This article provides a review of papers relating to long-term trend analyses that have been published between 2018-2022, focusing on research related to the Earth’s mesosphere, thermosphere and ionosphere.



As in the case of the author’s prior review papers, this latest manuscript serves as a good primer on the state of research relating to observed and modelled trends to date. The author has evidently put in a significant amount of effort to synthesize, organize and contextualize often very disparate research papers. Although not presenting original work, this manuscript provides a useful summary guide on the main progress made in the field, and it will be useful and helpful to a broader scientific audience. As such it is relevant for publication in ACP.

However, the article needs some work prior to publication. General and specific comments are listed below. Due to the nature of the article, that is, it is an extended literature review, I feel that the volume and type of edits needed, prompt me to return for “major revision” rather than simply for “minor revision”. Some of these changes are quick changes, while others will require re-reading of cited papers in order to pull out additional necessary information. Once these changes and improvements are made, I recommend swift publication.

A general comment is that the manuscript text still needs some polishing and refinement in terms of language use and sentence structuring. Specific examples are provided below along with suggested changes. However, this is non-exhaustive. A strength of a review paper such as this would be to allow a reader to get an overview of a field of research, and to provide enough definitions and context to act as a standalone and informative piece. This review paper covers studies across a range of different foci; it would be helpful to define certain terms on first usage to make the paper more accessible to readers who may be experts on one part of the upper atmosphere, yet unfamiliar with another. 

Some sections of the manuscript currently read far too much like a list and would benefit from some additional explanatory text to help the reader place the results into context, to understand the gist of any issues/caveats mentioned in the study, and to improve the flow of the article. The author must make sure to sufficiently detail the latitude location/range, time period and altitude range covered in each of the studies cited. This is not always consistently done. 

Specific comments/suggestions:

Line 13: change “reviews main progress in investigations” to “reviews THE main progress MADE in investigations”. Similar editorial changes should be made throughout much of the manuscript to improve the flow and readability. I’ll include some suggestions below, but these are non-exhaustive.

Line 18-19: “The results on [..] detailed them further”. This sentence is currently a little unclear and hard to understand. I suggest providing clarification here on what “but develop and detailed them further” refers to. Alternatively I would suggest changing this to “The results on temperature trends in the MLT region are generally consistent with THOSE FROM PREVIOUS STUDIES. HOWEVER, NEW SIGNIFICANT FINDINGS INCLUDE…”.

Line 27: “evidence of clear” —> “evidence of A clear”

Line 28: “results in increasing” —> “results in AN increasing”

Line 29: need to define “foE” as first usage here.

Line 31: it would be perhaps useful to define the approximate latitude, as not all readers will be familiar with the latitude range that Europe extends across.

Line 39: Define “foF2, hmF2, TEC and Te” as first usage here.

Line 44: define time period of “Holocene”.

Line 55: “affect also” —> “also affect”

Line 101: “in ionosphere” —> “in THE ionosphere”

Line 101: Define “middle atmosphere” here.

Line 108: please provide additional explanation of what “lower levels” refers to.

Line 115-116: please provide additional explanation what “with some military consequences” refers to.

Line 117: provide explanation of what “intensification of “meteorological control”” refers to.

Line 121: define “ionospheric slab thickness”.

Line 124-128: the use of “deals with” and “treats” is very informal. Suggestion to change these to words/phrases like “examines”, “describes”, “provides an overview of”, etc.

Line 137-138: unsure what the tense of this sentence is. Suggest either changing to “which leads to” or “which led to” depending on the intended tense.

Lines 157, 163, 168 (and throughout the manuscript): please list the latitude of these listed places (i.e., Juliusruh, Pruhonice, Rome, Slough/Chilton, Boulder, etc.); it should not be assumed that a reader will necessarily know where these stations are.

Line 173: define “NmF2” as first usage.

Line 179: replace “Main progress was reached in…” with “SIGNIFICANT progress was MADE in...”

Line 180: “removing/suppression of effect of solar cycle” —> “REMOVAL/suppression of THE effect of THE solar cycle”

Line 196: change “mostly” to “most” or perhaps “main”?

Line 227: suggest removing “carefully”. As it was not similarly used on line 221 for the Li et al. study, it could unintentionally suggest that the Venkat Ratnam et al study was “carefully merged” but the Li et al. study was perhaps carelessly merged?

Line 230: suggest changing “over more than 25 years” to “ACROSS more than 25 years”.

Line 240: define latitude of “Moscow region”.

Line 255: “on most heights by 1-2 K/decades” —> “AT most heights by 1-2 K/DECADE”

Line 257: define “NH” and “SH” here as first usages.

Line 262: “with mean value” —> “with A mean value”

Line 268-269: define altitude ranges of “lower mesosphere” and “upper mesosphere”.

Line 273: “20002-2020: —> “2000-2020”?

Line 273: “20-110 km (middle atmosphere)” — unclear if this is Zhao et al. definition of middle atmosphere, but it is worth clarifying as this definition may differ from what others understand the altitude range of the middle atmosphere to comprise of.

Line 287: “estimated -7.5 K” —> estimated TO BE -7.5 K”?

Line 289: provide latitude of Observatories de Haute Provence.

Line 293-294: Odd standalone sentence. Please better integrate this into the surrounding text to improve flow.

Line 298: “reaching maximum of about” —> “reaching A maximum of about”

Line 298: state the “middle and lower thermosphere” definition that the authors used.

Line 299: “near none” —> “near ZERO”

Line 300: suggest giving an example/s of the “dynamic effects” to help the reader understand why the temperature trend may be near-zero/slightly positive.

Line 309: define “middle atmosphere” used by Venkat Rattan et al. (2019).

Line 313: “decade” —> “decadeS”

Line 328: “near altitude” —> “near AN altitude”

Line 340: Sentence beginning “Very important parameter…” needs rewording. Example could be: “Atmospheric waves, namely gravity waves, planetary waves and tides, are a very important vertical coupling mechanism between the upper atmosphere and ionosphere, and the lower atmosphere below.”

Line 356: suggest changing “(= polar mesospheric) clouds” as it looks too informal. Could change last part of sentence to “…related trends in polar mesospheric clouds (sometimes known as “noctilucent” clouds if observed during nighttime).” Or similar.

Line 354: “near surface” —> “near THE surface”.

Line 365-366: “with merged” —> “with THE merged”

Line 366: “over” —> “BETWEEN” or “ACROSS THE PERIOD”

Line 372-373: provide latitudes for “California, Hawaii and New Zealand”.

Lines 377-378: Clarify what “Such a trend is within trends” means. Do you mean the trends from Nedoluha et al. (2022) are consistent with the findings of other independent studies?

Line 391: “of impact” —> “of THE impact”

Line 393: Not critical, but I suggest that it may be helpful to a reader to state how increasing methane concentration results in an increase of water vapor.  This is later stated in lines 470-472, but it would be more useful if explained here (i.e., earlier in the manuscript).

Line 402: “reported update” —> “reported AN update”

Line 402: state latitude of  “Moscow region”

Line 406: can you explicitly define “summer length”?

Line 414: state latitude of “Zvenigorod”

Lines 421-422: “obtained after eliminating effects” —> “obtained after eliminating THE effects”

Line 425: “over 2004-2020” —> “BETWEEN 2004-2020” or “ACROSS THE PERIOD 2004-2020”

Lines 424-428: This paragraph needs further detail. Please explain some of the different definitions of “summer length” used by Jaen et al. (2022). Which definition of summer length resulted in a positive trend? Please further clarify what is meant by “break point and non-uniform trend of summer length”.

Line 436: “…increasing concentration of CO2 is cooling…”. It is very important to specify at what altitude range that CO2 acts as an IR cooling, as this otherwise runs the risk of being misquoted/misunderstood by those that may not understand that it acts as an important IR heater in the lower atmosphere.

Line 439: “partially maybe due to insufficient…” —> “LIKE due to THE insufficient”

Line 444: “near none” —> “near ZERO”

Line 445: again suggest giving a few examples of what these “dynamic effects” may be.

Line 449: Final sentence beginning “Summing up we may say that long-term trends…” needs rewording. Currently it is too informal and quite vague. Suggest at least rewording the  beginning to “In summary, it is clear that long-term trends…”. It would be helpful to the reader to state 1-3 clear ways (examples) in which “our knowledge broadened and it is more detailed”.

Line 452: “Other important group” —> “ANOTHER important group”. This sentence needs strengthening/rewording. Presumably winds and atmospheric waves are the “dynamical parameters”? One could reword to something like: “Dynamical parameters, such as winds and atmospheric waves, play a critical role in the MLT region.”

Line 456: “The limited activity in the area of” —> suggested clarifying that this limited activity refers to the amount of published papers on this topic between 2018-2022.

Lines 467-468: Suggest rewording sentence beginning “Water vapor trends…” to “Water vapor trends in the mesosphere are generally positive; it is only in the equatorial region that there are very little or near-zero trends.”

Line 469: provide latitude of Switzerland.

Line 470: “with unknown” —> “with AN unknown”

Line 479-480: Sentence beginning “Midlatitude partial..” requires an earlier definition of what is meant here at lines 424-428.

Lines 493-395: provide more context of what “their method might be questioned” means.

Line 500: “in the deep solar” —> “DURING the deep solar”

Line 496: provide more details. Satellite drag data from where?

Lines 501-503: provide more details about the Mlynczak et al. (2022) study. What altitude range? What latitude range?

Lines 506-511: what latitude range? Is this a globally averaged simulation?

Lines 512: state what altitude range is used by Liu et al. (2020).

Line 517-523: what altitude range? Define latitudes of all stations, but particularly Sodankyla as it hasn’t formerly been mentioned. What time range was covered by the six solar cycles? Can you elaborate on what the impact of not considering particle precipitation could be?

Line 533: “since review” —> “since THE review”

Line 535-538: what time period and altitude range was covered by this study?

Line 539: define station latitudes, particularly for Wakkanai. This is defined later in the paragraph (line 545) but it would be more useful near the start. Also include the time period covered by the study.

Line 546-548: Unclear what is meant by this sentence. Please reword and clarify.

Line 588-590: Need to reword/restructure the sentence “As for hmF2” as unclear what is meant by the second half of the sentence. Do you mean that the average trend in hmF2 was comparable to the trends observed in middle-upper atmosphere CO2 and also that observed in the Earth’s magnetic field?

Line 620:621: restate the sentence beginning “Some problems..” It would be helpful to the reader if you give examples of what some of these problems were.

Line 633: define mesopause altitude used here.

Line 636-637: elaborate on how the behavior of NO and O(3P) differs from that of CO2.

Line 643: “as in historical period” —> “COMPARED TO THE historical period”. Also please define what “historical period” is being referred to. 

Lines 654: suggest changing “violent oscillations” to “SIGNIFICANT oscillations”, or perhaps “DRAMATIC oscillations”.

Line 668: “during several last decades” —> “during the last several decades”.

Line 691: suggest changing “suffers with the” to “EXPERIENCES the”

Line 700: define Te and Ti here. I note that you later define them on lines 701-702, but it would be helpful to do so on their first usage.

Line 721: briefly explain what “vertical E x B drift” is for those that may be unfamiliar.

Line 737: unclear what “higher under low than under high levels” means. Please clarify.

Lines 763-764: make sure that the tense of “contractions” and “was” is consistent. Suggest either changing to “contraction” and “was”, or alternatively to “contradictions” and “were”.

Line 773: “near none” —> “near ZERO”

Line 775: “develop and detailed them further” —> “WERE DEVELOPED and detailed further”.

Line 805: “Important part of long-term trend investigation is specification of roles of individual trend drivers” —> “AN important part of THE investigation of long-term trends is THE specification of THE roles of individual trend drivers.”

Line 818: “from surface” —> “from THE EARTH’S surface”.

Line 828: “removing/suppression of effect of solar cycle” —> “THE REMOVAL/suppression of THE effect of THE 11-YEAR solar cycle”.

Line 834: “Despite of evidence progress, various challenges and open problems remain.” —> “Despite evident progress having been made, it is clear that various challenges and open problems still remain.”.

Line 835: “in activity” —> “in THE activity”.

Line 836: “which are very” —> “which are A very”

Line 845: “understood and related” —> “understood, and related”

Line 847: “needs to be more specified” —> “needs to be BETTER specified”.

Line 848: “not mutually sufficiently consistent” —> do you mean “still not in consistent agreement with one another”?

Line 850: Sentence beginning with “Summing up..” Suggest changing this to something less informal. A possible suggestion is something like “In summary, although there has been significant progress made in studies published between 2018-2022, it is clear that there is still much work to be done in reaching scientific closure on these outstanding issues.”

This paper presents a review of trends in the mesosphere, thermosphere and ionosphere based on results published during the period 2018-2022. It is important to understand how the upper atmosphere is modified by global climate change and such a study can be an interesting contribution to the topic. However, a major revision of the paper is needed before it can be published in ACP. 

A long list of publications has been reviewed, but the paper appears to be a long list of results obtained by the authors. It is difficult for the reader to extract the main results and understand how recent publications have improved our understanding of trends in the mesosphere, thermosphere and ionosphere. At the end of each section, a summary of the main results should be provided. This is particularly true for the very long section on the mesosphere and for the section on the ionosphere. 

Further remarks are listed below.

Section 2, problems in calculating long-term trends.

This section focuses mainly on the impact of solar activity in the calculation of long-term trends. This is an important contribution to consider because of the strong response of the upper atmosphere to the 11-year solar cycle. However, other problems may arise when calculating long-term trends, in particular the impact of atmospheric tides where the local time of measurement is not fixed or where there are trends in the tides that make the trend dependent on the local time.

Section 3: Trends are generally given in K/decade or %/decade but in some cases in K/year (lines 262 and 263) or %/year (lines 377 and 416). Please homogenise.

Line 289: Observations at the Observatoire de Haute Provence are made by Raileigh lidar, not by Rayleigh radar.

Lines 424-428 and 452-462 concern trends in wind measurements. Please group the two sections together.