Scientists use climate proxy records like coral skeletons, tree rings, glacial ice cores, and sediment layers. For example, the levels of oxygen 16 in a layer of ocean debris and fossils go up as temperatures rise. So a high level of oxygen 16 in sediment from one layer tells scientists that the planet was hot and watery when the sediment was laid down.

[+] marmo7ade@lemmy.world -6 points 1 year ago* (last edited 1 year ago) (3 children)

The presence of oxygen 16 tells you the planet was warm. It does NOT tell you atmospheric temperature.

The claim made in the image is fear-mongering non-sense. The earth is 4 billion years old and was almost certainly hotter in the past, and within the last 100,000 years.

Scientists need to stop being deliberately melodramatic - to the point of lying - to make a point. It is counter-productive.

[–] bzah@discuss.tchncs.de 2 points 1 year ago

It seems the temperature has been slightly hotter about 6500 years ago for a period of around 2 centuries with temperature estimated between +0.8 and +1.8 °C compared to 19th century, but this is subject to debate, (see for example https://www.nature.com/articles/s41597-020-0530-7).

Before that, we have to go back to a period where most Homo Sapiens were living in Africa about 125,000 years ago, where warming was likely +0.5 to +1.5°C compared to the same 19th century baseline.

Regardless if there was periods much hotter in the long past, the big difference with today's situation is the rate at which this warming is taking place. For example, for the "6500 years ago" period, it took about 3000 years of warming to go from +0 to it's maximum (which is between +0.8 and +1.8 °C). Today we are at about +1.1°C and it took us only 100 years, through fossil fuels burning and farming to reach that and most of which happened in the past 50 years.

Sources:

IPCC WG1, figure2_1
https://www.nature.com/articles/s41597-020-0530-7

Also, about oxygen 16 and oxygen 18:

The water remaining in the ocean develops increasingly higher concentration of heavy oxygen compared to the universal standard, and the ice develops a higher concentration of light oxygen. Thus, high concentrations of heavy oxygen in the ocean tell scientists that light oxygen was trapped in the ice sheets. The exact oxygen ratios can show how much ice covered the Earth. Sources:

https://earthobservatory.nasa.gov/features/Paleoclimatology_OxygenBalance which is based on https://hal.science/hal-03334828/file/jgrD1994Jouzel25791.pdf
You may also find this wikipedia article useful https://en.wikipedia.org/wiki/%CE%9418O#Extrapolation_of_temperature

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