How do we know the climate is warming?

Scientists with NASA measure the changing climate through a set of vital signs, similar to the way doctors monitor a patient in a hospital. Those data, compiled from millions of global measurements per year, clearly show that fossil fuel burning has given Earth a long-running fever. 

The concentration of carbon dioxide in the atmosphere has increased by about 50 percent since 1880, raising the average global surface temperature by 2.2 degrees Fahrenheit since then. Based on studying air bubbles trapped in ancient ice and other fossil records, several studies estimate the CO2 level hasn’t been this high since the Pliocene era, from 5.3 million to 2.6 million years ago, when the average global temperature was about 5.4 degrees Fahrenheit warmer than today.

NASA’s vital signs also show that Arctic sea ice has declined 13.1 percent per decade since 1979, shifting the region toward an “unprecedented state,” scientists said in a key 2019 report. Polar ice sheets, as well as mountain glaciers are melting quickly and sea level is rising about 1 inch every eight years, and not just because of the melting ice—the world’s oceans have absorbed about 93 percent of the heat trapped by greenhouse gases, and that warming expands the water, bulging it shoreward. 

Researchers also have calculated that all the planetary warming since the late 1800s has been caused by human activity, mainly fossil fuel burning, but also by changes to land areas from logging, farming and urban development. Without the human-caused changes, Earth would have cooled slightly since then based on factors like volcanic eruptions and slight changes in the planet’s orbit. 

— Bob Berwyn

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