Why do floods happen?

Submitted by Matthew on 2 February, 2011 - 12:25

Why do floods happen? And why so fast rising? In Queensland, it had rained fairly continuously for a long time before the floods suddenly arrived. Their depth, some 5m in Brisbane, was also far greater than the depth of the rainfall.

Are the recent floods in Australia, Brazil, Sri Lanka and Pakistan, the forest fires in Russia and so on, symptoms of CO2-induced global warming and climate change? I don’t know, and neither does anyone else. The science of climate is an inexact one, being better with long-term general predictions than short-term ones relating to quite small areas of the Earth.

It is interesting, nevertheless, to look at some of these long-term predictions:

1. Rising air temperature (near surface); 2. rising atmospheric moisture content; 3. rising ocean heat content; 4. rising sea level; 5. rising sea surface temperature; 6. rising temperature over oceans; 7. rising temperature over land; 8. loss of snow cover; 9. loss of glaciers; 10. loss of sea ice; 11. latitudinal shift of the jet-stream; 12. changes in soil moisture content; 13. increases in drought events and severity; 14. increases in flood events and severity; 15. reduced crop production capacity due to precipitation and drought events.

And, according to the OSS foundation, each of these is actually occurring.

This does not amount to conclusive proof. It is true that CO2 absorbs heat radiation and, all other things being equal, it logically follows that increased atmospheric CO2 would lead to increased average temperatures.

Earth benefits from a substantial “greenhouse effect”, explained by the Irish physicist (and pioneering alpinist) John Tyndall, who showed in 1863 that water vapour in the atmosphere absorbed infra-red (heat) radiation. He found that the contribution of other gases, such as CO2, was negligible. At that time, average CO2 levels were about 25% lower than today. This effect keeps the Earth about 30 °C warmer than it would otherwise be and prevents substantial day-night fluctuations, clearly making the Earth much friendlier to life.

Around 1900, the Swedish physical chemist Svante Arrhenius studied the absorption of infra-red by CO2, predicting that doubling current levels would lead to an average rise of about 2°C. This compares with the 2-4.5°C predicted by the IPCC2. He estimated that it would take 3000 years for this to occur but, at present-day rates, it will occur in about 100 years.

Throughout the 20th century, CO2 emissions grew as fossil fuels were burnt at an increasing rate. It was assumed that most of the extra CO2 would be absorbed by the oceans. In 1957 oceanographer Roger Revelle showed that the ability of the oceans to do this was lower than thought.

Earth’s climate is very complex, depending on energy from the Sun, the Earth’s rotation, the tilt of its axis, and the unequal distribution of land and sea. Australia is affected by periodic warming and cooling of the Pacific Ocean due to El Niño and La Niña effects. Rainfall is extremely variable from year to year and decade to decade. Occasionally, extreme rainfall with flooding is to be expected. This is because the ground can only absorb so much water and subsequent rain runs straight into rivers.

How can this cause the enormous depths of flooding seen in Queensland recently? Well, if 21cm of rain fell on Queensland in December and this was to run straight into the 6.5% of the state which is water (rivers and lakes), their depth would increase by a factor of 100 ÷ 6.5 or about 15-fold. That’s about 3m. Water running downhill to lower-lying areas will be concentrated in smaller areas and therefore rise higher. The speed of rise will be limited by the speed of the flow into the rivers. This is where human activity can have an effect.

Australia has long had a policy of deforestation and brush clearance. This increases the rate of run-off, while decreasing the ability of the soil to absorb water. Also, people have been placed in the target area by policies of building on flood plains. The degradation of Australia’s environment since colonisation by Europeans is discussed in Jared Diamond’s book Collapse 3.

It is said that ocean surface temperatures were particularly high at the time of the rains and that this would have contributed to their amount, by causing more water to evaporate. This does not amount to proof that global warming made the floods worse but it adds to the circumstantial evidence.

Not everyone is convinced, though. Brendan O’Neill4, eminence grise of sp!ked5, a compendium of contrarian thought, wonders whether environmentalists, with their “obsession with global warming”, might have “exacerbated the impact of the flooding in Brisbane.” This, he claims, is because Australian politicians believe that the problem is “increased heat, droughts and a lack of rainfall.”

O’Neill is clearly not aware that low rainfall characterises the Australian climate, except in narrow coastal areas. He criticises the water policies of the Queensland government, which built dams to collect water, not realising that between 1991 and 1995 Queensland suffered its worst drought on record, severely affecting agriculture. He is not aware of the effects of El Niño and La Niña events on droughts and flooding He is also not aware that global warming models predict increased droughts and floods.

Diamond describes how Australian farming policies are quite inappropriate for the climate, a deeply unwelcome message for sp!ked, whose writers reject any suggestion that the world’s resources may be limited and believe that all problems can be solved by technological progress.

1. Open Source Systems, Science, Solutions http://www.ossfoundation.us/

2. IPCC = Intergovernmental Panel on Climate Change http://www.ipcc.ch

3. Collapse: How societies choose to fail or survive, Jared Diamond, Penguin 2006

4. Brendan O’Neill, former journalist for LM (Living Marxism) magazine, journal of the Revolutionary Communist Party, now finds himself blogging for the Telegraph: http://blogs.telegraph.co.uk/news/

5. www.spiked-online.com/

Add new comment

This website uses cookies, you can find out more and set your preferences here.
By continuing to use this website, you agree to our Privacy Policy and Terms & Conditions.