WHY IS THE STRATOSPHERE WARM?
By Paul Homewood
This is a guest post by Erl Happ.
It is the second in a series on the role of ozone in the Earth’s climate.
WHY IS THE STRATOSPHERE WARM?
Energy arrives from the sun in the full gamut of wave lengths documented above. It is emitted by the Earth in a relatively narrow range in the infra-red between 1-125 um.
Ozone is made possible by the splitting of the oxygen atom by short wave radiation in the ultraviolet spectrum at wave lengths shorter than 250 nanometres, equivalent to 0.250 um. Once formed ozone can be broken down by wave lengths between 0.3 and 0.4 um in the ultraviolet.
Ozone is a greenhouse gas absorbing some of the very considerable sum of energy emitted by the Earth in the infra-red spectrum instantaneously transmitting that energy to adjacent molecules. But does this absorption in the infra-red contribute to the warmth of the stratosphere? You can survey the literature and find nary a reference to this phenomenon. The literature is dominated by the environmental concerns as to whether ozone is a significant pollutant in the troposphere and secondly, the degree to which its presence in the stratosphere enables the screening out of short wave energy that is harmful to plants and animals. The ‘ozone hole scare’ gave rise to the Montreal Protocol for the elimination of the use of certain gases including the commonly used ‘Freon’. Apart from that, the stratosphere is seen as largely irrelevant to the concerns of man, a place where the atmosphere is relatively static because ‘stratified’ with warmer air above colder air.
Now let’s get to grips with the question posed in the heading:
WHY IS THE STRATOSPHERE WARM?
On a website prepared by the US Earth Sciences National Teachers association with the support of NASA we have this statement:
Temperatures rise with increasing altitude in the stratosphere. Ozone molecules in the stratosphere absorb ultraviolet radiation coming from the Sun. The energy from the UV radiation is transformed into heat. The heating is most intense near the top of the stratosphere, so that is where the stratosphere is warmest.
From Wikipedia we have:
Within this layer, temperature increases as altitude increases (see temperature inversion); the top of the stratosphere has a temperature of about 270 K (−3°C or 26.6°F), just slightly below the freezing point of water. The stratosphere is layered in temperature because ozone (O3) here absorbs high energy ultraviolet (UVB and UVC) radiation from the Sun and is broken down into the allotropes of atomic oxygen (O1) and common molecular oxygen (O2). The mid stratosphere has less UV light passing through it; O and O2 are able to combine, and this is where the majority of natural ozone is produced. It is when these two forms of oxygen recombine to form ozone that they release the heat found in the stratosphere. The lower stratosphere receives very low amounts of UVC; thus atomic oxygen is not found here and ozone is not formed (with heat as the by product).
So, the question arises: Is the energy absorbed at short wave lengths that is involved in both the creation and destruction of ozone sufficient to explain the warmth of the stratosphere? Well, for a start we can note that the stratosphere is warmest at 1 hPa and not at 30 hPa where the Wikipedia article suggests that the heat is released in the creation of the ozone molecule. Secondly, we can note that the reversal of the lapse rate (rate of temperature change with elevation) at the tropopause requires heating at and below that elevation to produce the reversal of the lapse rate. It’s obvious therefore that the notion that the heat of the stratosphere is due to interception of short wave radiation is deficient.
The profile of Earth emitted outgoing long wave radiation as measured by satellites reveals a deficit at 9-10 um reflecting infra-red energy lost to ozone in heating the stratosphere. The deficit in outgoing radiation at 9-10 um represents a hole in the energy emitted that is close to the peak of the spectrum carrying the bulk of the energy emitted by the Earth. The energy at these wave lengths is abundant. That energy is absorbed by ozone in the troposphere (contains about 10% of total ozone) and the stratosphere. There is no shortage of energy in the infra-red wavelengths to enable atmospheric warming whereas the energy available in the ultraviolet spectrum is but a tiny portion of the entirety of incoming radiation.
The effort to map the distribution of ozone in the atmosphere by satellites can utilize the attenuation of radiation in the ultraviolet spectrum OR the infra-red spectrum to infer the presence and concentration of ozone. The Toms satellite utilized the former while more recently the availability of Japanese developed instruments abroad the Gomes satellite enables the measurement of ozone utilizing the attenuation of the infra-red spectrum by ozone. The Japanese maintain a healthy interest in the ozone content of the air and matters stratospheric.
The source of that information is the following expert review:
The attenuation by ozone, water vapour and CO2 in the region of 9-10um is graphed:
We see that the attenuation of the energy available in certain wave lengths between 975 um and 1100 um rises to as much as 50% of the total with considerable energy imparted to the atmosphere measured in terms of watts per square centimetre.
There is another consideration that is very important. It is conveyed in the following statement that can be accessed in the original here
This is a very important dynamic that influences how and where the stratosphere absorbs outgoing infra-red. Consider this:
The amount of radiation absorbed by ozone is pressure dependent. The lower ozone is present in the atmospheric column the greater its heating capacity. This is of particular interest given the fact that ozone is ubiquitous throughout the atmospheric column in high latitudes and richly so in winter. The closer the poles are approached, the greater is its heating power, helping to explain the velocity of the jet stream that develops at the conjunction of dense ozone deficient air of mesospheric origin and warmer air that is ozone rich on the equatorial side of the polar vortex.
The greater opacity of ozone in the troposphere, to the point that the troposphere absorbs as much energy as the entire stratosphere goes a long way to explaining the attenuation of the lapse rate at very low altitudes as we approach the poles. See here for lapse rates.
This phenomenon helps to explain the origin of the energy behind the generation of so called ‘cold core’ cyclones that are in reality cold core only below 500 hPa , the origin of the uplift being an extensive warm core in the atmosphere manifesting strongly above 400 hPa.
In fact the bulk of short wave radiation from the sun is fully absorbed by nitrogen and oxygen above 1 hPa while only a relatively small portion remains to directly energise oxygen and ozone in the stratosphere. The following table from here details the particular spectra absorbed at various elevations.
According to this table, it is the Herzberg continuum between 200 and 242 nm (0.200 um to 0.242 um) that impacts oxygen in the stratosphere whereas longer wave lengths up to 0.850 um impact ozone.
The question must be posed again: How much of the heating of the atmosphere by short wave radiation at between 0.200 um and 0.850 um is due to the absorption of short wave energy from the sun? How much is the result of heating by long wave energy at 9.6-11 um that is emitted by both the sun and the Earth but overwhelmingly the latter? Fortunately, observation can inform us as to the source of the warmth in the stratosphere.
Above we have a hovmoller diagram showing us the evolution of surface temperature from 2005 through till 2011 at 20-40° south latitude. On the right there is another hovmoller showing the evolution of temperature at 10hPa over the same time interval. At 10 hPa the partial pressure of ozone in the air in relation to the other gases peaks. It peaks there in part because it is gathered together in the lower atmosphere in low pressure cells of its own creation and lifted to the top of the atmospheric column. Only 1% of the atmosphere lies above the 10 hPa pressure level. The maths is (10/1000)*100/1 = 1%
10 hPa represents an elevation of 30 km. For surface dwellers that’s just a five hour walk. Fifteen minutes on the free-way. In terms of radiation from the Earth the transit from surface to 10 hPa takes no time at all.
The surface at 20-40° south latitude emits long wave radiation according to the season of the year and the distribution of land and sea. At this latitude there is a relatively invariable level of radiation from the sun all year round.
At 10 hPa the signature of surface temperature in winter and summer is plainly evident. There is a obvious warming above the very cold waters up-welling from the deep near Chile in South America where there is very little moisture in the atmospheric column to deplete outgoing infra-red radiation. It is obvious that the temperature of the stratosphere at 10 hPa responds very strongly to variations in the emissions of long wave radiation from the Earth. In fact the differences in temperature at 10 hPa relate to the pattern of long wave radiation from the surface of the Earth.
It is plain that the temperature of the stratosphere is in large part due to the emission of infra-red radiation from the Earth, an entirely different story to that pedalled by NASA, the teachers association and Wikipedia.
There is another way to look at this question of the source of energy that heats the stratosphere and that is in terms of the diurnal range between day (abundant short wave radiation) and night (no short wave radiation from the sun at all).
Siedel et al in a paper that you can access here investigated the diurnal range of temperature at different elevations from the surface through to 10 hPa. The diurnal range at 10 hpa was found to be about half the range of that at the surface and one fifth of the range of actual skin temperature. Plainly, the small diurnal response to the presence of sunlight points towards a large role for outgoing long wave radiation in determining the temperature of the stratosphere. Between day and night there is an ON/OFF relationship between the availability of short wave radiation from the sun. The emission of long wave radiation continues across the 24 hour interval. Siedel et al found a smaller diurnal range of temperature over the ocean than over the land and a larger range in the summer when the diurnal temperature variation at the surface is greater, than in the winter. The timing of the peak in the daily temperature becomes less defined and is more variable as altitude increases. This indicates that the circulation of ozone, as a source of local heating is more important in determining atmospheric temperature than is the 24 hour rotation of the Earth on its axis and the resulting flux in the availability of short wave solar radiation. It confirms that the warmth of the stratosphere is primarily a direct response to the ability of ozone to intercept outgoing long wave radiation rather than direct heating in the process of photolysis or energy release via recombination phenomena.
Why worry about this you ask? The origin of the energy to heat the stratosphere is a matter of considerable importance because ozone heating in the winter hemisphere is the origin of the variation in surface weather and climate on all time scales. If one is not cognizant of the source of heating that gives rise to the stratosphere it is hard to conceive that the winter stratosphere can play a major role in the evolution of weather and climate. If one imagines that the temperature of the stratosphere is due to incoming short wave radiation alone then the source of the so called ‘coupling of the troposphere and the stratosphere’ becomes a baffling mystery. To be charitable, the misunderstanding of atmospheric processes amongst those who write UNIPCC reports has its roots in misconceptions, a failure to observe and disinterest based upon unshakable belief patterns. Science thrives on scepticism, curiosity, observation and checking. Dogma is the companion of superstition and fear. Dogma is also the source of inappropriate responses to natural phenomena. It has led to the burning of witches. It has led to the demonization of carbon dioxide, a trace constituent in the atmosphere that represents plant food, a substance at the base of the food chain for which demand is so voracious that it has never been anything but a trace constituent of the atmosphere or the ocean. Today it stands at 400 parts per million in the atmosphere, the advance from 300 ppm leading already to a marked greening of semi arid climates. Give a plant more CO2 and it requires less water.