# Locating a carbon sink

Easter Island is now home to an extraterrestrial/futuristic/handwavey device. It continuously and instantly removes CO$_2$ from the atmosphere above the island. To avoid harmful effects on plants, it excludes the 30m of atmosphere closest to the land. If it is unable to process enough atmosphere to remove 40 billion tons of CO$_2$ per year (current global production) it will increase the strength of the prevailing winds within 1000km until it is processing enough.

People will very quickly notice that global CO$_2$ levels are not increasing.

How and how soon will they notice where it is going?

• They'll notice the machine before they notice a CO2 discrepancy. – Frostfyre Jun 1 '15 at 14:37
• @Frostfyre It's underground. Deep underground. – frodoskywalker Jun 1 '15 at 14:41
• @Frostfyre If I knew how extraterrestrial/futuristic/handwavey technology worked, I'd be too busy with my yacht full of money to ask this question. It works as described. When it's located, the puzzle will naturally stop being "Where is the CO$_2$ going?" and start being "How does this work?" – frodoskywalker Jun 1 '15 at 15:00
• It's currently being converted to free oxygen and subterranean graphite. – frodoskywalker Jun 1 '15 at 16:31
• It's an ironic choice for location of the device, given Easter Island's past interaction with the environment. – Samuel Jun 1 '15 at 17:04

The Orbiting Carbon Observatory-2 will spot it.

There would be an anomalous section over Easter Island. Especially if it was affecting weather patterns, it would show up in an amazing video like this.

Its spatial resolution is under 2 km. This would be enough to spot Easter Island (163.6 km²) on its own, but especially if the machine is affecting wind from 1000 km away.

• The science lead of the OCO mission spoke about it as part of the Von Kármán lecture series – Nick T Jun 2 '15 at 0:16
• @NickT If you ever link me a video more than three or four minutes long I'm probably not going to watch it. For an hour and sixteen minutes you can be sure I won't, especially with such a vague description. But, perhaps others will, thanks for the link. – Samuel Jun 2 '15 at 0:23
• I wasn't trying to foist it upon you to watch. It's a (very informative, i.e. from the science lead) lecture about the OCO mission: why it's important, how the spacecraft works, the instruments, etc. Lectures in general are about an hour long, and the Von Kármán lectures offer tons of digestible (they're targeted at the general public, not fellow scientists) information that you can't really get anywhere else (reading a Wikipedia article or watching a few 2-minute videos pale in comparison). – Nick T Jun 2 '15 at 1:19
• @NickT Oh I don't doubt its value. I've had enough education to appreciate the information that can be packed into a lecture. My rule is just a matter of free time. – Samuel Jun 2 '15 at 1:22

$400\;\text{ppm CO}_2$ in air

1 cubic mile of air: $1.275×10^6$ tons

$\text{CO}_2$ is slightly heavier but for sanity sake I'm not going to get into that and just treat as equal in weight to the other $\text{ppm}$.

So let's go with $510$ tons of $\text{CO}_2$ in one cubic mile of air.

So we need to empty about $78431372$ cubic km of air of $\text{CO}_2$ in a year.

To simplify things, lets treat Easter Island as a square about $13\;\text{km}$ on a side rather than the shape it really is and I'm going to imagine that the wind always goes in one direction and that the machine harvests $\text{CO}_2$ from the first 10(ish) miles of atmosphere and I'm going to ignore the air thinning or assume it's a little higher than $10\;\text{km}$ to make up for the air thining.

(I don't like how many caveats I'm needing to include)

divide $78431372$ by $8766$ hours in a year.

$8947$

divide by the size of our harvesting zone:

$8947/130$ gives a little over $68\;\text{km}$ per hour winds which is gale force but not huricane speeds.

So someone probably would notice constant gale force winds toward the island even if they didn't notice anything on satellites as Jimmy360 said.

• The CO$_2$ is 50% heavier than bulk air, so we can reduce the wind speed by one third. Still very noticeable though. Good answer! – frodoskywalker Jun 1 '15 at 16:34

Quickly... There are many satellites in orbit which measure the temperature of the atmosphere. The big $CO_2$ hole would show up as a thermal abnormality. Also, there is a satellite in orbit which observes $CO_2$ levels.

• More directly, the recently-launched OCO2 measures atmospheric CO2 globally, and would probably be the first to notice. – 2012rcampion Jun 1 '15 at 16:06

Actually only $1/2$ of the question has been answered. The effects of this device on the local weather would be noticeable (a constant wind of gale or near gale force proportions would certainly have quite a noticeable effect), but where is the $\text{CO}_2$ actually going?

If it is being pumped into the ocean, the deep cold waters might absorb it, but the acidity of the local oceanic environment is going to increase. As well, if there is any upwelling, the $\text{CO}_2$ will rapidly fizz out of the water much like a carbonated drink. Liquifying the $\text{CO}_2$ requires a lot of energy to refrigerate the gas and compress it past 5 atmospheres, so Easter Island will also become rather hot as the waste heat from these activities is released into the gale force winds. IF you simply create dry ice and deposit it on the ocean floor, the pressure and cold should keep it in place, and the energy bill will also be much lower. Injecting the $\text{CO}_2$ into a subduction zone will dispose of the gas for a few tens of thousands to millions of years, but it will eventually return as part of the carbon cycle in a volcanic eruption.

So the disposal of the $\text{CO}_2$ will also need to be factored into your plans.

• The question specifies (indirectly, true) that it doesn't matter where the carbon is going. – Mark Jun 2 '15 at 8:51

Small point to add, but 40 billion tons of $\text{CO}_2$ is 9 billion tons of Carbon. The energy needed to extract that much carbon from $\text{CO}_2$ would be close to all the energy use in the world (not really felling the urge to do the calculation), but it would be in that range.

But let's ignore that, lets assume the 9 billion tons of carbon is somehow converted/stored mostly as graphite - about a specific gravity of 2. That's $4.5\;\text{km}^3$ volume - a bit over a cubic mile. On an island roughly $63$ miles square, a cubic mile of graphite would be noticeable. Spread over the island it would be $90$ feet high. Piled up in a pile it would be much higher than the highest point on the island, which is some $511$ meters above sea-level. If we had the means (we don't), but if we did, storing a few cubic miles of graphite might be a nice solution to climate change. I doubt Easter Island is where they would store it. :-)

• Yeah, it will of course be pretty close to all the energy use in the world - most of the energy we "create" is released from fossil fuels. Something that basically reverses that process must take the same amount of energy. Accounting for waste, this should well offset nuclear and other sources of energy. It also means that the island is going to get quite hot, even if the technology is close to perfect efficiency... – Luaan Jun 2 '15 at 7:02