6.3 Climate Change

The changing climate has an uncertain impact on peatland systems since peatlands have developed within a particular hydrogeomorphic setting as a consequence of the local climate that has prevailed since deglaciation. Water exchanges within peatlands, and between them and their surrounding landscape and/or fluvial systems, are a product of the climate that prevailed over their development. The rates of their carbon sequestration and decay, which dictate the rates of peat accumulation and their eventual form, are closely tied to climate (Figure 21). Climate change affects peatland—groundwater interactions indirectly through changes to floral and faunal communities, food webs, nutrient availability, the hydraulic structure of peat, its thermal state, or other ecosystem properties. Given the importance of groundwater to the water balance in many peatland settings, and the sensitivity of peatlands to water balance changes, even small changes to peatland— groundwater interaction could alter the greenhouse gas flux to the atmosphere (Figure 21), such as increasing the CO₂ emission for drying scenarios and increasing the CH₄ flux for wetting, potentially amplifying climate warming (Tarnocai, 2006). However, ecohydrological feedback processes that may dampen or amplify greenhouse gas fluxes are poorly understood. In large part, this is due to the lack of clarity on the rate, pattern, and trajectory of change of peatland ecosystems, their surrounding landscapes, and their interactions with groundwater systems.

Figure 21 – A simplified representation of carbon exchanges at a) present and b) in a warmer climate. a) Carbon dioxide (CO₂) is sequestered from the atmosphere through plant photosynthesis. These plants form the peat deposit. CO₂ is released by the decaying plant litter and peat, especially in the aerobic zone above the water table (WT). Smaller quantities of methane (CH₄) are also released from the saturated, anoxic peat below the water table, although some of this is oxidized as it moves through the aerobic zone. b) A changing climate that results in a lower water table accelerates CO₂ loss to the atmosphere from the thicker aerobic zone, which is a positive feedback to climate warming. CH₄ is more likely to be oxidized under the low water table scenario, and since CH₄ is an important greenhouse gas this represents a negative feedback to climate warming. In most peatlands the larger CO₂ efflux will have a greater impact on global warming than reduced CH₄. Dissolved organic carbon (DOC) produced by decaying peat moves offsite via groundwater, and can be an important carbon source for downstream aquatic environments. A lower water table will reduce groundwater outflow, hence DOC export. Diagram from Renou-Wilson and others (2011).

Climate warming has the potential to transform peatlands because it increases the availability of energy needed to drive the hydrological, meteorological, and ecological processes that control their form and function (Carpino et al., 2021). In tropical regions, climate warming is expected to increase the frequency and severity of drought (and wildfire) and flooding events. Climate warming is generally thought to increase precipitation, since warmer air can hold more precipitable water. This could lead to the introduction and substitution of plant species, which would then lead to changes in local water and nutrient cycling and an alteration of the type of organic material for peat formation. For example, a lowered water table promotes the growth of woody vegetation, itself a form of carbon storage, although likely only important in its initial establishment. Ongoing effects of forest growth, however, include higher transpiration losses and precipitation interception that enhance drying of the peat. Over periods of decades to centuries, this could alter groundwater interactions through changes to peat hydraulic properties.

A climate warming-induced increase in peatland water temperature has the potential to increase the frequency and duration of hypoxic and anoxic conditions, which can reduce the growth rates of peat-forming species, although this is potentially offset where groundwater contributes significantly to peatland water balances. Sea-level rise driven by the melt of the Greenland and Antarctic ice caps will result in the loss of coastal wetlands in the tropics and elsewhere.

In the temperate region, climate warming is expected to increase the frequency and duration of droughts, putting more pressure on groundwater systems to maintain peatlands. Warmer winters would produce a higher frequency of mid-winter melt events, reducing the amount of snow on the ground at the end of winter. This would reduce the magnitude of the annual end-of-winter moisture recharge to peatlands. The absence of significant changes to total annual precipitation in some temperate regions can mask significant changes in the temporal distribution of precipitation. For example, Shook and Pomeroy (2012) reported an increase in the frequency of large, convective storms in the Canadian prairies and, as a result, greater flooding and hydrological connectivity of sloughs and other wetlands (Hayashi et al., 1998). Such changes have the potential to disrupt the recharge of local groundwater systems and thereby alter their role in sustaining peatland systems.

Perhaps the greatest impact of climate change on peatlands is expected in the boreal and subarctic regions, since it is these regions that contain most of the world’s peatlands, have higher than global average projected temperature rise, and because the peatlands of these regions developed and function in the presence of seasonal ground ice and/or permafrost. The loss of ground ice/permafrost can profoundly alter the hydrological functioning of peatlands. For example, ground ice, whether seasonal or interannual, can impound water, thus limit drainage; however, as it thaws and the overlying ground surfaces subside, hydrological connections develop between previously impounded wetlands, allowing them to cascade shallow groundwater from one wetland to the next (Connon et al., 2015).

Seasonal ice and permafrost can impede flow between groundwater and surface water systems; the thaw of such impeding layers can increase groundwater interaction with wetlands, a process often referred to as groundwater reactivation (St. Jacques and Sauchyn, 2009). In permafrost regions, the thaw of ground ice can produce taliks (Connon et al., 2018; Devoie et al., 2019) where the depth of summer thaw exceeds the depth of winter re-freeze. Such layers provide a conduit for suprapermafrost groundwater exchange even during winter. The thaw of permafrost impoundments and the development of taliks can lead to the dewatering of wetlands (Haynes et al., 2018).Permafrost thaw and resulting land cover subsidence can also dramatically alter the local environment for peat formation and decay processes (Swindles, 2015), thus peatland hydrology (St. Jacques and Sauchyn, 2009). Additional information on groundwater in permafrost peatlands is provided in Box 2.

In short, it is difficult to predict the changes that will occur to any particular peatland as a result of climate change. Given the strong ecohydrological feedbacks in peatland systems, caution is advised in generalizing outcomes without a thorough understanding of ecosystem processes.