How many times do you see a headline or read an article about climate change, where you are told that threat of some scary extreme event (flood, heat waves, cold wave, etc.) has increased five or ten or 100 times due to global warming?

But as we will see,

**such scare statistics may not mean as much as the headlines suggest.** In fact, they can be quite deceptive. Let me explain.

Many of the scary stories talk about the frequency of exceeding some extreme threshold, such as the number of times temperatures exceed 90F in Seattle during the summer, or the frequency of daily precipitation exceeding 20 inches during the winter in the Cascades--that kind of thing.

**The nature of extremes is that they are unusual.** So a very small increase of their numbers can result in the number of occurrences above some threshold increasing radically.

So if there are normally one day a year with temperatures over 90F during the summer and there is two one year, the frequency has DOUBLED. A 100% increase! Huge.

**Let me do this a bit more quantitatively.** The climatological distribution of temperature is often Gaussian (also called the normal and bell-shaped distribution).

Below is an example of such a distribution, whose mean and most probable value is 75F. The x-axis is temperature and the y-axis is frequency. A measure of the spread or width of the distribution is the

*standard deviation* (greek letter sigma is often used to denote it). 67% of the observations should be within one standard deviation of the mean (I have assumed a 3F standard deviation in the figure below)

You notice the frequency (or probability) of observations drops rapidly for values much larger and smaller than the mean.

**Now let's think about extremes.** What is the probability of experiencing a temperature more than 85F? According to the calculation shown above: .00043 (.043%) Not much

OK, now lets warm things up by 1F, so the mean becomes 76F (I am keeping the shape the same). You might not even notice that. Here is the new distribution.

The probability has increased to .00135 (.135%). OMG! The probability of exceeding 85F has gone up by 3.14 times!

**But think about it a bit more. ** Instead of the scary increase of 3.14 times, think about the actual increase of probability of getting about 85F.

**The increase is ONLY .1%.** Yes, a tenth of a percent increased risk of such warm temperatures. Doesn't seem so scary all of sudden.

I could give a dozen other examples of this...but hopefully you get the idea.

So be very cynical when you read about increases of extreme events due to global warming (or anything else) and particularly when the increases are for exceeding some threshold and given as a factor (5 or 10 or 100 times more).

**Such numbers can be extremely deceptive and imply a big increase in risk in situations with minor changes. **

And there is more. Even if global warming results in a higher frequency of exceeding some threshold, its contribution might be quite small. Consider Hurricane Harvey. Global warming may have increased the precipitation by a few percent, but nearly all of the event was the result of natural processes. When folks hear that global warming has increased the probability of an event by some factor they tend to assume that most of the event was due to global warming, when that is generally not the case.

**Finally, thresholds are generally arbitrary and subjective.** For example, why is 90F more special than 91F? The use of thresholds for such climate communication is essentially deceptive and should be used far less.

What really counts is not the frequency of crossing some threshold but the increase of the amount of the threat (e.g., the increase in high temperature, precipitation). For example, by the end of the century, the Northwest snowpack will probably be down by 30-50%.

** That is scary. **Or the heaviest precipitation in atmospheric rivers could be 20-40% larger. Again, a major issue.

**Global warming is too serious of an issue for us to use questionable, if not deceptive, statistics for communications of its impacts.**