Trees, Tailpipes, and CO2 “Offsets”
Stephen Q. Shafer, Saugerties NY, Sept 16 2018

Trees sequester carbon (taking it up and storing it long-term) so well that they are often cited as way to offset releases of carbon to the atmosphere due to combustion of fossil fuels, A statement on the admirable website equates the CO2 sequestered by an acre of trees in a year with the tailpipe emissions of a gas-powered car driven 26,000 mi. The concept that trees can "offset" tailpipes is not wrong, but the equivalencies proposed can vary wildly. The comparisons depend on what values are used for the following five variables:

Planting trees and preserving woodland is extremely worthwhile, and warrants strong emphasis in a climate smart community.
Left out of most such "offset" calculations is that motor vehicles don.t emit just CO2 but other gases like carbon monoxide and oxides of nitrogen that are, unlike CO2, called pollutants by EPA. Fossil fuel combustion also produces particulates. Trees can.t handle any of these as they do CO2.

Also neglected in most "offset" computations is that greenhouse gases are emitted at many steps earlier in the life cycle of a gallon of petroleum than its arrival in a vehicle.s fuel tank. Exploration, extraction, transport to the refinery, refining and transport to the pump all release to the atmosphere CO2 and methane as well as other air pollutants.

If we shortsightedly look only at CO2 from a vehicle tailpipe, the above 26000 mile statement is correct, even conservative for the following specific values of the five variables, taken from the table below

The car emits 23,400 lb CO2/yr while an acre of fast-growing hardwood trees with average age of 20 years sequesters more, 30066 lbs CO2/yr. If the car.s mileage beats 22 mpg, the trees excel even more. At 44 mpg the car emits in 26,000 miles only 12,700 lb CO2 . An acre of two hundred 20-year old slow-growing conifers could sequester that amount of CO2, while two hundred 20-year old fast-growing hardwoods could offset that car's going 61,552 miles. If the tree density is higher than 200/acre, the acre of trees looks better yet.

HOWEVER, if the mpg is lower than 22 or the trees are not all fast-growing hardwoods, an acre of trees no longer appears to balances the 26,000 mi. A car with 11 mpg emits 19.6/11 = 1.78 lbs CO2/ mi. 26,000 mi at that rate releases 46,280 lbs CO2. It would take 1.5 acres, not just 1, of fast-growing 20-year old hardwoods to offset that and 8.5 acres of slow-growing 20 year old conifers.

NOTE ALSO: the rate of C-sequestration per year increases for most tree species as the tree ages until it is past 30. After about that age the annual sequestration rate declines rapidly for most species. All the comparisons in this essay are based on an acre of imaginary woodland in which all 200 trees are living, all are the same general type (e.g. hardwoods of fast growth rate) and are all 10-30 years old. Thus, if someone were to hold (and the line on the web site does not so hold) that having planted let's say 200 eastern white pine 1 yr seedlings last year offsets 26,000 miles of a 22 mpg car, that would be flat wrong. If all 200 survived to age 23 they would offset 26,000 mi; not before then.

Planting trees and preserving woodland is extremely worthwhile, warrants strong emphasis in a climate smart community. Seeming precise comparisons like "trees" to "tailpipes" can be instructive, however, only when we know the assumptions that underlie them and remember the factors not in the calculation like methane vented or flared in recovery of shale bed oil.



The examples of rates of carbon sequestration by trees are taken from This EPA report.

To convert mass of C (carbon ) to that of CO2 multiply by 44/12

Type Rate of
Age Example Lb C/yr
per tree
Lb C/yr
per acre
Lb CO2/yr
per acre
200 H S 5 Sugar maple 3.2 640 2347
200 H M 5 Norway maple 6.1 1220 4473
200 H F 5 Sweetgum, Red oak 10.1 2020 7407
200 H S 10 Sugar maple 5.5 1100 4033
200 H M 10 Norway maple, Paper birch 11.2 2240 8213
200 H F 10 Sweetgum 19.3 3860 14153
200 H S 20 Sugar maple 10.8 2160 7817
200 H M 20 Norway maple 23.2 4640 17013
200 H F 20 Sweetgum 41 8200 30066
200 C S 5 Scotch pine 1.9 380 1393
200 C M 5 Red cedar 3.7 740 2713
200 C F 5 Eastern white pine 6.4 1280 4583
200 C S 10 Scotch pine 3.5 700 2567
200 C M 10 Red cedar 7.4 1480 5324
200 C F 10 Eastern white pine 13.2 2640 9680
200 C S 20 Scotch pine 7.4 1480 5427
200 C M 20 Red cedar 16.7 3340 12247
200 C F 20 Eastern white pine 30.8 6160 22587
Type: H = hard wood, C= conifer
Rate of Growth: S = slow, M = moderate, F = fast

This US Energy Information Administration report shows CO2 emissions from electric power generation. A modern natural gas fired plant emits about 0.9 lb CO2/kwh, coal about twice that!