Question

How do we know that global warming is happening? And that human activity is the primary cause?

Answer 1

1. Global warming is the long-term heating of the earth’s climate system observed since the pre-industrial period (between $1850$ and $1900$) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in earth’s atmosphere.
2. It is most commonly measured as the average increase in the earth’s global surface temperature.
3. Since the pre-industrial period, human activities are estimated to have increased Earth’s global average temperature by about $1°C$, a number that is currently increasing by $0.2°C$ per decade.
4. It leaves no doubt that human influence has warmed the atmosphere, ocean, and land.