It’s a tricky business, dealing with events that happen at the same time. Are they linked, or just coincidental? Case in point: climate change.
Humans have raised the atmosphere’s carbon dioxide to 400 parts per million: a higher level than anytime in at least the past 800,000 years. Some data suggest that our planet last saw the current level 15 million years ago. So, no question: We’re living in an unusual situation.
Now let’s add some other unusual event – the recent severe California drought, say. Or the current odd rains in that same state, which caused widespread flooding. Are they linked to the carbon?
Answering such questions necessitates the use of statistics, which (as we all know) are often misused, either deliberately or through ineptitude. Then all sorts of mischief bubbles up. For example, males are five times more likely to be hit by lightning than females. Is it because Mother Nature favors her own sex, as she does with infant survival rates? No, it’s simply because men and boys are more often out fishing or playing golf in open areas.
The most basic error in statistics is assigning probability after an event has occurred. This mistake is seen frequently in pseudoscientific presentations, and has been deliberately utilized by demagogues throughout history.
To illustrate how this works, assume that you’ve just watched three women with red hats cross a street just as the town’s siren conducts its daily noontime test. You could maintain that the odds of witnessing this exact sequence are millions to one against. Did you therefore see something very improbable?
Not at all. Once an event has happened, its chance of occurring becomes 100 percent. It is no longer unlikely in the least. Only a prediction made in advance can assign meaningful odds.
Unusual weather is the most common area where this error pops up, since odd periods of heat, cold, snow or rain bring out the “expert” in all of us. These days, unusual conditions are reflexively seen as signs that the climate’s changing. Trained researchers, however, find it much more difficult to tell whether anything odd has actually occurred. The problem with weather is that the unusual is normal.
A good analogy involves flipping a coin twice. Getting two heads is unlikely, and so is two tails, or first a head then a tail, or the other way around. But while a particular outcome is improbable, the odds are 100 percent that one of them must occur. Thus, an improbable event must happen every time a coin falls to the floor twice.
So it’s reasonable that we’ll often find ourselves in the midst of some unusual condition. Floods, drought, unusual heat or cold are a daily certainty somewhere in the world.
The interesting thing about this way of thinking is that we then live in a universe where everything’s fascinating, but nothing’s completely surprising.