We all need some base numbers to work from.
On average, whatever that means, an acre of mature forest contains about fifty tons of carbon. The figure will go well over one hundred tons in special circumstances, like the Amazon. About two thirds of that carbon content will be held in the soils and one third will be contained in the trees themselves.
What is not so obvious, is that deforestation results in a drop of two thirds, through the elimination of the trees(one third) and soil quality reduction(one third). It appears that crops do not require the sames mount of soil and lose what is not used. Historically, this soil loss has been blamed on bad agricultural practices, which has a certain merit. After all, if you are dumb enough to raise goats and pigs who destroy the sod, you will eliminate a lot of soil in a hurry. However, traditional cattle husbandry is very supportive to soil maintenance. This suggests that most of this lost topsoil is lost into the atmosphere because it is simply not needed.
The obvious conclusion is that simple restoration of natural forest cover were practical is a very good start on solving the global warming problem and something every local government globally can attend to with their own resources. After all, it takes mostly labor to rebuild a woodland.
That leads us to the next question. How many acres of land globally are grossly under utilized, primarily due to the unavailability of ground water?
The lands in question cover the temperate zone and the tropics. The northern arboreal forests are actually maxed out and have seen little human interference and are simply unsuitable for any form of crop growing system.
We are describing Western North America, including the Basin and Range region, North Africa including the Sahel, all the Middle East and all Western China, Australia and a small parts of South America and Southern Africa.
Recall that trees grown on mountain slopes support valley bottom agriculture.
Globally, it is accepted that we have 46 million square kilometers of which 15 million square kilometers are defined as desert, half of which is the Sahara. At least half of this is reasonably accessible to early exploitation and development
Knowing that each square kilometer contains about 250 acres, we can state that a square kilometer of forest land will contain around 12,500 tons of carbon.
Therefore, if all the arid lands were to be forested, the earth will be able to sequester at least an additional 575 billion tons of carbon. We currently are producing between 3 to 5 billion tons per year. Over the last 100 years we have saturated the environment's ability to absorb this carbon and we are now storing the overage in the atmosphere.
This obviously has to end, and the only way it can end properly is to build out the forests to absorb the ongoing fossil fuel economy.
Curiously we can burn all the available fuels for 200 years and in the process completely reforest the earth while moderating the climate. The economy can even stay on a carbon based transport system thereafter by the expedient of converting carbon waste back into oil which is a very convenient energy storage mechanism.
In other words, this may all actually turn out to be a good thing.