In a previous post we learnt that a common way scientists test climate models is to compare the model’s output to historical climate data . If a model can reproduce historical climate data like temperatures, it is more likely to be able to make accurate predictions of future temperature .
Our understanding of the history of our climate system back to around 1850 is from data recorded by people . But how do we know what aspects of our climate system (temperatures, sea levels, CO2 concentrations) were like before then?
Well, in fact there are many different ways [4,5,6,7] of estimating what the climate was like hundreds or even a million years ago .
For example scientists can learn about aspects of the ancient climate by looking at the width of rings in tree trunks (see the image!)[4,5], analysing the oxygen content and thickness of layers of snow in ancient ice [4,5,6] or by finding fossils and minerals deep under the sea bed [5,7].
There are many positives to using indirect data from times before 1850. For example, it includes climate changes which were much larger  and faster  than any in the past 150 years. If models have data of huge rapid changes in climate it means their predictions of the abnormally fast and big changes in climate we are currently facing will be improved .
 https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter09_FINAL.pdf (See: FAQ9.1 Paragraph 4)
(See: Section 4 bullet point 3)
 https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-0870.2006.00211.x (See: Section 5.1 Paragraph 1)
 https://www.earthobservatory.nasa.gov/features/Paleoclimatology_CloseUp(See: Paragraph 1)
https://www.earthobservatory.nasa.gov/features/Paleoclimatology_IceCores (See: Paragraph 5)
https://www.earthobservatory.nasa.gov/features/Paleoclimatology_SedimentCores (See: Final Paragraph)
 https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_TS_FINAL.pdf (See: TS.2.1 Paragraph 1)
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