re: If we cover the desert in solar panels, what happens to all the reflected sunlight?

Any time you try to store energy or use that you previously stored, some of it will be wasted as heat. This is true of refining and the burning fossil fuels and also of storing and then using solar power.

However, this wasted energy as heat IS NOT the primary driver of anthropogenic global warming.

When a photon from the sun interacts with a solar panel, some of the energy is stored and some goes to thermal energy. Similarly, when you combust a hydrocarbon in your car's engine, hydrocarbon molecules and O2 molecules from the air react and produce H2O and CO2 molecules plus energy. Some of that energy goes to moving your car, charging your battery, etc, and some is wasted as heat.

But in both cases, that energy that's wasted as heat is not enough to drive global warming. The thing that causes global warming is that CO2 molecule that was produced. That molecule enters the atmosphere where it will remain for decades trapping thermal energy radiated from the earth and re-emitting it.

The energy wasted as heat from the initial reaction, both in the case of solar and hydrocarbon power, was a one time thing. However, only in the hydrocarbon case did we produce a molecule of greenhouse gas (CO2 in this case) that will continue trapping heat and re-emitting it back toward earth for decades and decades.

This was a good question, and the answers illustrates well why some forms of energy production cause anthropogenic global warming while others have a negligible impact. I hope you and the others here will read the answer with an open mind.