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Evidence Linking Global Warming and Extreme Weather

 “All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be….  ”  – Trenberth


Kevin Trenberth, senior scientist at the National Center for Atmospheric Research, explains that asking for proof that global warming causes severe weather, is asking the wrong question. “All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be. The main way climate change is perceived is through changes in extremes because those are outside the bounds of previous weather. The average anthropogenic climate change effect is not negligible, but nor is it large, although a small shift in the mean can lead to very large percentage changes in extremes. Anthropogenic global warming inherently has decadal time scales and can be readily masked by natural variability on short time scales.”

Scientists  have been very cautious about linking severe weather events to global warming, but the link is getting stronger each year. The Earth has warmed an average of 0.82 over the last century, which doesn’t sound like much, but it means that some places have warmed much more than in the past. Since the amount of moisture the air can hold depends on the temperature, the air can now hold about 6% more moisture. Before 2010, scientists would cautiously point out that higher temperatures lead to the likelihood of drought, and that more energy and moisture in the atmosphere was a recipe for severe weather. But how is it possible to establish that weather events were becoming more extreme?

There are many reports like the interim report by the Climate Council in Australia which found that, in the period between 1971 and 2008,  heatwaves in Australia were becoming more frequent, increasing in intensity and are lasting longer. The report said climate change was  having a key influence on a trend that has seen the number of hot days in Australia double and the duration and frequency of heatwaves increase. Reports like that were not good enough for the skeptics. By 2011 a good case was established that global warming was causing heat waves and droughts in the U.S., but the case was not strong enough to overcome the Skeptics objection, even when in 2012, a definite probability link  was established for  extreme temperatures and droughts. 

To understand whether a weather event is extreme, it must be compared to the norm. This can most easily be done for temperatures, as we have over a century of temperature records from almost all parts of the world.

Example of a Normal Distribution

Example of a Normal Distribution – Click to Enlarge

There is enough temperature data that normal distributions can be graphed, which allows us to quantify  the probability of a temperature event. In the example at the right, the maximum in the curve is the mean of the data. The probability of the occurrence of an event can be measured by the number of standard deviations, sigma(σ), a particular value is from the mean. The values within 2σ of the mean, blue, are considered to be in a mostly normal range. Those from 2 to 3σ, yellow, are considered to be exceptional events, and those beyond 3σ, red, are considered to be extreme. Those yearly events that fall in the yellow range are considered to be 100 year events while those that fall in the red are 1000 year event.

As an example, the normal distribution graph to the right is for the temperatures in Moscow since 1950.  The maximum in the curve is the average temperature, which is set to zero, and the temperature for other other year is described as a temperature anomaly, i.e., how far it is above or below the average value.Moscowjulytempanomaly2010 The curve approximates a normal distribution so the standard deviation of the temperature anomalies can be used to decide whether an event is extreme. The temperatures for 1972  and 2001 fall in the hundred year event range, while  that for 2010 would only be likely to occur only once in every hundred thousand years, unlikely, but still possible.

The Skeptics would still not be convinced, claiming that the link to global warming climate change causes severe weather was not proven, but proof is not necessary when probabilities for a large number of events are involved. For instance, you have only a 50% chance of calling a coin toss correctly, but you can likely guess the number of heads on 1000 flips with less than 1% error. Small differences in probabilities lead to big outcomes. The rules of blackjack give the house a 50.5% to 49.5% advantage, and though some players may win thousands on a lucky streak, considering all the bets placed, the house will make millions from that small difference in probability. And, probabilities are useful for predictions. A 0.270 hitter may get the game winning hit at his next bat while a 0.300 pinch hitter may strike out, but with the game on the line, the coach will likely pinch hit. If trying to predict the future, it is better to go with the probabilities. Though  it is not possible to prove that any one weather event is caused by global warming , scientists have observed a change in probabilities of severe weather events over long periods of time. With the thousands of weather events that occur on the Earth each year,  a small change in probability can cause an definite change in the number of severe weather events.  


An even more convincing argument can be made that global warming causes severe temperatures if the normal distribution is examined as a function of time. Research by James Hansen has established the link by showing that the normal distribution has changed since 1951. The curves show that beginning in about 1970, the mean begins to move to the right and the the curves flatten, showing that the probability of extreme temperatures increase greatly from 1950 to 2011.  His work shows that the probability of extreme temperatures is 10 times as great as for the 198o to 2010 years.

It should also be noted that the left side of the graph flattens, but that the probability of extremely cold temperatures is not zero. There is still a significant likelihood of cold temperatures -and a cold winter now and then does not disprove global warming.

The Skeptics are still claiming that is not proof enough, and that the data says nothing about droughts and wildfires.  There are still some Skeptics who argue that this does not mean  heat waves necessarily related to droughts or that the droughts are causing the increase in wildfires we have experienced, but their arguments seem to be improbable. It should be clear by now that no amount of evidence will convince Skeptics who wish to ignore probabilities.

(C) 2014  J.C. Moore



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    […] Trenberth, senior scientist at the National Center for Atmospheric Research, explains that asking for proof that global warming causes severe weather, is asking the wrong question. – See more at: http://jcmooreonline.com/2014/05/08/evidence-linking-global-warming-and-extreme-weather/#sthash.upNu… […]

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