S3C3: The Atmosphere & Climate

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a) Physics of the atmosphere

In Chapter 1 on geology, we saw the solid and liquid body of the Earth was formed by the aggregation of particles under the effect of gravity. As it grew in size, the planet eventually cleaned its orbit around the Sun and its volcanic activity subsided over time though it still had to face meteorite impacts.

It was not explicitly mentioned in Chapter 2 but the fact that our hydrosphere, whether it is oceans, glaciers or clouds remain within the confines of our Blue Planet instead of escaping into outer space is also a consequence of gravity. And so it is for the presence of atmosphere. And just like heat, gravity and pressure all combine to stratify both the oceans and the body of the Earth, so they do with the atmosphere, the layer of gas enveloping us.

All of these natural features are not exclusive to Earth and are shared by many terrestrial planets, those with sufficient gravity and enough distance or protection from the solar winds to retain their gaseous layer. Or should I say layers because the atmosphere is far from being uniform with dramatic changes in molecular concentration and temperatures.

The interface between interplanetary space and the thermopause or exobase is called the exosphere and reaches up to 10,000km in altitude or even more depending on the criteria used and it is not uniformly regarded as being part of the atmosphere proper. Molecular density there is so scant that collisions are few and far between, yet the molecules are still gravitationally bound to Earth. Thus the more formal boundary is the exobase, located at an altitude of around 700km, +/-200km depending on factors such as time of the day, season and solar activity.

Below this threshold is the thermosphere where particles tend to absorb solar radiation so their temperature can reach levels in excess of 2,000 °C, not that you would feel it due to the feeble molecular density that is insufficient to conduct heat. This exposure to high energy radiation, including UV and cosmic rays, leads to an ionization of many particles in a process known as photoionization and defined by the Encyclopedia Brittanica as “the interaction of electromagnetic radiation with matter resulting in the dissociation of that matter into electrically charged particles.” We’ll revert to this in a moment in the context of the magnetosphere and the ionosphere.

Under the thermosphere, with a boundary at around 80km of altitude called the mesopause, lies the mesosphere. Under the combined effects of low gas density, lower concentration of UV-absorbing ozone and the cooling effect from CO₂ radiated emission (electrons transitioning to a lower energy state emit photons), temperatures in the mesosphere get markedly colder with altitude until they reach the coldest part of the entire atmosphere, down to -100 °C at the mesopause.

At the stratopause, around 50km above sea level, the atmosphere temperature is at its highest level, even though the pressure is only a thousandth that of the surface of our planet. Below this threshold, the mesosphere becomes the stratosphere and temperature starts dropping again. This unexpected state of affairs is due to the presence of the ozone layer (O₃), at around 20-35km of altitude, which absorbs most of the ultraviolet radiation coming from our star, or rather most of the UV-B and UV-C with wavelength ranging from 315-280nm and 280-100nm respectively. As the high-energy photons are absorbed, they break down the covalent bonds of the ozone molecules into a dioxygen molecule (O₂) and an excited oxygen atom. This process called photodissociation is exothermic, meaning it releases energy in the form of heat.

Finally, the temperature gradient reverses again at the tropopause, below which is the troposphere, the inner-most layer of our atmosphere. This is the layer richest in water vapour and benefitting the most from the heat radiation emanating from the Earth itself, and so it is where most of the weather phenomena take place and the temperature gradients change both with altitude and latitude. As a result, there is a very significant variation in the location of elevation of the tropopause, it is as high as 17km above the equator and as low as 9km above the geographic poles. The fact that most of the heat originates from below and through the surface of the Earth and then ascends through the troposphere due to buoyancy promotes what is called “vertical mixing”, thus creating atmospheric turbulences compounded by the rotational friction created by the Earth’s spin.

As promised, before looking at the interplay between the atmosphere and life, we should also understand the nature of two other layers: the magnetosphere and the ionosphere. The nature of magnetic fields was explained in S1 Section 8.e and in S3 Section 1.c we saw the Earth’s magnetic field was the result of the mantle convection current. The main effect of the magnetosphere, the surface created by the magnetic field lines, is to shield the planet from the high-energy particles carried by the solar wind and cosmic radiation by deflecting them for the reason that they are electrically charged, regardless of whether this charge is positive (protons and alpha particles for instance) or negative in the case of electrons. I will not go into the detailed physics of it but you are welcome to help yourself by reading about Lorentz Force, I have included a link to the Wikipedia entry in the last section of this chapter. By the way, this dynamic is what gives rise to the phenomena of aurora borealis and aurora australis in the northern and southern hemispheres, respectively. The impact of solar wind compresses the magnetosphere to about 65,000km from the surface of the Earth on the side exposed to the Sun and so the magnetosphere is highly asymmetrical in its contours.

In comparison, the ionosphere can be found at a much lower altitude, all the way from 80km to 600km and more. Like I wrote earlier, this is the zone where photoionization takes place and it is responsible for the faint emission of light in our atmosphere even at night as the excited particles shed some energy. Interestingly, this layer of charged particles also acts as a refractory surface for high frequency radio waves and this property has historically been put to good use by radio operators to propagate shortwave radio beyond the horizon.

The absorption, refraction and scattering properties of the atmosphere are not limited to electrically charged particles and radio waves, they also affect light in the visible spectrum. For example, refraction causes the bending of light rays, which sometimes enables us to see objects, Sun included, that are actually below the horizon. This property was leveraged by George Everest to improve the breadth of its measurements when carrying out the Great Trigonometrical Survey in India, as described in The Great Arc. As for the scattering of light, because blue light with short wavelength is proportionally more subject to this optical property, we end up with a blue sky and the blue light of the setting Sun being mostly scattered away as it travels a long distance through the atmosphere leaves us with the red-end of the spectrum to admire.

b) Relationship with the biosphere

From what we have just seen, it is very clear the combined existence of the atmosphere (including the ozone layer) and the magnetosphere would have provided an indispensable set of shields permitting the eventual emergence of life near the surface of oceans and on land. This was compounded by the thermoregulation properties of the atmosphere and oceans.

And of course, we should consider the beneficial aspects of the gaseous content of the atmosphere in supporting life. However, the reason I have not properly mentioned composition until then is because the relationship between atmospheric content and the biosphere is a two-way street and it hasn’t always been smooth sailing. Time to turn the clock back. In the beginning…

4.6 billion years ago there was no atmosphere to speak of, mostly hydrogen molecules, and it is on the back of continued meteorite collisions and volcanic activity that gas trapped within the molten rocks, metals and silicates was released, a process called outgassing. The incremental cooling of the Earth’ crust resulted into the condensation of vapour and the creation of primordial oceans from which life emerged about 3.5 billion years ago. The atmosphere then consisted mainly of nitrogen, carbon dioxide and methane compared to the 78.1% nitrogen (N₂), 21% oxygen (O₂), 0.9% argon (Ar) and 0.04% carbon dioxide (CO₂) we currently have – all expressed by volume of dry air, i.e. excluding water vapour.

How did we go from one atmospheric content to the other? The answer is that life itself was the primary driver in the rise in oxygen concentration, starting from negligeable levels. Therefore it would be wrong to think life was made possible by oxygen, rather some type of simple life forms released oxygen as by-product, a useful but also greatly oxidizing molecule – effectively a double-edge sword, which ultimately assisted the development of more complex life forms. This process bears the name of the Great Oxidation Event (or Great Oxygenation Event) and its impact was not linear because, as O₂ production by cyanobacteria started around 2.45 billion years ago, most of it reacted with rock surfaces within the oceans then on land or remained dissolved in the oceans. It wasn’t until 850 million years ago that the pick-up in atmospheric concentration took place and it would have initially led to the extinction of many species.

Nowadays, the main source of oxygen and consumption of carbon dioxide is from plants and phytoplankton with animals doing just the opposite, and a species among them, us humans, dialling up the emission of CO₂ through industry. As for nitrogen, it is indispensable for plants but quite difficult to access in its gas form so they have to make do with compounds like NO₃^– that can be found in the soil.

c) Key climate variables and classification

We have now explored Earth’s geology, its hydrosphere and its atmosphere; we just need to take a quick look at its rotation and path around the Sun and we’ll have all the key elements at a macro level to understand its climate as well as weather phenomena caused by temperature changes, currents and wind.

The hour was originally defined as 1/24^th of a day, being the time it takes for our planet to rotate once along its North-South axis. This rotation means we have diurnal cycles with different degrees of illumination as well as temperature changes since solar radiation is a crucial source of heat. It also means the points at the surface of the planet travel pretty fast, 1674km/h to be precise on the equator and less as you ascend or descend across latitudes. If you attempt the calculation, make sure to use the sidereal day since every time the planet orbits around the Sun it would have completed one more rotation than the number of days we observe – the concept of sidereal day will be explained properly in S3 Section 5.b. This surface speed creates air friction in the atmosphere as well as the Coriolis effect, a fictitious force that seems to bend trajectories when observed from a rotating (non-inertial) frame of reference. I have included a link to the Wikipedia entry for Coriolis Force in section g) and we’ll see it becomes relevant when studying weather systems.

The orbiting of our planet around the Sun is comparatively less impactful in terms of climate in equatorial or tropical regions but as one travels away from the equator and towards the poles, the effect of seasonality becomes much more pronounced. The main reason for seasonality is actually not the elliptical shape of the orbit, it is the fact that the axis of rotation of Earth is tilted – quite a lot in fact at 23.43 degrees – combined with axial parallelism, which is the maintenance of the same axis direction throughout the orbital plane. As a consequence, the average daily exposure varies quite dramatically away from the equator and creates what we call the seasons, and those are opposite in the northern and southern hemispheres. It varies to the point where for several days every year all points above a certain northern latitude and below the corresponding southern latitude will experience either no sun in the sky or the midnight sun, days without setting and rising of the sun. In the northern hemisphere, this threshold is called the Arctic Circle and its latitude is 66.57° N, being 90° minus 23.43° whereas the Antarctic Circle is at 66.57° S. The same 23.43° is the latitudes of the tropics, that of Cancer in the north and Capricorn in the south – those names refer to star constellations. You may come across the numbers 66°34′ and 23°26′ instead of those written above, don’t worry they are the same, one degree of angle comprises 60 minutes (noted ′ ) which itself comprises 60 seconds (noted ″ ).

Climate is a rather ill-defined term that generally refers to a set of weather patterns over several years or even decades. In that sense, weather is short term events showing substantial variability whereas climate would be the equivalent of a last-30-years average exhibiting yearly cyclicality. This seasonal cyclicality is driven by average sun exposure and therefore temperatures, the latitude, proximity or not to oceans, water precipitation levels, atmospheric pressure and therefore altitude, as well as the lay of the land. Furthermore, regional climates are not isolated systems and have impacts on one another though this tends to fall more into the realm of weather than that of climate.

There is an element of subjectivity in selecting variables and quantifying applicable thresholds, including how much granularity one wishes to map out, so there isn’t a definite, universally-stamped climate classification. Nevertheless, some classifications are more equal than others and the system devised by Köppen is widely used – I have included a link to the relevant Wikipedia entry at the end of this chapter. It categorizes the climate on Earth into five main groups based on the levels of temperature and precipitation experienced with further distinctions made depending on seasonal patterns of those two variables. The five top-level climate groups are as follows:

Tropical climates (noted as “A”) have significant precipitations and average temperatures equal or superior to 18°C. The group mostly covers the equatorial regions up and down until the tropics and is subdivided into savanna, monsoon and rainforest climates.
Arid climates (B) can be hot or cold and desertic or semi-arid. They encompass non-polar areas with very low levels of precipitation such as the Sahara, the Namib and Kalahari, most of inland Australia and parts of the Andes, Rocky Mountains, Patagonia, Tibet, Mongolia and Central Asia.
Temperate climates (C) are neither too hot nor too cold with large variability in terms of precipitation patterns. Almost all of the regions classified in this group have a maritime façade with the notable exception of Northern India and the parts of Angola and Mozambique located north of the Kalahari. These include most of Western Europe, the greater Río de la Plata estuary region, New Zealand and South-eastern Australia, South-eastern China, and the South-eastern United States.
Continental Climates (D) are non-polar exhibiting temperature variability with at least one month averaging under 0°C and at least one month above 10°C; so there is still room for quite distinct sub-divisions according to how cold the winters and how hot the summers are, and the levels of precipitation. We thus have the tundra and taiga areas of Siberia, Alaska and Northern Canada belonging to the “subarctic climate”, and a large part of Eastern Europe, Western Russia and Southern Canada under the name of “humid continental”.
Polar climates (E) are solely defined based on temperature, the criteria being to have no month with an average exceeding 10°C. So it’s either tundra or ice cap there and geographically this corresponds to the extreme south of Patagonia, Antarctica, Greenland, Iceland, the very north of Siberia and North-eastern Canada.

d) Weather systems and phenomena

Temperatures and precipitations are the key variables in the categorization of climates as well as important factors in our day-to-day experience of local weather and they are also crucial in determining the type of fauna and flora that can survive and thrive in a given climate, which will differentiate into various ecosystems based on topography.

But what drives specific weather patterns and phenomena? One could argue temperature is as much a cause as it is an effect. I think here we need to take a step back and consider the underlying reasons behind the temperature level in a specific location on a given day. When we looked at the SI base units of measurement in S1 Section 1.f, we used the following definition for the kelvin: the thermodynamic temperature, related to the energy embedded in the motion of particles such as the atom. And as we have already seen earlier in this chapter, there are two ways temperature can increase: the heating of parts of the atmosphere by solar rays or by convection, that is heat moving from one place to another. Conversely, temperatures decrease by dissipation of the kinetic energy, at night when there is no solar ray energy being absorbed in particular, and also by convection with the denser cooler air moving down and the hotter one up.

The direct consequence of air heating up it that is expands and not only tends to move up because it is more buoyant but its density also decreases, resulting into a volume with lower pressure. The opposite effect takes place with cool air, which creates differences in atmospheric pressure and contributes to the mixing of air, a phenomenon we call wind. The same process also occurs in oceans where areas with higher solar radiation exposure heat up and move up. Other factors come into play in the case of oceans as well such as difference in salinity that creates density differentials, tides, seafloor topography and winds near the surface. The evaporation of water in the atmosphere will thus be enhanced above areas of warm water oceans and load the atmosphere with moisture.

All the ingredients are in place to explain weather phenomena and yet, predicting those remain a challenge models and supercomputers have not overcome, though it must be acknowledged they are getting increasingly good at it, regardless of what some people think about weather forecasting. The reason it is such a challenge has to do with the sheer size of the data that is required as well as the countless feedback mechanisms occurring all over the place so that, to get a proper forecast in a precise location, one would theoretically need to have exhaustive and accurate information about the entire system. We may be looking at deterministic laws of physics and still be unable to tell the future; this smacks of Laplace’s demon and the name for this is a chaotic system.

That said, if we can’t confidently predict weather phenomena several days ahead of time, we are able to explain them. Here is a very short list that may prove of interest:

The jet streams are high-speed westerly winds (i.e. coming from the west) produced at the interface of different atmospheric circulation systems called cells. There are three of them in each hemisphere: the Hadley cell from the equator to about 30°, the Ferrel cell between 30° and 60° and the Polar cell from 60° to the respective North and South Poles. The masses of air with different pressure and temperature produce wind and the Coriolis force ensures these are deflected to move in the direction in which the Earth rotates, frow west to east, instead of following longitudinal lines (vertical on a map). This means there are two jet streams in each hemisphere, a Polar Jet and a Subtropical jet. Planes use them to save on fuel when travelling eastward and make sure to avoid them when flying westward.
The polar jets wrap around circumpolar vortices, one at each pole, extending until about 60°-50° of latitude. They are at their strongest during their winter, when temperature is the lowest because of the low or lack of exposure to solar radiation. Occasionally, this contained system breaks down and large masses or freezing air escapes toward temperate latitudes where populations are often ill-equipped to deal with such low temperatures.
Trade winds are part of the Hadley cell atmospheric circulation system but, unlike the jet stream, they generally blow from the northeast and from the southeast in the Northern and Southern Hemispheres, respectively. That is opposite the direction in which the Earth spins and diagonally towards the equator. They were named thus for they were put to good use during maritime exploration and trading. If you want an interesting account of how counter-prevailing wings played havoc with the Spanish attempts at establishing their own trade in the Spice Islands, I recommend the book Spice by Roger Crowley. It is really good.
Anticyclones generally bring good, stable weather; they form in zones of high pressure where a downdraft of this higher-pressure air results in lower humidity. Conversely, extratropical cyclones form around low-pressure systems and draw in air closer to the surface, which then moves up, taking moisture with them and thus forming clouds when condensation occurs. They can give rise to thunderstorms or blizzards – if you wish to know more on this topic, I have included the relevant Wikipedia link in the last section.
Saving monsoon for the end. The Asian monsoon showering the Indian subcontinent might be the best known but it isn’t the only one. The term refers to a global seasonal phenomenon where a pressure differential between the air above lands and oceans produces winds carrying moist air inland and as elevation rises the moisture condensates and eventually precipitates. The reason for the pressure differential is that liquid water has a higher heat capacity so its temperature doesn’t oscillate as much as that of the air above landmasses. Hence, as summer comes and solar exposure increases, the temperature above land rises much more than it does in the ocean, resulting in masses of air that expand and become less dense, i.e. they exhibit lower pressure.

e) Causes and effects of climate change

We have already briefly looked at some of the effects of climate change on the cryosphere in S3 Section 2.c, so I will not reiterate those and simply mention the main drivers, be they anthropogenic or natural, and quickly discuss their existing and expected future impacts.

Climate has always oscillated, mostly as a function of the atmospheric content and the geological activity within the planet, the two being linked via outgassing, in particular through volcanic activity and the biosphere has also left its signature by altering the composition of the atmosphere, as we have seen in section b). However, never before has this relationship had such a short-term impact, one that can be measured in decades rather than tens of thousands of years. The main driver of change is the man-made emission of greenhouse gases, methane and carbon dioxide in particular. Planets receive heat by radiation and also lose heat by radiation, except that a lot of the latter is in the form of longer wavelength radiation and those tend to be absorbed by those gases, so they act as a lid and participate in a global warming of the atmosphere.

In turn, as we have seen when discussing weather phenomena, because this increase in temperature is not distributed smoothly across the surface of the globe, this leads to increased differentials in pressure which cascade into more extreme weather phenomena, be it huge precipitation volumes in one place, long dry spells in others, cold waves and heat waves, and of course the rise in sea levels. The data in support of the anthropogenic causes is overwhelming, and yet even if it was not so, there would be an equally strong case to try to mitigate the impacts by removing greenhouse gases from the atmosphere or reducing their emission, ideally both.

f) Trivia – the Antarctic Circumpolar Current

I initially thought about writing about the Southern Oscillation, also known as El Niño and La Niña, before I realized it was going to get quite technical and long so, instead, I opted to take a look at the world’s strongest current system: the Antarctic Circumpolar Current (ACC).

The ACC is located in the Southern Ocean and surrounds the Antarctic landmass, flowing from west to east and linking in the process the Indian, Pacific and Atlantic Oceans. It is caused by the southwestern westerly winds, the vertical movements of water within the ocean depending on salinity and temperature gradients, and the underwater topography (also called bathymetry). The boundary it creates is extremely rich in nutrients, especially phytoplankton, and supports a large food chain including seals, penguins, whales and a few bird species. If you ever wondered why sailing races such as the Vendee Globe go this way around, this is it.

It is in this part of the world that the densest water resides, a function of its cold temperature and high salinity. For this reason, the ACC is a key component in ocean circulation at a global scale, a concept captured under the name of thermohaline circulation (“háls” refers to “salt” in ancient Greek). I have included a link to the relevant Wikipedia entry in the next section.