Rainforests and Climate Change

Tropical deforestation is one of the largest causes of global greenhouse gas (GHG) emissions. Without conserving these forests, it may be impossible to stabilize atmospheric carbon and avoid the catastrophic impacts of global warming.

Measures to conserve tropical forests must be included in any future climate change agreements: it is not an exaggeration to say that if we lose the rainforests, we lose the fight against climate change.

CO2 Emissions from Deforestation

Forest ecosystems draw down atmospheric CO2 through photosynthesis and store it in biomass and other carbon stocks. Tropical rainforests are one of the world's primary carbon sinks, storing vast amounts - around 610 billion tonnes of CO2.

Deforestation and forest degradation - through the decomposition and burning of plant matter and the oxidation and burning of soils, especially peatlands - release this carbon into the atmosphere.

The Intergovernmental Panel on Climate Change (IPCC) estimates that the global forest sector accounts for 17% of manmade GHG emissions, approximately 7-8 gigatonnes (Gt) of CO2 each year. Forest emissions are greater than the entire transport sector, and larger than the annual emissions of the USA or China [1].

Although there is some debate over the share attributable to tropical forests, there is little doubt that it is very high: one study estimates that tropical deforestation accounts for 96% of global forest-related emissions [2]. Rainforests account for most of this, as mature rainforests carry very high carbon loadings (up to 800 tonnes of CO2 per hectare).

In aggregate, there is more carbon stored in rainforests than there is in the atmosphere. Just one day of tropical forest emissions from deforestation is equal to 12.5 million people flying from London to New York [3].

Amplification Effects

But rainforests are important for another reason. Mature tropical rainforests continue to sequester carbon at a rate of a few tonnes of CO2 per hectare each year. One study estimates that old-growth tropical forests absorb 4.8 billion tonnes of CO2 equivalent (Gt CO2e) each year through this sink effect, or up to 15% of annual manmade GHG emissions [4]. So, in addition to the 17% of global GHG emissions resulting directly from tropical forest loss, tropical deforestation produces an ‘amplification effect', because the stock of natural forests remaining to absorb carbon dioxide out of the atmosphere is reduced.

In addition, the cloud cover over tropical rainforests provides an insulating belt around the planet, reflecting sunlight and evaporating moisture: this can reduce the ground temperature by as much as five degrees Celsius. This insulating effect is lost after forests are cleared.

Recent scientific research indicates that deforestation in the Amazon region could also lead to droughts, which would trigger the release of carbon dioxide from vegetation and perhaps lead to a massive die-off in the world's largest rainforest. These positive feedback effects could greatly accelerate global warming.

Vital Role in Climate Stabilization

It will be extremely difficult to develop an adequate response to climate change that does not include slowing deforestation. The IPCC concluded that to avoid warming greater than two degrees Celsius, and therefore the worst effects of climate change, the global concentration of atmospheric greenhouse gases (GHG) needs to stabilize between 445 and 490 parts per million (ppm) CO2e.

The present atmospheric CO2e level stands at 433ppm. Recent research indicates that to get on the pathway to 450ppm the world must reduce its global CO2e emissions, relative to Business-As-Usual, by 17 Gt by 2020. Action must be taken immediately: each year of delayed action will ‘cost' another 3-5ppm increase.

This same research concludes that the forest sector offers one of the largest opportunities for carbon abatement. Significantly reducing tropical deforestation could contribute over 5 Gt CO2e per year of avoided carbon emissions. It could also do so rapidly and at a low cost relative to other measures. Without addressing the issue of tropical deforestation, it is difficult to see how the world can achieve its 450ppm goal and avoid the worst impacts of climate change [5].

Download Climate change: a short summary (PDF)

Sources

1 IPCC, AR4 Synthesis Report (2007)
2 Houghton (2003)
3 Mitchell, A. W. et al, ‘Forests NOW in the fight against climate change', Global Canopy Programme (2008)
4 Lewis study in Science Daily (20 Feb 2009). See also IPCC (2007) WG1 Ch. 7
5 McKinsey & Company, ‘Global GHG Abatement Cost Curve v2' (2009); ClimateWorks Foundation / McKinsey & Company ‘Project Catalyst'