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)?
In: 4028
There are two main lines of evidence: oxygen isotopes in ice cores from Greenland and Antarctica and oxygen isotopes from the shells of carbonate critters (mainly foraminifera) preserved in ocean sediment cores. Evidence from ice cores is the gold standard for atmospheric temperature reconstruction. In addition, ice cores preserve bubbles of air that provide direct measurements of greenhouse gas concentrations deep in Earth’s past, which is how we build equations to relate GHGs to temperature. Ice core evidence can provide annual resolution, which is powerful. The evidence from ocean sediments is helpful and goes back a very long time (longer than ice cores), but the temporal resolution is poor, like millennia to tens of millennia, and they don’t do a good job of recording atmospheric and land temperatures (it turns out they record sea temperatures, which can be quite out of sync with air and land). It took decades of trying to do these reconstructions with ocean cores before we realized that it wasn’t great for air or land. Luckily we have the ice cores now.
Edit: how do we know “July” temps… we really don’t, but we can get summer vs. winter data from the ice core evidence. Think of annual deposition of oxygen isotopes in precipitation as a sine wave with crests and troughs. Those crests and troughs represent the highs and lows, i.e., summer and winter. We can interpolate (read between the lines) to get spring and fall. The amplitude, or height/depth of the crests and troughs represent the magnitude of temperature variability.
Source: my PhD specialty is Quaternary environments and I teach a graduate-level course on this topic.
Latest Answers