Climate has always changed, so humans aren't the cause
Past climate variability is well-documented and driven by orbital cycles, volcanic activity, and solar output. None of these natural drivers can explain the rapid warming observed since 1950; human CO2 emissions are the primary identified cause.
What we know
It is scientifically accurate that Earth's climate has shifted many times throughout its history, driven by Milankovitch orbital cycles, continental drift, changes in solar output, and large volcanic episodes. Acknowledging this does not undermine the evidence for human-caused warming - it strengthens it, because scientists have measured and accounted for all known natural drivers.
The IPCC Sixth Assessment Report (2021) states that it is unequivocal that human influence has warmed the atmosphere, ocean, and land. Observed warming of approximately 1.1 degrees Celsius above pre-industrial levels cannot be explained by natural forcing alone. Natural factors over the same period show a slight cooling trend in solar output and a neutral volcanic contribution. When climate models include only natural factors, they reproduce pre-industrial climate well but fail to replicate the warming observed after 1950. When human greenhouse gas emissions are added, the match is close.
A 2021 study in Environmental Research Letters analyzed over 88,000 peer-reviewed papers and found greater than 99 percent consensus that recent climate change is human-caused. The logic that 'climate changed before therefore humans cannot be causing it now' is equivalent to arguing that because wildfires have always occurred naturally, arson is impossible.
Common claims
- Climate changed before humans existed, so current change is naturalFalse - natural drivers cannot explain post-1950 warming
- Scientists agree human emissions are the main driver of recent warmingTrue - greater than 99% scientific consensus
- CO2 levels have been higher in Earth's historyTrue - but past high-CO2 periods caused major extinctions and occurred over millions of years, not decades