Comment by Qem 2 days ago
> The United Nations (UN) believes there are 4B buildings on Earth. This week, a dataset called "GlobalBuildingAtlas" (GBA) was published by researchers at the Technical University of Munich (TUM) that attempts to estimate this number at being closer to 2.75B.
If we painted the roofs on all of them white, by how much would the temperature of the planet drop?
There was a good study on this a few years ago that ran the numbers on this and landed on white paint for residential homes as the best option, for a few reasons, if I remember correctly:
- Installation, maintenance and transmission costs are lower when solar is aggregated on farms - Solar offsets air conditioning, but that moves the heat outside. White roofs reduce the need for AC, which helps significantly with urban heat scenarios
A quick search yields a UCL study, which supports the lower claim: https://phys.org/news/2024-07-roofs-white-city.html
A good percentage of Pakistan did this recently and removed 35% of demand off of their grid https://www.ted.com/talks/jenny_chase_solar_energy_is_even_c...
I would love to have an air conditioning / cooling solution that is directly linked to solar panels with no batteries involved. Like the sun shines, we get electricity, we do the work. My main goal for this thought experiment is to come up with uses of solar electricity that is resilient to the unpredictable and unreliable energy generation from solar. Thoughts?
Supposedly water heaters are an amazingly good target for variable power. Water heating takes 18% of home energy use[0] and is already a well insulated storage device. Just heat it up as you are able with the day's sun.
If you are looking for a cooling solution, you could go the other way and make water chillers through a dedicated water tank. You would tie the HVAC to pipe air through a heat exchanger. Seems like all of that is well established engineering.
[0] DOE link on water heating https://www.energy.gov/energysaver/water-heating
> I would love to have an air conditioning / cooling solution that is directly linked to solar panels with no batteries involved.
I think that would be very viable with fridges, that represent a large share of electricity consumption among the poorest. Before electricity, people powered fridges by constantly buying ice blocks. They were just isolated boxes where food was stored together with the blocks. Perhaps it's just necessary to go back at the roots, and make fridges that take energy from solar panels and generate a lot of ice by day, and uses it to keep cold at night, with no need for batteries.
I'm now wondering if painting a roof white is better than covering the same roof with solar panels and using that to drive air conditioning (in the house). My intuition is that painting it white must be vastly more environmentally friendly, although it probably doesn't reduce the temperature very much compared to aircon.
There are certain materials that aren't just white but also specifically radiate heavily in IR bands that the atmosphere is particularly transparent to and so can actually stay sub-ambient temperature while in sunlight. See, for example, https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12284
But adding conventional AC is never going to cool down the world.
AC doesn't make it cool it merely transfers the heat to the outside in an energy efficient manor. The emerging tech is special coatings that convert heat to an infrared frequency that easily escapes the Earth's atmosphere.
In cooler climates, you are now taking a hit on heating costs. That now blocked Winter sun would have provided some amount of free warmth.
there is a cool project that explores this topic: https://coolcolors.lbl.gov/
I think we should do more albedo engineering. like white roads and car roofs.
Back-of-the-envelope - Road area. World road length ≈ 60–70 million km. Using an average paved width of ~8–10 m ⇒ area ≈ (0.9–1.3)×10¹² m². Earth’s surface is 5.1×10¹⁴ m², so roads cover ~0.09–0.13% of the planet.
- Albedo change. Dark asphalt is ~0.05–0.10. “White” coatings can push toward ~0.4–0.6 (fresh), but weathering quickly dulls them. So a plausible Δalbedo for roads is +0.2 to +0.5.
- Global albedo change. Δα_global ≈ (road fraction) × (Δalbedo_road) ≈ (0.001)×(0.2–0.5) ≈ +0.0002 to +0.0005.
- Radiative forcing. Globally averaged incoming sunlight ≈ S₀/4 ≈ 340 W m⁻². Forcing from an albedo change is ΔF ≈ −Δα_global × 340 ≈ −0.07 to −0.17 W m⁻².
- Temperature response. Using a standard sensitivity ~0.8 °C per W m⁻² (≈3 °C per CO₂ doubling): ΔT ≈ −0.05 to −0.14 °C at equilibrium.
I'm thinking about light colored roads that seem to be made of concrete. See them here and there. Seemed to be more of them when I was a kid.
Wonder if that would make a substantial difference? Much brighter than asphalt but not bright enough to bother drivers.
There are materials that do what you're looking for surprisingly effectively, staying many degrees sub-ambient even in direct sunlight. (See https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12284 for example). But a roadway is a really difficult use case for a surface coating. With something like 50% of the surface area of many American cities being road or parking, there's a lot of potential room for effectively mitigating the urban heat island effect, but I think roofs are a better target. They don't have cars sitting on top of them blocking sightlines nearly as often.
Painting roofs (or even all man-made surfaces) white wouldn't do much globally at all, but it might reduce the urban heat island effect. But probably (speculating a bit here) not quite so much as creating more green spaces (or green roofs, for that matter).
I would guess not that much - certainly fractions of a degree - because the proportion of the planet's surface is still relatively low, and as much as 60% of that reflected light would be scattered and absorbed by the atmosphere.
It'd have a much bigger impact if all those roofs had solar panels, and the resulting electricity was used to replace carbon-emitting energy sources.
Probably not as much as if we “painted” them black with solar panels and used the resulting electricity to displace fossil fuel burning.