cooling has several fundamental consequences. First, the larger equator to pole gradient in cooling by NO, and its variability, implies a potential link to dynamics and transport as the equator to pole gradient in net heating influences the largescale circulation. The observed weakening of the equator to pole gradient in radiative cooling over these 7 years is a strong indication that the large scale thermospheric dynamics have also weakened. In addition, the larger cooling at the poles than at low latitudes in the thermosphere is consistent with the observed cooling in the mesosphere and stratosphere. 3. Discussion and Conclusions [17] In the tropics, trend community has been enriched with the results reported from quite a few reliable data sets from lidar and satellite for the past 2 decades. These new tropical trend results, obtained mainly from the data of past 2 decades, indicate a cooling trend, but magnitude is found to be smaller than the earlier results reported mainly from rocketsonde measurements carried out during the late 60s to early 90s. This cooling trend increases with height in the mesosphere and becomes around 3 K/decade around 70 km but with an uncertainty of about ±1 2 K. The mesospheric temperature trends for the midlatitudes are consistent with the earlier conclusion of a cooling of a few degrees per decade in the lower mesosphere and midmesosphere. The most noticeable addition in the mesospheric temperature trend research is the robust analysis of HALOE data for the entire duration of 14+ years. The cooling trends from the HALOE data set are now found to be significant at most 6of9

7 latitudes of the middle and lower mesosphere. They range from 1 K/decade at low latitudes to about 3 K/decade at the middle latitudes [Remsberg, 2008, 2009]. Values of the order of 1 K/decade are reasonably consistent with those reported from the lidar measurements at low latitudes. At middle latitudes, there is reasonable agreement in the middle and upper mesosphere with published values from rocketsondes and lidar measurements of the preceding 2 decades. However, the HALOE trends are smaller in the lower mesosphere. This later may be due to the fact that earlier comparative results were obtained when the decreasing upper stratosphere ozone was an added factor for the total radiative cooling response at those altitudes. The cooling rates diagnosed from HALOE are generally larger than those from models for the middle latitudes of the upper mesosphere. [18] Quite a few important advancements have been made after Beig et al. [2003] on the mesopause region temperature trend. (1) There is an indication of weak negative trends in the mesopause region temperature, which is a drift from earlier perception of no trend feature in this region. (2) This result is more consistent with model simulations, the first indication of a break in linear trends at the mesopause region and a signal that trend is likely to be more stronger (toward negative) in recent time as compared to the past. (3) The trend in the entire MLT region may not be identical in all seasons and is likely to have even monthly variability on long term scale. (4) The final advancement is the importance of dynamics, particularly the role of gravity waves in understanding the variability in mesopause region trends, parameterization of which in the upper atmospheric new generation model still remains a potential issue. These issues are elaborated further below. [19] In the mesopause region, a noticeable development during the past decade has been the tendency of shift from near zero trends to negative trend. Growing number of model studies during the past decade have advanced to a level where they started to reproduce near zero trend with an inclination toward negative trends near mesopause region temperature. The modeling community was working hard to reproduce the zero trend features as emerged from a majority of observations until recently but was unable to confirm it fully. However, now since most recent data started to show the tendency of shift from near zero to negative trends in the mesopause region, model results can be considered as relatively more consistent with data. Another critical aspect which should always be kept in mind is that trend analysis is a snapshot of the time period covered. Hence, while comparing various long term trend results, it should be noted that the period of measurements is not identical in all results reported by different workers as summarized in this paper. It has also been common practice to analyze the data with a term that varies monotonically with time. The linear regression coefficient is what is referred to as long term trend. Strictly speaking, trends are often not quite linear. If the long term behavior is substantially nonmonotonic or oscillatory, the term long term change is used. However, in many cases, the linear trend approximation is sufficient, and in other cases, linear approximation is used for easier comparison with trends in other parameters. A linear trend may not continue indefinitely, and in reality a quasi stable trend should begin and may end within a certain period of time. Recently, Offermann et al. [2010] have shown that there is a break in mesopause region trends in Similarly, trend results reported using Russian data [Golitsyn et al., 2004; Semenov et al., 2002] at the MLT region are obtained in linear approximation for the period from the end of the 1950s until present time, while the majority of the other data are obtained in recent decades. Analysis of such a data series may be characterized by a sophisticated mathematical curve but may often be approximated by the so called piecewise contiguous linear trend model [Weatherhead et al., 2002; Reinsel et al., 2005], wherein essentially different linear trends are fit to the data within different time intervals, the latter also being model parameters. The upper atmospheric linear trends should probably be understood in a similar way. However, it is a little difficult to account for the possible tendency of variation in rate of change in MLT temperature over the recent years as compared to initial data because of limited sampling and limitation in length of time series. Application of a break analysis trend approach like the wavelet transform technique or any other has to be made with care, as the trend necessarily has a noninfinite period limited by the length of the data set or the length of the interval of the stationary linear trend existence. [20] Similarly, the seasonal differences near the mesopause region with convincing interpretation still need to be understood if some of the observational records contradicting the majority of trend results are to be believed. The role of dynamics, particularly the role of gravity wave and its breaking, is very important to better understand the temperature trend features reported in this paper. Our present knowledge on the parameterization of gravity wave in the mesospheric region is far from satisfactory. Gravity wave (GW) breaking influences effectively the distributions of chemical species and the vertical diffusive transport of heat in the upper mesosphere lower thermosphere. Strengthening of the GW drag and related diffusion leads to a cooling of the upper mesosphere and of the mesopause. The combined effects of the greenhouse gas concentration increase and of the GW drag and diffusion strengthening may be a major cause of the observed distinct long term trend in temperature of the mesospheric region. [21] Satellite measurements of polar mesospheric clouds have the potential to address some of the open issues discussed in this paper. It may be possible to use satellite data to investigate long term changes in the start or length of the PMC season or the lowest latitude at which PMCs are regularly detected. A better understanding of the magnitude and phase of diurnal variations is necessary to understand variations in PMC occurrence and brightness between different data sets. Further study of long term PMC trends is needed to identify the most useful way in which to present these results, as well as the appropriate numerical values. A careful analysis of the historical PMC and NLC data record for evidence of additional PMC enhancements due to rocket launches is also needed to validate the magnitude of derived trends. Identification of secular trends in PMC source mechanisms is more problematic than the determination of solar cycle effects because the data set length required for statistical confidence is inversely proportional to the magnitude of the trend. 7of9

8 [22] The MLT temperatures as provided by SABER data of the TIMED mission are up to 4 K cooler in 2006 than in 2002 [Mlynczak et al., 2010]. These results are coincident with the declining phase of the present solar cycle and are suggestive of MLT cooling associated with the solar cycle. More definitive results that account properly for the temporal sampling pattern of the TIMED satellite are emerging to support this tendency and hence are likely to be supportive of a cooling trend as revealed by majority of the long term trend results reported for this region. Recent GCM calculations as well as some data sets [Offermann et al., 2004; Bremer and Berger, 2002] indicate dynamical and thermal changes in the middle atmosphere as consequences of anthropogenic CO 2 and O 3 changes. Some models indicate that the trend seen before the year 1980 or the 1990s was different from the trend after that time period; such a possibility is even supported by the observational data in recent time [Offermann et al., 2010]. This is difficult to understand with anthropogenic influences, and it therefore remains to be determined as to what extent the longterm variations seen may be man made or are part of a natural climate fluctuation. Hence, trend results are likely to be dependent on the time interval chosen for analysis, which could be one of the possible reasons for the different trend results obtained by different investigators. An attempt has also been made by us to examine the possible impact on mesospheric temperature trends which might have been caused by ozone recovery in the stratosphere. The answer to this question is in nonaffirmative so far, and we do not find any convincing signal which can be correlated with ozone trend reversal in the stratosphere. However, it could be a potential scientific problem as the ozone recovery become faster with elapsing time. [23] Understanding and interpreting the causes of atmospheric trends requires a fundamental understanding of the energy budget. This is essentially the focus of the entire field of tropospheric climate science, which is seeking to determine the extent to which human activities are altering the planetary energy balance through the emission of greenhouse gases and pollutants even in the upper atmosphere. The TIMED mission is fast emerging to quantitatively assess the energy budget in the MLT for the first time and to help us to provide the answers to some of the outstanding questions in this direction. We really need a consistent picture from the modeling community to back our future measurement recommendations. As the TIMED mission continues, data derived from SABER will become important in assessing long term changes due to the increase of carbon dioxide in the atmosphere. Hence, the topic of MLT trends and climate change continues to remain a potential problem to explore more rigorously in future. [24] Acknowledgments. The author is grateful to the temperature trend community for providing their valuable input. Helpful discussions with Jürgen Scheer are gratefully acknowledged. [25] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. References Azeem,S.M.I.,G.G.Sivjee,Y. I. Won, and C. Mutiso (2007), Solar cycle signature and secular long term trend in OH airglow temperature observations at South Pole, Antarctica, J. Geophys. Res., 112, A01305, doi: /2005ja Batista, P. P., B. R. Clemesha, and D. M. Simonich (2009), A fourteen year monthly climatology and trend in the km altitude range from Rayleigh lidar temperature measurements at a low latitude station, J. Atmos. Sol. Terr. Phys., 71, , doi: /j.jastp Beig, G. (2006), Trends in the mesopause region temperature and our present understanding An update, Phys. Chem. Earth, 31, 3 9, doi: /j.pce Beig, G., and S. Fadnavis (2001), In search of greenhouse signals in the equatorial middle atmosphere, Geophys. Res. Lett., 28, , doi: /2001gl Beig, G., et al. (2003), Review of mesospheric temperature trends, Rev. Geophys., 41(4), 1015, doi: /2002rg Bittner, M., D. Offermann, H. H. Graef, M. Donner, and K. Hamilton (2002), An 18 year time series of OH rotational temperatures and middle atmosphere decadal variations, J. Atmos. Sol. Terr. Phys., 64, , doi: /s (02) Bremer, J., and U. Berger (2002), Mesospheric temperature trends derived from ground based LF phase height observations at mid latitudes: Comparison with model simulations, J. Atmos. Sol. Terr. Phys., 64, , doi: /s (02) Clemesha, B. R., P. P. Batista, and D. M. Simonich (1997), Long term and solar cycle changes in the atmospheric sodium layer, J. Atmos. Sol. Terr. Phys., 59, , doi: /s (96) Clemesha, B. R., D. Simonich, P. Batista, T. Vondrak, and J. M. C. Plane (2004), Negligible long term trend in the upper atmosphere at 23 S, J. Geophys. Res., 109, D05302, doi: /2003jd Clemesha, B., H. Takahashi, D. Simonich, D. Gobbi, and P. Batista (2005), Experimental evidence for solar cycle and long term changes in the lowlatitude MLT region, J. Atmos. Sol. Terr. Phys., 67, , doi: / j.jastp DeLand, M. T., E. P. Shettle, G. E. Thomas, and J. J. Olivero (2006), Aquarter century of satellite PMC observations, J. Atmos. Sol. Terr. Phys., 68, Espy, P. J., and J. Stegman (2002), Trends and variability of mesospheric temperature at high latitudes, Phys. Chem. Earth, 27, French, W. J. R. (2010), Solar cycle and linear trend analysis of the 15 year hydroxyl rotational temperature record over Davis station, Antarctica, Geophys. Res. Abstr., 12, Abstract EGU French, W. J. R., and G. B. Burns (2004), The influence of large scale oscillations on long term trend assessment in hydroxyl temperatures over Davis, Antarctica, J. Atmos. Solar. Terr. Phys., 66, French, W. J. R., and F. J. Mulligan (2010), Stability of temperatures from TIMED/SABER v1.07 ( ) and Aura/MLS v2.2 ( ) compared with OH(6 2) temperatures observed at Davis Station, Antarctica, Atmos. Chem. Phys., 10, 11,439 11,446, doi: /acp French, W. J. R., G. B. Burns, and P. J. Espy (2005), Anomalous winter hydroxyl temperatures at 69 S during 2002 in a multiyear context, Geophys. Res. Lett., 32, L12818, doi: /2004gl Gadsden, M. (1998), The north west Europe data on noctilucent clouds: A survey, J. Atmos. Sol. Terr. Phys., 60, , doi: / S (98) Gadsden, M. (2002), Statistics of the annual counts of nights on which NLCs were seen, paper presented at Mesospheric Clouds 2002, Br. Astron. Assoc., Perth, Scotland, Aug. Golitsyn, G. S., A. I. Semenov, N. N. Shefov, and V. Y. Khomich (2004), The response of middle latitudinal atmospheric temperature on the solar activity during various seasons, paper presented at 3rd International Workshop on Long Term Changes and Trends in the Atmosphere, Int. Assoc. of Geomagn. and Aeron., Sozopol, Bulgaria. Keckhut, P., F. J. Schmidlin, A. Hauchecorne, and M. L. Chanin (1999), Stratospheric and mesospheric cooling trend estimates from U.S. rocketsondes at low latitude stations (8 S 34 N), taking into account instrumental changes and natural variability, J. Atmos. Sol. Terr. Phys., 61, , doi: /s (98) Kirkwood, S. (2004), Trends in PMSE and noctilucent clouds, paper presented at 3rd International Workshop on Long Term Changes and Trends in the Atmosphere, Int. Assoc. of Geomagn. and Aeron., Sozopol, Bulgaria. Kirkwood, S., and K. Stebel (2003), Influence of planetary waves on noctilucent cloud occurrence over NW Europe, J. Geophys. Res., 108(D8), 8440, doi: /2002jd Kutepov, A. A., A. G. Feofilov, B. T. Marshall, L. L. Gordley, W. D. Pesnell, R. A. Goldberg, and J. M. Russell III (2006), SABER temperature observations in the summer polar mesosphere and lower thermosphere: Importance of accounting for the CO 2 n 2 quanta V V exchange, Geophys. Res. Lett., 33, L21809, doi: /2006gl of9

9 Laštovička, J., R.A. Akmaev, G. Beig, J. Bremer, and J. T. Emmert (2006), Emerging pattern of global change in the upper atmosphere, Science, 314(5803), , doi: /science Laštovička, J., R. A. Akmaev, G. Beig, J. Bremer, J. T. Emmert, C. Jacobi, M. J. Jarvis, G. Nedoluha, Y. I. Portnyagin, and T. Ulich (2008), Emerging pattern of global change in the upper atmosphere and ionosphere, Ann. Geophys., 26, , doi: /angeo Lowe, R. P. (2002), Long term trends in the temperature of the mesopause region at mid latitude as measured by the hydroxyl airglow, paper presented at the 276th WE Heraeus Seminar on Trends in the Upper Atmosphere, Leibniz Inst. of Atmos. Phys., Kühlungsborn, Germany. Lowe, R. P. (2004), The temperature trend near the mesopause as measured using the hydroxyl airglow, Eos Trans. AGU, 85(17), Jt. Assem. Suppl, Abstract SA52A 03. Mertens, C. J., M. G. Mlynczak, M. Lopez Puertas, and E. E. Remsberg (2002), Impact of non LTE processes on middle atmospheric water vapor retrievals from simulated measurements of 6.8 mm Earth limb emission, Geophys. Res. Lett., 29(9), 1288, doi: /2001gl Mertens, C. J., et al. (2004), SABER observations of mesospheric temperatures and comparisons with falling sphere measurements taken during the 2002 summer MaCWAVE campaign, Geophys. Res. Lett., 31, L03105, doi: /2003gl Mlynczak, M. G., et al. (2010), Observations of infrared radiative cooling in the thermosphere on daily to multiyear timescales from the TIMED/ SABER instrument, J. Geophys. Res., 115, A03309, doi: / 2009JA Offermann, D., M. Donner, P. Knieling, and B. Naujokat (2004), Middle atmosphere temperature changes and the duration of summer, J. Atmos. Sol. Terr. Phys., 66, , doi: /j.jastp Offermann, D., P. Hoffmann, P. Knieling, R. Koppmann, J. Oberheide, and W. Steinbrecht (2010), Long term trends and solar cycle variations of mesospheric temperature and dynamics, J. Geophys. Res., 115, D18127, doi: /2009jd Reinsel, G. C., A. J. Miller, E. C. Weatherhead, L. E. Flynn, R. M. Nagatani, G. C. Tiao, and D. J. Wuebbles (2005), Trend analysis of total ozone for turnaround and dynamical contributions, J. Geophys. Res., 110, D16306, doi: /2004jd Reisin, E. R., and J. Scheer (2002), Searching for trends in mesopause region airglow intensities and temperatures at El Leoncito, Phys. Chem. Earth, 27, Reisin, E. R., and J. Scheer (2009), Evidence of change after 2001 in the seasonal behaviour of the mesopause region from airglow data at El Leoncito, Adv. Space Res., 44(3), , doi: /j.asr Remsberg, E. E. (2007), A reanalysis for the seasonal and longer period cycles and the trends in middle atmosphere temperature from the Halogen Occultation Experiment, J. Geophys. Res., 112, D09118, doi: / 2006JD Remsberg, E. (2008), On the observed changes in upper stratospheric and mesospheric temperatures from UARS HALOE, Ann. Geophys., 26, , doi: /angeo Remsberg, E. E. (2009), Trends and solar cycle effects in temperature versus altitude from the Halogen Occultation Experiment for the mesosphere and upper stratosphere, J. Geophys. Res., 114, D12303, doi: / 2009JD Remsberg, E. E., and L. E. Deaver (2005), Interannual, solar cycle and trend terms in middle atmospheric temperature time series from HALOE, J. Geophys. Res., 110, D06106, doi: /2004jd Remsberg, E. E., P. P. Bhatt, and L. E. Deaver (2002), Seasonal and longerterm variations in middle atmosphere temperature from HALOE on UARS, J. Geophys. Res., 107(D19), 4411, doi: /2001jd Romejko, V. A., P. A. Dalin, and N. N. Pertsyev (2003), Forty years of noctilucent cloud observations near Moscow: Database and simple statistics, J. Geophys. Res., 108(D8), 8443, doi: /2002jd Semenov, A. I. (2004), The long term temperature trends in the MLT region at the mid latitudes, paper presented at 3rd International Workshop on Long Term Changes and Trends in the Atmosphere, Int. Assoc. of Geomagn. and Aeron., Sozopol, Bulgaria. Semenov, A. I., N. N. Shefov, E. V. Lysenko, G. V. Givishvili, and A. V. Tikhonov (2002), The seasonal peculiarities of behavior of the long term temperature trends in the middle atmosphere at the mid latitudes, Phys. Chem. Earth, 27, She, C. Y., and A. Krueger (2004), Impact of natural variability in the 11 year mesopause region temperature observation over Fort Collins, CO (41 N, 105 W), Adv. Space Res., 34, , doi: /j.asr She, C. Y., J. Sherman, J. D. Vance, T. Yuan, Z. Hu, B. P. Williams, K. Arnold, P. Acott, and D. A. Krueger (2002), Evidence of solar cycle effect in the mesopause region: Observed temperatures in 1999 and 2000 at 98.5 km over Fort Collins, CO (41 N, 105 W), J. Atmos. Sol. Terr. Phys., 64, , doi: /s (02) She, C. Y., D. A. Krueger, R. Akmaev, H. Schmidt, E. Talaat, and S. Yee (2009), Long term variability in mesopause region temperatures over Fort Collins, Colorado (41 N, 105 W) based on lidar observations from 1990 through 2007, J. Atmos. Sol. Terr. Phys., 71, , doi: / j.jastp Sigernes, F., N. Shumilov, C. S. Deehr, K. P. Nielsen, T. Svenøe, and O. Havnes (2003), Hydroxyl rotational temperature record from the auroral station in Adventdalen, Svalbard (78 N, 15 E), J. Geophys. Res., 108(A9), 1342, doi: /2001ja Sridharan, S., P. V. Prasanth, and Y. B. Kumar (2009), A report on long term trends and variabilities in middle atmospheric temperature over Gadanki (13.5 N, 79.2 E), J. Atmos. Sol. Terr. Phys., 71, doi: / j.jastp Weatherhead, E. C., A. J. Stevermer, and B. E. Schwartz (2002), Detecting environmental changes and trend, Phys. Chem. Earth, 27, G. Beig, Indian Institute of Tropical Meteorology, Pune , India. (beig@tropmet.res.in) 9of9