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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How can they accurately know it 120 years from now?
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Buy combining lots of data points. Ice cores can show how much snow fell in that year and since layers of snow formed at different temperatures have distinct chemical properties, all of these layers can give us some insight to the existing temperature conditions of that time.
Tree rings can tell you how much the tree grew that year, the bigger the rings the warmer the year. So a fossil of a tree can tell you about the growing conditions, plus the location of the tree. If you find a tree that will only grow in a range of temperatures in a location, you can assume that location was within those ranges. For example, you might find a tree the only grows in hot climates in the Greenland fossil record, so you can infer that Greenland was a lot hotter in the past.
Sediment on the bottom on lakes may contain shells of small, surface-living animals that are deposited over millions of years. Oxygen isotopes present in these sediments, again provide a range of temperatures
The more datasets that you combine will narrow the range that the temperature could be.
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What Michael E Mann said was, from his Twitter:
> [July will be the warmest month this planet has seen in modern history (and plausibly the past 120,000 years)](https://twitter.com/MichaelEMann/status/1681664587697889282)
It is so far the hottest global average temperature since we have been keeping accurate records using thermometers. That part is certain.
Climatologists have accurate global coverage of temperatures measured by thermometers since 1970, [and thermometer measurements for limited locations (mainly developed nations) since the late 1800s.](https://www.metoffice.gov.uk/hadobs/hadcrut5/)
Earlier than that they have to use things called proxy measurements. They’re “proxy” because they are measurements that are strongly correlated to temperature and they have a good understanding of that correlation.
Others have mentioned isotope records. Oxygen has 2 dominant isotopes, O-16 and O-18. The both occur naturally, and the ratio between O-16 and O-18 is dependent on temperature. So climatologists can measure that ratio inside ancient ice and fossils to determine the temperature. There’s also other chemical-based proxies, called ‘[Paleothermometers](https://en.wikipedia.org/wiki/Paleothermometer#)’ like magnesium-calcium ratios, or TEX86 and UK37 (chemicals in cells).
Glacial ice also preserves bubbles of air from the time the ice solidified. Very carefully they can extract the ancient air and get a measurement of the atmospheric composition from back then. Using the concentration of CO2 and other greenhouse gases they can base a temperature on that too.
[Another proxy is literally tree rings](http://www.climatedata.info/proxies/tree-rings/). Biologists know how wide the gap between rings are for certain environmental conditions, and climatologists look at old trees and measure the gap to get the temperature.
For ocean temperature measurements they can use an “assemblage” proxy. Based on what we know of how different plankton species are distributed across different sea temperatures today, they can search for these plankton fossils in ocean sediments and get an estimate of the temperature then.
Climatologists won’t just rely on a single proxy. All of these have their limits and uncertainties, so they will combine all of them to eliminate errors to get that 120,000 year timespan.
The problem with going back in time is that you lose “time resolution” as the scale gets squished. Most proxies are naturally based on seasons, so you get a one record of winter conditions, and another for summer, at best. So basically beyond what they have directly measured, it’s nearly impossible to be certain whether or not this July is the hottest July in 120,000 years.
But it is definitely the hottest since we kept direct temperature measurements.
https://theconversation.com/is-it-really-hotter-now-than-any-time-in-100-000-years-210126
The key word in your question is accurately. Accuracy can have many different resolutions such as 100:1 or 1,000,000:1. If you are meaning accuracy to the 8th digit or 36.12345678 degrees celsius at noon on a random Tuesday 120,000 years ago, the scientist cannot do that with the data we have.
What can be said with a great amount of certainty is the global average temperature in a given year or even greater amounts of years the further back the record is studies. Global average temperature is currently gathered at many, many sites around the world including land, sea, and ice masses. Imagine 1,000,000 sites gathering data every second for a whole day. There are ~~32,587,200~~ 86,400 seconds in a day. This comes to ~~32,587,200,000,000~~ 86,400,000,000 data points. This will give you a high degree of accuracy and mankind has been doing this for decades.
Next compare the known data to factors affected by temperature such as tree growth rings from around the world to polar ice samples that date back hundreds of thousands of years. The ice history can be quantified to correlate to specific global events such as the eruption of Mount Vesuvius. A period representing 100 years of average global temperature can easily be identified. Place the extrapolated data for as far back as we can and you will have a simple chart showing all annual global average temperature.
The global average temperature of June is the highest point on the graph dating back at least 120,000 years.
edit to correct maths. Point still valid. We have a lot of data.
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.
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.
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