December 13, 2013
In their paper, Mackey and colleagues question "to what extent [land-based mitigation] can be legitimately considered an 'offset' for fossil fuel CO2 emissions."
According to Mackey, several of the most convincing effects of increased CO2 emissions in the atmosphere are reductions in the Greenland ice mass and Arctic sea ice cover , which have caused a capital investment and industrial exploitation boom of the new land and sea territories that have become available and the significant global increases in surface land temperatures.
A ceiling of the rain forest in Java. Photo by Rhett A. Butler / mongabay.com
The burning of fossil fuels is the largest global source of anthropogenic (human-caused) CO2 emissions. Human land-use activities such as deforestation, logging, and soil disturbance make up the second largest source.
Mackey and his team estimate that, on average, 0.16 million square kilometers of forests are cleared annually. About half of the world's forests have already been cleared, leaving 40 million square kilometers still standing. Of this, only 36 percent are primary forests (i.e., old-growth forests), which have not been significantly disturbed to the point at which bio-diversity is altered.
But carbon isn't released only when trees are cut down or burned, or when the soil is moved, but also from machinery such as trucks, bulldozers and chainsaws used to carry out these activities. Mackey and his colleagues report that it's hard to know the precise amount of global carbon emissions produced from forest degradation, but estimates suggest that these activities together account for a 50 percent increase in regional emissions compared to deforestation alone.
Giant rainforest dipterocarpus. Photo by Rhett A. Butler / mongabay.com
"These estimates highlight the very modest scope for reforestation to reduce (atmospheric) CO2 compared with both the magnitude of fossil fuel CO2 emissions and emissions from deforestation and degradation," Mackey and his colleagues write in the paper. "Moreover, complete reforestation of previously cleared land is an implausible scenario due to competing land uses — especially from food production and the need to feed a human population predicted to surpass nine billion by 2050 — along with projected demand for land to produce transport biofuel."
When asked specifically about the relationship between the amount of carbon released into the atmosphere and the amount of carbon stored in the ground and in vegetation, Mackey clarified that the amount of organic carbon that is grown and stored in a forest ecosystem is determined primarily by rainfall and temperature. These climatic variables affect the rate of plant growth and decay. Other factors impacting the amount of carbon stored are soil characteristics and groundwater accessibility by plants.
"When you log or clear a forest, the biomass and soil carbon is oxidized and released as carbon dioxide into the atmosphere," said Mackey. "If the forest is allowed to regrow (assuming it has not been logged or cleared such that it is destroyed) then theoretically it should be able to grow back the same amount of carbon. However, in the case of a primary forest this can take hundreds to thousands of years."
A reforestation project in the Dominican Republic. Photo by Tiffany Roufs / mongabay.com
"If carbon is to be usefully stored (on land, in the ocean or in geological repositories), it must remain stored not just for 100 years, but for more than 10,000 years…Indeed it is accepted de facto in many policy contexts that it is sufficient to maintain stores for 100 years," reads the paper.
Solving climate change is not going to be as easy as countries planting trees, according to the paper. rapidly decreasing greenhouse gas emissions—whether from fossil fuels or land use changes—is far more important.
"No amount of reafforestation or growing of new trees will ultimately off-set continuing CO2 emissions due to environmental constraints on plant growth and the large amounts of remaining fossil fuel reserves," Mackey says. "Unfortunately there is no option but to cut fossil fuel emissions deeply as about a third of the CO2 stays in the atmosphere for 2 to 20 millennia."
- Nature Climate Change: "Untangling the confusion around land carbon science and climate change mitigation policy" by Brendan Mackey (Griffith Uni.), I. Colin Prentice (Macquarie Uni. & Imperial College), Will Steffen (ANU), Joanna I. House (Bristol Uni.), David Lindenmayer (ANU), Heather Keith (ANU) and Sandra Berry (ANU). (doi:10.1038/nclimate1804)
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