How we might ultimately cool the Earth

Because of our increasing energy use, we might eventually need to refrigerate the planet. Here are a few half-nutty ways we might do it, and maybe one not so nutty.

[Hello readers: Sorry for long delay from my last post, caused by the need to write some money-making articles. Other stuff too, such as me paying too much (??) attention to worrying trends in global politics. I’m sure we all have the same struggles!]

I’ve written plenty about human waste heat and how it will eventually cause us some major problems. If and when we solve the greenhouse gas warming problem, we won’t have much time to relax because our waste heat alone will be causing significant heating of the Earth. If we go on using ever more energy, as we have through human history, this is a certainty.

In the past few years, as I’ve pondered our predicament, I’ve often wondered about active measures we might ultimately take to remove this waste heat. We do this all the time on smaller problems — we refrigerate food, cool our houses, etc. Why not so the same to the Earth? It’s an interesting question, and the answer is — it is possible, in many ways, which vary in their practicality.

One key issue is that refrigerators themselves use and dissipate energy. You can’t just build some huge refrigeration factories around the world and expect them to cool the planet down. You can’t cool your kitchen by leaving the fridge door open, because the cooling achieved by the fridge in its interior will be more than compensated by the extra heat generated and dumped out to the rest of the kitchen. The kitchen will just get warmer, not cooler.

By the same reasoning, you can’t cool the Earth by sucking air into some machine, cooling it down, and releasing it again. You’ll only end up making the Earth warmer than it was, and wasting energy in the process. Any refrigerator acting to cool the Earth and its atmosphere will have to be located outside of the Earth, outside of the atmosphere, so it can dump the extra heat it generates into space. Equivalently, most of the Earth system has to be inside of the refrigerator, as in the photo above, so the heat generated gets dumped outside of the Earth.

That was my thinking at least, and it is generally correct, although with some minor exceptions, as we’ll see.

As a first thought, one obvious way to cool the planet is just by reducing the amount of sunlight reaching it. You can do this in various ways, most easily by injecting aerosol particles into the upper atmosphere where they’ll reflect some of the sunlight back out to space. This is one approach currently under examination for responding to greenhouse gas warming, and it has the advantage that it’s relatively cheap and easy to do, although it also has a number of serious problems. One is that the consequences for global weather are hard to predict, and may well help some nations while afflicting others with worse droughts or flooding. These risks are serious enough that some experts think pursuing this method of climate control will likely trigger serious international conflicts.

But there are other problems too, perhaps most importantly that reducing the amount of incoming sunlight means that we are actually reducing the amount of solar energy we can collect at the Earth’s surface. But if we do that, whatever extra bit of solar energy we lose will have to be replaced with energy we generate in some other way, which will add more waste heat to the environment. Hence, trying to cool the Earth by blocking out useful incoming energy is almost certainly just counterproductive. Whatever we seem to save we will immediate replace with some other energy source, most likely with a net addition to planetary heating.

But there are other ways to cool the Earth environment, many covered in a recent well-produced video by the physicist Sabine Hossenfeld. I’ll run through a few of the ideas very quickly, but highly recommened the video, which only runs 22 minutes.

One idea is called cirrus cloud thinning. Cirrus clouds are very high in the atmosphere and consist mostly of microscopic ice particles. Like all clouds, cirrus clouds to two things — they reflect some sunlight back out to space, thereby cooling the Earth, and they also absorb infrared thermal radiation traveling upwards, thereby warming the Earth. But these particular clouds on average do more of the latter then the former, and so give a net warming contrbution to the planet.

Cirrus clouds at high altitude

Hence, the Earth could be cooled a little if these clouds could be systematically thinned out. This might be done in various ways, which have been under research for more than a decade. Not surprisingly, thinning the clouds would involve injecting some kinds of tiny particles into the atmosphere to influence how the clouds form or how long they last. Some added particles would tend to enhance the nucleation of ice crystals, which would grow until falling out of the clouds, thinning them out. A study in 2013 using a global climate model assessed that optimal addition of particles could achieve sufficient thinning to entirely counteract all greenhouse gas warming to date, which sounds incredibly promising. At the same time, it acknowledged anything less than optimal particle seeding could either lead to very little effect, or make planetary warming worse.

We just don’t know enough yet. IPCC scientists considering this method in 1921 concluded that so little is currently known about how these clouds work that they had low confidence it would be useful.

Another way to refrigerate the planet would be to change the nature of the waste heat energy itself, improving it ability to escape the Earth. This waste heat is thermal energy at the temperature of the Earth’s surface and atmosphere, which puts it in the broad infrared frequency range. Here the atmosphere is mostly absorbing, so this energy cannot escape out to space. But there is a small transparency window in the wavelength range of 8-13 micrometers, and researchers have demonstrated a device which takes advantage of this window to effectively pump heat out into space.

This was demonstrated in 2014 using a metamaterial composed of multiple thin layers of specially designed photonics materials. These layers were designed to do two things: 1) be extremely reflective to incoming sunlight and 2) to emit energy strongly in the particular wavelength band of 8-13 micrometers. In this way, if you put a sheet of this material in the sun and exposed to the sky in the afternoon, it reflects all the incoming light away, and also radiates other energy through the atmosphere and into space through the transparency window.

As if by magic, this research showed that a sheet of this material in the mid-day sun naturally cools down to about 5 C cooler than the surrounding environment. It acts as a refrigerator — and dumps the excess energy to space.

Hence, it might be possible to cool the Earth by using lots of such materials to cover surfaces, cooling those surfaces down below the temperature of the surrounding air. The surfaces would then in turn cool the air. There’s a company already selling such panels to cover the roofs of businesses to they can reduce electricity used for air conditioning. It may seem impractical to cover much of the Earth’s surface with such panels, but I can imagine it might be possible to engineer large arrays of such cool surfaces, perhaps in dry hot windy deserts, where the rapid flow of air over the arrays could produce significant atmospheric cooling.

Clearly, this idea needs further research and development. I also like the final sentence of the paper, which concludes that “our results point to the largely unexplored opportunity of using the cold darkness of the Universe as a fundamental renewable thermodynamic resource for improving energy efficiency
here on Earth.”

The cold darkness. It’s out there to be used.

There are some further ideas, discussed briefly in Hossenfelder’s video, which start to sound like science fiction. One is the idea of building “super chimneys” which would take hot waste heat in low lying air and effectively pump it upward through huge chimneys. This energy would be exchanged into the upper atmosphere, raising its temperature, which would then increase the rate at which it radiates energy into space. In effect, the super chimney transports surface heat up to where it can radiate heat away without having to go through the atmosphere.

Unfortunately, it seems that such a chimney would need to be about 5 km high and 1 km in diameter, and it not likely to be built any time soon.

There are variations on this idea, such as trains of balloons carrying the warm air upwards in some cyclic fashion, or even artificial tornadoes. A tornado is a natural mechanism which pumps warm air upward as cold air plummets down. They don’t rise up to the upper atmosphere, but maybe we could engineer artificial tornadoes in many parts of the world to do our cooling for us. Might also be risky. Again, I’m not expecting to see this anytime soon.

And yet — the problem of warming from waste heat is a problem of the future, not urgent now, but it will be in two hundred years. If we haven’t destroyed much of the planet by then, we may find some of these crazier ideas aren’t so infeasible. If we’re to go on using ever more energy, one thing is clear — we will one day need to refrigerate the Earth.