Examining Earth’s Lungs

Several studies released this year and last have focused on the health of the world’s forests. Considered the Earth’s “lungs”, forests, both around the equator and away from it, are responsible for removing immense amounts of carbon dioxide from the atmosphere while returning oxygen to it.

The studies have returned mixed results.

African Rainforests Growing

A study published in February in the journal Nature found that equatorial rainforests in Africa have grown larger, increasing the amount of CO2 sequestered in these environments. Studying 79 forest areas across Africa between 1968 and 2007, researchers found that, on average, trees in intact African forest areas soaked up an additional 0.63 tons of carbon per hectare per year. Extrapolating these results and incorporating measurements from 156 forest areas from 20 countries around the world, the researchers estimate that African rainforests absorb 1.3 billion tons of CO2 a year and that global rainforests take in 4.8 billions tonnes per year, a substantial portion of the estimated 32 billion tons of CO2 that is emitted by the burning of fossil fuels each year.

However, another study has shown the rainforests to be particularly sensitive to climate conditions, specifically drought.

Amazon Rainforests Susceptible to Drought

In 2005, while the Atlantic Ocean was setting records for hurricane activity with 27 named storms, 15 of which became hurricanes including Hurricane Katrina, the Amazon rainforest was experiencing its worst drought in recorded history. In October of that year Brazil declared a state of emergency for the state of Amazonas due to record low water levels. As rivers turned to mud leaving millions of dying fish behind, tens of thousands were left without primary sources of food, water, and transportation.

A study released this week in the journal Science found that the drought also had a significant impact on the Amazon’s ability to absorb carbon dioxide from the atmosphere. Utilizing data obtained from 55 forest areas throughout the region, researchers found that 76% of the plots were absorbing 0.5 tonnes of carbon per year per hectare prior to the drought with the remainder somewhat less. However, during the drought, while 51% continued to absorb some carbon, the remainder actually became carbon sources as trees died and released their sequestered carbon back into the atmosphere at an alarming rate as high as 6 tons per year per hectare.

There were some concerns away from the equator as well.

Canada’s Forests Become Carbon Sources

While not as effective as the equatorial rainforests, Canada’s forest lands have been considered a strong and reliable sink for atmospheric carbon. But at least one study published early last year in the journal Nature found that large scale insect outbreaks, worsened by rising temperatures that prevent cold winters from killing off large portions of the insect populations, were actually turning forests in British Columbia from a small net carbon sink to a large net carbon source.

Components of the Kyoto protocol allow subscribing countries to record forest lands as offsets to industrial carbon emissions. However, the results of such studies against Canadian forests were significant enough that Canada removed forest management as an offset as concerns rose that these environments were now becoming sources of emissions rather than sinks for them.

At about the same time last year, another study published in Nature raised concerns about the impact of climatic changes related to warming during the spring and autumn seasons across northern ecosystems.

What Does It All Mean?

Researchers involved in all of the studies have uttered a similar refrain, that while the Earth’s forests have been and continue to be an immense buffer against industrial emissions of carbon dioxide from mankind, these environments have demonstrated themselves to be very sensitive to climatic changes, and what historically have been restraints on atmospheric CO2 content could instead become accelerating contributors to it.

Additional Reading

• Old-growth forests as global carbon sinks – Nature (July 7, 2008)