 Imagine you're hiking deep in the wilderness with a friend of yours, the one who's a lot of fun but doesn't always have the best judgment. As you turn a corner on the trail, you come across an enormous bear asleep on the path of head. You could turn around, you could walk around it. But instead, to your horror, your friend says at full volume, Hey, look at that bear! You whisper at him to shut up! But it's too late. The bear opens its eyes, looks at you and growls. We got to get this bear out of our way, he says. He picks up a stick and walks towards the bear. Renowned climate scientist Wally Broker has said, The climate system is an angry beast and we're poking at it with sticks. And like our friend poking the bear, humans have been poking the climate system by adding heat-trapping gases into our atmosphere. Wally Broker studied the climate of the Earth's past before records were kept. This branch of science is called paleoclimatology. Paleoclimat researchers use techniques from chemistry to infer what temperatures, sea level that make about the atmosphere and other environmental variables were like. Not just before modern records, but before modern humans, growing back hundreds of thousands and even millions of years. As far back as we can reconstruct conditions on Earth, one thing is clear. The climate changes significantly when the amount of energy in the system changes. Like changes in energy received from the sun or the amount of greenhouse gases in the atmosphere. It does so because changing CO2 levels changes the energy balance of the system. The physics behind this have been understood for more than a century and a half and are completely uncontroversial. However, what we've learned recently is that the climate system is pretty sensitive to changes in its energy balance. Reinforcing feedbacks amplify any initial change in temperature. That means the total amount of temperature change in response to a change in energy is several times greater than what you would see if there were no reinforcing feedbacks. We now have paleoclimate reconstructions going back hundreds of thousands to hundreds of millions of years of Earth's history. They suggest that the sensitivity of Earth to changes in energy, from changes in CO2 levels for example, is similar to what our most advanced climate models predict. Paleoclimate reconstructions also tell us that when CO2 levels change rapidly, not only did the climate rapidly change, but this had big impacts on life at the time. Changes in greenhouse gas levels are associated with the worst of Earth's mass extinction events. But it's important to keep in mind that greenhouse gases like CO2 are not the only factor in climate. The sun's energy is obviously a big player as well. You might not know it, but stars like our sun get brighter and hotter over time. In the ancient past, we received a significantly lower amount of energy from the sun, which meant CO2 levels could be higher without boiling away the ocean. We know that the position of Earth's continents also has changed a lot over time. It turns out that this can have a significant effect on climate. On average, land is a lot more reflective to sunlight than the ocean is, and the sun is much more intense near the equator. That means if the continents are clustered more near the equator, a lot more of the energy sun gets reflected back out to space before it can warm the planet. And when the continents tend to be spaced away from the equator, a lot more solar energy gets absorbed by the ocean. Other factors, like the presence or absence of plants and ice sheets, can also change how much of the sun's energy makes its way into the climate system. When looking at the relationship between greenhouse gases like CO2 and climate, it's important to control for all of the other variables. When we look over Earth's history using our best available data, the relationship is clear. When greenhouse gases change, the climate changes in response. There's one myth that argues that because CO2 levels got so high in the atmosphere in Earth's past without the climate becoming super hot, then that must mean that CO2 can change a lot with just a small amount of change in climate. In other words, the myth uses past CO2 levels of evidence that the CO2 warming effect is not that strong. However, this myth uses faulty reasoning. It is in effect cherry picking. CO2 levels are important in shaping the climate, but the other factors are as well. We have to consider factors like the changing sun, how much sunlight gets reflected back into space. Ignoring other important factors and ignoring the best estimates of past CO2 levels and climate changes is an oversimplification. CO2 and climate are strongly related, but that's not the whole story. And finally, this oversimplification results into jumping to a conclusion that turns out to be wrong. Failing to consider all the relevant factors leads to the false conclusion that pumping out enormous amounts of CO2 won't have an impact on the climate. But evidence from Earth's history shows just the opposite. Our climate changes when greenhouse gas levels change. These climate changes can happen very quickly, causing some of the largest extinctions in Earth's history. And now we've increased levels of greenhouse gases higher than they've been in over a million years. And as best we can tell, we're increasing levels of greenhouse gases faster than at any time during Earth's history. Faster even than during those mass extinctions.