Weather extremes and climate change

That global warming could lead to extreme weather events seems reasonable to me. And such events can cause large losses of life and property – made worse by their unpredictable nature.

So there is scientific debate on whether the extreme events we have seen over recent years resulted from climate change or not. Some scientific work suggests “yes,” others “no.” (And you can guess which work the climate denial echo chamber amplifies).

Now, James Hansen and two co-authors are publishing work suggesting “yes” – we are already seeing extreme events resulting from the currently rather small amount of global warming – and we expect to see more extreme events in future. They base their conclusions on the statistical distribution of all weather events (measured by temperature) and the change in that distribution over recent years.

You can see the basic idea in the graph below taken from their paper (Climate variability and climate change: The new climate dice).

Credit: Climate variability and climate change: The new climate dice.

Here the frequency of the event (temperature anomaly  measured against the 1951-1980 average) is plotted (Y-axis) against the size of the anomaly (expressed as a ratio of the anomaly divided by the standard deviation of the distribution σ). The black curve shows the normal distribution expected from such data As the authors describe it:

“A normal distribution of variability has 68 percent of the anomalies falling within one standard deviation of the mean value. The tails of the normal distribution . . .  decrease quite rapidly so there is only a 2.3% chance of the temperature exceeding +2σ, where σ is the standard deviation, and a 2.3% chance of being colder than -2σ. The chance of exceeding +3σ is only 0.13% for a normal distribution of variability, with the same chance of a negative anomaly exceeding -3σ.”

As you can see the variability in anomalies for the periods 1951 – 1961 (red), 1961-1971 (yellow), and 1971-1981 (green) are very similar to the normal distribution for the whole period 1951-1981. But in the periods (1981 – 1991 (light blue), 1991 – 2001 (dark blue), and 2001 – 2011 (violet), the distribution has shifted to higher, more positive, values of the anomaly.

This means that those more extreme temperature events have become more common in the last 30 years. In the above figure the chance of temperature exceeding +2σ has moved from 2.3% to greater than 15%. And the chance of exceeding +3σ from only 0.13% to around 10%.

The paper presents the effects of the movement in climate of different regions and different seasons. Here are maps for June-July-August in the years 1965 and 2010. The colour code indicates the size of the temperature extremes in terms of σ.

Credit: Climate variability and climate change: The new climate dice.

Notice the larger regional occurrence, of more extreme events (brown=>+3σ and red=+2-+3σ) in 2005.

As they say in their abstract:

“These extremes were practically absent in the period of climatology, covering much less than 1% of Earth’s surface. Now summertime extremely hot outliers, more than three standard deviations (σ) warmer than climatology, typically cover about 10% of the land area. Thus there is no need to equivocate about the summer heat waves in Texas in 2011 and Moscow in 2010, which exceeded 3σ – it is nearly certain that they would not have occurred in the absence of global warming. If global warming is not slowed from its current pace, by mid-century 3σ events will be the new norm and 5σ events will be common.”

So, that’s another study to add to our assessment of the effects of climate change. Given the diversity of conclusions around it may not be the clincher. I guess we will have to wait for the next review from the Intergovernmental Panel on Climate Change (IPCC). After all, that is the body that will thoroughly review all that has been published and draw conclusions. I think it is likely that the next review will conclude we are already seen the effects of global warming in these extreme weather events.

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