A deeper understanding of climate change...
After the IPCC report published last month, climate change and what could be done against it was yet again all over the media again.
But how does climate change actually work and precisely what are Greenhouse Gases (GHGs) and what do they do?
The Sun
The temperatures we experience on Earth are due to solar radiation.
The sun emits electromagnetic radiation ranging from radio-waves (long wavelength - low energy) to ultraviolet light (short wavelength - high energy).
Three things can happen to the radiation coming from the sun.
1. It gets reflected/deflected.
This happens to just under a third of the incoming radiation.
2. It interacts with the atmosphere.
This happens to just under a quarter of the incoming radiation.
3. It does not interact with the atmosphere and hits the Earth's surface.
This happens to just under half of the incoming radiation.
Our atmosphere
All these factors taken into account, the Earth receives about 340 Watts per square meter on average. Closer to the poles, this number decreases.
On the other hand, the number is quite constant all year round.
Our atmosphere consists of various gases, most notably nitrogen, oxygen, carbon dioxide, argon, and water vapour.
Out of these, oxygen, nitrogen, and argon do not cause heating due to their chemical characteristics.
Carbon dioxide, water vapour, and methane, on the other hand, do get warmer when hit by sunlight.
The mechanism
In all atoms, electrons form a cloud around the nucleus.
What a tiny electron can tell us about the structure of the universe
Molecules are combinations of atoms, and here the electrons form a cloud around the nuclei.
Electromagnetic radiation (such as sunlight) causes changes in the electron clouds of atoms and molecules; incoming radiation can 'knock' an electron onto a higher energy level, leaving a 'hole' in its original level.
After a while the elevated electron will plunge back to its original level.
Both these processes cause slight changes in the molecules' geometry, and since the flux of radiation from the sun is continuous so is this conformational change - the molecules vibrate and bend continuously.
When an electron is 'knocked' onto a higher energy level, it absorbs electromagnetic radiation.
When it falls back, it emits electromagnetic radiation.
Due to thermodynamics, the emitted radiation has to be of lower energy than the absorbed radiation. (Otherwise we could build a perpetual motion machine.)
While the incoming radiation that promotes electrons to a higher energy level is ultraviolet (i.e. of wavelengths < 400 nm) the radiation emitted is infrared (i.e. of wavelengths > 800 nm) which we experience and measure as heat.
So much for the fundamentals.
Which GHGs are there?
The one everybody has heard of is of course carbon dioxide, CO2.
However, there are others as well, such as methane (CH4), nitrous oxide (N2O), ozone (O3), water vapour (H2O), and various halogenated gases (CFCs etc.).
All these gases contribute to global warming but to a varying degree.
Compared to the main constituents of our atmosphere (oxygen (O2, about 21%) and nitrogen (N2, about 78%)) the concentrations of the Greenhouse Gases are exceedingly small, e.g. the concentration of CO2 is less than 0.0002%.
However, their effect is undeniable.
The reason for this is that the main constituents of our atmosphere do not act as Greenhouse Gases, they are climatically neutral.
Hence, it is the rest that makes the difference.
In this context it should be mentioned that there are gases which have a cooling effect, most notably nitrogen oxides (other than the above mentioned N2O) and sulphur dioxide (SO2), even though they get much less press coverage.
These gases are generally acid anhydrides which is to say that they form an acid when they come into contact with water.
If they remain in their anhydrous form they can cause respiratory problems animals and humans.
If they react with water vapour in the atmosphere, such airborne acids precipitate with the rain and cause all manner of harmful effects on earth.
Anybody who has consciously lived through the eighties will remember the term 'acid rain' and pictures of forests dying of it.
Where do the GHGs come from?
For the purpose of this article, CO2 has always been present in our atmosphere.
CO2 is the final oxidation product of any hydrocarbon.
A forest fire generates CO2 because of all the carbon contained in the trees.
This is an example of a carbon-neutral process, because the forest came into existence because plants sequestered CO2 from the atmosphere in the process of photosynthesis.
Burning oil or coal also releases CO2.
This, however, is not a carbon neutral process; it is the reintroduction of carbon that was extracted from our atmosphere so long ago that it bears no effect on our current atmospheric composition.
Reintroducing it into our atmosphere today does have an effect.
Methane (CH4) is formed by certain microbes who 'breathe' CO2 and in so doing transform CO2 into CH4.
This is called methanogenesis and it is the principle source of methane.
Large cattle herds are blamed for their methane emissions.
True, but it is methanogenic bacteria in their guts that actually produce the methane.
The same goes for methane emissions from rice paddies, landfill sites, and from the ocean floor.
What happens to atmospheric methane?
Anybody who has had a little bit of chemistry at school (and didn't sleep through it!) will remember that methane is a reactive gas, it burns.
Does atmospheric methane get 'burned' to CO2? Not quite.
In the higher altitudes of our atmosphere chemistry works in different ways to what we are used to down here on Earth.
The reason for that is the much higher UV radiation which leads to species like long-lived radicals.
Methane is oxidised but not to CO2 but only to formaldehyde (H2CO).
This reaction also produces water vapour and sometimes ozone, depending on the reaction pathway. Halogenated gases (such as CFCs) are entirely anthropogenic in nature.
CFCs in particular have become known for their detrimental effect on the ozone layer and were banned under the Montreal Protocol.[REF01]
Halogenated gases are diverse class of compounds and would merit an entire article in their own right.
Nitrous oxide (N2O) is probably best known as 'laughing gas'.
It is used as an anaesthetic and is listed on the World Health Organisation's List of essential medicines.[REF02]
N2O also finds application in rocket fuels.
N2O results from both natural (~ 2/3) and anthropogenic processes (~ 1/3). Like methane, N2O is generally of microbial origin.
Anthropogenic emissions are due to activities such as livestock manure, nitrogen-based fertilisers, and the burning of biomass.
Like CFCs, N2O has a detrimental effect on the ozone layer.
The differences between various Greenhouse Gases
In order to compare the individual contributions of the various GHGs to global warming scientists have chosen CO2 as the reference point.
GHGs differ in their Global Warming Potential (GWP) as well as their lifetime.
The GWP is defined as the heat absorbed by any Greenhouse Gas (bar CO2) as the multiple of the heat absorbed by the same mass of CO2.
As it turns out, the GWP of all Greenhouse gases other than CO2 is invariably higher than that of CO2; by up to five orders of magnitude.
Lifetime may be visualised as the mass of GHG X in a box of volume V divided by the sum of the flow of X out of the box, the chemical loss of X, and the deposition of X.
Put in very simplistic terms: It's In divided by Out.
The lifetime of CO2 itself is a topic of considerable debate. Again, put in very simplistic terms: Due to the enormous variations of the In the mathematical models lose their reliability.
A few practical examples: Methane has a GWP of 28 and lifetime of 12.4.
That means, release of 1 kg of CH4 absorbs 28 times the heat 1 kg of CO2 would absorb and will continue to do so for 12.4 years.
One of the halogenated hydrocarbons, Tetrafluromethane (CF4) on the other hand has a GWP of 50,000 and lifetime of 6,630.
That means, release of 1 kg of CF4 absorbs 50,000 times the heat 1 kg of CO2 would absorb and will continue to do so for 6,630 years.
The term Global Warming Potential implies a time-factor.
This takes into account that GHGs are also removed from the atmosphere.
As methane is converted into other species its effect on global warming decreases.
Over the time-frame of 100 years the GWP of methane is 25.
Over the first 20 years, however, its GWP is significantly higher, 86.
The time-frame is often indicated as a suffix, GWP100 indicates that the time frame for which the GWP has been calculated/estimated is 100 years.
The carbon dioxide equivalent (CO2e) of a gas is simply the mass of CO2 which would warm the atmosphere by the same amount.
There are other ways of expressing the same concept placing different emphasis on different aspects.
Where the Global Warming Potential estimates the heat absorbed by Greenhouse Gases, the Global Temperature Change Potential (GTP) estimates the change in surface temperature of the earth.
Another metric is Radiative Forcing Capacity. It is the amount of energy (per time and area) that would otherwise be lost to space by reflection or deflection.
In the context of the IPCC reports: It is proven beyond reasonable doubt that the current climate change is man-made and that the consequences of continued unchecked release of Greenhouse Gases into the atmosphere will bring disastrous consequences and the forthcoming report of Work Group III on possible countermeasures will make for a very interesting read.
For a selection of similar articles, subscribe to our blog.
01) https://en.wikipedia.org/wiki/Montreal_Protocol
02) https://www.who.int/medicines/publications/essentialmedicines/EML2015_8-May-15.pdf