It is well known that climate change will alter the occurrence of extreme weather events like heat waves, droughts, and severe storms. But weather is unpredictable and naturally variable, so how can we be sure climate change is happening today?

Climate change attribution

Scientists have recently developed tools for so-called event attribution, to say (through the use of statistics) whether a particular extreme weather event is caused by climate change. The fourth annual report on event attribution was just published in the journal Bulletin of the American Meteorological Society (BAMS). Researchers around the globe used different methods to assess 28 events that occurred in 2014. They found that some of these events probably would not have happened without climate change.

Why ‘probably’?

Any individual weather event is a part of a chaotic and complex system (yes, those are the technical terms). Because of this, it is theoretically impossible to predict weather over any meaningful timescale. So scientists turn to probabilities.

When your local weather forecaster tells you there’s a 30 percent chance of rain, that number doesn’t come out of a hat. The percentage comes from many weather models run over and over again. A 30 percent chance of rain tomorrow means that for every 100 model simulations of the weather tomorrow, 30 had rain.

To attribute an event to climate change, scientists run climate models many times both with and without the effects of greenhouse gases. They then compare the model results to observations. If the observed event, say a major heat wave, occurred often in the models that included greenhouse gases but only rarely in the models without them, they would conclude that the heat wave can probably be attributed to climate change. There’s still a chance the event would have occurred anyway – but you wouldn’t bet on it. Just like if your local weather forecast called for a 90 percent chance of rain, you’d probably leave home with your umbrella.

2014 events attributable to climate change

The BAMS study was not an exhaustive study of all extreme weather events in 2014, so it is possible that more events can be attributed to climate change than just those listed here. The researchers found that climate change is responsible for:

• An increased risk of fire in California (though not the 2014 fire season, specifically)
• The major heat wave that hit Argentina in December 2013
• Record warmth observed in Europe in 2014 (as measured by the annual average temperature over the region)
• The 2014 drought in East Africa (drought is caused by many factors – several, but not all, factors in this event were caused by climate change)
• The broiling spring of 2014 in Korea (spring temperatures in northern China were also higher than average, though the climate change influence was not as strong there)
• A greater probability of warm surface ocean temperatures in parts of the Pacific Ocean
• The increased frequency of hurricanes near Hawaii, as was seen in 2014
• Increasing the likelihood of several hot spells across Australia in 2014

The significance of climate change

What’s significant about the BAMS study is not which particular events in 2014 were probably caused by climate change but that any at all were probably caused by climate change.

If greenhouse gases weren’t changing the climate, then the simulations with and without greenhouse gases would give equal probabilities of a particular event. But they don’t.

To say it another way, if greenhouse gases weren’t changing the climate, then the model simulations without greenhouse gases would be just as successful as the others at predicting the observed events. But they’re not.

Extreme weather events have always happened, and always will. But the fact that the natural variability in climate models no longer predicts all extreme events is strong evidence that greenhouse gases are changing the climate right now. And the models tell us that the impacts will only grow in the future.