Upthread Steve mentioned that the waste heat from human-produced energy is only a tiny fraction of greenhouse heat from human-produced CO2.
But in
this video, Sabine Hossenfelder tells us "I recently learned that waste heat will boil the oceans in about 400 years." If trends continue the waste heat will dwarf the greenhouse heat! Is this correct?
No.*
What assumptions are involved?
Dunno, haven't watched. Do you have a link to a transcript of what she says in the video?
She does say that wind- and solar-produced power do not generate new waste heat: They harness energy that would otherwise have been "wasted" (turned to heat) anyway.
Well that's certainly not true of solar power, which converts light into heat (mostly via electricity and whatever we use that electricity for).
If it hadn't encountered a solar panel, some of that light would have been reflected back into space.
Every solar panel on Earth reduces the planet's albedo, and causes some conversion of incident sunlight into heat.
I haven't done the maths, but I would bet dollars to donuts that this albedo effect is utterly minuscule - as is the effect of waste heat vs. heating due to atmospheric changes.
Waste heat isn't anywhere close to sufficient to boil even a small ocean in only a few centuries. She's dropped a significant number of zeros in whatever calculations misled her to that conclusion. Or perhaps has assumed that waste heat is somehow completely 100% contained by the atmosphere and cannot radiate out into the cold of space. Perhaps the atmosphere knows the difference between nasty artificial waste heat, and nice, natural, solar heat?
*Back of the envelope - solar irradiance is around 1.3kW per m2. If we aren't generating over a W of power per m2, we aren't adding as much as 0.1% to that heating.
US land area is ~10 million km2, at 1W per m2 that's 1012W, or 10TW of power generation required to produce 0.1% of additional heating. US power generation is around 0.45TW, so to add 0.1% to the heat delivered by the sun, the entire surface of the world, including the oceans, would need to generate electricity at more than twenty times the rate that the USA currently does.