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Max/min Thermometer readings from a weather station with Stevenson screen are subject to noise from many sources and represent the max/ min at that one point in space.

It's well known that underground temperatures are stable at, I believe, the average year-round temperature. They would then make a much better way of measuring average temperature over years.

Do any underground instrumental temperature records exist?

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    $\begingroup$ Instrumental, I don't know, but speleothems are used as climate proxies: en.wikipedia.org/wiki/Speleothem#Climate_proxies $\endgroup$ Jul 24, 2022 at 10:56
  • $\begingroup$ Thanks, I wonder whether oxygen-18 in speleotherms is a proxy for temperature underground in the cave, or for the whole earth (as assumed in ice core reconstructions) $\endgroup$
    – Peter A
    Jul 25, 2022 at 8:16

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There are many underground temperature measurements. Monitoring of permafrost is one instance where underground temperature has been monitored for many years. In Europe there are many boreholes reaching about 100 m under ground that measure temperature. A project called Permafrost and Climate in Europe (PACE) established a network of bore holes that monitor ground temperatures from Svalbard to Spain (Harris et al. 2009). Results from one of these boreholes in Tarfala, Sweden, (67.9098° N, 18.6101° E) has shown warming trends in the upper parts of the bore hole since establishment in the year 2000 (Bolin Centre, no year).

enter image description here

The upper pane in the figure shows the air temperature (in degrees Celsius) at the PACE bore hole at 1650 m a.s.l. (red) as well as the monthly average temperature at the Tarfala Research Station (1130 m a.s.l.; blue) in the northern Swedish mountains. The lower three graphs show the temperature (again in degrees Celsius) at the depths of 25, 30 and 40 m below the ground surface. As can be seen warming occurs at depth. In this particular bore hole the seasonal effects of summer and winter reaches about 20 m below ground level so the presented curves shows the longer term trends in ground temperature. There are some spurious deviations likely caused by water leaking into the bore hole and refreezing causing momentary increases in temperature due to the release of heat as the phase change from liquid to solid occurs.

The extrapolated depth of the transition from negative (permafrost) to positive temperature is about 300 m below the ground at this site.

References

Bolin Centre for Climate Research: Tarfala Data, Permafrost. (Data base) https://bolin.su.se/data/tarfala/permafrost.php

Harris et al 2009. Permafrost and climate in Europe: Monitoring and modelling thermal, geomorphological and geotechnical responses. Earth-Science Reviews. https://www.sciencedirect.com/science/article/pii/S0012825208001311

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    $\begingroup$ Excellent answer! It needs to be mentioned that one possible reason for the boreholes only doing down to 100 m depth is that beyond that, increases in ground temperature will most likely be due to the geothermal gradient. $\endgroup$
    – Fred
    Jul 24, 2022 at 17:01
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    $\begingroup$ It is interesting to note how the ground temperature down to 30 m experiences a noticeable increase from 2008 or 2009, but there was no such increase at 40 m depth. $\endgroup$
    – Fred
    Jul 24, 2022 at 17:03
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    $\begingroup$ @fred There is a general geothermal flow which is constant (over human time perspectives) but the temperature curve is determined by the average annual temperature at the surface so with a change at the upper boundary, warming will start at the surface and affect the temperature curve at progressively increasing depth. In a steady state situation the curve will be constant and therefore the change will trend toward a new steady state (although that is not likely a natural condition). The temperature conditions in the ground is a very interesting topic that cannot be fully explained here $\endgroup$ Jul 24, 2022 at 17:30
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    $\begingroup$ Thank you for the excellent and very helpful answer $\endgroup$
    – Peter A
    Jul 24, 2022 at 21:03

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