Large scale heat management: controlling or influencing temperature flows on a geographic (regional or global) scale. Heat management is one of the deep fundamental problems in life and engineering, but humans have never tried to do anything smarter or more ambitious in this area than standard HVAC stuff.

Humans like moderate temperatures, say 55-75 F, but we spend quite a lot of our time in discomfort or even pain because the actual temperature is outside this range. But the problem isn't that heat (or cold) is in short supply, it's just distributed unevenly. This fact hit home for me when I was riding in an Uber because terrible winter weather knocked out Boston's subway system, and the driver told me she had just returned from a trip to Brazil, which was mostly unpleasant because the heat made it impossible to do anything outside.

Here are some options:

• Heat banking: store heat during the summer in large reservoirs of water. Release it during the winter.
• Heat trade: hot regions send heat to cold regions; both sides are happier.
• Heat sequestration: there are huge pools of cold water about 1000m underneath the ocean surface, when your city is too hot, send some heat down there.

I'm actually quite confident some version of this idea will work, because there are two vastly powerful forces working in its favor:

• Economics: in heat trade, both parties feel that they are exchanging a good for a bad. This kind of exchange almost never happens, most normal trade relies on the parties valuing something at a different magnitude of positive or negative value, but with the same sign.
• The Great Second Law: Humans suffer from temperature unevenness but Nature actually prefers temperatures to equilibriate. We just have to help Nature do what it already wants to do.