Why Can’t Scientists Say the Recent Extreme Weather Events Are ‘Proof’ of Climate Change?

By Nicole Heller, Claudia Tebaldi, and Phil Duffy, Climate Central Research Scientists

Massive flooding and extreme heat in Pakistan, devastating heat and wildfires raging across Russia, a giant iceberg calving off Greenland, a wicked hot summer on the U.S. East Coast: this is exactly the stuff scientists such as ourselves have been warning would be a likely consequence of climate change. So, it might seem frustrating when you read a news report saying something along the lines of “while these events are consistent with climate change, no single event can be directly linked to or regarded as proof of climate change.”

If that is the case, you may be wondering when we are going to get that definitive proof – the smoking gun, if you will – that links today’s weather events to climate change. The thing is that if we continue to look for proof in those terms, it may take a very long time.

The reason is that the extreme weather events we are seeing today are theoretically possible with or without climate change. That’s why these events don’t prove the existence of human-caused climate change any more than last winter’s snowstorms disproved it. While there’s overwhelming observational evidence showing that humans are affecting climate, this evidence comes from long-term trends, rather than individual events.

Extreme events are related to climate change, however: the odds of them happening are much greater with climate change.

Satellite image showing the swollen Indus, Jhelum and Chenab Rivers in Pakistan after unusually heavy rainfall, taken on Aug. 11 from NASA's MODIS instrument aboard the Terra Satellite. Image credit: NASA.

Take the deadly 2003 heat event in Europe that killed an estimated 40,000 people. To explore links between climate change and that heat wave, Dr. Peter Stott of the UK Met Office and colleagues ran two types of climate simulations. One replicated conditions of a natural climate (unmodified by human influences) and the other included both natural influences and the effects of human emissions of greenhouse gases from burning fossil fuels and other activities. The authors estimated the probability of exceeding an extreme temperature threshold, measured as the degrees above the historic summer mean, in both simulated climates. In their paper, published in Nature in 2004, they wrote: “According to our calculation, there is a greater than 90% chance that over half the risk of European summer temperatures exceeding a threshold of 1.6K  (which is the same as 1.6 degrees Celsius, or 2.9 degrees Fahrenheit) is attributable to human influence on climate.”

In other words, Stott and colleagues asserted with very high confidence that the chances of having a heat wave of a magnitude similar to the 2003 event had doubled because of human-induced warming. But notice what their analysis did not find. It did not conclude that the heat wave would not have occurred, absent human influences on climate. That is because that heat event could still have taken place without human-induced climate change, although with relatively lower likelihood.

We can estimate probabilities, but rarely can it be asserted with 100 percent confidence that there is a causal relationship between variables. That might be how some findings by scientists get interpreted, but scientists don’t tend to talk that way. So we could run computer models today and try to determine how likely the extreme weather of this summer was with and without human influences. And we could estimate by how much human-induced climate change has elevated the odds of these events happening. But we cannot say in a scientifically rigorous way that the event was definitely due to climate change. 

The point is that while it is a perfectly reasonable question to ask: “Was this event due to climate change?” it would more useful to ask a related question: “are we putting ourselves at greater risk of experiencing this kind of event?” And to that scientists can answer with high confidence: yes!

Now, you might think this question is less interesting or useful, and perhaps not as worth asking than the first one. But we would argue that, in fact, it is very important to pose this question, and to carefully consider its answers.

Think of smoking, sun bathing without sunscreen, eating lots of junk food and so on. You may not be able – ever – to unequivocally attribute one person’s problem to the effects of these activities: people develop lung cancer without smoking, for example, but as a population we know we are better off wearing sunscreen, watching our cholesterol, and not smoking, since all of these actions have been shown to make the chances of harm to our health lower.

So, although climate scientists cannot give us the comfortable certainty of a yes or no answer to the “who’s to blame question,” that doesn’t mean people should roll their eyes and discard the issue altogether. Rather, we should confront the reality that we are further escalating the risk of these extreme weather events every day that we postpone action to reduce greenhouse gas emissions.