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 4.5-billion-year history, driven by Milankovitch orbital cycles occurring over tens of thousands of years, continental drift over millions of years, changes in solar output, and large volcanic episodes that can alter climate for a few years at a time. Acknowledging this long history of natural variability does not undermine the evidence for present-day human-caused warming, it actually strengthens the case, because scientists have specifically measured and modeled each of these known natural drivers and found that, together, they cannot account for the pattern and speed of warming observed since the mid-20th century.
The IPCC Sixth Assessment Report (2021) states unequivocally that human influence has warmed the atmosphere, ocean, and land. Observed warming of approximately 1.1 to 1.2 degrees Celsius above pre-industrial levels, most of it accumulated since 1970, cannot be explained by natural forcing alone. Measurements of solar output over the same period show a slight cooling trend since roughly 1980, and the net contribution from volcanic activity has been close to neutral or slightly cooling given the absence of any climate-altering supervolcanic eruption in this period. When climate models are run using only natural factors, orbital, solar, and volcanic, they reproduce the relatively stable pre-industrial climate quite well but fail to replicate the sharp warming trend that begins around 1950 and accelerates thereafter. When human greenhouse gas emissions are added into the same models, the simulated temperature record closely matches what has actually been observed, which is one of the strongest lines of evidence for attribution.
A 2021 study in Environmental Research Letters analyzed more than 88,000 peer-reviewed papers on climate change and found greater than 99 percent consensus that recent climate change is human-caused, a level of agreement comparable to other settled areas of science. NASA's own summary of the evidence points to the same conclusion using independent datasets and methods. The core logical error in the "climate always changed" argument is a non sequitur: the fact that a process has occurred naturally in the past does not mean every future instance of that process must also be natural. The same reasoning would suggest wildfires cannot be started by arson because wildfires have always occurred naturally, or that heart disease cannot be caused by diet because hearts have always eventually failed for other reasons throughout history.
Paleoclimate records also show that even the largest natural CO2 fluctuations in Earth's deep history unfolded over many thousands to millions of years, allowing ecosystems time to adapt or migrate. The current rate of CO2 increase, driven by fossil fuel combustion, is occurring roughly ten times faster than the fastest known natural increases in the geological record, which is itself a significant part of why the current episode of change is considered exceptional rather than simply another turn of a familiar natural cycle. This distinction between rate and magnitude is often lost in public debate but is central to why scientists consider the present warming episode unusual: it is not simply that CO2 levels have changed before, but that the current speed of change gives species and human infrastructure far less time to adapt than comparable natural episodes allowed.
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