A chemist inside the greenhouse

THE WORLD is warming up, according to the experts who assiduously watch their thermometers. The weather will show more extremes, rainfall will shift around the country, and those living to close to the sea will be flooded.

It is all because of the greenhouse effect.


If there were no greenhouse gases the mean temperature on earth would be a bleak 18 degrees below zero. With them, it is a comfortable 33 degrees higher.


Before there were plants on earth, the atmosphere was almost entirely carbon dioxide and the earth was as hot as hell.


Well, at least as hot as Venus (450C) where the atmosphere is 96 per cent carbon dioxide today. Plants keep us cool.


It was the French mathematician Fourier in 1827 who first described how traces of carbon dioxide (and water vapour, variable from 1 to 4 per cent) in the atmosphere caused it to warm up and warned that human activity could affect climate. In 1896 the physical chemist Arrhenius coined the phrase "greenhouse effect", believing that the gases behaved like glass in a greenhouse in providing the warming filter, letting sunlight in to warm the earth, but blocking infra-red (heat) radiating back out again to cold outer space.


The optical physicist Robert W. Wood in 1909 made two identical greenhouses, one with glass and the other with rock salt (which is transparent to both infra-red and visible light), and showed they both reached very nearly the same temperature.


Today thin plastic sheets (which do not absorb much infrared) are often used instead of glass, and work just as well because it is actually the suppression of ventilation that causes hot houses to be hot. Nevertheless, the term has stuck, like so many others in science that were assigned for totally the wrong reasons (like oxygen and helium). This constant reminder keeps us humble.


Earth's dry atmosphere contains roughly 78 per cent nitrogen, 21 per cent oxygen, and 0.9 per cent argon, several other gases making up the last 0.1 per cent. So why have scientists picked on the miserly (0.03 per cent, give or take) amount of carbon dioxide?


To answer that, we need to note that radiation (radio, microwave, infra-red, visible light, x-ray and such) all belong to the same big family, differing only in the length of their waves.


Transmitted radio waves are picked up by a radio receiver antenna by inducing an electric dipole in the antenna. The plus/minus of a dipole is the electric equivalent of the north/south of a magnet. This extracts some of the energy from the wave that the radio converts back to sound.


In the same way, water in a food in a microwave oven extracts heat from the microwaves. Some container materials do the same, while others microwave-safe do not. Melamine becomes much hotter than polythene or glass.


Some molecules in the atmosphere act the same way, in that they extract energy from infra-red radiating out from the warm earth to cold space and so keep the earth warmer than otherwise. Molecules that either have a dipole because the atoms joined together are very different electrically, or are easily induced to have one, are those that are most effective. This puts water at the top, followed by methane, then carbon dioxide.


It leaves out oxygen, nitrogen, and argon. It includes to a very minor extent ozone and CFCs, gases which are much more important for UV protection of the earth.


Methane is produced by rice paddy fields and belching cows and released from coal, oil and gas fields but it is the carbon dioxide produced from burning these fossil fuels that is the main focus of concern.


We get most of our day-to-day energy from burning carbon-based fuel to convert it into carbon dioxide. We rely on plants (including most importantly marine ones), to use the sun's energy to recycle carbon dioxide back to fuel again, but that doesn't work unless the plant material is allowed to build up to the equivalent of the fossil plants we have burnt. In the West we have not been doing that for a hundred years or so.


We can't turn carbon dioxide back again (or into something not damaging) on the huge scale needed, because that would take more energy than we got out in burning, unless we are much smarter than plants, and we aren't.


In limestone territory, carbon dioxide taken up in water dissolves limestone and releases it again somewhere underground to form stalactites and stalagmites. That's pretty. But nearly all our excess atmospheric carbon dioxide is taken up by the oceans, where it dissolves the limestone shells of corals and mussels and so on, and is not released. These sea creatures then find it much harder to maintain their shells. That isn't so pretty.


One hope is for us to become smarter in burning fossil fuels. We can try to decarbonise coal in the mine before we burn it as fuel. We can try to capture and compress carbon dioxide at the power station and store it somewhere safe underground (compressed for ever), rather than release it into the atmosphere. If any of this works it will add significantly to the cost. And that is a good thing because the freeloading on the Commons by adding carbon dioxide as waste to the atmosphere is what has stopped really major investment in renewable technologies such as wind, tidal and solar (well, these are all in the end solar or lunar) and has kept nuclear energy uncompetitive in Australia.


The major challenge is for large-scale, efficient and transportable energy sources. Chemical energy stored as petrol (and other fuels) is just brilliant. We are also well designed because we eat and store really efficient chemical energy such as sugar and fats. Our body releases and uses this energy with a biochemical fuel-cell operation involving a smart (rechargeable energy) card called ATP. The hope is for the development of an equivalent fuel-cell technology for our cars.


Nuclear energy in Australia is currently "of middle eastern appearance", suffering a prejudicial stereotype. It is a technology that has made mistakes in the past and is linked to weapons of mass destruction and waste storage.


All these aspects can be dealt with intelligently.


Like most really thoughtful environmentally concerned scientists, I'd rather a tiny amount (in metric tonnes or cubic metres, after decades of use) of stored radioactive waste than the unmitigated disaster of millions of tonnes of carbon dioxide released into the atmosphere. And renewables are not realistically and politically going to fill the gap any time soon.


So next time you fill balloons with helium gas, remember this gas is just compressed dead alpha particles emitted by radioactive elements such as uranium that have kept the interior of the earth warm and collected in oil and gas wells from deep down-under. And it has been released along with the oil and gas to provide your petrol. So be kinder to nuclear.


Professor Selinger is a former professor of chemistry at the ANU.


http://chemistry.anu.edu.au/Staff/BKS/home.html