 One of my favorite quotes about science comes from John Reisman. He says, science is not a democracy. It is a dictatorship. It is evidence that does the dictating. Science isn't based on a show of hands. It's based on evidence. The more lines of evidence we have, the more confident we are that our scientific understanding is correct. So what is our scientific understanding of climate change? In simple terms, it's this. When we burn fossil fuels like coal and oil, we send carbon dioxide into the atmosphere. Carbon dioxide is a heat trapping gas. This process of trapping heat in the atmosphere and oceans is called the greenhouse effect. As we emit more and more greenhouse gases, more and more heat is being trapped, increasing the greenhouse effect. This is making the world warmer and that's our scientific understanding of what's causing global warming. Now the beauty of science is we can check whether we got it right by collecting evidence. Warming from an increased greenhouse effect should show a number of different patterns. We can look for those patterns by taking measurements. Let me give you a few examples of the observed patterns of human cause warming or human fingerprints. One of the strongest human fingerprints is being measured by satellites. Greenhouse gases are trapping some of the heat that would otherwise escape out to space. That means that satellites orbiting the earth should see less heat escaping to space. And for over 40 years now, a series of different satellites have measured exactly that, a human fingerprint in outgoing heat. If less heat is escaping to space, more heat should be radiating back to earth. Measurements taken at the earth's surface find exactly that, just as we expect. Scientists are measuring more infrared radiation or heat coming down from the atmosphere to the earth's surface. Another powerful prediction was made before the satellite era in the 1960s. The prediction was that if greenhouse gases are trapping heat in the lower atmosphere, then we should see cooling in the upper atmosphere. Over the last few decades, that's exactly what satellites have observed. A cooling upper atmosphere at the same time as a warming lower atmosphere is a distinct human fingerprint. Because the upper atmosphere is cooling, it's also shrinking. We're actually changing the structure of our planet's atmosphere. The fact that the upper layers of the atmosphere are falling inspired climate scientist Gavin Schmidt to once joke that yes, the sky is falling. There are a number of other human fingerprints being observed throughout our climate. All these lines of evidence from many independent sources point to the same conclusion. Humans are adding heat trapping greenhouse gases to the atmosphere. As a result, we are causing global warming. And here's another thing. If you take all the different climate drivers such as greenhouse gases, the sun, volcanoes, or internal variability driven by ocean cycles and cross-check them against the observed climate patterns, only the warming from our greenhouse gas emissions match all the climate patterns. The human fingerprints not only give us evidence that we're causing global warming, they also rule out other possible causes of climate change. They rule out the sun. They rule out volcanoes. Could internal variability be causing global warming? All the observed climate patterns rule it out. Scientists call this a conciliance of evidence. Personally I like to call it a consensus of evidence because when I use the word conciliance people give me blank stares. There is one myth about climate change that there's no real evidence for human cause global warming. The argument often goes that the case for climate change is all based on computer models rather than empirical evidence. Now for starters, I don't want to dismiss climate models. They give us many great insights into how our climate works but even if you threw out every climate model in the world our confidence that humans were causing global warming will be just as strong. That's because we have many human fingerprints all adding to a great pile of evidence. So that myth misrepresents the science. It doesn't take into account the full body of evidence. In fact, it ignores the evidence entirely. The reality is that our scientific understanding of global warming is based on fundamental physics that have stood the test of time for more than a century and our understanding of the science has been confirmed over and over again by many lines of evidence.