<p>One kilometer of rock is actually a lot to dig through to get to that very warm condition.</p>
<p>The key quantity here is <strong>heat flux.</strong> The crux of the matter is:</p>
<p>Geothermal heat flux is only a small fraction of a percent of solar heat flux.</p>
<p>Solar heat flux balance terms are on the order of tens to hundreds of watts per square meter <a href="https://www.azimuthproject.org/azimuth/show/Solar+radiation" rel="noreferrer">source</a>:</p>
<blockquote>
<p>The earth’s surface absorbs 156 W/m² from the sun (as a global average) and emits 55 W/m² long-wave energy to the atmosphere. The atmosphere absorbs 84 W/m² and emits 185 W/m² as longwave radiation to space.</p>
<p>The above figures are from Barry and Chorley, 2003[¹]. The account in Kiehl and Trenberth’s 1997 paper[²] is more complicated, but it can be partially summarized by this picture:</p>
<p>In particular, note that they list:</p>
<p>342 W/m² (341 W/m²) average power hitting the Earth’s atmosphere.</p>
<p>198 W/m² (184 W/m²) average power hitting the Earth’s surface.</p>
<p>168 W/m² (161 W/m²) average power being absorbed by the Earth’s surface.</p>
<p>Bracketed figures are from the Trenberth et al 2009 paper[³].</p>
</blockquote>
<p>In contrast, geothermal heat flux is measured in tens of <em>milliwatts</em> per square meter:</p>
<blockquote>
<p>Heat flows constantly from its sources within Earth to the surface. Total heat loss from Earth is estimated at 44.2 TW (4.42 × 10¹³ watts).[⁴] Mean heat flow is 65 mW/m² over continental crust and 101 mW/m² over oceanic crust.[⁴] This is 0.087 watt/square metre on average (0.03 percent of solar power absorbed by Earth[⁵]), but is much more concentrated in areas where the lithosphere is thin, such as along mid-ocean ridges (where new oceanic lithosphere is created) and near mantle plumes.[⁶]</p>
</blockquote>
<p><strong>Cited References</strong></p>
<ol>
<li><p>R. G. Barry and R. J. Chorley (2003), <em>Atmosphere, Weather and Climate</em>, Routledge, London, 2003.</p>
</li>
<li><p>J. T. Kiehl and Kevin E. Trenberth (1997), &quot;Earth’s annual global mean energy budget&quot;, <em>Bull. Amer. Meteor. Soc.</em> <strong>78</strong>, 197–208.</p>
</li>
<li><p>K. E. Trenberth, J. T. Fasullo, and J. Kiehl (2009), &quot;Earth’s global energy budget&quot;, <em>Bull. Amer. Meteor. Soc.</em> <strong>90</strong>, 311–323.</p>
</li>
<li><p>Pollack, Henry N., et.al. (1993) &quot;Heat flow from Earth's interior: Analysis of the global data set&quot;, <em>Reviews of Geophysics</em>, <strong>31</strong>, 3, p. 273. Archived 2011-08-11 at the Wayback Machine. <a href="https://doi.org/10.1029/93RG01249" rel="noreferrer">doi:10.1029/93RG01249</a></p>
</li>
<li><p>&quot;Climate and Earth's Energy Budget&quot;. NASA. 2009-01-14.</p>
</li>
<li><p>Richards, M. A.; Duncan, R. A.; Courtillot, V. E. (1989). &quot;Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails&quot;. <em>Science</em> <strong>246</strong> (4926): 103–107. Bibcode:1989Sci...246..103R. <a href="https://doi.org/10.1126/science.246.4926.103" rel="noreferrer">doi:10.1126/science.246.4926.103</a>. PMID 17837768. S2CID 9147772.</p>
</li>
</ol>

CC BY-SA 4.0
2023-03-16T23:48:01.503Z
2023-03-17T16:27:45.830Z
2023-03-17T16:27:45.830Z
Oscar Lanzi
<p>I believe the temperature gradient underground is as high as 30°C per kilometer of depth. Thus, it is very warm only 1 kilometer below the Earth's surface. Yet, it is commonly stated that geothermal energy is insignificant to the Earth's surface temperature. How can this be?</p>

CC BY-SA 4.0
2023-03-17T12:44:59.477Z
1km of rock is a damn good insulator. Very little energy reaches the surface.
Michael
<climate><temperature><geothermal-heat>
Why is geothermal heat insignificant to surface temperature?
Peter A
# Why is geothermal heat insignificant to surface temperature?

(Asked by: username_0 on 2023-03-16)

I believe the temperature gradient underground is as high as 30°C per kilometer of depth. Thus, it is very warm only 1 kilometer below the Earth's surface. Yet, it is commonly stated that geothermal energy is insignificant to the Earth's surface temperature. How can this be?

**Tags:** climate, temperature, geothermal-heat

> **Comments:**
> **username_1**: 1km of rock is a damn good insulator. Very little energy reaches the surface.  

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## 1 answers

### Answer by username_2 on 2023-03-16. Score: 37 | Accepted answer

One kilometer of rock is actually a lot to dig through to get to that very warm condition.

The key quantity here is **heat flux.** The crux of the matter is:

Geothermal heat flux is only a small fraction of a percent of solar heat flux.

Solar heat flux balance terms are on the order of tens to hundreds of watts per square meter [source](https://www.azimuthproject.org/azimuth/show/Solar+radiation):

> The earth’s surface absorbs 156 W/m² from the sun (as a global average) and emits 55 W/m² long-wave energy to the atmosphere. The atmosphere absorbs 84 W/m² and emits 185 W/m² as longwave radiation to space.
>
> The above figures are from Barry and Chorley, 2003[¹]. The account in Kiehl and Trenberth’s 1997 paper[²] is more complicated, but it can be partially summarized by this picture:
>
> In particular, note that they list:
>
> 342 W/m² (341 W/m²) average power hitting the Earth’s atmosphere.
>
> 198 W/m² (184 W/m²) average power hitting the Earth’s surface.
>
> 168 W/m² (161 W/m²) average power being absorbed by the Earth’s surface.
>
> Bracketed figures are from the Trenberth et al 2009 paper[³].

In contrast, geothermal heat flux is measured in tens of *milliwatts* per square meter:

> Heat flows constantly from its sources within Earth to the surface. Total heat loss from Earth is estimated at 44.2 TW (4.42 × 10¹³ watts).[⁴] Mean heat flow is 65 mW/m² over continental crust and 101 mW/m² over oceanic crust.[⁴] This is 0.087 watt/square metre on average (0.03 percent of solar power absorbed by Earth[⁵]), but is much more concentrated in areas where the lithosphere is thin, such as along mid-ocean ridges (where new oceanic lithosphere is created) and near mantle plumes.[⁶]

**Cited References**

1. R. G. Barry and R. J. Chorley (2003), *Atmosphere, Weather and Climate*, Routledge, London, 2003.
2. J. T. Kiehl and Kevin E. Trenberth (1997), "Earth’s annual global mean energy budget", *Bull. Amer. Meteor. Soc.* **78**, 197–208.
3. K. E. Trenberth, J. T. Fasullo, and J. Kiehl (2009), "Earth’s global energy budget", *Bull. Amer. Meteor. Soc.* **90**, 311–323.
4. Pollack, Henry N., et.al. (1993) "Heat flow from Earth's interior: Analysis of the global data set", *Reviews of Geophysics*, **31**, 3, p. 273. Archived 2011-08-11 at the Wayback Machine. [doi:10.1029/93RG01249](https://doi.org/10.1029/93RG01249)
5. "Climate and Earth's Energy Budget". NASA. 2009-01-14.
6. Richards, M. A.; Duncan, R. A.; Courtillot, V. E. (1989). "Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails". *Science* **246** (4926): 103–107. Bibcode:1989Sci...246..103R. [doi:10.1126/science.246.4926.103](https://doi.org/10.1126/science.246.4926.103). PMID 17837768. S2CID 9147772.


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