No climate event of the last 2,000 years looks like humanity’s

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Some people who reject climate science seem to think climate scientists have never heard that the climate has changed in the past-as if scientists weren't the ones who discovered those events in the first place. In reality, researchers are intensely interested in past climates because there is a lot to learn from them. You can see how sensitive Earth's climate is to changes, for example, or how variable things can be even when the long-term average temperature is steady.

("Climate has changed without humans before, so humans can't be changing it now" is not a logically valid argument, FYI. It's the equivalent to arguing that we can't cause forest fires, since they occurred before we were around.)

Searching the past

Some historical records show evidence of a cooler period between the 1400s and early 1800s, which has come to be called the "Little Ice Age." We know that glaciers from the Rockies to the Alps expanded during this time period, leaving piles of rocks behind when they eventually retreated. But was this really a global event? Or were there just regional downturns in temperatures that we erroneously connect out of a desire to make a simple story?

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