Consider the science behind the reality of climate change - Yorkshire Post Letters

Once again, Michael Carter (YP Letters, December 7) tries to convince Clark Cross that a "mere" 0.04 percent concentration of CO2 in our atmosphere can be responsible for the global warming that is disrupting our weather so blatantly.

At the risk of being accused of ‘blinding with science’, I wonder if a little technical explanation might help clarify why this apparently tiny proportion could be so powerful?

Energy arrives on our planet as solar radiation - both visible sunlight and throughout the wider electromagnetic spectrum - including ultraviolet and the infrared frequencies that are familiar in the operation of heat lamps, etc.

The frequencies of light are not present equally across the solar spectrum and they interact variously with different substances.

For example, the sky looks blue because certain gases in the atmosphere absorb that frequency from the sunlight and re-emit it in all directions, including back to space, so the light 'bounces around' on its way to our eyes. The sun shines yellow and red because that light comes straight through - more intensely red at sunrise and setting as more atmospheric distance is crossed, so scattering more blue.

All substances absorb and emit energy to different extents, according to their molecular structure and the varying frequencies of radiant energy. Incident energy absorbed by a molecule may be re-emitted at a different frequency.

Ground substances warmed by incident infrared, that has also passed directly through the atmosphere, cool down again by emitting other frequencies which, it so happens, can be absorbed by atmospheric CO2, methane and water vapour, but not by nitrogen and oxygen.

Why so? The more complex the molecule, the greater the number of frequencies it may absorb and emit, since the range of vibrational resonances in a molecule relates to the number of atoms it has and the pattern of their bonding together - just as the note of a guitar string changes with length.

Nitrogen and oxygen, the two most abundant atmospheric gases, have two atoms each per molecule. CO2 has three atoms, as does water vapour (H2O), while methane (CH4) has five.

Thus methane has much greater potential absorbance, being at least 30 times more powerful than CO2 per molecule as a greenhouse gas. However, being of course readily reactive with oxygen, methane does not stay long in the atmosphere and at just two parts per million it has less than one 200th the concentration of CO2, so relatively less warming influence.

Water vapour concentrations in the atmosphere are highly variable geographically - highest in the tropics, much less at the poles - but typically averaging around 0.4 per cent worldwide.

Hence water vapour has around ten times the concentration of CO2, but due to a much lower molecular absorbency at the relevant frequencies, it contributes just half of our planet's temperature moderation - much as it has always done.

The rest is largely due to the influence of CO2, so the 50 per cent increase in its concentration over the past 200 years is the predominant cause of current excess global warming. Nitrogen and oxygen make up some 99 per cent of the atmosphere between them, but contribute nothing in this respect.