Atmospheric carbon dioxide (CO₂) has been increasing since 1700, but the amount of CO₂ produced in that time by burning fossil fuels should have resulted in a much greater increase than has been observed. Plant ecologist Allen Auclair claims that the woodlands of the Northern Hemisphere have been acting as a carbon sink, absorbing carbon from the atmosphere and turning it into wood. Auclair uses measurements of factors affecting the area and density of a forest—such as logging, fires, and pests — and estimates of tree growth rates to argue that increases in the growth rates of individual trees in these forests since 1920 have created a large volume of wood that accounts for the missing carbon.

To determine whether the woodlands as a whole are releasing or absorbing carbon, the volume of wood added to the woodlands must be compared with the wood lost. Auclair's analysis of the past hundred years shows the woodlands changing from a carbon source to a carbon sink. Before 1890, northern woodlands were a source of CO₂ mainly because of forest fires and logging. Such deforestation releases CO₂ because debris from the forest floor rots more quickly when the trees are cleared. After 1920, the steep increase in tree growth rates surpassed the losses stemming from fire and logging, turning the northern forests from a carbon source into a carbon sink and storing CO₂ from fossil fuel over the next fifty years.