Earth’s climate response to a changing Sun

Coordinated by: Jean Lilensten, Thierry Dudok de Wit,
Katja Matthes

Editors: Thierry Dudok de Wit, Ilaria Ermolli,
Margit Haberreiter, Harry Kambezidis,
Mai Mai Lam, Jean Lilensten, Katja Matthes,
Irina Mironova, Hauke Schmidt,
Annika Seppälä, Eija Tanskanen,
Kleareti Tourpali, Yoav Yair

COST (European Cooperation in Science and Technology) is a pan-European intergovernmental framework. Its mission is to enable break-through scientific and technological developments leading to new concepts and products and thereby contribute to strengthening Europe’s research and innovation capacities. It allows researchers, engineers and scholars to jointly develop their own ideas and take new initiatives across all fields of science and technology, while promoting multi- and interdisciplinary approaches. COST aims at fostering a better integration of less research intensive countries to the knowledge hubs of the European Research Area. The COST Association, an International not-for-profit Association under Belgian Law, integrates all management, governing and administrative 10 functions necessary for the operation of the framework. The COST Association has currently 36 Member Countries.

HYPERLINK “http://www.cost.eu/” www.cost.eu

“COST is supported by the EU Framework Programme Horizon 2020”

Cover illustration: Sunset observed from the International Space Station - expedition 15 (NASA). Aurora at Svalbard, by Cyril Simon Wedlund (Aalto-University (Finland) and IPAG-CNRS).

Printed in France

ISBN: 978-2-7598-1733-7

DOI: 10.1051/978-2-7598-1733-7

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broad-casting, reproduction on microfilms or in other ways, and storage in data bank. Duplication of this publication or parts thereof is only permitted under the provisions of the French Copyright law of March 11, 1957. Violations fall under the prosecution act of the French Copyright law.

© EDP Science, 2015

Contents

Foreword

Preface

PART I. INTRODUCTION TO THE SUN-CLIMATE CONNECTIONS

1.1   The Earth’s atmosphere: an introduction

1.2   The impact of solar variability on climate

1.3   The Sun-Earth connection, on scales from minutes to millennia

1.4   The role of the Sun in climate change: a brief history

1.5   The role of the Sun in climate change: a societal viewpoint

1.6   The debate about solar activity and climate change

References of Part I

PART II. SOLAR AND SPACE FORCING

2.1   Basics of solar and heliospheric modulation

2.2   Solar radiative forcing

2.3   Variability of solar and galactic cosmic rays

2.4   Variability and effects by solar wind

2.5   Variations of solar activity

2.6   Understanding solar activity

INFOBOX 2.1 Orbital forcing of glacial - interglacial cycles

INFOBOX 2.2 Grand minima and maxima of solar activity

INFOBOX 2.3 A practical guide to solar forcing data

References of Part II

PART III. DETECTING SOLAR INFLUENCE ON CLIMATE

3.1   Observations on paleoclimatic time scales

3.2   Ground-based observations

3.3   Satellite observations

3.4   Reanalysis data

3.5   Uncertainties and unknowns in atmospheric observations: How do they affect the solar signal identification?

3.6   Numerical models of atmosphere and ocean

3.7   From climate to Earth system models

3.8   Uncertainties in the modeling of the solar influence on climate

3.9   Detection and attribution: How is the solar signal identified and distinguished from the response to other forcings?

INFOBOX 3.1 Why are models needed in the first place, and can they be trusted?

INFOBOX 3.2 Model Equations and how they are solved

References of Part III

PART IV. IMPACTS ON THE EARTH SYSTEM

4.1   Direct impact of solar irradiance variability

4.2   ‘Top-down’ versus ‘bottom-up’ mechanisms for solar-climate coupling

4.3   Interactions of different sources of variability

4.4   Impact of solar variability on the magnetosphere

4.5   Atmospheric ionisation by solar energetic particle precipitation

4.6   Impact of energetic particle precipitation on atmospheric chemistry and climate

4.7   The impact of cosmic rays on clouds

4.8   Impact of solar variability on the global electric circuit

INFOBOX 4.1 Modeled impact of total solar irradiance (TSI) forcing

INFOBOX 4.2 Lightning, cosmic rays and energetic particles

INFOBOX 4.3 The influence of solar variability on extreme weather

References of Part IV

PART V. CONCLUSION

Conclusions

References of Part V

Glossary

The authors

FOREWORD

Roger-Maurice Bonnet¹

1    Why did I accept to write this foreword?

As former solar astronomer, and member of the Service d’Aéronomie of CNRS in France (now renamed LATMOS), I am since long deeply involved in – and motivated to – understanding the role of the Sun on the Earth’s climate. However, when Thierry contacted me for writing this foreword, I had some hesitations before answering positively. The response of our climate to the changing Sun has been passionately debated at length among the concerned scientists, solar physicists, meteorologists, climatologists, and many non-specialists, and these debates often made me uncomfortable. The topic deserves indeed a lot of care and its scientific analysis an utmost rigor, both too easily overwhelmed by irrationalism and passion. Why that passion? The issue is not the Sun, a modest G-type star fairly well understood, but rather the climate of our Earth, the most complex of all planets in the Solar System because it is a water planet, because it has an atmosphere and hosts life, particularly human life, the whole set resulting in an incredibly complex system permeated by complex interactions, which make other planets, like Mars for example, look very simple. That is where the problem lies!

The 20th century has witnessed a quasi-exponential growth of the world population followed by a similar growth of the demand on energy necessary to sustain its needs, and of course on fossil fuels, the cheapest source and admittedly the largest producer of CO₂, whose efficient greenhouse effect might be the most powerful cause of the changes observed in the recent evolution of the Earth’s climate. Hence the debate! Those who trust in a solar effect are right: nobody would argue that without the Sun, life would not exist on Earth in its present form at least. Nobody would argue against Milankoviimages’s theory that describes the collective effects of changes in the Earth’s orbital movements upon the climate, and that winters are colder than summers. On the other side, few would argue against paleoclimate studies concluding that without the greenhouse effect of methane and carbon dioxide in the atmosphere of the young Earth, when the Sun output was 70% fainter than at present, our planet would have remained a cosmic snowball for ever. Those who doubt that the present growth in the concentration of atmospheric greenhouse gas is largely responsible for the observed changes, naturally receive the support of those who do not like the idea that Man’s activities might be the cause of the present climate problem, because the remedy to anthropogenic forcing would responsibly imply adopting profound changes in our dealing with industrial activities, fossil fuel production, economy, and in our social systems. The “business-as-usual” attitude is indeed less disturbing in the short range than accepting the necessity of such changes!

In the course of last century and also recently, nearly all the wars on the planet have oil in their background. The fight for oil is indeed a serious matter: oil is relatively cheap and a source of immense revenues. The scientists therefore bear a crucial responsibility in placing the debate between natural and anthropogenic causes of climate change on solid irrefutable rationale grounds. As a scientist accepting that responsibility, I consider that careful attention should be given to the reasoning of the skeptics on both sides, in their expressing doubts about the other camps arguments. Not only because Thierry is a dear friend and colleague but also a man of high scientific rigor whom I fully respect, and because of what precedes, I eventually accepted to write these lines.

2    A surprising document

This monograph, built from some 30 contributions involving more than 60 authors, is surprisingly diverse and non-uniform. That is probably the usual form of reporting that characterises European COST actions, which intend to offer means of intellectual exchanges, be they of a scientific, technological, sociological or political nature using all possibilities of networking between selected members of the respective communities. The topic of this work responds quite exactly to that definition! Its final success would be reached when those involved would eventually concur that solar variability in all its forms does have effects on the Earth’s climate, agree on what these effects are, and what are their intensity. To reach that goal, interdisciplinarity is essential and that might not have been so easy to implement, remembering the idiosyncrasy that for a long time has characterised parts of the communities involved. Undoubtedly, this report testifies that this goal was achieved and all the participants to this impressive effort must be congratulated. In that respect, it is important to appreciate the participation of a relatively large number of young scientists whose opinions are remarkably open and far from being biased by corporate protective behavior. They undoubtedly contributed in no small way to approach the Earth not anymore as a set of independent layers, an onion, but rather, as the great French poet Paul Eluard would say, like a “blue orange” inside which all layers interact, forcing the fall of the tight barriers that for a long time made the science of solar variability influences on Earth not fully effective. That courageous approach persuaded me to look closer at the diverse pieces of that colorful stain glass window and to appreciate the result of that confrontation between individual view points, opening new avenues for further discussions, and potentially leading to practical progress.

3    What do I get from this book?

The four main parts of this book follow a logical – nearly Cartesian – order, from a general introduction, followed by a documented analysis of the factors related to solar forcing, and a discussion of detection methods, data analysis techniques and related uncertainties, to terminate with a concluding review of the impacts of solar variability on the Earth System. It is probably the first time that we do have in hand a most detailed analysis so far of the mechanisms potentially contributing to the natural variability of our climate. Even though none of these seems yet to trigger a profound revolution in our understanding of the causes of climate change, many contribute to progress and advances, which justify the genuine value of this interdisciplinary effort. Improving the accuracy in reconstructing the variability of solar phenomena, and more particularly the still-too-low accuracy of UV spectral irradiance measurements, can be recognised as an important message to the scientists and their funding agencies for establishing a network of space-based instruments certainly reinforced by the perspective of a necessary long-term continuity. Recognising the importance of the Global Electric Circuit in modeling the atmospheric phenomena linking the ionosphere to the Earth’s surface is another important output which opens a set of research perspectives to answer many questions related to the influence of solar activity in the generation of solar high energy particles and in the modulation of galactic cosmic rays that influence the circuit. Meanwhile, reaching the conclusion that Galactic cosmic rays are unable to significantly alter global cloud cover, nor explain recent global warming, is pleasant to read, remembering the rather long-lasting controversy raised by that possibility. Generating an open scientific discussion to address controversial issues is fine (that is exactly the rationale for creating the International Space Science Institute, in Bern), provided however that at some stage, conclusions should eventually be drawn and if not, agreements be reached on what should be done to later achieve that goal.

4    Reaching the readers, the political world and the social targets

Therefore, I cannot end this preface without raising a question and a concern. The question is: to whom is this book addressed? Is it just a tool, a step in the understanding of the Earth’s climates natural variability, a motivation for scientists to initiate more specific studies, furthering their analysis and calibrating their progress? The concern has to do with how will policy makers interpret the document. As a positive response to the question above, or as a justification for them to conclude that because science has not yet delivered its last word about the main causes of climate change, it is too early to take the necessary decisions on capping CO₂ emissions? Because the complexity of the factors intervening in climate forcing is so large, and has not allowed yet to properly quantifying their effects relatively to other sources of forcing, it might indeed be interpreted by some that there is no urgency to act, thereby further delaying the necessary remedies to the deterioration of our atmosphere and of the climate, sacrificing for centuries to come the future of our descendants. That would be regrettable to say it mildly, and using stronger words, highly irresponsible making the present work a very counterproductive and dangerous exercise.

Without any doubt, a positive answer to the question is exactly what should be the conclusion of this work, opening a set of new research avenues leading to a more precise quantification of the relative importance of solar variability with respect to the most powerful anthropogenic forcing mechanisms, and to new discoveries in the understanding of our changing climate. To further engage in both theoretical and observational researches, aiming at improving the precision of the data and of their use in modeling the various forcing mechanisms, while ensuring their continuity over the remaining decades of this century, would be highly gratifying for all those involved in this important work, and would undoubtedly represent the most positive breakthrough of this COST action. It would fortunately bring to a halt the skepticism and “wait-and-see” attitude that in the recent past has often resulted from an improper communication from the scientists involved in this research to explaining their science while identifying the limits of their convictions. This book does offer a rich menu of possible actions that should be pursued with utmost vigor and determination. It possesses all the tools and justifications to add an essential and strong support to the understanding of climate change. The readers cannot interpret its conclusions in a different way.

¹ International Space Science Institute, Bern, Switzerland

PREFACE

It’s the Sun, stupid! The Sun has indeed come under close scrutiny as a possible cause for global warming observed since the 1950s. Over 99.96% of the energy entering the Earth’s atmosphere comes from it, which would make the Sun a natural culprit for the changing terrestrial climate.

Although the Sun is a remarkably stable star over the timescale of a human life, many scientists have wondered whether its variability could affect climate. These studies really gained interest in the 1980s only. This growing awareness coincided with the first direct satellite observations of the amount of energy radiated by the Sun, which indeed turned out to vary.

The Sun–climate connections are one of the most challenging problems faced today in climate science. There are two reasons for this. The first one has to do with the high complexity of the underlying physical and chemical processes at the Sun, and in the Earth’s atmosphere. One may imagine these processes as multiple links of a long chain that extends all the way from the solar interior to the Earth’s surface. For many years, it was believed that this chain could be approximated by one single quantity, ironically called the solar constant, whose main impact is a direct heating of land and oceans. Today we know that this chain has numerous more subtle aspects: the impact of changing ultraviolet radiation on ozone production, changes in upper atmospheric composition induced by high-energy particles, fluctuations in the solar wind that eventually affect aerosol production by means of the Earth’s global electric field, and more. All these mechanisms are strongly interrelated, and affect all layers of the Earth’s atmosphere, thereby making it extremely difficult to investigate them one by one, and quantify their impact. A paradigm is shifting: instead of understanding a system by dissecting it into its individual parts, we need a more holistic approach that focuses on the interactions between the different elements of the chain.

The second challenge encountered in the study of Sun–climate connections is the fragmentation of the scientific communities. For many decades, scientists were organised as communities that were specialised on specific aspects, with limited consideration for the full picture. The authors of this handbook have, individually, strong expertise on specific parts of the Sun-climate connections, such as solar physics, high-energy particle physics, geomagnetism, climate modelling, atmospheric chemistry, and more. They have tried to overcome this fragmentation of their communities by interacting and working more closely together. As they were doing so, a global picture of the chain gradually took shape.

The framework that has enabled such multidisciplinary interactions is a pan-European COST action named TOSCA (Towards a more complete assessment of the impact of solar variability on the Earth’s climate¹), which lasted from 2011 until 2015. COST stands for Cooperation in Science and Technology², and is a framework supporting cooperation among scientists and researchers across Europe. TOSCA involved over one hundred scientists, both young and senior, and this handbook is its main outcome. COST actions are fully open to participation, and the texts indeed reflect the diversity of the scientists involved. We deliberately allowed the contents of this handbook to reflect this diversity.

Our prime objective with this handbook is to show science in action on a highly challenging topic with major societal consequences. Today, there is strong evidence for solar variability to represent a minor contribution to global warming, as observed since the 1950s. However, small as the solar forcing may be, many open questions remain regarding the mechanisms by which it may affect climate, and the magnitude of their forcing. For that reason, this handbook concentrates on the impact of solar variability on climate, rather than on the role of the Sun in global warming.

The results collected represent the state of the art of what we know today about the Sun–climate connections, also raising many open questions. Some of the results shown here may be refuted tomorrow, or new mechanisms may be discovered. However, progress on such a highly multidisciplinary topic will be achieved only if the new evidence is supported by facts that can be tested, and explained in terms of physical mechanisms, and not just beliefs or speculations.

How to read this handbook

This handbook consists of five parts, each of which is subdivided into chapters: part one is a general introduction to Sun–climate connections, part two addresses solar and space forcing, part three is about detecting solar influence on climate, part four addresses impacts on the Earth’s system, and part five ends with a conclusion, and a glossary. We intentionally structured this handbook as a mosaic of short chapters that reflect the multiple facets of Sun–climate connections. Each chapter may be considered as a piece of stained glass that fits in a multi-coloured window. We encourage the reader to exercise his/her judgment, and imagine what the global picture may look like.

The chapters are self-contained and can be read independently. Some recurrent technical words are explained in the glossary. Most chapters provide some references for further reading, while more specific and technical references are all grouped at the end of each part.

The number of authors involved in this handbook is large. At the end of it, you will find for each of them a short biography, enabling you to appreciate the diversity of the communities involved.

Acknowledgements

This handbook would never have seen the light of day without an active editorial team that played a major role in harmonising all the different chapters. We gratefully thank the members of this team: Ilaria Ermolli, Margit Haberreiter, Harry Kambezidis, Mai Mai Lam, Irina Mironova, Annika Seppälä, Hauke Schmidt, Eija Tanskanen, Kleareti Tourpali, and Yoav Yair.

We also thank Agnès Henri (EDP Sciences) for her assistance in helping us make a handbook out of such a large ensemble of chapters, each of which has a history.

Finally, COST, by funding our Action TOSCA made this whole endeavour possible.

Thierry Dudok de Wit, Jean Lilensten, and Katja Matthes,
August 2015

¹ http://www.tosca-cost.eu

² http://www.cost.eu

Part I

INTRODUCTION TO THE SUN-CLIMATE CONNECTIONS

CHAPTER 1.1

THE EARTH’S ATMOSPHERE: AN INTRODUCTION

Kleareti Tourpali¹, Jean Lilensten² and Roxana Bojariu³

1    Introduction

The Earth’s atmosphere, the thin envelope of air that surrounds our planet, consists of a mixture of gases, extending from the surface of the Earth to the edge of space and tied to the Earth mainly by its gravitational force, and partly by the magnetic field at high altitude. It comprises mainly molecular nitrogen (78% in mass) and molecular oxygen (21%), with the other 1% composed of trace gases, named so because their concentrations are very small. Despite their small amount, carbon dioxide, ozone and water vapor are very important to the Earth’s climate and can have a large impact on atmospheric processes. Above about 200 km, atomic oxygen, although tenuous, becomes the major species while atomic hydrogen is the most abundant species above typically 1000 km. Depending on the approach taken, there are various ways one can consider a distinction between atmospheric layers: composition, temperature, electromagnetic properties.

Although pressure and density decrease with altitude in the atmosphere, temperature remains relatively constant or even increases with altitude at certain regions, so it may well be used to distinguish between atmospheric layers. Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere and thermosphere, as illustrated in Figure 1. Based on composition and mixing, the atmosphere can be divided into two major layers, the homosphere and heterosphere, defined by whether the atmospheric gases are well mixed. The homosphere, starting from surface, includes the troposphere, stratosphere, mesosphere and the lowest part of the thermosphere. From there on lies the heterosphere where the chemical composition varies with altitude, and gases are stratified due to their molecular weight. In this section, we discuss the properties of the atmospheric layers distinguished by both composition and temperature profiles.

Fig. 1. Structure of the atmosphere and its layers based on temperature distribution during summer (orange line) and winter (blue line). (Adapted from http://www.athena-spu.gr/).

2    The homosphere

The amount of solar radiative energy that arrives perpendicularly to the Earth is on average 1361 Wm⁻²; this quantity is called Total Solar Irradiance (TSI). However, taking into account the spherical shape of the Earth, this value must be divided by 4 (i.e., 340 W m⁻²) to obtain the overall mean amount of energy that reaches the Earth.

About 50% of this energy is used to heat the surface, 20% is absorbed in the atmosphere and 30% is reflected back to space (the albedo is the ratio between the amount of energy reflected or diffused and the amount received. It approaches 1 for snow, for instance, and therefore amounts to 0.3 on average on the surface of the Earth).

The energy absorbed on the surface of the Earth heats the ground and then, by conduction and convection, the air in the lowest layer of the atmosphere. It also leads to considerable evaporation from the oceans and lakes. This phenomenon, which is amplified by the transpiration of plants, results in a rate of relative humidity of more than 50% on average on the globe in the troposphere, which contains 99% of atmospheric water vapor.

2.1    The troposphere

The term troposphere is a Greek-derived term (tropein – to change, circulate or mix), indicating the turbulent nature of this layer where weather systems circulate air masses and move around almost all content of atmospheric water vapor. Water vapor concentration varies from trace amounts in polar regions to nearly 4% in the tropics. Most clouds are present in this layer, too. The troposphere contains around 75–80% of the total mass of the Earth’s atmosphere. As mentioned above, the main constituents of the atmosphere are nitrogen, oxygen, argon and traces of ozone, and other gases and particles. Carbon dioxide is also present in small amounts, but its concentration has increased by more than 40% in comparison with the pre-industrial level (e.g., 1750) due to human activities. Like water vapor, carbon dioxide is a greenhouse gas, which traps a part of the Earth’s heat in the troposphere.

The troposphere extends upward to about 16–20 km above the Equator, and to about 7–8 km above the poles. The tropospheric thickness depends on latitude, season, and diurnal cycle. It is highest in summer and lowest in winter. This seasonal effect is strongest at mid-latitudes.

The troposphere is mainly heated from the Earth’s surface warmed by solar radiation. The uneven heating of the Earth surface by the Sun (which warms land and ocean at the Equator more than at the poles) leads to large-scale patterns of winds that move heat and moisture around the globe. Atmospheric circulation is deflected by the Earth’s rotation as it goes between the poles and Equator, creating weather systems and belts of surface winds. The warmed air tends to rise leading to convection which is the mechanism responsible for the vertical transport of heat in the troposphere while horizontal heat transfer is due to advection. Air convection generates clouds and ultimately precipitation when liquid or solid water particles grow large enough in size to fall toward the surface. Temperature, pressure, air density and water vapor content in the troposphere decrease with height. Air temperature decreases with height at a mean lapse rate of about −6.5 C km⁻¹, up to the tropopause (the boundary between the troposphere and stratosphere). However, sometimes temperature can increase with height within limited tropospheric layers leading to temperature inversions, which prevent the vertical mixing of air. Such atmospheric stability can lead to air pollution episodes at ground level. The temperature is constant in the tropopause. In the troposphere, winds increase with height leading to jet streams which usually meander in the upper parts, moving air at speeds of about 100 km h⁻¹, just below the tropopause. Atmospheric circulation in the upper troposphere strongly influences weather systems in the lower troposphere, too.

The troposphere influences the stratosphere – the next layer beyond tropopause – through atmospheric waves that propagate upward. A number of studies show that this wave forcing leads to slowly evolving changes in stratospheric circulation, affecting polar areas (polar vortex; see next paragraph), which in turn can feed back to tropospheric phenomena (such as the Arctic/North Atlantic Oscillation) that shape anomalous weather regimes in winter.

2.2    The stratosphere

The stratosphere is a region where the air temperature increases gradually to the stratopause (found at an average height of 50 km), the area marking the border to the mesosphere (Figure 2). Due to the increase of temperature with height through the whole stratosphere, warmer air lies above colder air. Hence, convection is not favored. The atmospheric conditions are stable and vertical mixing is suppressed. Only small amounts of water vapor enter the stratosphere (due to their freeze-drying at the tropopause), so the stratosphere is a very dry region.

Ozone plays the major role in regulating the thermal regime of the stratosphere. The stratospheric region with the highest ozone concentration is known as the “ozone layer” or “ozonosphere”, located at heights roughly between 15 and 35 km extending over the entire globe, with some variation in altitude and thickness. Its peak concentration occurs at heights from 26 to 28 km (or slightly higher) in the tropics and from about 12 to 20 km toward the poles. Typical ozone concentrations at its peak are about 10 parts per million by volume (ppmv), reaching to higher values over the tropics. Temperature increases with ozone concentration, as solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation, resulting in heating of the stratosphere. Temperature increases also due to thermal energy released during the ozone formation cycle. The reaction of oxygen (O) and oxygen molecules (O₂) that produces ozone is exothermal; heat is released and the stratosphere warms. As ozone is the main gas involved in radiative heating of the stratosphere, its solar-induced variations can directly affect the radiative balance of the stratosphere with indirect effects on circulation. A detailed account of solar-induced variations in the stratosphere and the ozone layer is given in Chapter 4.1.

Intense interactions take place among radiative, dynamical and chemical processes, in which the horizontal mixing of gaseous components proceeds much more rapidly than in vertical mixing. The horizontal structure of the stratosphere and the stratospheric circulation present a number of interesting features. Air rises in the tropical region from the troposphere into the stratosphere, is transported upward and poleward in the stratosphere, and descends in stratospheric middle and polar latitudes. This circulation, called the Brewer-Dobson Circulation, is strong in the respective winter hemisphere and transfers ozone and other trace constituents from the tropics to the polar regions, as illustrated in Figure 2. The polar vortex is a large-scale circumpolar cyclonic circulation (strong west-to-east winds that circle the polar regions). The polar vortex extends from the upper levels of the troposphere through the stratosphere and into the mesosphere (above 50 km) and is associated with low temperature and ozone values. Polar vortices occur over both hemispheres, strongest during the respective winter season, when the pole-to-equator temperature gradient is strongest. They play an important role in the coupling between the troposphere and stratosphere (e.g., Arctic Oscillation) and in ozone depletion (e.g., Antarctic Ozone Hole).

Other important features of the stratosphere are the quasi-biennial oscillation (QBO) and the Sudden Stratospheric Warmings (SSWs), both of which affect circulation and transport of tracers, such as ozone and water vapor (see the Glossary for their brief descriptions).

2.3    The mesosphere

The mesosphere’s (middle sphere) average location is from about 50 to 90 km above the Earth’s surface, and is characterised by temperature decreasing with height. Thus, the mesosphere is warmer at its lowest level (just above the stratopause), and becomes coldest at its highest level. The upper limit of the mesosphere, the mesopause, is at the altitude at which the temperature reaches a minimum. The mesopause, a region that experiences large height variations, is located at altitudes between 85 and 100 km, depending on latitude and season. The average mesopause temperature is about −90 °C (Figure 1), but the lowest temperatures are found over the poles during the summer season, where the temperature may often drop below −100 °C (at the same time, the other (winter) pole may have higher temperatures by several °C), so the summer polar mesopause is the coldest level in the whole atmosphere.

Fig. 2. Schematic representation of the Brewer-Dobson Circulation and Middle Atmosphere Transport. Arrows show the different pathways of air mass transport in the stratosphere and the stratosphere-troposphere exchange. Retrieved from Brewer-Dobson Circulation, (2011) http://www.eoearth.org/view/article/150779/.

The percentage of oxygen, nitrogen, and carbon dioxide in the air in the mesosphere is almost the same as that in the lower atmospheric levels, but the density of air is much less. The water vapor content is very small, and the mesosphere contains higher percentages of ozone than the lower levels. The mesosphere is the coldest among the atmospheric layers, so that the small amounts of water vapor that are present there are frozen into ice crystal clouds. These are the highest clouds in the Earth’s atmosphere, called Noctilucent Clouds and can be observed after sunset at high latitudes during summer.

The mesosphere is a region where solar radiation and energetic particles contribute to downward energy transfer, and gravity and planetary waves along with the atmospheric tides transfer energy upwards from the stratosphere. Strong variations occur in the mesosphere as a function of height, both in the physical processes as well as in the chemical composition, especially in the vicinity of the mesopause.

The mesosphere together with the lower thermosphere (sometimes referred to as Mesosphere-Lower Thermosphere or MLT) is the region where vertical turbulent mixing is gradually replaced by molecular diffusion as we move to higher altitudes. The amount of solar radiation available to photolyse atmospheric molecules increases rapidly; molecules dominate the atmospheric composition at lower levels and atomic species, which are lighter, are abundant in the upper layers.

The major contributions to the energy budget are the absorption of solar radiation by ozone and molecular oxygen, the emission of infrared radiation by CO₂, dissipative processes from the interaction with gravity waves, and transport of heat through advection. The mesosphere is also the layer in which many meteors burn up when they enter the Earth’s atmosphere, as a result of the collision with some of the gas molecules present in this layer.

The stratosphere and mesosphere together are in many cases referred to as the “middle atmosphere”. In all the “low” layers we have just described, molecules and atoms mingle, producing a homogeneous gas. The concentration is roughly 10²⁵ particles per cubic meter at ground level and 10¹⁹ particles per cubic meter at the homopause (or turbopause), which is its upper limit.

3    The heterosphere, the thermosphere, the ionosphere

The heterosphere begins above the homosphere. It only became possible to explore the properties of this part of the atmosphere with the advent of radio communications in the twentieth century. Subsequently, sophisticated radar techniques and measurements by satellite revealed a complex, dynamic medium, a gas consisting of a mixture of electrically charged particles and neutral particles. This sheath still raises many questions about the part it plays in the ecosystem of the Earth and in the emergence of life on Earth.