The Science behind the climate debate

By: David Walker
August 3, 2009

There have been a couple of areas of the science upon which the premise that our climate is changing more rapidly than it should be that have puzzled me.

The first of these is how can we tell the concentration of atmospheric CO₂ back to 10,000 years ago? The answer is that in parts of the world that are covered with snow, yearly snowfalls accumulate as a sequence, and can be identified much like the growth rings on a tree. Air is trapped in the deposited snow and ice, and the annual snow deposits can then be drilled, removed as an ice core, and analysed. It is a relatively simple procedure to measure the concentration of CO₂ (and other gases) in the air. Analysis of samples up to 400,000 years old has been made.

I also questioned how we can get an accurate measurement of the temperature over such an extended period. The science here is a little more sophisticated. Most oxygen has an atomic weight of 16 (¹⁶O), but there is also an isotope ¹⁸O with an atomic weight of 18. Water molecules with ¹⁸O require more energy (higher temperatures) to evaporate them. Thus snow/ice laid down in periods of higher temperature contains more ‘heavy oxygen’ in its H₂O. The ratio of the 2 isotopes in the snow enables the calculation of temperature of the atmosphere all those years ago!

The accompanying chart from CSIRO shows that CO₂ concentrations (the top line on the chart) over the last 1000 years have generally ranged around 270 parts per million (ppm). In recent years, however, the concentration has increased by about 30% over previous levels. Over the same period, temperatures (bottom line on the chart) have started to increase.

Another question that I had concerned the prediction that hotter temperatures meant less rainfall. My thought was that higher temperatures would mean more evaporation, more cloud and more rain. However, the problem is that over hot land masses, higher temperatures mean high pressure cells, which would tend to block moist air. Indeed the weather patterns associated with the drought that has had such a huge impact on South East Australia over the last eight or so years have been dominated by highs. 

The effect that warm surfaces have on air circulation is also the basis for El Niño and its associated droughts over Australia. When the sea surface temperatures in the mid Pacific are warmer than those to the north of Australia, the air over that warmer water tends to rise, and the air from the Australian region flows eastwards to replace it. Thus the Trade Winds, which normally come from the central Pacific towards Australia, are reversed, and lower rainfall eventuates. 

Similarly it is ‘sea surface temperature anomalies’ off NW Australia that have influenced the record low rainfall that fell across the wheatbelts of Western and eastern Australia in recent years. According to the WA Dept of Agriculture and Food and the University of NSW, a strong high pressure system became established in the eastern Indian Ocean. Hence storm activity in the systems travelling from west to east was forced below Australia, only moving north again when they got to New Zealand and the central South Pacific. Moisture movement and cloud activity from the North West was virtually non-existent and, in combination with weaker cold fronts, meant that rainfall was well below average.

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