The Pew Center is teaming up with Scientific American to explain the link between climate change and extreme weather. In a new three-part series featured on Scientific American.com, award-winning science journalist John Carey dissects the science, impacts, and actions to take regarding the record-breaking floods, heat waves, droughts, storms, and wildfires experienced across the United States and the world in the past year. The first installment appears today. (Part two. Part three coming Thursday.)

The Pew Center commissioned the article, and the author and Scientific American had complete editorial control.

Also today, the Pew Center releases a new white paper explaining that the recent spate of extreme weather events is part of a longer trend in rising extremes that reveal a pattern of increasing risk as the planet warms. That paper and a variety of other extreme weather resources are available on a new web page.

The record-breaking flooding in North Dakota over the weekend and record-breaking drought that grips Texas and other southern-tier states are part of this trend. Many reports pin the current extremes on a strong La Niña pattern in the Pacific Ocean. Texas doesn’t always dry up during La Niña; yet, we know the risk of drought is higher during La Niña even if it doesn’t always pan out. In other words, we know that La Niña is a risk factor, just as a high-salt diet is a risk factor for heart disease.

“Risk factor” is also the right way to think about the link between climate change and extreme weather. Climate statistics show that there are long-term trends of increasing heat waves and heavy downpours in the United States, and climate models simulate these trends when greenhouse gases are elevated. And we understand the physics behind the trends. That’s strong stuff, and it gives us confidence that the risk of extreme events is rising because of human-induced global warming.

So La Niña + global warming = even more risk of extreme weather. Patterns of natural climate variability—La Niña, El Niño, the Pacific Decadal Oscillation, etc.—will always be with us, and the risk of various types of extreme weather will bob up and down with these cycles. But there is never just one cause of extreme weather. Instead, there are multiple risk factors, just as there are for heart disease. Over the past 30 years, global warming has emerged as an ever-present risk factor for extreme weather. The difference between the natural oscillations and global warming is that the oscillations come and go, while global warming is ever present and is getting stronger over time.

When people get caught in a flood or a drought, they naturally want to know what caused that particular event. Unfortunately, the answer is never going to be perfectly clear—not because scientists don’t know enough, but because the climate just doesn’t work that way. Consider a parable:

Long ago on planet Cyclon, there was one category 5 hurricane per year. Then the climate began to warm up, and after a few decades, there were two category 5 hurricanes per year. When the Pan-Cyclonian Council on Climate Assessment (PCCA) reported that global warming had doubled the odds of powerful hurricanes, the public was initially very concerned and demanded to know which of the two monster storms they endured that year was caused by climate change. Scientists explained that neither of the two storms could be attributed to climate change. Rather, the fact that there were now two storms per year instead of one meant that the risk of powerful hurricanes was twice what it used to be. The headline in The Cyclon Evening Press read: “No particular hurricane linked to climate change, says PCCA.” The citizens of Cyclon breathed a collective sigh of relief.

The moral of the story is that focusing on the unanswerable question of single-event attribution creates a paradox:  Scientists are clear that climate change has caused certain kinds of extreme weather events to occur more frequently, yet they can’t pin climate change to any single event. So, in spite of solid scientific evidence, the public rarely gets the clear message that climate change IS increasing the risk of extreme weather.

To summarize, the climate is already changing, the odds of extreme weather have increased because of global warming, and that means the risk of future extreme weather events will continue to rise. The correct response is to manage the risk by adapting to unavoidable changes that are already underway and by reducing greenhouse gas emissions to keep the climate from changing so much that we can’t adapt.

It’s good sense informed by good science.

Photo by sebadanon.