Peatlands change as a result of both internal ecological processes and external forcing such as climate change and human disturbance. The relative influence of these factors depends on the magnitude of forcing and the type and stage of development of the peatland. In the past, climate has been the most important influence on changes in peatlands. In their natural condition, peatlands behave almost like whole organisms – they grow upwards and expand outwards and go through successional stages over time. The detailed changes in vegetation and surface topography are also partly a result of competition, small-scale hydrological changes, and chance. Overlain on these ‘internal’ processes is the over-riding effect of climate variability. Changes in the character of the peatland and its continued existence are due to a combination of the complex internal mechanisms and climate change. Human influences are a further ‘external’ forcing factor that, when severe enough, may over-ride both internal ecological factors and climate change to alter the system. However, under natural conditions, climate is the most important determinant of changes over time (e.g. Barber 1981).

Peatlands have experienced many climate changes in the past and in their natural state, peatlands are often resilient to such changes. Changing climates over the Holocene have been a major influence on peatland condition. Because of the preservation of biological and geochemical evidence in peat, quite a lot is known about the responses of peatlands to climate change over several centuries and millennia. However, these studies are sometimes limited in the range of changes they can detect. In northwest Europe, a series of fluctuations from wet to dry conditions have occurred in response to past climate changes. Under natural conditions, peatlands are self-regulated systems that have developed different mechanisms to resist changes in hydrological regime caused by climatic fluctuations. During drought, evaporative loss is reduced due to the formation of a ‘skin’ of dried surface vegetation. Changes in moisture content result in ‘mire breathing’ where the surface of the peatland rises and falls depending on the amount of available water, assisting the maintenance of the water table at a high level during drought periods.

Palaeoecological records show that fire frequency is increased during periods of warm, dry climate, but much greater changes are brought about as a result of human activity. Although fires occur under natural conditions, there is evidence that fire frequency has been altered by human activity in both prehistoric and historical times. Although peatlands generally have water tables near the surface and are therefore less susceptible to fire than dryland ecosystems, natural fires do occur during dry periods. The relationship between fire, climate and human activity varies between regions and peatland types, although in general peatland fires are more frequent during warm, dry periods.

Average rate of carbon accumulation in the three raised bogs in southern Finland (Mäkilä and Saarnisto 2005) and lake-level fluctuations in Finland and in Sweden. The 2a line indicates modified Holocene lake-level fluctuations according to Digerfeldt (1988) and 2b line indicates the fluctuations according to Sarmaja-Korjonen (2001). Lake-level positions are expressed only as high or low by Sarmaja-Korjonen (2001). The quantitative annual mean temperature reconstruction based on pollen is also shown (Heikkilä and Seppä 2003).

Rates of carbon accumulation have varied in response to past climate change. The nature of the response depends upon the initial climate conditions and the type of peatland. Peatland response depends on the balance between plant productivity and decay rates in the surface peat layers. Rates of peat accumulation in peatlands have varied through the Holocene period. Some of these changes are due to successional change, typically with high accumulation rates in phases of terrestrialisation. However, where there is stability of peatland type through a period of climate change, palaeoecological studies show that the rate of carbon storage changes. In northern Finland, aapa mires show reduced carbon accumulation after 7000 years BP, followed by an increase at around 4500 years BP, thought to be related to the change from a relatively warm, dry mid-Holocene period to a cooler and moister later Holocene (Makila et al. 2001).

Peatland response to climate change may not occur smoothly; sudden changes may occur when specific climate and/or ecological thresholds are reached. Palaeoecological research demonstrates that shifts in peatland condition do not necessarily occur smoothly.

Transitions between relatively wet and relatively dry conditions can occur over periods of decades rather than requiring centuries to change (e.g. Figure 4). It is not clear whether these shifts are attributable to abrupt climate change or whether they represent crossing of hydrological thresholds as a result of longer term changes in prevailing climate. In either case, it demonstrates that we should not expect future changes to be gradual; sudden shifts in peatland condition are to be expected.