Many of these problems don't seem scientific at all, but rather a problem of political will.

As you said, Solar power is incredibly economical. There are plenty of ideas around putting them over farms, or parking lots en-masse to provide cleaner energy.

Access to clean drinking water, while certainly scientific in some situations, is also a problem of political will and money.

Restore and Improve Urban Infrastructure - It's infrastructure week!

Seems to have been developed in 2008 (continuing through 2017), which explains the "economical" framing: https://en.wikipedia.org/wiki/National_Academy_of_Engineerin...

At this point the Hard Problem is policy to get out of solar's way.

In some regions, but it would be great if it was economical in cloudy Seattle and not just the sunbelt

  3. Develop Carbon Sequestration Methods