To document recent changes in both climate and peatland systems, direct observations and measurements can often be used rather than the geological record, although direct observations on peatlands are limited in timescale and scope. The main changes in climate are highlighted in the Summary for Policymakers of the IPCC Fourth Assessment Report (IPCC 2007). The key findings of most relevance to peatlands are summarised here.

Global temperatures have risen by approximately 0.74^oC during the last 100 years (years AD 1906-2005). Temperature changes in the last 50 years are very likely explained by anthropogenic greenhouse gas emissions. It is likely that the 1990s and 2000s were the hottest decades for at least the last 600 years.

 

    

 

Projected changes in temperature in 2020-2029 (left) and 2090-2099 (right) compared to the period 1980-1999, based on the multimodel ensemble for the IPCC A2 emissions scenario. This scenario is chosen to illustrate the spatial pattern of temperature change compared to the global average. It is striking that key peatland areas in the northern high latitudes are projected to experience some of the largest temperature changes, much greater than the global average temperature rise.

 

Global changes in precipitation over the last 100 years are harder to detect but there is strong evidence to suggest changes in total precipitation, seasonality and extremes in some regions where peatlands are found. Global sea levels have risen at an average rate of 1.8 ± 0.5 mm p.a. over the period 1961 to 2003 Regional rates of sea-level rise are moderated by land surface movements which may increase or decrease these global averages. Recent observed changes show that peatlands have already responded to 20th century climate change. Direct observations of changes in peatlands indicate that recent climate changes may already be having an impact. However, in some cases it is difficult to attribute changes to climate alone because of alterations in atmospheric pollution (especially nitrogen deposition) and management (grazing, fire). The extent and duration of permafrost in northern peatlands has decreased. In northern peatlands a rise in late 20th century temperatures is linked to a reduction in the extent of permafrost. In northern Manitoba, a regional warming of 1.32oC has caused accelerated permafrost thawing (Camill 2005). These patterns are repeated across much of Arctic Canada where the southern limit of permafrost in peatlands has moved north by 39km on average and as much as 200km north in some places (Beilman et al. 2001).

Many natural peatlands appear timeless and unchanging, but recently it has been shown that even on pristine peatlands, there have been changes in plant communities over the last 50 years recorded by directly by plant ecologists. There are only a few places in the world where direct comparisons of plant inventories over long periods of time are possible. In Sweden, some species disappeared and others colonised one site where comparative data were available from 1954 and 1997 (Gunnarsson 2002). In northern Britain species changes have also been recorded since the 1950s (Chapman 1991). In many cases, it is impossible to separate the effects of successional processes from those ‘external’ factors such as atmospheric pollution, subtle changes in human management and climate change. However, recent work suggests that the impact of climate change on mid-latitude peatlands may already be one of surface drying and water-table decline (Smith et al. 2003, Hendon and Charman 2004).

Strong El Nino events have produced a larger number of severe fires in Southeast Asian peatlands. The smoke from burning forests and peatlands that has become a regular occurrence in Southeast Asia is a severe threat to peatlands in the region as well as to human health and economic growth. The fires are started for forest or scrub clearance and are made much worse by drainage and previous damage making them more susceptible to burning. However, meteorological conditions play an important part in determining the frequency and severity of fires. During ENSO events, Southeast Asia experiences much drier than usual conditions and moisture levels in peatlands are much lower than normal. Thus some of the most severe fires occur during ENSO years. For example in 1997/98, it is estimated that between 0.81-2.57 Gt of carbon was lost from burning peatlands in Indonesia – equivalent to 13-40% of global annual fossil fuel emissions (Page et al. 2002).

Increasing aridity and associated drought frequency and intensity has led to degradation of peatlands in some steppe and mountain regions. There is evidence of increased aridity in steppe and mountain regions of Central Asia and some other parts of the world. The last 10 years have been drier than average (especially 2000-2002) and this, combined with overgrazing, has led to the loss of extensive areas of peatland in Mongolia. During the extended droughts, the peatlands become much drier and the growth of dryland grasses is encouraged, changing vegetation to meadows and steppe ecotypes. These effects are particularly noticeable in the Orchon River and Ider valleys and the Darchat inter-montaine basin.

Drier peat surfaces have experienced erosion during storm rainfall. Increased aridity and consequent drying of peat surfaces makes peatlands more susceptible to erosion because the structure of the peat is weakened, especially where vegetation cover has been reduced or removed altogether. In Mongolia, erosion of dried sloping peatlands was observed in mountain regions during 2004-2005 (Minayeva et al. 2004, 2005).

Coastal peatlands have undergone marine transgressions during periods of past sea-level rise and new peatlands have formed in areas where sea level has fallen. In Finland and Sweden new land surfaces are being exposed creating areas for new peatland formation and a succession of peatlands of differing ages at different elevations above current sea level (Merila et al. 2006). In subsiding coasts such as southern England, sea-level transgression has occurred at various times in the past and there is a current threat to coastal wetlands only held in check by coastal defences.

In many cases, human activity has exacerbated impacts associated with climate change phenomena. Disentangling the effects of climate change from those of human activity is not always possible, but it is clear that in many cases human actions have increased the vulnerability of peatlands to climate changes during the last 100 years. The impacts of climate change in many of the above examples are much worse where drainage, burning and over-grazing are also involved. In Indonesia, peat fires are always more severe in drained, logged peatlands than in pristine areas. Likewise, peatland damage in Mongolia from increased droughts is exacerbated by over-grazing. Erosion of peat from high intensity rainfall is more likely when vegetation has been reduced or removed by grazing and pollution (Warburton et al. 2004).