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How do we know that human activity is responsible for global warming?

I’ve frequently been asked how we know that human activity is responsible for a warming earth. Of course there are natural factors that are influencing the climate, such as sunspots (on time scales up to a decade), orbital variations around the sun (on time scales of 10,000 to 100,000 years), and volcanic eruptions (effects usually last no more than 5 years). There are several factors which affect climate where you are, including land use changes such as agriculture and irrigation and increasing urbanization. But to affect the entire earth, the changes have to be in the atmosphere where they can spread their effect across the whole world. And there are no natural changes that can do that on the time scale that we are seeing the temperature change, although long-term variability in the ocean probably has some impact on that as well. Here is a good article which provides detailed information about how we know the warming is from humans and is not natural.

Source: NASA