Underground temperatures as indicators of surface temperatures – part 2

This article was originally written and published by me in Finnish in Ilmastotieto-blog and this is just an English version of it.

Continued from part 1.

Subsurface temperature

There are two main factors affecting the subsurface temperature: the temperature changes in the surface and the heat from the Earth’s core. The heat from the Earth’s core varies very slowly over million year timescales. Surface temperature on the other hand varies in relatively short timescales. These two factors can be separated from each other because the changes in the heat from the Earth’s core are so slow. When temperature evolution is being studied in time periods of hundreds or thousands of years, the heat flux from the Earth’s core can be assumed to be constant, which enables us to separate the short time variations as changes due to surface temperature variations.

The surface temperature variations are being transferred to deeper ground as heat waves. The amplitude of the waves decreases strongly when going deeper and the decrease of amplitude depends on the frequency of the wave; fast changes decrease more quickly than slow changes. It is because of this that the annual temperature changes show deeper than daily temperature changes. Also because of that, eventhough the daily and annual changes are much bigger than the long-period climate changes, the daily and annual changes are not seen deeper than few tens of meters while the long-period climate changes are seen deeper. The surface temperature changes penetrate deep ground at such speed that the temperature changes of thousand years are typically seen in upper 500 meters.

So the steady influence of the heat from the Earth’s core must be separated from the measured temperature profile. After this a profile is left that is called the ‘‘residual’’ temperature profile. The final surface temeprature reconstruction is based on the residual temperature profile. Reconstructions have been done by two models; forward models and inverse models. In a forward model certain history is first assumed for the climate and from that it is computed how the subsurface temperature profile should look. The result is compared to the measured profile. The assumed climate history is then corrected to more suitable one and the comparison is performed again. Like that the model that fits to the measurements is sought and the output of that model is then the surface temperature reconstruction. In inverse models the surface temperature is derived directly from the observations.

When a reconstruction is made from a combination of several boreholes, many disturbing factors can be eliminated at least partly. Disturbing factors are mostly characteristic to the borehole and are not seen in many boreholes simultaneously, so the features changing simultaneously in several boreholes can be thought to describe changing climate. In addition to that, the reconstruction is usually compared to the surface temperature record measured from the region, so that it is certain that the reconstruction really shows surface temperature at least during that time when there’s data from both sources available.

The usage of subsurface temperatures is restricted by their bad resolution in time, i.e. the fact that they don’t show fast variations in same manner as for example tree-rings, which show annual variation. Good aspect is that reconstructions from subsurface temperatures represent longer time temperature averages well. There are also lot of measurements available with good geographical coverage. For example from Southern Hemisphere there are lot of reconstructions from subsurface temperatures while there are only few of other reconstructions from there. Even in Finland some reconstructions based on subsurface temperatures have been done [this note was included because the article was originally to a Finnish audience].

Global surface temperature from borehole reconstructions

Also global analyses have been made from borehole reconstructions. As in other reconstructions and in surface temperature analyses, also in global borehole reconstructions it must be solved how to combine the data from different sources in a meaningful way. There is lot of research on the subject. In addition to some global analyses presented below, for example Mann et al. (2003) ja Pollack & Smerdon (2004) are noteworthy. “Global” is here like in other reconstruction methods rather relative concept because borehole measurements made from ocean floor are not being used (ocean floor measurements are not made deep enough – they are usually only few meters deep) and the global distribution of boreholes is not evenly spread even in land boreholes.

Here are some global reconstructions and their results briefly:

Huang et al. (1997) use subsurface heat flux measurements to make a 20,000 year surface temperature reconstruction. Their results show that early and middle Holocene were warmer than present and additionally there was a warmer period than present also about 500-1000 years ago. But their most important result is probably this:

Although temperature variations in this type of reconstruction are highly smoothed, the results clearly resemble the broad outlines of late Quaternary climate changes suggested by proxies.

Pollack et al. (1998) – global analysis of 358 boreholes for the last 500 years shows that:

…in the 20th century, the average surface temperature of Earth has increased by about 0.5°C and that the 20th century has been the warmest of the past five centuries. The subsurface temperatures also indicate that Earth’s mean surface temperature has increased by about 1.0°C over the past five centuries.

Huang et al. (2000) – also for 500 years but from 616 boreholes:

The results confirm the unusual warming of the twentieth century revealed by the instrumental record, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies.

Pollack & Huang (2000) – as a review article mostly reviews the results of other works from 600 boreholes for last 500 years:

Taken as a global ensemble, the borehole data indicate a temperature increase over the past 5 centuries of about 1 K, half of which has occurred in the twentieth century alone (Figure 7). This estimate of twentieth century warming is similar in trend to the instrumental record of surface warming determined from meteorological stations (Jones et al 1999b). When this trend is added to the more gradual warming in the previous centuries, the twentieth century stands out as the warmest century of the past five, a result similar to many recent multi-proxy reconstructions (Overpeck et al 1997; Jones et al 1998; Mann et al 1998, 1999) that did not include any geothermal component.

Beltrami (2002) – 500 year global reconstruction from 826 boreholes:

Results indicate that the global average ground temperature and ground heat flux have increased an average of 0.45°K and 18.0 mWm² respectively over the last 200 years, and 0.9°K in the last five centuries.

Huang et al. (2008) – modern version of Huang et al. (1997). This study also is from last 20,000 years and it uses subsurface heat flux measurements and subsurface temperature measurements combined with modern surface temperature record. Their reconstruction is shown in Figure 2. The results of the study are broadly similar as in their 1997 study, but the details have changed a little:

The reconstructions show the temperatures of the mid-Holocene warm episode some 1–2 K above the reference level, the maximum of the MWP at or slightly below the reference level, the minimum of the LIA about 1 K below the reference level, and end-of-20th century temperatures about 0.5 K above the reference level.

Figure 2. Reconstruction of global surface temperature anomaly from subsurface temperature and heat flux measurements combined with modern surface temperature record. Y-axis is the surface temperature anomaly in kelvins and X-axis is years in thousands of years ago. The highest point is the Holocene climate optimum (about 6000 years ago). Lowest point is the latest ice age. Other points worth mentioning are the medieval warm period (slight peak about 1000 years ago), little ice age (couple of hundred years ago) and the late 20th century (at the point zero years ago). The zero point of surface temperature anomaly is the 1961-1990 mean. The data is from NOAA Paleoclimatology website and is originally from Huang et al. (2008).

Thanks to Kaitsu, Esko and Jari for good comments on this article.

References

Beltrami (2002), “Beltrami, H. (2002), Climate from borehole data: Energy fluxes and temperatures since 1500, Geophys. Res. Lett., 29(23), 2111, doi:10.1029/2002GL015702, [abstract, full text]

Huang et al. (1997), “Late Quaternary temperature changes seen in world-wide continental heat flow measurements”, Geophys. Res. Lett., 24(15), 1947–1950, [abstract, full text]

Huang et al. (2000), “Temperature trends over the past five centuries reconstructed from borehole temperatures”, Nature 403, 756-758, doi:10.1038/35001556, [abstract, full text]

Huang et al. (2008), “A late Quaternary climate reconstruction based on borehole heat flux data, borehole temperature data, and the instrumental record”, Geophys. Res. Lett., 35, L13703, doi:10.1029/2008GL034187, [abstract, full text]

Mann et al. (2003), “Optimal surface temperature reconstructions using terrestrial borehole data”, J. Geophys. Res., 108(D7), 4203, doi:10.1029/2002JD002532, [abstract, full text]

Pollack et al. (1998), “Climate Change Record in Subsurface Temperatures: A Global Perspective”, Science 9 October 1998:
Vol. 282. no. 5387, pp. 279 – 281, DOI: 10.1126/science.282.5387.279, [abstract, full text]

Pollack & Huang (2000), “Climate Reconstruction from Subsurface Temperatures”, Annual Review of Earth and Planetary Sciences, Vol. 28: 339-365, doi:10.1146/annurev.earth.28.1.339, [abstract, full text]

Pollack & Smerdon (2004), “Borehole climate reconstructions: Spatial structure and hemispheric averages”, J. Geophys. Res., 109, D11106, doi:10.1029/2003JD004163, [abstract, full text]