Peatlands have been recognised for their high carbon storage and their potential to emit a huge amount of carbon emissions once they are drained. Their annual carbon emissions of 2 billion tonnes are a paramount issue in the global effort to mitigate climate change. However, the perilous and far-reaching consequences from peat drainage activities are not limited to carbon loss only. Another key concern that so far has been given insufficient attention is the issue of ‘subsidence’.
Soil subsidence is the lowering of the soil surface as a result of compaction, shrinkage and loss of substance (carbon) through oxidation and erosion. It is a well-known phenomenon in all places in the world where peatlands have been drained, sometimes for over periods of several centuries. It becomes especially worrisome if the peatland lies with its lowest peat layers under sea level (in coastal peatlands) or below the water levels of the adjacent rivers.
“The current situation of the Netherlands is a good example to illustrate what will happen if you drain down to the drainage base and continue draining these peatlands with pump-operated systems,” says Marcel Silvius, Head of Programme and Strategy for Wetlands and Livelihood at Wetlands International. Due to extensive and continuous drainage activities for a period of more than 300 years, almost half of the country is now lying several metres below sea level. “Although the country is small and rich enough to invest in dikes and pumping capacity to avoid the loss of its habitable and productive land, it has condemned all future generations to a system that is costly and inherently unsustainable”.
But, what about countries in Southeast Asia where the drainage base of about 70 per cent of coastal peatlands lies below sea level, and where there are extremely high annual precipitations? “As a result of higher temperatures, subsidence there will go 5-10 times faster,” says Silvius.
In general, the initial subsidence in newly drained areas will be up to 50 cm per year, depending on the drainage level and type and depth of the peat. After several years, the rate will drop to around 5 cm per year. In the case of drained tropical peatlands, especially with rainfall over 2000 mm annually and often falling in concentrated periods (rainy season), severe flooding will be inevitable. “Mitigation through investments in dikes, dams and pumping capacity can be effective for some plantations, but will hardly be feasible at landscape scale due to the huge capacity required,” continues Silvius.
It is estimated that Sarawak – where about half of its 1.2 million hectares of coastal peat swamps have been converted into oil palm plantations and the remainder destined to follow suit – will eventually lose around 10 per cent of its coastal areas to flooding as a result of soil subsidence. This is further exacerbated by reduced freshwater supply to downstream areas and hence increased periods of drought and salt water intrusion in coastal areas. This will culminate in the loss of key land and water resources that are important for the economy of Sarawak and the livelihoods of its people.
In other words, when you drain peatlands, you risk losing not only their valuable ecosystem service of carbon storage, but also many other life-support services. “This should be a key concern for governments to reflect on,” adds Silvius. “Do they really want to lose their land just for short-term economic profits?”