Global Warming, or Global Cooling? I guess it depends on how you look at it 
True. They are just heat exchanger. . Not really that much different then the air to air ones in principle. beyond one get it from the air, and one gets it from the ground temp water.
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The horizontal loops tend towards the annual average temperature of the region. The natural slow heat conduction of the dirt and the surface with ambient air will pull the loop material back to average annual temps. As Sean mentions… temperatures do rise as you get deeper but the dominant factor in “recharging” a ground loop is surface temperatures.
The action of the heat pump will push the ground loop temperature higher or lower based on use. The loop just has to be sized so that the temperatures swings are not too big and the pull to back to annual averages can keep up reasonably over the full cycle of the seasons. You don’t want big swings because heat pumps are more efficient when the temperature difference between input and output is smaller. This is the opposite of Stirling heat engines (and all heat engines) which become more efficient as the temperature difference becomes greater.
Vertical bore loops have less heat exchange with the surface them and need a better balance of heat in and heat out to avoid getting ever hotter or colder. Horizontal loops are close to the surface and over a wide area so it “returns to mean” much more easily and quickly. There are web forums devoted to ground source heat pump systems that explore all this in depth.
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