But how do scientists know global warming is caused by humans and that the observed warming isn’t a natural variation in Earth’s climate? Scientists use three closely connected methods to understand changes in Earth’s climate. They look at records of Earth’s past climates to see how and why climate changed in the past, they build computer models that allow them to see how the climate works, and they closely monitor Earth’s current vital signs with an array of instruments ranging from space-based satellites to deep sea thermometers. Records of past climate change reveal the natural events—such as volcanic eruptions and solar activity—that influenced climate throughout Earth’s history. Today, scientists monitor those same natural events as well as humanreleased greenhouse gases and use computer models to determine how each influences Earth’s climate.