Photograph of a piece of coal being held in front of a coal-fired power plant.

How Do We Know That Humans Are The Major Cause of Global Warming?

Quick Answer

We know that humans are the major cause of global warming in two ways. First, calculations show that natural factors alone come nowhere near explaining the observed warming over the last 150 years. Second, direct chemical evidence in the atmosphere shows that the majority of the carbon dioxide that has been building up in the atmosphere comes from fossil fuel combustion.

Image above: A piece of coal being held in front of a coal-fired power plant in the Netherlands. Photograph by "Adrem68" (Wikimedia Commons; Creative Commons Attribution-Share Alike 4.0 International license).

Carbon emissions from human activity and concentrations of CO₂ and other greenhouse gases in the atmosphere have risen dramatically since the industrial revolution began in the early 1800s. Earth’s average temperature has been rising in a way that cannot be accounted for by natural variation alone. Climate models that incorporate increasing CO₂ from human activities explain this warming trend better than any models based on only natural variation. For an example of a model that separates out and quantifies natural and human factors, see the video below: “What is Causing the Earth to Warm?”

"What is Causing the Earth to Warm?" Paleontological Research Institution (YouTube)
But we don’t have to only rely on models. Measurements of carbon isotopes in atmospheric carbon dioxide carry the “fingerprints” of fossil fuels. This is explained in the Digital Encyclopedia chapter “Evidence for and Causes of Recent Climate Change” (see Section 2: Causes of Recent Climate Change), and in the video below.

"The Culprit(s) Behind Our Changing Climate," Paleontological Research Institution (YouTube)