9 ways we know humans triggered climate change

Most Americans recognize climate change, but some are still unsure about its causes.

Tens of thousands of scientists in more than a hundred nations have amassed an overwhelming amount of evidence pointing to a clear conclusion: Humans are the main cause.

We're the ones who burn fossil fuels and clear trees that absorb carbon dioxide, sending heat-trapping gases into the atmosphere.

It's like the smoking-cancer link

No one questions 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 state of human activities and climate change as no different than smoking and cancer.

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

Scientists are more confident than ever that humans are causing global warming.

Ilissa Ocko
Ilissa Ocko, Climate Scientist

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 1900s)
  2. Basic accounting of what we burn, and therefore how much CO2 we emit (data collection beginning in 1970s)
  3. Measuring CO2 in the atmosphere and trapped in ice to find that it's increasing, with levels higher than anything we've seen in hundreds of thousands of years (measurements beginning in 1950s)
  4. Chemical analysis of the atmospheric CO2 that reveals the increase is coming from burning fossil fuels (research beginning in 1950s)
  5. Basic physics that shows us that CO2 absorbs heat (research beginning in 1820s)
  6. Monitoring climate conditions to find that recent warming of the Earth is correlated to and follows rising CO2 emissions (research beginning in 1930s)
  7. Ruling out natural factors that can influence climate like the sun and ocean cycles (research beginning in 1830s)
  8. Employing computer models to run experiments of natural versus human-influenced simulations of Earth (research beginning in 1960s)
  9. Consensus among scientists who consider all previous lines of evidence and make their own conclusions (polling beginning in 1990s)

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