Scientist claims that July 2023 is the hottest July in 120,000 years.
My question is: how can scientists accurately and reproducibly state this is [the hottest month of July globally in 120,000 years](https://www.theage.com.au/environment/climate-change/the-hottest-july-in-120-000-years-what-s-in-store-for-australia-this-summer-20230719-p5dpm3.html)?
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There’s a lot of good comments here about different paleoclimate proxies. A lot of them mention “oxygen isotopes”. But what does that mean? I’ll explain a little more how that works to help us understand how temperatures fluctuated in the ancient past.
Water molecules can come in a couple different varieties depending on which isotope of oxygen happens to be attached to the hydrogen. In simple terms, there’s a heavy isotope of oxygen called O-18 and there’s a lighter one called O-16. So some molecules of water are ever so slightly heavier than others depending on which variety of oxygen they have as the O in H2O.
Now imagine you have a box filled with ping pong balls and golf balls. The golf balls are a little heavier than the ping pong balls but otherwise they’re pretty much the same. Suppose you start gently shaking the box up and down. The ping pong balls are going to be jostled more, and more of them will fall out of the box than the golf balls. Now pretend you start shaking the box much harder. Lots of ping pong balls will still fly out, but now lots of the golf balls will fly out too.
When the earth’s temperature is cool, it’s like when you’re shaking the box only gently; mostly it’s just the lighter molecules of ocean water that get evaporated while the heavier molecules stay behind. When the temperature rises the water molecules are being jostled harder so relatively more of those heavier molecules are evaporated into the atmosphere. Eventually that water vapor forms clouds, and some of those clouds eventually fall as snow into glaciers. When global temperatures are warm, that snow has relatively more of the heavier molecules compared to snow that falls in colder climate conditions. In reality there’s a lot of complicated factors that have to be considered when studying this stuff but that’s the basic idea.
When scientists study ice cores, they’re analyzing how the proportions of the heavy vs light isotopes of oxygen changed in the layers of snow that fell thousands of years ago, and with that they can work out a very precise picture of how global temperatures have changed over time.
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