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La Niña chief culprit in flooding rains, global warming a co-conspirator

La Niña brings us rain - global warming makes it worse. AAP

As six weeks’ rain gets dumped on Sydney in a single day and Queensland cleans up after shocking floods, it’s not surprising people ask if global warming is to blame for our weather woes.

But following the summers’ floods, we heard from scientists that the torrential rains were mainly caused by an unusually severe La Niña event, a natural phenomenon that has occurred as far back as we can measure.

Other scientists told us that, as far as we know, climate change is not having any real effect on La Niña/El Niño episodes. This year may have been extreme, but next year is a new roll of the dice. Ergo, climate change has nothing to do with these heavy rains, right?

Well, no. It is true that La Niña is the main factor that made this summer so wet, and this La Niña was probably not unprecedented. La Niña warms the waters just north of Australia, while cooling the waters farther east near the equator.

The result is that the atmosphere north of Australia warms, rises, sucks in an outsized share of the water vapour that clings near the surface of the tropical oceans, and unceremoniously dumps it nearby. During the Austral summer monsoon, “nearby” very often means northern coastal regions of the continent.

The question is: if the same La Nina had happened without global warming, would the rains have been less severe?

The most straightforward way to answer this would be to obtain perfect observations of two identical La Niñas, one now and one in a cooler (or warmer) global climate. Unfortunately, no two La Niñas (or snowflakes, or just about any atmospheric event) are the same.

But two studies recently appearing in Nature have done the next best thing. And what they conclude is: yes, all other things being equal, global warming is causing the most severe storms to dump more rain than they used to, something like 5-10% more than fifty years ago.

The first of these studies simply looked at trends in the amount of rain recorded in the most severe rain events throughout the Northern Hemisphere. Data were insufficient in the Southern Hemisphere, but there is no reason to expect things to be any different here. The researchers found upward trends nearly everywhere, and the geographic pattern of the trends bore no relation to an El Niño or La Niña pattern.

Many of these upward trends had already been documented and tentatively attributed to global warming. They are expected because in a warmer climate more ocean water evaporates and the atmosphere becomes more humid; this means more water available for the atmosphere to dump on us.

What made the new study novel is that the researchers were, for the first time, able to rule out alternatives such as natural variability. They also found that the trends appeared to be several times faster than indicated by climate models.

Research is now underway to get to the bottom of this worrying finding, which is one of a number of recent cases where models have under-predicted observed rates of change.

The second study took a more theoretical approach. Investigators repeatedly simulated weather events under global circulation conditions that caused severe flooding in Wales in 2000. They did this thousands of times, by making the code available to run on idle desktop computers, with today’s climate and the climate we would have had without any human influences. What they found is that human-caused warming has made these severe floods about twice as likely as they would have been without global warming.

Could this be true in Australia? The laws of physics are the same in both hemispheres, so the answer is almost certainly yes. I suspect that before long, a similar study will be done for eastern Australia, and will come up with a similar answer.

While a 5% increase in rainfall may not sound like a big deal, it translates into a much larger relative increase in floodwater that exceeds what soils can absorb. This extended the devastating consequences we saw this summer to places that might otherwise have been spared.

Australia has always had unusually variable rainfall. Because of this, a 5% increase in extreme rain rates due to global warming is hard to spot, even more so when many regions of the country are getting rain less frequently than they used to.

But future warming is expected to far exceed what has occurred so far. If this comes to pass, soon it won’t be so hard to spot the impact even in Australia. In the mean time, the main factor to watch will continue to be the mercurial siblings, El Niño and La Niña.

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