 Climate scientists are like detectives at a crime scene. They want to solve the case of what's causing global warming, and they found human fingerprints all over the climate. We call them fingerprints because they're a unique pattern of changes. In one case, we're changing the very structure of the atmosphere. Now, how are we doing this? Well, when we burn coal, oil and gas, we release greenhouse gases and strengthen the greenhouse effect. Greenhouse gases allow sunlight to pass through and warm the Earth's surface, but they absorb infrared light given off by the Earth. Greenhouse gases also have their own infrared glow. This glow goes in all directions and sends some heat back down to the Earth's surface. As we release more greenhouse gases, the atmosphere gets better at absorbing infrared. At the same time, the greenhouse infrared glow gets stronger. Near Earth's surface, greenhouse gases absorb a bit more of the infrared that passes through the air. The greenhouse glow sends some of this trapped heat back to the surface where it's absorbed again. Heat cycles between the surface and the atmosphere. With more greenhouse gases, more heat gets trapped in the cycle and temperatures rise. High up in the atmosphere, about 20 kilometers and above, we are much closer to space. Outer space barely absorbs or emits infrared at all. There is basically no heat coming from above. Meanwhile, greenhouse gases boost the greenhouse glow. The extra glow or heat that goes upwards can escape. Above about 20 kilometers, adding greenhouse gases cools the sky. This happens even while the lower atmosphere warms. Scientists predicted this pattern before 1970. Now, satellite measurements confirm the prediction. So what does this pattern look like? Here are two different simulations of warming through the atmosphere. The pictures go from the Earth's surface at the bottom to space at the top. Red means getting warmer, while blue means getting cooler. On the left is what greenhouse warming should look like. There is cooling at the very top and warming everywhere else. We mentioned greenhouse warming has this unique fingerprint. For example, if the sun were to get hotter, the pattern of warming would look more like the box on the right. Here we see warming at the top of the atmosphere as well as at the bottom. Satellites have measured the cooling in the upper atmosphere, so the pattern that we expect to see from greenhouse warming has been confirmed. One myth distorts the fingerprinting evidence. It uses a red herring by focusing on this hotspot about six kilometers up in the tropical atmosphere. Simulations expect this area to warm faster than the surface, but real-world measurements have not yet conclusively proven whether this hotspot exists. The myth says that the lack of conclusive proof of this hotspot casts doubt on greenhouse warming, but this is a red herring because the hotspot is irrelevant to greenhouse warming. It is simulated for natural warming as well as greenhouse warming, but let's see why we expect the hotspot to exist at all. Earth's surface can cool by sweating. Water evaporates from the surface and it carries heat with it. As it rises, the air cools, and cooling with height is called the lapse rate. It's why it's colder on the top of mountains. As moist air rises and cools, some of the water condenses out. If it condenses into a big enough blob, it falls and that's what we call rain. When it condenses, it dumps the heat that's been carried up by evaporation. Warming means more evaporation and more rising vapor. When it condenses, it releases its heat, and this has the largest effect starting about six kilometers up in the tropics. This is where we expect to see the tropical hotspot. Now this is difficult to measure and it hasn't yet been proven whether it's there or not. If it's not there, scientists will have to explain why. But using the hotspot to cast doubt on greenhouse warming is a red herring. It's a sign of changing moisture in the tropics, not of greenhouse gas warming. A real fingerprint of greenhouse warming is warming near the surface while the atmosphere above about 20 kilometers height cools. And this has already had a spectacular effect. The cooling upper atmosphere has contracted like a balloon in a freezer. All lights that brush the top of the atmosphere have literally felt it falling away.