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MYSTERIES OF THE UPPER ATMOSPHERE

Posted on: August 23, 2020

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THIS POST IS A CRITICAL REVIEW OF THE ARTICLE

ANTARCTIC RESEARCH UNLOCKS MYSTERIES OF THE UPPER ATMOSPHERE, Home News and media 2020 Antarctic research unlocks mysteries of the upper atmosphere 27 JULY 2020

A long-term study in Antarctica of the upper atmosphere in the coldest place on Earth reveals several new discoveries about how the region is responding to increasing greenhouse gases:

PART-1: WHAT THE ARTICLE SAYS

  1. The upper atmosphere is cooling 10 times faster than the average rate of global warming at the Earth’s surface. The temperatures change is in response to both the solar activity cycle and carbon dioxide emissions from human activities. Temperatures also vary in the upper atmosphere on a cycle of roughly four years.
  2. The findings also promise to shed new light on the variability of the highest clouds on Earth, the spectacular noctilucent or ‘night shining’ clouds that are seen as a ‘miner’s canary’ of climate change.
  3. Scientists with the Australian Antarctic Division (AAD) have recorded more than 600,000 measurements over the last 24 years, 87 kilometres above Australia’s Davis research station. These comprise one of the longest temperature records available for this region of the atmosphere.
  4. Known as the hydroxyl airglow layer, it contains molecules formed by the reaction of hydrogen and ozone and emits a continuous ‘airglow’ in the night sky.
  5. The measurements of the infrared spectrum from the airglow determine the layer’s temperature and are recorded every seven minutes throughout the Antarctic night.
  6. Conversely to the warming that occurs in the lower atmosphere, the upper atmosphere cools in response to greenhouse gases increases.
  7. The density of the upper atmosphere is so low, carbon dioxide actually radiates heat away into space, rather than trapping it.
  8. The climate of the Antarctic upper atmosphere is particularly sensitive to changes in atmospheric composition.
  9. The hydroxyl airglow layer provides a natural means of sampling the upper atmosphere from the ground and enables us to monitor the rate of cooling induced by carbon dioxide increases.
  10. The study’s measurements date back to the early 1990s and have used the same, robust, precisely calibrated scanning spectrometer to scan the infrared spectrum produced by hydroxyl airglow.
  11. The Davis research station measurements show that the cooling rate was 1.2C per decade over the last 24 years, approximately 10 times the rate of average global warming of 0.1C per decade. The upper atmosphere has already cooled 3C in the 25 years since measurements began in 1995.
  12. The Davis record clearly shows long-term temperature variation in the upper atmosphere caused by the sun’s activity cycle, but more importantly, overlying this is an unequivocal cooling trend that is consistent with the effect of increasing carbon dioxide emissions from human activities.
  13. The duration of these continuous measurements and their location in Antarctica make them unique. There are many challenges in keeping the instrument operating, supported and well calibrated over a quarter of a century now, but it is something the AAD has done well. The results match those from climate model simulations and satellite measurements and provide a more precise, complete picture of the long-term temperature trend.
  14. Another revelation from the study is that the polar atmosphere above Davis has been undergoing a roughly four-yearly temperature fluctuation cycle of three to four degrees Celsius. This finding is significant. This variation, called the Quasi-Quadrennial Oscillation or QQO, appears to be linked to interactions between the ocean and atmosphere in the Southern Hemisphere, and its effects are apparent in the upper atmosphere of both the Antarctic and Arctic. The QQO discovery further highlights how interconnected the global atmosphere is and the importance of long-term and precise measurements for monitoring and understanding the climate system.
  15. This was the first time the effects of the oscillation had been identified in the global upper atmosphere, and it had implications for scientists’ ability to model climate processes, detect long-term change, and understand its effects on other upper atmosphere phenomena such as noctilucent or ‘night shining’ clouds.
  16. Noctilucent clouds are ice clouds that glow in the dark. At a height of 83 km, noctilucent clouds (also known as NLCs) are the highest in the atmosphere, on the edge of space. Composed of ice crystals, they need extremely low temperatures (around minus 130°C) to form, and only become visible after the sun has set on the lower levels of the atmosphere. They appear to glow a pale blue, still illuminated by the sun, in a night sky. Measurements from the long-term study provide new precise information on changes in the upper atmosphere to test climate models, and suggest that the NLCs in this region are likely to show particular variability in extent and brightness that has not previously been examined. Enhanced cooling in the upper atmosphere increasingly provides suitable conditions for NLCs to form, so they are expected to become brighter and more extensive in the years ahead.
  17. With the cooling conditions linked to carbon dioxide emissions that drive warming elsewhere, these clouds, although spectacular, are also harbingers of change and have been referred to as the canary in the coal mine of climate change.
  18. Website for the Network for Detection of Mesospheric Change: https://ndmc.dlr.de/
  19. The second of two papers on the research has just been published in the European journal, Atmospheric Chemistry and Physics.

PART-2: CRITICAL COMMENTARY

  1. The bibliography provided below shows that cooling of the mesosphere has been studied for the last 40 years with multiple authors making the connection to atmospheric CO2 concentration in climate models while others paying more attention to natural factors. In any case, what we find in the bibliography is that the novel discovery by Australian scientists of mesosphere cooling and its possible relationship to GHG warming of the lower troposphere by rising atmospheric CO2 concentration has been studied for the last 40 years.
  2. Significantly, what we find in these studies is the acknowledgement of large uncertainties not only in the amount of cooling involved but also as to the relative role of the various possible causes that include the claimed role for fossil fuel emissions as well as natural factors. The pretense to certainty in the Australian study is inconsistent with the literature unless it is a novel discovery not well known by previous researchers.
  3. Even as we extend the assumed impact of fossil fueled growth in atmospheric CO2 concentration found in climate models to the upper atmosphere, it is worth noting that the assumed global warming effect of rising atmospheric CO2 concentration is itself not well understood as seen in the pre-occupation of climate science with the “emergent constraint” issue.
  4. The emergent constraint literature reveals an unsettled struggle with large uncertainties in the values of the key parameters in this theory, namely the equilibrium climate sensitivity (ECS) and transient climate response to cumulative emissions (TCR). This issue is discussed in a related post. https://tambonthongchai.com/2020/08/22/emergent-constraints-on-ecs-tcr/ . The emergent constraint issue shows that the AGW theory of fossil fuel driven global warming is an unsettled work in progress.
  5. A further note of caution in the interpretation of fossil fuel emissions at the root of the causation chain of all of these effects is the absence of empirical evidence that the observed changes in atmospheric CO2 concentration are caused by fossil fuel emissions. This issue is discussed in related posts on this site.
  6. LINK#1: https://tambonthongchai.com/2018/12/19/co2responsiveness/
  7. LINK#2: https://tambonthongchai.com/2018/05/31/the-carbon-cycle-measurement-problem/
  8. LINK#3: https://tambonthongchai.com/2020/06/10/a-monte-carlo-simulation-of-the-carbon-cycle/
  9. LINK#4;
  10. https://tambonthongchai.com/2018/08/22/stratospheric-cooling/

IN CONCLUSION: Before extending the assumed effect of fossil fuel emissions to the mesosphere by way of the warming effect of rising atmospheric concentration caused by fossil fuel emissions, empirical evidence must be provided to show that fossil fuel emissions cause atmospheric CO2 concentration to rise and that rising atmospheric CO2 concentration causes the lower troposphere to warm. As seen in the related posts linked above, no such evidence exists and climate scientists acknowledge the uncertainty in these relationships in terms of their “emergent constraint” research agenda. Cooling of the upper atmosphere and warming of the lower atmosphere may well be causally related but that does not imply a role for fossil fuel emissions

It is shown in a related post that the assumed cooling response of the stratosphere to warming in the lower troposphere is not supported by the data https://tambonthongchai.com/2018/08/22/stratospheric-cooling/

DET-TROP-STRAT

PART-3: THE RELEVANT BIBLIOGRAPHY

  1. Hernandez, G. “Climatology of the upper mesosphere temperature above South Pole (90 S): Mesospheric cooling during 2002.” Geophysical research letters 30.10 (2003). Polar mesospheric temperature measurements obtained above South Pole from 1991 through 2002 are shown, and their long‐term behavior ‐climatology‐ has been deduced. The variation of this temperature during the polar night shows an annual amplitude of 14C about the mean yearly value. A strong and highly significant correlation is found between the mean yearly temperature and the solar activity ‐expressed as the F10.7 flux‐, giving a 13 K change per 100 unit F10.7 change. The mean measured 2002 polar night temperatures are about 35C cooler than the mean for the previous 11 years and well outside their excursions about this mean. This observed mesospheric cooling is in keeping with the disturbed lower atmosphere reported for this period. Other effects from the latter include both a smaller austral ozone depletion hole than has been recorded in previous years and higher than average minimum temperatures in the south polar region. The observed mesospheric cooling resembles the cooling observed at Northern midlatitudes during stratospheric warmings. LINK TO FULL TEXT: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003GL016887
  2. Dickinson, Robert E. “Infrared radiative cooling in the mesosphere and lower thermosphere.” Journal of atmospheric and terrestrial physics 46.11 (1984): 995-1008. This paper discusses the current status of calculating infrared cooling by CO2 in the mesosphere and lower thermosphere. It is desirable to have fast but accurate procedures for use in dynamic models. The most difficult region is from 70 to 90 km, where cooling rates are strongly influenced or, in the case of the summer mesopause region, dominated by the absorption of radiation emitted by underlying layers, with the hot bands and isotopic bands playing a significant role. A three-energy-level model is derived for the excited population levels of a CO2 molecule. Vibrational-vibrational coupling between isotopes is also included as significant. Results from model calculations for cooling rates and NLTE source functions are presented. Global average infrared cooling rates appear to be in reasonable balance with solar heating rates, considering the uncertainties in calculating both these terms. Radiative cooling rates by CO2 above 100 km are strongly dependent on atomic oxygen concentrations and on the rate of energy exchange between atomic oxygen and CO2. Likewise, NO cooling, which is important above 120 km, is proportional to atomic oxygen concentrations. Since CO2, NO and O concentrations can all vary with motions, these dependencies suggest interesting feedbacks to atmospheric dynamics. FULL TEXT: https://www.sciencedirect.com/science/article/abs/pii/0021916984900060
  3. Liu, Han‐Li, and Maura E. Hagan. “Local heating/cooling of the mesosphere due to gravity wave and tidal coupling.” Geophysical research letters 25.15 (1998): 2941-2944. Numerical experiments in this study show that the tidal wind may have strong impacts on the stability of the gravity wave and therefore significantly affects the breaking of the gravity wave. This enhances the local dynamical cooling and turbulence heating, and produces descending heating/cooling structures, which are similar to recent lidar observations. The propagating phase of such structures is dependent on the descending phase of the tidal wave and the gravity wave breaking level. The maximum heating corresponds closely with a negative or positive shear of the accelerated flow, depending on the gravity wave propagation direction. FULL TEXT PDF https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/98GL02153
  4. Akmaev, R. A., and V. I. Fomichev. “Cooling of the mesosphere and lower thermosphere due to doubling of CO 2.” Annales Geophysicae. Vol. 16. No. 11. Springer-Verlag, 1998. A new parameterization of infrared radiative transfer in the 15-μm CO2 band has been incorporated into the Spectral mesosphere/lower thermosphere model (SMLTM). The parameterization is applicable to calculations of heating rates above approximately 15 km for arbitrary vertical profiles of the CO2 concentration corresponding to the surface mixing ratio in the range 150–720 ppm. The sensitivity of the mesosphere and lower thermosphere (MLT) to doubling of CO2 has been studied. The thermal response in the MLT is mostly negative (cooling) and much stronger than in the lower atmosphere. An average cooling at the stratopause is about 14 K. It gradually decreases to approximately 8 K in the upper mesosphere and again increases to about 40–50 K in the thermosphere. The cooling and associated thermal shrinking result in a substantial density reduction in the MLT that reaches 40–45% in the thermosphere. Various radiative, chemical, and dynamical feedbacks potentially important for the thermal response in the MLT are discussed. It is noted that the results of simulations are strikingly similar to observations of long-term trends in the MLT. This suggests that during the last 3–4 decades the thermal structure in the real upper atmosphere has undergone substantial changes driven by forcing comparable with that due to doubling of CO2.
  5. Vineeth, C., et al. “Highly localized cooling in daytime mesopause temperature over the dip equator during counter electrojet events: First results.” Geophysical research letters 34.14 (2007).  The first observations of lowering of mesopause temperature during Counter Electrojet (CEJ) events over a narrow region of ∼ ±150 km centered at around magnetic equator are presented. The daytime mesopause temperature is measured over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), India using the ground based Multiwavelength Dayglow Photometer. The unique meridional scanning capability of the instrument is extensively used in this study. A lowering of temperature by as much as ∼25 K has been observed during certain CEJ events, which includes a few partial CEJs. The gravity wave tidal interaction through vertically upward wind is proposed to be manifesting as lowering in the mesopause temperature and also as CEJ. These observations are ‘new’ and address to the issues concerning the vertical coupling processes prevailing in the equatorial Mesosphere Lower Thermosphere Ionosphere (MLTI) region. FULL TEXT https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007GL030298
  6. Allen, D. C., et al. “Radiative cooling near the mesopause. Nature 281.5733 (1979): 660-661. The heat budget of the region of the upper atmosphere near the mesopause at ∼85 km is determined by a balance between radiative, photochemical and dynamical effects leading to a very cold polar mesopause in the summer and a comparatively warm mesopause in the winter. There is a temperature minimum at the mesopause primarily because of the strong radiative cooling which occurs due to thermal emission by carbon dioxide in its v2 vibrational band at 15 µm wavelength. Above 80 km this band is no longer in local thermodynamic equilibrium (LTE) so the amount of cooling depends critically on the rate at which CO2 molecules are excited or relaxed by collision. Any estimate of radiative cooling for this region of the atmosphere, therefore, relies on knowledge of the collisional relaxation time, τ. The importance of the temperature dependence of τ was pointed out by Houghton1. Previous calculations used a wide range of values because no direct measurements of τ had been made in the appropriate temperature range. We show here that the effect on cooling rate calculations of using values of τ measured at temperatures down to 175 K. We also estimate a radiative relaxation time for the atmosphere near the mesopause and are able to draw conclusions as to the natural lifetime of any temperature perturbation at this altitude.
  7. Apruzese, John P., Darrell F. Strobel, and Mark R. Schoeberl. “Parameterization of IR cooling in a Middle Atmosphere Dynamics Model: 2. Non‐LTE radiative transfer and the globally averaged temperature of the mesosphere and lower thermosphere.” Journal of Geophysical Research: Atmospheres 89.D3 (1984): 4917-4926. An efficient technique for including non‐LTE radiative transfer effects has been added to our LTE IR cooling model. The physical processes of thermal and turbulent conduction, odd oxygen transport, gravity wave dissipation, and NO cooling are incorporated into the radiation model to calculate the globally averaged equilibrium temperature profile up to 120 km. The quantitative importance of each process is discussed. We find that gravity wave effects are crucial in cooling the lower thermosphere, which would otherwise be 15–100 K warmer. The magnitude of required gravity wave cooling provides a stringent upper limit on the globally averaged turbulent diffusion coefficient in the mesopause region of <106 cm2 s−1. In the 65‐ to 75‐km mesospheric region, IR cooling rates are uncertain primarily due to the lack of accurate CO2 hot band collisional activation rates. Without any gravity wave heating in this region, total IR cooling rates of 2 K d−1 are required for the computed equilibrium temperature profile to agree with observations, in contrast to the 3.5 K d−1 obtained with a two‐level‐atom non‐LTE source function. Gravity wave heating in this region is estimated at 0.5 K d−1. FULL TEXT PDF https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/JD089iD03p04917
  8. Akmaev, R. A., and V. I. Fomichev. “A model estimate of cooling in the mesosphere and lower thermosphere due to the CO2 increase over the last 3–4 decades.” Geophysical research letters 27.14 (2000): 2113-2116. Long‐term observations indicate a substantial cooling in the mesosphere and lower thermosphere (MLT) over the last 3–4 decades. Available model studies have primarily considered the effects of CO2 doubling expected to occur in the future. We present a benchmark estimate of radiative forcing in the MLT due to the increase of CO2 mixing ratio from 313 ppm to about 360 ppm (or by 15%) observed over the last four decades. The Spectral Mesosphere/Lower Thermosphere Model is employed for “retrocasting” the atmospheric response. As expected, the thermal response is predominantly negative. As a function of altitude, the cooling maximizes in the mesosphere at about 3 K, practically vanishes at 100–120 km, and grows to 10–15 K in the thermosphere. Although this vertical shape is remarkably consistent with various sets of observations, the magnitude of the cooling rate is smaller by about a factor of 2–10. This suggests that other mechanisms, e.g., the ozone depletion, might have contributed substantially to the negative temperature trend. FULL TEXT PDF: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GL011333
  9. Beig, G., et al. “Review of mesospheric temperature trends.” Reviews of Geophysics 41.4 (2003).  In recent times it has become increasingly clear that releases of trace gases from human activity have a potential for causing change in the upper atmosphere. However, our knowledge of systematic changes and trends in the temperature of the mesosphere and lower thermosphere is relatively limited compared to the Earth’s lower atmosphere, and not much effort has been made to synthesize these results so far. In this article, a comprehensive review of long‐term trends in the temperature of the region from 50 to 100 km is made on the basis of the available up‐to‐date understanding of measurements and model calculations. An objective evaluation of the available data sets is attempted, and important uncertainly factors are discussed. Some natural variability factors, which are likely to play a role in modulating temperature trends, are also briefly touched upon. There are a growing number of experimental results centered on, or consistent with, zero temperature trend in the mesopause region (80–100 km). The most reliable data sets show no significant trend but an uncertainty of at least 2 K/decade. On the other hand, a majority of studies indicate negative trends in the lower and middle mesosphere with an amplitude of a few degrees (2–3 K) per decade. In tropical latitudes the cooling trend increases in the upper mesosphere. The most recent general circulation models indicate increased cooling closer to both poles in the middle mesosphere and a decrease in cooling toward the summer pole in the upper mesosphere. Quantitatively, the simulated cooling trend in the middle mesosphere produced only by CO2 increase is usually below the observed level. However, including other greenhouse gases and taking into account a “thermal shrinking” of the upper atmosphere result in a cooling of a few degrees per decade. This is close to the lower limit of the observed nonzero trends. In the mesopause region, recent model simulations produce trends, usually below 1 K/decade, that appear to be consistent with most observations in this region. FULL TEXT https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2002RG000121

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