measuring the planet

How do we know anything about the Earth’s past climate? Discussions about climate change—its extent, its causes, and what to do about it—often hinge on what we know about our planet’s temperature history. Climate scientists and policymakers routinely talk about the Earth’s “global mean temperature” and compare today’s temperature to a record dating back hundreds of thousands of years. But where does that record come from? And what does it even mean for a single figure to represent the temperature of our entire planet, with its regional diversity and dynamic atmosphere? Scientists have devised ingenious techniques to peer into our planet’s past temperature record, but the picture they give us is a blurry one.

If today you decided that you wanted to know how the climate changes at a certain location, say the base of the Statue of Liberty, you could put a thermometer there and record a measurement at noon every day—or at the beginning of every hour or second, if you want finer resolution. This would ensure that you have a thorough record of fluctuations in temperature at that particular site, from this day forward.

Thanks to scientists (and scientifically-minded amateurs), we have such temperature records dating back more than two centuries for some particular sites. But in discussions of global climate change, the figure of interest is not just the temperature at the feet of Lady Liberty—no single site is wholly representative of the Earth’s complicated climate system—but rather a number representing the Earth’s temperature as a whole. This figure is often cited but rarely explained.

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