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Another form of proxy temperature measurement

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is called bore-hole 
temperatures. 

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What you do is you drill a hole in an 
ice sheet, or in a soil column. 

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And you measure the temperature as a 
function of depth. 

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What is happening with the temperature 
is that, in equilibrium,

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if the temperature at the surface stays 

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constant for a very long time, 
the temperature will 

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increase with depth because of the 
generation of heat below the earth's surface. 

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This is called the geothermal temperature 
gradient. 

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The temperature goes up by about 30 

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degrees Centigrade per kilometer in the 
earth,

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more or less. 

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If you have a sudden change in 

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the temperature at the surface, it takes 
awhile for 

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that news to difuse down into the 
ice core 

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or the rock column that 
you're measuring. 

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What would happen to the temperature 
inside is initially it 

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would only know about the warming 
right up at the surface. 

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And then as time passed the temperatures 
would relax toward 

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a new equilibrium where it's, again, along 
the geothermal gradient, 

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just going 
monatomically upward from the 
surface condition. 

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If you go out and you measure 

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the temperature today and you happen 
to see 

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something that looks like that, you can 
use what is known as an inverse method...

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to try to back out how much the 
temperature changed, and when. 

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It's called an inverse method because, 
while reality goes from cause to effect,

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the change in temperature at the surface

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leads to this distribution of 
temperatures below. 

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The inverse problem has to 

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go the other direction, and go from the 
effect, which is this snapshot 

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of temperatures in the bore holes down 
inside the ice ice sheet,

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and back out what the cause was. 

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How much the the temperature changed and 
when. 

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In general, borehole temperatures give 
you a much 

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smoother estimate of how the temperatures 
change with time. 

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Because if there's fast 

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temperature changes here, you'll never see 
that down in the interior. 

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Basically what you see is the 
same longer time-scale

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overall patterns (of temperature change).