 We know that humans are burning fossil fuels and are releasing billions of tons of carbon pollution every year. This has increased the greenhouse effect, trapping more heat in the Earth's atmosphere. That leads to an important question. How much will global surface temperatures increase because of this extra heat? If we double the amount of carbon dioxide in the atmosphere, the heat trapped will cause a direct warming of about 1.2 degrees Celsius or 2.2 degrees Fahrenheit. But there are also effects called feedbacks that can either dampen or amplify that warming. For example, when ice melts, the Earth's surface gets less reflective, causing it to absorb more sunlight and warm even further. A warmer atmosphere also holds more water vapor. Since water vapor is another greenhouse gas, this increases the greenhouse effect and causes further warming. These are amplifying feedbacks. Clouds are especially complicated. Changes in cloud cover can mean more reflection of sunlight, a dampening feedback. But changing clouds can also trap more heat since they also contribute to the greenhouse effect, which is an amplifying feedback. Once the total effect when you add up all the different feedbacks. The end result is called climate sensitivity, the increase in global temperature from a doubling of the amount of carbon dioxide in the atmosphere. There are several ways we can calculate climate sensitivity. We can look at how Earth's climate has behaved in the past. How do global temperatures change in response to a change in heat? We can use complex climate models that simulate all the different climate feedbacks. Or we can combine modern measurements with simpler mathematical models. All these independent methods find a fairly consistent answer. If we double the amount of carbon dioxide in the atmosphere, global temperatures will rise between 1.5 and 4.5 degrees Celsius, or between 2.7 and 8 degrees Fahrenheit. The best estimate for climate sensitivity when combining these methods is around 3 degrees Celsius. Remember, direct warming from the increased carbon dioxide greenhouse effect is only about 1.2 degrees Celsius. Reinforcing feedbacks roughly double or triple that warming. Three degrees warming may not sound like very much. But the difference between an ice age and the current warm period is only about 4 to 5 degrees Celsius, or 8 to 9 degrees Fahrenheit. Seemingly small temperature changes make a big difference when you're talking about the whole planet. However, there's one myth that argues that climate sensitivity is low, therefore global warming is nothing to worry about. This myth actually contains two fallacies. Firstly, it cherry-picks among the different methods used to estimate climate sensitivity. It only looks at some of the attempts using modern measurements and ignores estimates using past climate change or global climate models. Each method has its strengths and weaknesses. The strength of looking at past climate change is that it gives us estimates based on actual past events in Earth's history. However, the weakness is that these changes happened under conditions that aren't identical to today's climate. So they're not perfect analogs. The strength of global climate models is that they let us estimate how sensitive the climate is under today's conditions. The weakness is that they're limited by how well we understand all the processes that influence the first complex climate system. For example, most climate models don't take into account factors like the response of permafrost and the Arctic. As the Arctic warms, melting permafrost will release trapped methane, which will increase the greenhouse effect and amplify global warming. But because the amount and timing are uncertain, this effect is usually left out of the models. The strength of using modern measurements with simple models is that they're relatively straightforward. We don't need to understand all the intricacies of the Earth's climate that way. One weakness is that this approach relies on the accuracy of our recent climate measurements. For example, if we've underestimated the amount of heat going into the oceans or overestimated the size of the global energy imbalance, this method will underestimate the climate sensitivity. Another weakness with this method is that we don't know if the climate feedbacks will behave the same way in a hotter world decades from now as they do today. Scientists using this method assume that's true, but it might not be. Climate models suggest it's not. And if it's not, estimates using modern measurements with simple models will be wrong. Studies using historic climate change and global climate model methods have arrived at consistent results, estimating the climate sensitivity between about 2 to 4.5 degrees Celsius global surface warming if we double the amount of carbon dioxide in the atmosphere. In recent years, a few studies using modern measurements with simple climate models have arrived at somewhat lower estimates, around 1.2 to 4 degrees Celsius. Considering the full body of evidence and research into the short coming of this third approach, those lower estimates may very well be overly optimistic. But even if the lower estimates of climate sensitivity are right, we're still on track to experience warming at dangerous levels. It just means that we'll reach the most dangerous levels of global warming about a decade or two later than we expected. That's not enough to justify refusing to take action to reduce carbon pollution and slow global warming.