Re: How many trees need to absorb CO2 from burning 1 million barral crude oil?

Hi Sultan:

That's kind of a tough question, it depends on many things! Like the kind of tree or where it is in the world (because things like light and temperature determine the rate of photosynthesis). As near as we can tell, the amount of carbon uptake by forests is very large (see here for an article on the subject) relative to the amount of carbon input by burning oil.

There isn't any way to calculate that to any certainty, but I can provide you with a rough back of the envelope calculation. I'll caution you though, that this will be only a very rough estimate.
This table from this report gives the density of crude oil as about 31 API gravity units, which works out to about 0.87 kg per litre (using this conversion calculator). A barrel of oil is about 120 litres, so that works out to about 104.4 kg of oil per barrel, of which about 85% is carbon (from that same table). So, there is about 88.7 kg of carbon in one barrel of oil, and if we multiply that by one million barrels, we wind up with 88740 metric tons of carbon in a million barrels.

Carbon is taken up by plants in the process of photosynthesis, and that rate depends on a lot of things, like the amount of light and temperature. According to this paper, the net photosythetic rate (the amount of photosynthetic carbon fixation over and above any carbon produced by the tree by respiration) for several deciduous trees is from about 2 to 10 micromoles carbon per square meter of forest, per second. For the purposes of our calculation, we'll use a value of 5 micromoles per square meter per second.

First, we need to convert our tons of carbon into moles of carbon. The molar mass of carbon is 12 g/mole, which gives us 7.4x10⁹ moles (8.874x10¹⁰ grams divided by 12 grams per mole). Next, since photosynthesis is a rate, we need to choose a time value over which we'll calculate photosynthesis - I'll use one year. If we multiply our 5 micromoles per square meter per second by the number of seconds in a year (about 3.2x10⁷), we come up with a value of about 157 moles per square meter per year. To find out the number of square meters to use up the 1.23x10¹⁰ moles, we divide them, and come up with 4.69x10⁷ square meters, about 47 square kilometers.

So, according to those calculations, it would take a forest of about 8 kilometers by 8 kilometers square a year to take up the amount of carbon in one million barrels of oil. That is an extremely rough number, and it seems pretty low to me. It probably isn't truly applicable to the real world - for instance, crude oil is usually refined before it is burned. As well, the article I cited at the top notes that the amount of respiration by forests may be much higher than we think, so that any carbon assimilated by photosynthesis may be released very quickly again by respiration. I have given you all the links for the data I used, and you can try redoing the calculations for something like petroleum (in cars) or light gas (natural gas and propane), or different net photosynthetic rates.

I hope that helps,
Rob Campbell, MAD Scientist