First of all, a gigaton is one billion metric tons. One metric ton (2,200 lbs.) is what a cubic meter of water weighs. One billion metric tons is what one cubic kilometer (one billion cubic meters) of water weighs, and it is called a gigaton.
Next, remember atmospheric CO2 includes two oxygen atoms, and weighs 3.7x the carbon feedstock. So if there are 70 gigatons of carbon in the Amazon, for example, burning the remaining Amazonian carbon will release 2.7x that many gigatons of CO2 into the atmosphere (ref. Amazon Ecology Project). So far, tropical deforestation alone has resulted in the release of about 475 gigatons of CO2 into our atmosphere.
So how many gigatons of CO2 are we contending with, anyway, in our atmosphere? Referencing and extrapolating from J. Schlorrer’s 1994 study, “Why Does Atmospheric CO2 Rise?”, there are probably about 3,000 gigatons of CO2 in the earth’s atmosphere right now.
Forests are at best carbon neutral, they grow, absorbing CO2, and they expire in various ways, releasing it again. It is the permanent removal of forests, and the permanent addition of carbon mass to the atmosphere, that matters. Temperate zone forests don’t store nearly the carbon mass per area compared with tropical forests, more than negating the greater area of temperate forest that has been lost. And loss of temperate forests has far less impact on thermal or hydrological conditions – not nearly as much as tropical forests.
Clearly if total tropical rainforest restoration (impossible) were to be implemented, the permanent addition of fast growing trees permanently removing 475 gigatons of CO2 from the earths atmosphere (every 7.8 gigatons of carbon removed lowers CO2 concentrations by one PPM) would be a very good thing. But compared to CO2 impact, the hydrological and thermal impacts of adding or removing tropical rainforest is far more significant.
Each year, nearly 15,000 gigatons of H2O, that’s 15,000 cubic kilometers of water, is evaporated from what remains of our tropical rainforests. For perspective, consider there are only about 12,900 (ref. Nasa Earth Observatory) cubic kilometers of water in the entire atmosphere at any given time, and that each year only about 50,000 cubic kilometers of water rain onto the continents.
Where there is no longer tropical rainforest, and it is well over 50% gone, there is proportionally reduced evaporation, less rainfall, and complete loss of the reflective cloud cover that perennially forms over tropical rainforest. Add these even more significant hydrological and thermal effects of tropical deforestation to the 475 gigatons of atmospheric CO2 that will either be added or deleted based on whether or not we remove what’s left, or replace what’s gone.