If Earth has warmed and cooled throughout history, what makes scientists think that humans are causing global warming now?
The main reason that scientists think humans caused warming since 1950 is that none of the natural processes that influence Earth’s climate have changed enough during that time period to explain the warming. Over the past thousand years, temperatures have been preserved in natural records like tree rings, ice cores, and coral reefs. Many independent estimates of temperatures from these sources show that while global average surface temperatures varied, at no time were they warmer or did they climb more quickly than during the latter half of the 20th century. Three things can alter global temperatures over this short period: changes in the Sun’s activity, volcanic eruptions, and human emissions of greenhouse gases and aerosols. During the twentieth century, the average amount of energy coming from the Sun either remained constant or increased slightly. Major volcanic eruptions temporarily cooled temperatures by pumping reflective gases into the atmosphere. At the same time, the burning
