The corpse was riddled with stab wounds. Beside the body lay a bloody knife, and painted on the wall, a message: I, A MURDERER, HAVE COMMITTED THIS MURDER WITH A KNIFE. Below the message were the name and address of the killer, along with his tax returns, several photographs, and details on how to follow him on social media.
The detective frowned. This would be a tough case to crack. He turned to his partner. “How,” he said, “can we be sure it’s not the Sun?”
Sometimes I feel like climate scientists live in an alternate-universe Law and Order, where the detectives are less attractive and every episode begins with an argument over whether objective reality exists. Imagine how boring crime dramas would be if the police were forced to spend the first 45 minutes explaining that yes, they are aware that people die of natural causes all the time.
In reality, the facts are pretty clear: our world is changing, and there are fingerprints all over the scene. We’ve collected plenty of evidence. And we know, beyond any reasonable doubt, who did this.
But climate has always changed, say people to the scientists who figured that out. And it’s true: many things can affect the Earth’s temperature. A large volcanic eruption can spray so much gas and dust into the atmosphere that it blocks the sun. Natural variations in the Sun’s brightness or the Earth’s orbit can make it warmer or cooler. We can measure these things—if the Sun suddenly got much brighter, I’m pretty sure someone at NASA would notice. And we can use our understanding of how the Earth system works to see how they might affect the climate.
A few years ago, we worked with Blacki Li and Eric Roston of Bloomberg News to explain why we’re so sure humans are causing global warming. Using a sophisticated computer model of the Earth system, we calculated the effect of each individual factor in isolation. The graphic they made is wonderful, and whenever it starts trending on Twitter again it’s a good sign some prominent politician has said something silly about climate change. It’s the clearest explanation I’ve ever seen of who’s responsible for rising temperatures.
The data we used for this project extended only to 2005. But warming has continued through the 21st century. 2015 was the hottest year on record… until 2016. Last year was the hottest year ever measured to not have an added boost from a natural El Niño event. Greenhouse gas emissions have continued to increase, and they continue to be the prime suspect behind global warming.
Climate, of course, is more than just the average temperature. We’ve seen human fingerprints all over the planet. The oceans are warming and becoming more acidic, the atmosphere contains more water vapor, and heat waves are becoming more frequent and intense. The science of climate change detection and attribution—climate detective work—has advanced to the point that we can now confidently blame human activities for some individual extreme events.
It’s always possible scientists have made a serious mistake here. But if you don’t believe people have affected the climate, you need a coherent alternate explanation for the changes we’ve seen. Moreover, you need to explain why carbon dioxide doesn’t act the way we believe it should. It could be that we’ve all been wrong about the greenhouse effect this whole time. But overturning that understanding would take a radical paradigm shift that would call into question much of what we know about physics and chemistry. It’s possible, but it’s a tall order.
We have the evidence, and we’ve done the forensics. The culprit has been identified beyond a reasonable doubt. It doesn’t strike me as very useful to argue that because people die of natural causes all the time, murder can’t possibly exist. That’s a terrible premise for a TV show, and a strange way to think about the world. But it’s never enough to collect evidence—we have to argue our case and convince a jury. And that jury? It’s us.
But with climate change, we’re not only judge and jury—we’re the victims, and the culprits too. We can reduce our sentence through good behavior—radically slashing greenhouse gas emissions and decarbonizing our economy. But we can’t pretend no crime has been committed. The case against us is too clear.