Tropical forests can play a major role in the reduction of atmospheric carbon and achieve a below 2 degrees C rise in global warming by 2050, says a recent study published in the journal Nature Climate Change.
As the transition from total reliance on fossil fuels to that on renewable sources of energy is expected to take place over the next 35 years, the study finds that tropical forests can be the bridge to this transition by causing uptake of atmospheric carbon, provided, astute forest management combined with fossil fuel use reduction are effected in a speedy manner.
According to Dr. R. A. Houghton, the first author of the paper from Woods Hole Research Center, Massachusetts U.S., enhancing carbon uptake and reducing emissions could account for as much as 50 per cent of total carbon emissions.
Explaining the math behind this conclusion, he wrote in an email to this Correspondent: Total carbon emission works out to 13 units — emission from fossil fuels accounting for 9 units and gross emissions from land use (emissions due to deforestation) working out to 4 units. Since growing of forests will help in sequestering 3 units of carbon from the atmosphere, the total carbon present will be 10 units. While the total carbon present can be further reduced to 6 units by eliminating land-use emissions, the uptake of carbon by reforested large areas (1 unit) would further cut the total emissions to 5 units (50 per cent of the initial total, 10).
To achieve a 75 per cent likelihood of avoiding warming in excess of 2 degrees C through changes in fossil fuel emissions alone, such emissions would have to be eliminated over the next 20 years or less. In contrast, the same effect could be achieved if, first, tropical forest management removed 5 petagram of carbon per year (1 petagram is equivalent to 1 billion metric tons) from the atmosphere, phased linearly over the next 10 years, and, second, fossil fuel emissions were held constant for the next 10 years and then reduced linearly to a level equal to 20 per cent of 2014 emissions by 2050 before further linear reduction to zero by 2100. This estimate was reached based on the gross sources and sinks of carbon associated with three types of forest management in the tropics. First, deforestation and degradation of forests in tropical regions currently release into the atmosphere about 1 Pg C/yr. If these were stopped, those emissions would cease. Second, carbon is removed from the atmosphere by secondary forests that are recovering from harvests and from ‘slash and burn’ shifting agriculture at rates as high as 3 petagram of carbon per year. This gross rate of uptake is taking place now.
“The landscape with shifting cultivation has 10 per cent of the land in crops, say, and 90 per cent of the land in fallows. The landscape with permanent agriculture has 10 per cent of the land in crops and 90 per cent of the land in forests. On average, forests hold 2-5 times more carbon than fallows. If shifting cultivation were replaced by permanent cultivation, the fallows would return to forests, taking up carbon as they grow,” Dr. Houghton notes. If fallows were to return to forests, up to 3 Pg of carbon per year will get accumulated in growing forests and would continue for decades before declining as the forests mature.
A third activity, more difficult to achieve than the first two because of higher costs per hectare, is the re-establishment of forests on lands previously forested but not currently used productively.
Estimates of the areas available for reforestation vary, but re-forestation of 500 million hectares could sequester at least 1 Pg C /yr for decades. According to the study, the three activities together would reduce total emissions by as much as 5 Pg C/yr( a reduced source of 1 Pg C/yr and increased sinks of 4Pg C/yr). Though the rate of carbon accumulation in forests diminishes with time, the forest sinks (totally 4 Pg C/yr) would conservatively last for approximately 50 years (to 2065) before declining linearly to zero by 2095.
As the large trees containing most of the above-ground forest carbon tend to be absent in degraded forest, and large trees can take well over a century to mature fully, the absorption is likely to continue at a high level well beyond a 50-year period.
The study is especially significant because forest management can be much more easily and quickly implemented than development of alternate renewable energy technologies and would account for much of the reduction in atmospheric carbon until reliable and efficient renewable energy technologies are put in place.
