9 ways we know humans caused climate change

Tens of thousands of scientists in more than 100 nations have amassed an overwhelming amount of evidence pointing to a clear conclusion:

Humans are the main cause of climate change — burning fossil fuels, producing livestock, clearing trees and otherwise increasing the amount of heat-trapping gases in the atmosphere.

It’s like the smoking-cancer link

Few people would question the link between smoking and cancer, because the science was settled in the 1960s after more than 50 years of research.

We can think of the link between human activities and climate change in a similar way — there’s a clear connection.

In fact, we are as confident that humans cause climate change as we are that smoking causes cancer.

So what’s the evidence?

The research falls into nine independently studied, but physically related, lines of evidence:

1. Simple chemistry: When we burn carbon-based materials, carbon dioxide (CO2) is emitted (research beginning in the late 1800s).

2. Basic accounting of what we burn, and therefore how much CO2 we emit (data collection beginning in the 1970s in the United States, for example).

3. Measuring CO2 and other greenhouse gases in the atmosphere and trapped in ice to find they are increasing, with levels higher than anything we’ve seen in nearly a million years (measurements beginning in the 1950s).

4. Chemical analysis of the atmospheric CO2 that reveals the increase is coming from burning fossil fuels (research beginning in the 1950s, PDF).

5. Basic physics that shows us that CO2 absorbs heat (research beginning in the 1820s).

6. Monitoring climate conditions to find that the air, sea and land is warming, as we would expect with rising greenhouse gas emissions; as a response, ice is melting and sea level is rising (research beginning in the 1930s).

7. Ruling out natural factors that can influence climate like the sun cycles (research beginning in the 1910s).

8. Employing computer models to run experiments of natural versus human-influenced simulations of Earth (research beginning in the 1960s).

9. Consensus among scientists who consider all previous lines of evidence and make their own conclusions (surveys and analyses beginning in 2004).

Learn more: 5 ways you can combat climate myths