[inspiring music]

(Describer) Title: PBS Digital Studios.

- 97% of climate scientists agree humans are causing global warming. It's one of the most famous statistics in all of science that experts, the people who know the most about Earth's climate agree almost universally that humans are warming the planet. Where does this 97% number come from?

[playful music]

(Describer) On a blueprint, title: Its Okay to Be Smart.

The most famous source of the 97% agreement comes from a study in 2013 by Australian scientist John Cook. He looked at almost 4,000 scientific papers that made some statement about whether humans were the main cause of climate change, and 97% of those papers agreed with the consensus. To make sure nothing was misinterpreted, he also asked scientists to rate the views of their own papers, and they found the same 97% agreement. It shouldn't surprise you to learn that people have attacked this study saying, it's methods were wrong, that it miscounted things. But even if we ignore Cook's study, turns out lots of other people have looked at this question and found a similar answer. Between 90 and 100% of experts agree that the climate is changing, Earth is getting warmer, and we are responsible for a lot of it. What does it mean to measure consensus? First, you identify the experts. In this case, the experts are thousands of scientists who study climate and publish their work in peer-reviewed journals. Peer review means that every finding that's published is analyzed by people working in the same field, people who really know what they're talking about. It's not flawless. Mistakes occasionally happen, but the system is built to correct those mistakes and it's by far the best process humans have ever come up with for doing good science. Once we find this group of experts, we analyze their opinion for or against a particular idea. Sometimes this is done by studying what scientists have written in their papers. Other times scientists are surveyed directly. This can even be done by listening to what scientists say in public. Now, some scientists don't explicitly express an opinion either way. They're not included in the analysis. Consensus is the fraction who support an idea, divided by the sum of those who support, plus those who reject the idea. All these different methods have ended up with the same conclusion. The people who know the most almost universally agree about what's causing global warming. But if you ask everyday people what they think the consensus is, they guessed that only 55% of climate scientists agree. That's way off from what experts actually think. Why does this gap exist? Because surprise, there are people out there who spend a lot of money and effort manufacturing doubt. In a lot of research papers, scientists never specifically write humans are causing most global warming. These papers are usually excluded from studies about consensus because they don't give a position either way. People have drawn different conclusions from this. Some say not giving a position is exactly what you'd expect from scientists who agree that something's basically settled. Like, how physicists don't write gravity is real in every single paper. Biologists aren't regularly citing Darwin and natural selection. They're accepted as true. But critics of global warming science claim that any scientist who doesn't specifically say in every paper that they agree with the consensus should be counted as uncertain or even counted as rejecting it. And this is a very strange claim to make, that any climate scientist who doesn't write, "I believe that humans are the main cause of global warming," is actually uncertain or doesn't believe that humans are the main cause of global warming. To show you why this is flawed, let's apply the same logic to another scientific idea. Plate tectonics is the theory that Earth's crust is made up of several large chunks that move over time and that new crust is made at some places and eaten up at others. Not a controversial idea today, but you might be surprised to learn that before the 1950s and '60s, most scientists didn't accept it. Researchers looked at recent geology papers, using the same criteria critics of global warming science claim should be used on climate consensus, that any paper that doesn't explicitly state that plate tectonics is real should be counted as uncertain or as rejecting it. Turns out not one single paper out of hundreds specifically endorsed the theory of plate tectonics. So clearly, plate tectonics is a hoax? Doubt about what's causing climate change really only exists among people who, how should I put this, aren't experts. And that's a big problem, because when we think scientists are divided on an issue we're less likely to think the issue is a problem. This also means if more people understood how much agreement there really is about humans causing climate change, we could start paying attention to more important questions like, what do we do about it? People sometimes say that science doesn't work by consensus or by agreement, and that every truth must be decided by experiment. But this is wrong. Consensus or agreement is a hugely important part of science. When scientists do experiments, they don't repeat or reestablish every single bit of knowledge that got them to that point. They, and I hope we, trust in the process of science. In every field there are things that are well enough proven, that are agreed upon, and these are where scientists start from to journey out into what's actually unknown. And when it comes to the science of global warming and climate change, experts agree overwhelmingly on the cause. I think it's time we start talking about what's really important. That's why I'm excited today to announce a new project here on YouTube, one I've been working on for most of the last year. It's a new channel dedicated to stories about climate change and climate science, called Hot Mess. Have a look.

(Describer) Title: youtube dot com slash HotMessPBS. Animations include wind turbines, planes and trains.

[dramatic music]

I'm Joe. - I'm Miriam. - And I'm Talia. - [Miriam] Let's discover how our climate works. - [Talia] How we can adapt to changes. - [Joe] And how we can solve this mess. Our planet's getting warmer and weirder. - [Talia] So let's get started. - It's brought to you by the same amazing people that make these awesome videos here on It's Okay to be Smart, along with a few new faces. Hot Mess is going to be the best channel on YouTube for stories about our planet's changing climate. I know that's something that a lot of you care about, so come join us over at Hot Mess. Stay curious.