Double trouble: subsidence and CO2 respiration due to cultivation of coastal peatlands

Coastal plains are amongst the most densely inhabited areas in the world. At the same time, inhabitants of coastal plains are facing progressive loss of land, and increasing risk of flooding. Many deltas are now drowning owing to increased compaction, sediment starvation, and global sea-level rise, trends that are all at least partly attributed to human impact. Drainage of peatlands to create arable land is common in coastal areas. This has very direct results for the inhabitants as the drained peat deposits oxidise, resulting in volumetric loss and carbon dioxide (CO2) emission. Quantification of drainage-related subsidence and CO2 emission is necessary because their impacts are large: i) locally it causes increased flood risk and ii) at the same time it causes increased atmospheric greenhouse gas concentrations on global scale. Yet, it is difficult to quantify subsidence and CO2 emissions as a result of cultivation of peatlands because the critical evidence (i.e. the peat) is usually missing as a result of oxidation. Although recently developed techniques allow measuring of subsidence (e.g. remote sensing) and CO2 emission (e.g. gas flux chambers) for the last decades, for time scales beyond the recent, possibilities for quantifications are limited. Nonetheless, the history of reclamation of peatlands is in many cases much older and can extend up to centuries. Possibly the longest record of extensive coastal peat drainage comes from the Netherlands, where the Dutch started cultivation already in medieval times. We applied a 3D approach that uses volumetric geological data to calculate the impact of intensive use and exploitation of the Dutch coastal plain since medieval times. We show that approximately 20 km3 of peat disappeared, which lowered the entire coastal plain surface (8000 km2) by 2.0 meters on average, bringing most peatlands below Mean Sea Level. Because subsidence is not solely the result of oxidation, we split this volume into an oxidation-component, a compaction-component and an erosion-component. The latter two components are only small and do not exceed 15 % of the total subsidence. The former component is in terms of CO2 respiration equivalent to a global atmospheric CO2 concentration of ~0.50 ppmv. While at least 64% of this value owes to the drainage of peatlands for agricultural use, we find that 36% is related to the excavation of peat for historical combustion purposes. As a result of the mining, in some locations the surface was lowered by almost 8 meters, creating lakes. The Dutch history of coastal peat exploitation serves as a prospect for other areas around the world. In many coastal and deltaic areas around the world, the shallow subsurface comprises thick peat sequences. We conclude that these coastal peatlands are extremely sensitive environments to human impact, because they lack sediment to compensate subsidence, and easily become sources for atmospheric CO2.