 In February of 2010, researchers in Germany announced the formal name for a recently created element. They called it copernicium. It's what's known as a synthetic element. That means for its entire four and a half billion year history, before the scientists created it in a lab, copernicium did not exist on earth. Copernicium is a very radioactive element and is unstable. It has a half-life measured in seconds. But it's a powerful demonstration that humans are capable of changing the world around them. Some of these changes, like the creation of copernicium, are fleeting. Others, like the way we're changing the earth's atmosphere and climate, can shape the world far into the future. The earth is heating up and human activities, mainly the burning of fossil fuels, are the cause. But how do we know we're responsible? Anytime scientists want to answer a question like this, they look at many different lines of evidence. When all of the lines of evidence point to the same conclusion, scientists can be confident about it. And this is exactly what's happening with climate change. First and foremost, we know humans are changing the climate through basic physical principles. Increasing greenhouse gases, like CO2, leads to more energy building up in the climate system. And this causes the planet to warm in response. We've understood the physics behind this since the 1800s. But we can be even more confident, because greenhouse warming will leave different fingerprints on the climate than other possible causes. We've known for many decades that increasing levels of CO2 in the atmosphere will produce a unique pattern of temperature change from the surface to the upper atmosphere. The surface and lower atmosphere will warm while the upper atmosphere will cool. By contrast, warming due to some natural factor like more sunlight reaching the earth would warm the surface and the upper atmosphere as well. The cooling of the upper atmosphere while the surface warms is a fingerprint of increased greenhouse warming. And it's exactly what we can see happening now. We can also look at the ways in which natural forces have changed the climate in the past. We can use this information to rule them out as causing the current change. We know that changes in the earth's position relative to the sun has caused climate changes in the past. But understanding these changes tells us that they aren't what's happening now. They occur far too slowly to be the present cause, and right now they would be acting to slightly cool the climate overall. We know that the sun's intensity can also change over time. But we have many instruments on the ground and on satellites monitoring solar activity. For the past several decades, the amount of solar energy reaching the earth has actually decreased, which would also act to cool the climate. Massive long-lasting volcanic eruptions caused by movement of the continents are capable of heating up the planet by increasing greenhouse gases. But we're monitoring volcanic activity, and volcanoes release a tiny fraction of greenhouse gases relative to human causes. And the greenhouse gases they do emit have a different chemical composition than gases produced by the burning of fossil fuels. The chemical signature of gases building up in the atmosphere tells us that they're from fossil fuels and not volcanoes. One by one, we can look at how natural changes could be affected in the climate and then rule them out. We can clearly see the human fingerprint of greenhouse warming in the pattern of temperature changes in the atmosphere. All of this agrees well with our expectations from basic physical principles. Humans are responsible for climate change. One myth ignores these lines of evidence and claims that because the climate changed in the past naturally, it must be changing naturally now. This is probably the most common myth about global warming, and it's dead wrong. This is like a defense lawyer who claims his client must be innocent of murder because humans have been dying from natural causes for 200,000 years. Meanwhile, the police caught his client at the scene of the murder with his fingerprints all over the murder weapon. Arguing that humans have been dying from natural causes for thousands of years isn't going to convince Jerry to let her murderer off the hook. And arguing that climate change naturally in the past doesn't negate the fact that humans are changing the climate now. The idea that because something happened in one way in the past, it will always happen that way, is faulty reasoning. Focusing on the way that things happened before and ignoring all of the possible ways they could happen oversimplifies the issue. This oversimplification leads to jumping to the wrong conclusion that the climate is changing due to natural causes, when in fact we know humans are responsible. Human activities are responsible for the changing climate. We understand how climate has changed in the past and this allows us to rule out natural causes, and our fingerprints are all over the warming climate.