**4.3 Borehole temperature profile**

The thermal history of the earth's subsurface offers a third evidence of global warming and global climate change. The subsurface stores temperature records over time that are related to the prevailing environment at the time. Responsive thermometers are used to calculate temperature profiles with depth in boreholes, caves, and deep mines. Temperature anomalies due to geological features, upward flow of warmth from the earth's interior, heat produced by crustal rocks, and variations in groundwater movement are generally adjusted for. Surface temperature oscillations propagate downward with depth, with shorter duration fluctuations attenuating more than longer period fluctuations. As a result, only long-term fluctuations in temperature penetrate great depths, with seasonal changes penetrating around 15 m until the signals fade. Century-long variations, in contrast to seasonal variations, can be observed to depths of about 150 m, and millennial cycles can be observed to depths of 500 m or more. These depths are easily attained by low-cost drilling. The subsurface serves as a selective filter, eliminating short-term temperature fluctuations and maintaining excellent records of global warming and, as a result, climate change (**Figure 6**) [7, 26].

The temperature profiles suggest substantial warming in the last century from 0.6°C in southeast Utah to more than 2.0°C in Alaska. Curves are arbitrarily offset for display purpose (see **Figure 6**). The temperature profiles of boreholes spread across a length of about 500 km of northern Alaska shows anomalous warming of 2 to 5°C in the upper 100 to 150 m of the permafrost and rocks [7]. Similarly, borehole temperature profiles in eastern Canada shows a less rapid warming of about 1.0°C. A warming of about 0.5 and 1.0°C were observed in Nebraska sites and Utah sites, respectively (see **Figure 6**). These results indicate that geothermal data mimicked the geographic variations of warming observed in weather station data. Baseline temperatures from previous centuries are often inferred from geothermal evidence, enabling researchers to date the start of the industrial revolution to this century and thus determine the effect of industrialization on global warming and climate change [7]. As a result of the data from the borehole temperature profile from various places, the temperature rises in the range of 0.5 to 5°C at 100 to 150 m. This hints to the likelihood of global warming.

#### **Figure 6.**

*Borehole temperature profile from sites in North America showing warmer temperatures within near-surface depths of 100–150 meters (source: Aizebeokhai [7]).*
