Impact of Global Warming on Wetlands

3.4 Impact of Global Warming on Wetlands

Wetland soils and sediments are considered to be among the world’s largest car- bon sinks. They have been accumulating carbon for between 4000 and 5000 years (Lloyd 2006), but are at risk of becoming an extremely large atmospheric carbon source because of climate change. Peatlands store an estimated one third of the world’s organic soil carbon (Gorham 1991; Weishampel et al. 2009). Weishampel et al. (2009) stated that carbon storage in peatlands is a consequence of long-term climate trends during which a positive water balance enables accretion of peat. Although open peatland productivity is low, all peatlands have acted as long-term carbon sinks for hundreds to thousands of years and store significantly more car- bon per unit area than is stored in uplands. Over long periods of time, natural wet- lands can be considered carbon stores (Kayranli et al. 2010).

3.4 Impact of Global Warming on Wetlands 139

There is a strong correlation between climate and soil carbon pools, where the organic carbon content decreases with increasing temperatures (Kirschbaum 1995; Rasmussen et al. 1998) due to decomposition rates doubling with every 10°C in- crease in temperature (Schlesinger 1997; Hartel 2005). If temperature increases continue and become more rapid, the decomposition of organic matter will increase, and wetlands will eventually become major sources of carbon. Some researchers (Gorham 1991; Hobbie et al. 2000; Davidson and Janssens 2006) point out that wetlands, which drain well and are therefore well aerated, will be associated with fewer fluxes of carbon dioxide in the event of warming. However, if wetland drain- age is poor and anaerobic conditions occur within the soil, wetlands may release considerable amounts of greenhouse gases. There is a general belief that increases in temperature and changes in water levels are important variables in the production of methane and carbon dioxide from wetlands (Moore and Roulet 1995; Updegraff et al. 2001). For example, as a result of a slight global temperature rise, parts of the tundra environment would act as a net source of carbon dioxide (Christensen 1993).

Trenberth et al. (2007) estimated that land warming in the Arctic is expected to

be twice as high as the global mean, and thus the effects of the observed and pre- dicted climate changes will be particularly strong in the Arctic. Methane emissions from Arctic wetlands are expected to increase (Wuebbles and Hayhoe 2002), and highly variable emissions, potentially indicating signs of climate change, have already been recorded for some sub-arctic wetlands. Ström and Christensen (2007)

reported between 0.2 and 36.1 mg CH 4 –C/m 2 /h for northern parts of Sweden. Dick and Gregorich (2004) carried out research to compare the decomposition rates of organic matter in tropical regions of Nigeria and cold dry climates in Canada, and concluded that decomposition rates were usually ten times faster in tropical re- gions than in cold and dry climates. Hence, global warming effects on tropical wetlands may also lead to increased decomposition and carbon fluxes, unless the corresponding temperature change is modest (Kayranli et al. 2010).

The impact of global warming on the economic exploitation of wetlands and on conservation policies is not well understood and is therefore often not considered in global models of climate-change effects (Clair et al. 1998). Much of recent terrestrial ecosystem modeling is aimed at estimating ecosystem carbon budgets and their future trends under a changing climate. Moreover, the current global financial crisis is likely to lead to reduced investment in wetland protection and conservation measures. The future of conservation wetlands should be secured by protecting their status (Kayranli et al. 2010).

The high initial global warming potential of increased methane emission in newly created wetlands means that many will have to establish themselves for over 100 years to be considered as carbon sinks (Whiting and Chanton 2001). However, improved design and management of constructed treatment wetlands, even after their decommissioning, should make a positive impact on long-term carbon storage. Changes in land use due to global warming such as increased drainage of wetlands for agricultural purposes could potentially lead to large car- bon dioxide and methane fluxes to the atmosphere, further accelerating climate change (Limpens et al. 2008). Destruction of wetlands is also likely to lead to

140 3 Carbon Storage and Fluxes Within Wetland Systems

secondary water pollution from the release of nutrients during wetland degradation due to lower water levels, as demonstrated at a peatland restoration project in Northern Germany (Scholz and Trepel 2004a, b).