Extreme floods and climate change – the science

Taken from climateactioncentre.org

Is there any relationship between the massive Queensland floods and global warming? Scientific opinion seems to be that warmer sea surface temperatures due to climate change (and hence more water vapour in the air), as well as a very strong LaNina with warmer waters off eastern and northern Australia, are both likely contributors to the record flooding.
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On Wednesday the Fairfax press reported that: “Australia has been known for more than 100 years as a land of droughts and flooding rains, but what climate change means is Australia becomes a land of more droughts and worse flooding rains,” David Karoly, from Melbourne University’s school of earth sciences, said. Professor Karoly stressed individual events could not be attributed to climate change, “But the wild extremes being experienced by the continent were in keeping with scientists’ forecasts of more flooding associated with increased heavy rain and more droughts as a result of high temperatures and more evaporation””On some measures it’s the strongest La Nina in recorded history … [but] we also have record-high ocean temperatures in northern Australia which means more moisture evaporating into the air,” he said. “And that means lots of heavy rain.
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“Further comments have been reported by Reuters:
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“I think people will end up concluding that at least some of the intensity of the monsoon in Queensland can be attributed to climate change,” said Matthew England of the Climate Change Research Center at the University of New South Wales in Sydney.
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David Jones, head of climate monitoring and prediction at the Australia Bureau of Meteorology in Melbourne: “The first thing we can say with La Nina and El Nino is it is now happening in a hotter world,” he told Reuters, adding that meant more evaporation from land and oceans, more moisture in the atmosphere and stronger weather patterns. “So the El Nino droughts would be expected to be exacerbated and also La Nina floods because rainfall would be exacerbated,” he said, though adding it would be some years before any climate change impact on both phenomena might become clear.
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Prominent U.S. climate scientist Kevin Trenberth said the floods and the intense La Nina were a combination of factors. He pointed to high ocean temperatures in the Indian Ocean near Indonesia early last year as well as the rapid onset of La Nina after the last El Nino ended in May. “The rapid onset of La Nina meant the Asian monsoon was enhanced and the over 1 degree Celsius anomalies in sea surface temperatures led to the flooding in India and China in July and Pakistan in August,” he told Reuters in an email. He said a portion, about 0.5C, of the ocean temperatures around northern Australia, which are more than 1.5C above pre-1970 levels, could be attributed to global warming. “The extra water vapor fuels the monsoon and thus alters the winds and the monsoon itself and so this likely increases the rainfall further,” said Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado.
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In The Guardian, Prof Vicky Pope, head of climate change advice at the Met Office, said that as a general point, a warmer world is a wetter world. “As the average global temperature increases one would expect the moisture content of the atmosphere to rise, due to more evaporation from the sea surface. For every 1C sea surface temperature rise, atmospheric moisture over the oceans increases by 6-8%. Also in general, as more energy and moisture is put into the atmosphere [by warming], the likelihood of storms, hurricanes and tornadoes increases.”
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At Climate Progress, Joe Romm tells of his recent interbiew with Kevin Trenberth: “I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.” That’s Dr. Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, on the warming-deluge connection.
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An ACF Fact Sheet reports: “Recent scientific advice to the Queensland Government warned that the state would be threatened by higher flood levels from intense torrential downpours brought on by climate change. In 2010 the Scientific Advisory Group to the Queensland Government’s Inland Flooding Study advised that “an increase in rainfall intensity is likely” and “the available scientific literature indicates this increased rainfall intensity to be in the range of 3–10 per cent per degree of global warming”.