How are humans responsible for global warming?

Deforestation and burning fossil fuels are chief culprits

Scientists have closed the case: Human activity is causing the Earth to get hotter.

How? Primarily by two actions: Burning fossil fuels, with a smaller contribution from clear cutting forests, known as deforestation.

Greenhouse gases trap heat

When we extract and burn fossil fuels such as coal or petroleum, we cause the release of carbon dioxide and other heat-trapping “greenhouse gases” into the atmosphere.
Though natural amounts of carbon dioxide have varied from 180 to 300 parts per million (ppm), today’s levels are around 400 ppm. That’s 40% more than the highest natural levels over the past 800,000 years.
We also can tell that the additional carbon dioxide in the atmosphere comes mainly from coal and oil because the chemical composition of the carbon dioxide contains a unique fingerprint.

Losing forests makes it worse

Clearing forests also releases large amounts of carbon dioxide. On top of that, plants and trees use it to grow. Worldwide deforestation means we don’t have as many trees to absorb the extra carbon dioxide.
This means more of it stays in the atmosphere, trapping more heat.

 Source:  https://www.edf.org/climate/human-activity-is-causing-global-warming