International audienceIn recent times it has become increasingly clear thatreleases of trace gases from human activity have a potentialfor causing change in the upper atmosphere. However,our knowledge of systematic changes and trends inthe temperature of the mesosphere and lower thermosphereis relatively limited compared to the Earths loweratmosphere, and not much effort has been made to synthesizethese results so far. In this article, a comprehensivereview of long-term trends in the temperature of the regionfrom 50 to 100 km is made on the basis of the availableup-to-date understanding of measurements and model calculations.An objective evaluation of the available datasets is attempted, and important uncertainly factors arediscussed. Some natural variability factors, which arelikely to play a role in modulating temperature trends,are also briefly touched upon. There are a growing numberof experimental results centered on, or consistent with,zero temperature trend in the mesopause region (80–100km). The most reliable data sets show no significant trendbut an uncertainty of at least 2 K/decade. On the otherhand, a majority of studies indicate negative trends inthe lower and middle mesosphere with an amplitude ofa few degrees (2–3 K) per decade. In tropical latitudesthe cooling trend increases in the upper mesosphere.The most recent general circulation models indicateincreased cooling closer to both poles in the middlemesosphere and a decrease in cooling toward the summerpole in the upper mesosphere. Quantitatively, thesimulated cooling trend in the middle mesosphere producedonly by CO2 increase is usually below the observedlevel. However, including other greenhouse gasesand taking into account a “thermal shrinking” of theupper atmosphere result in a cooling of a few degreesper decade. This is close to the lower limit of the observednonzero trends. In the mesopause region, recentmodel simulations produce trends, usually below 1 K/decade,that appear to be consistent with most observationsin this regio
We analyse data from weekly rocket temperature soundings up to 75 km carried out since mid‐1960s at polar (Heiss Island, 80.6°N and Molodezhnaya, 67.7°S), temperate (Volgograd, 48.7°N and Balkhash, 46.8°N) and tropical (Thumba, 8.5°N) latitudes. All records show significant cooling which is of the order of a few degrees K at 30–40 km, 10 K at 50 km and 20 K at 60–70 km. In the mesosphere the temperature trends are estimated from the hydroxyl rotational temperature records which start in 1957 at Zvenigorod (55.7°N) and Abastumani (41.8°N). These are related to the mesopause at 87 km and also show a cooling of about 30 K during the time. The cooling is qualitatively consistent with increasing concentration of greenhouse gases but may also reflect the changing chemistry of stratosphere and mesosphere, which is clearly seen in increasing emission intensities of hydroxyl during the last decades. Therefore the stratosphere and mesosphere may be regions with the strongest signals of global change.
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