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 statistically more confident that humans cause climate change than that smoking causes cancer.
So what's the evidence?
The research falls into nine independently studied, but physically related, lines of evidence:
- Simple chemistry – when we burn carbon-based materials, carbon dioxide (CO2) is emitted (research beginning in 1900s)
- Basic accounting of what we burn, and therefore how much CO2 we emit (data collection beginning in 1970s)
- Measuring CO2 in the atmosphere and trapped in ice to find that it is increasing and that the levels are higher than anything we've seen in hundreds of thousands of years (measurements beginning in 1950s)
- Chemical analysis of the atmospheric CO2 that reveals the increase is coming from burning fossil fuels (research beginning in 1950s [PDF])
- Basic physics that shows us that CO2 absorbs heat (research beginning in 1820s)
- Monitoring climate conditions to find that recent warming of the Earth is correlated to and follows rising CO2 emissions (research beginning in 1930s)
- Ruling out natural factors that can influence climate like the sun and ocean cycles (research beginning in 1830s)
- Employing computer models to run experiments of natural versus human-influenced simulations of Earth (research beginning in 1960s)
- Consensus among scientists who consider all previous lines of evidence and make their own conclusions (polling beginning in 1990s)
Addressing the skeptics
Skeptics sometimes point to the last two supporting lines of evidence as weaknesses. They're not.
But even if you choose to doubt them, it is really the first seven that, combined, point to human activities as the only explanation of rising global temperatures since the Industrial Revolution, and the subsequent climate changes (such as ice melt and sea level rise) that have occurred due to this global warming.
The sooner we agree the science is settled, the sooner we can work together toward addressing climate change – and a better future.
Act when it matters most