Spotlight on Climate: Recent warming is different

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Geoscientists use a number of tools to deduce what happened in the past, and they apply their techniques to some of Earth’s most astonishing features. They retrieve ice from kilometers-thick polar icesheets to analyze the composition of ancient air trapped in bubbles. They extract microfossils from mud lifted from the deepest ocean to decode past ocean temperatures. They sample biological archives from tree rings, bogs, and packrat middens to reconstruct climate changes on land.

This rich variety of evidence shows that recent global warming is distinct from previous warm periods in four ways:

(1) It’s been a long time since it’s been this warm. We need to look back to the warm period prior to the last major ice age, around 125,000 years ago, to find evidence for global temperatures that were warmer.

(2) Present warming reverses a long-term global cooling trend. Global temperature was decreasing slightly for thousands of years prior to industrialization, driven by slow changes in Earth’s orbit. Superposed on this cooling trend were centuries that were warmer or colder, but no changes were as prominent as the warming that began by the late 1800s.

(3) Earth is now warming very rapidly. At the end of the last major ice age, global temperature rose by about 5°C (9°F) between 17,000 and 12,000 years ago. The fastest warming rate averaged about 1.5°C per thousand years. This compares roughly to 1.1°C of global warming since the late 1800s. Higher-resolution records of global temperature show that the rate of global warming since 1970 has exceeded the rate during any other 50-year period over at least the past 2000 years.

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