So I thought "I wonder
how realistic this could be" - time to get the calculator and do some sums.
Estimates for the amount of carbon dioxide emitted
from "anthropogenic sources" (sources due to man's activities) - this is
mainly from fossil fuel burning - are around 22 billion
tons per annum.
The amount of carbon dioxide taken up and held by forest
in biomass of the trees is variable, but a figure of 120 tonnes of carbon
per hectare as suggested
here has been taken for the basis of the following calculations. This
is in line with the range of similar values for a number of species of forest
here and of Brazilian forest
120 tonnes of carbon per hectare corresponds to 440 tonnes
of carbon dioxide equivalent per hectare of mature forest trees.
Carbon - atomic mass = 12
Carbon dioxide - CO2 - molecular mass - 12 + 16 + 16 = 44
So a mature forest can soak up the equivalent of
440 tonnes of atmospheric carbon dioxide per hectare
in the 50-100 years it takes to reach maturity - that's a one-off total
- not per year and may take several decades to get there.
In order to deal with currently generated carbon dioxide,
an area of forest equivalent to:
22 billion tonnes divided by 440 tonnes
per hectare - is needed
= 50 Million hectares
per annum or 500 000 square kilometres per annum
...to be planted with forest and held in perpetuity
(not cut down or allowed to revert back to atmospheric carbon dioxide).
This is approximately equivalent to the
entire land area of Spain, twice as big as the United Kingdom and bigger
than any US state other than Texas (696,621 sq km.) or Alaska (1,717,854
sq. km.) To be forested anew each year and held as such forever.
Therefore planting trees alone other than on a
colossal scale is not going to even allow us to stand still
alone start to reverse the effects of global warming.