Separating Human and Natural Influences on Climate
Natural factors cannot explain the warming of recent decades. In fact, on their own, natural influences would have led to a slight cooling over this period.
Many lines of evidence show that the warming of the past 50 years is primarily due to human activity. First, basic physics explains that greenhouse gases trap heat and thus increase temperature. Since the industrial revolution and the rapid increase in the use of fossil fuels, humans have added more than a trillion tons of carbon dioxide to the atmosphere. As a result, atmospheric concentrations of CO2 have increased by about 35 percent since then.
Additional evidence comes from a number of so-called “fingerprint” studies. For example, the observed pattern of warming at the Earth’s surface and cooling in the upper atmosphere (stratosphere) is precisely what we’d expect to see from an increase in heat-trapping gases. In contrast, if the warming had been caused by an increase in the Sun’s output, Earth’s atmosphere would have warmed throughout.
Scientists have also found the human “fingerprint” in other parts of the climate system, including the dramatic decline of Arctic sea ice, increased moisture in the atmosphere, and changing precipitation patterns.
If human influences are removed from computer simulations of the last century, results suggest that Earth’s average temperature would have cooled slightly over the last 50 years, as shown by the blue bar on the right. When the human influence is added to these simulations, the results reproduce the observed climate change, as shown by the red bar and black line on the right.
The clear message from this entire body of work is that human activities are having an increasingly dominant influence on Earth’s climate.
