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This is where I get confused. I've seen online that about 3% of all CO2 emissions are attributed to humans. Is that wrong?

bryanlarsen 4y ago
The rate of absorption used to balance the rate of emissions.
margalabargala 4y ago
It's true that total emission and reuptake is quite large compared to human-caused emissions: about 29GT from humans, 750GT total in the carbon cycle annually. However, the systems that reuptake the CO2 are fairly sensitive and don't have a lot of slack in them, so adding that additional 29GT turns out to have an impact. The Earth's atmosphere weighs about 5.5 quadrillion tons. Ignoring the differences in weights of different air molecules, adding 5.5 billion tons of CO2 would increase the CO2 concentration by ~1PPM. So if all 29 billion tons of human emissions were not reabsorbed into the biosphere, we would expect to see global CO2 concentrations increase by between 5 and 6 ppm. In reality we see an increase of just over 2ppm per year, meaning that while much of human emissions are indeed reabsorbed into the biosphere, ~40% is not.
- margalabargala 4y ago
  To expand on this: the molar weight of dry air is ~28g/mol, while CO2 is ~44g/mol. So each increase in atmospheric CO2 by 1ppm corresponds to about 8.6GT of unabsorbed CO2. The Keeling Curve shows an increase of 1.86PPM from May 2021 to May 2022, meaning that the amount of CO2 emitted from all sources was greater than the amount absorbed by 15.7GT. So of the 29GT emitted by humans, 13.3 was reabsorbed. Obviously this is not the whole story. The Keeling Curve shows that worldwide CO2 levels were already rising in the 50s and early 60s, when total emissions were below the 13.3 our biosphere is absorbing annually. But this is sufficient to show that a (substantial) reduction in human emissions would reverse the currently increasing global CO2 levels, and the greenhouse warming that comes with it.
Imnimo 4y ago
Imagine you have a big tub, and it has a faucet which outputs 100 gallons per hour, and a drain which drains 100 gallons per hour. Over time, the amount of water in the tub will be stable. Now suppose I come by and start pouring in 3 gallons of water per hour. Now what happens to the amount of water in the tub over time? My contribution is only about 3% (a hair under that, even), but it can still change a stable tub to an overflowing tub.

bryanlarsen 4y ago

The rate of absorption used to balance the rate of emissions.

margalabargala 4y ago

It's true that total emission and reuptake is quite large compared to human-caused emissions: about 29GT from humans, 750GT total in the carbon cycle annually. However, the systems that reuptake the CO2 are fairly sensitive and don't have a lot of slack in them, so adding that additional 29GT turns out to have an impact. The Earth's atmosphere weighs about 5.5 quadrillion tons. Ignoring the differences in weights of different air molecules, adding 5.5 billion tons of CO2 would increase the CO2 concentration by ~1PPM. So if all 29 billion tons of human emissions were not reabsorbed into the biosphere, we would expect to see global CO2 concentrations increase by between 5 and 6 ppm. In reality we see an increase of just over 2ppm per year, meaning that while much of human emissions are indeed reabsorbed into the biosphere, ~40% is not.

margalabargala 4y ago
To expand on this: the molar weight of dry air is ~28g/mol, while CO2 is ~44g/mol. So each increase in atmospheric CO2 by 1ppm corresponds to about 8.6GT of unabsorbed CO2. The Keeling Curve shows an increase of 1.86PPM from May 2021 to May 2022, meaning that the amount of CO2 emitted from all sources was greater than the amount absorbed by 15.7GT. So of the 29GT emitted by humans, 13.3 was reabsorbed. Obviously this is not the whole story. The Keeling Curve shows that worldwide CO2 levels were already rising in the 50s and early 60s, when total emissions were below the 13.3 our biosphere is absorbing annually. But this is sufficient to show that a (substantial) reduction in human emissions would reverse the currently increasing global CO2 levels, and the greenhouse warming that comes with it.

Imnimo 4y ago

Imagine you have a big tub, and it has a faucet which outputs 100 gallons per hour, and a drain which drains 100 gallons per hour. Over time, the amount of water in the tub will be stable. Now suppose I come by and start pouring in 3 gallons of water per hour. Now what happens to the amount of water in the tub over time? My contribution is only about 3% (a hair under that, even), but it can still change a stable tub to an overflowing tub.