« Previous | Contents | Next »
Listen
Chapter 3 Climate change
This chapter examines how Scottish soils might change in response to potential changes in our climate, some of the implications of those changes for soil functions and land management and on feedbacks through GHG emissions.
3.1 Summary
- Although there are large uncertainties, it is expected that Scotland's climate will get warmer, drier in summer, wetter in winter and with an increased risk of storm events.
- The soil property most likely to be impacted is organic matter content but it is very difficult to be certain in which way and to what extent.
- Climate change may affect soil carbon turnover in a number of different ways and in different soils.
- Yields of agricultural and forestry products may be adversely affected, particularly if summer rainfall decreases and temperature increases thereby causing reductions in soil moisture content.
- The focus of this chapter is the impact of climate change on soils, but it is recognised that there are feedback loops e.g. increased emissions of N 2O from wetter soils, but also potential for mitigation through adoption of specific land management practices.
- Land management, for example trafficability and soil workability may be impacted at crucial times of the year, leading to a potentially increased risk of compaction.
- Erosion risk, dependent on other factors such as crop cover and soil type, may increase if storm events occur more regularly. This in turn would lead to more serious downstream effects such as increased siltation and/or eutrophication.
- Some valued soils and habitats may be disproportionately at risk, for example machair (from increased flooding) and montane soils (from warming).
3.2 Introduction and Description of Threat
There is an increasing consensus that human activities are causing considerable changes to the world's climate. This is largely through the emission of greenhouse gases ( GHGs) such as carbon dioxide, methane and nitrogen oxides that contribute to overall warming. Climatic conditions are factors which influence soil forming processes and also partly determine the extent to which soils can perform individual functions.
3.3 Policy
A report commissioned by Scottish Executive (Kerr and Allen, 2001) to review climate policy in countries with similar socio-economic and environmental characteristics to Scotland concluded that agricultural policy should be developed to encourage biomass production as an energy resource and to reduce nitrous oxide and methane emissions. Policies of this nature were found at that time in Ireland, Sweden and Denmark.
In 2006, the Scottish Executive set out its Climate Change Programme within which progress to date was described and further adaptation strategies outlined. The contribution that the agriculture and the land use change and forestry sector make to Scottish GHG emissions is significant and the role that soils play within it is highlighted.
3.4 Evidence
There is some evidence that Scotland's climate has warmed up marginally over the last century (Scottish Executive 2006). Since 1961, temperatures have increased in every season and in every part of Scotland. This has been the fastest period of warming identified in the period from 1914-2004. Rainfall in winter is almost 60% higher in northern and western Scotland over the last forty years although there have been no significant changes in summer rainfall. There are also shorter frosty periods and less incidences of frost and the growing season is approximately one month longer than 100 years ago. More information on climate trends can be found in a recent publication produced by Barnett et al (2006). On a global scale, the 1990s was the warmest decade in the last century. Extreme events and seasons are also occurring on a more regular basis and there is a body of evidence developing that is indicating some serious climate shifts have actually started.
Analysis of extreme rainfall data for the period 1961 to 2000 indicate that event magnitudes have significantly increased particularly for Scotland. For example the 50 year rainfall event in Scotland has become an 8 year event for eastern Scotland and an 11 year event for southern Scotland during the analysis period (Fowler and Kilsby, 2003). Black and Burns (2002) recently reassessed the flood risk for Scottish rivers using long-term flood records. They found that the frequency of peak over threshold events increased during the 1980s and 1990s especially in the west of Scotland. Recent frequencies may be the greatest within the last 100 years. However they found no similar evidence of increases in flood magnitude, although 8 out of 16 of Scotland's largest catchments have exceeded their previous flow maxima in the period since 1989 (Black and Burns, 2002). Another recent analysis predicts that for the UK as a whole, event magnitudes at a given return period will increase by 10% for short-duration (1-2 day) events and by up to 30% for longer frequency (5-10 day) events (Ekstrom et al., 2004).
We cannot be certain on the scale or indeed the direction of the climatic shift that might occur in Scotland but current models suggest a possible warming of up to 3.5 °C in summer and 2.5 °C in winter by the 2080s (Hulme et al 2002 quoted in Scottish Executive 2006) in a high emissions scenario. This would mean that Edinburgh might have a climate somewhat similar to that of London in the present day. More extreme events, both in frequency and magnitude, such as storm force winds and rainfall are also likely. Current predictions from the UK Climate Impacts Programme (Hulme et al 2002) suggest smaller changes in a low emissions scenario but with still a 1-2 °C increase in mean annual temperature. An alternative scenario is one where melting of the Greenland icecap and increased rainfall in Russia might lead to a dilution of the salt water in the North Atlantic leading to the Gulf Stream taking a more southerly route and in effect being 'switched off'. This would create much cooler conditions in Scotland, particularly in winter.
This short chapter will discuss some of the broader implications of a warming climate, with more extreme rainfall events, for Scotland's soils and for the ways that they might be managed. Clearly these must be seen as being speculative and cannot be viewed as evidence of change or threat, but should form a focus for further debate and to identify research and monitoring requirements.
3.5 Potential impacts of climate change on Scottish soils
A changing climate might impact on soils in two ways. Firstly there are the direct effects of changes to soil properties and secondly the indirect effects resulting from soil/climate interactions of for example soils becoming wetter or drier for longer or shorter periods of the year.
3.5.1 Direct effects
The soil property that is most vulnerable to climate change is soil organic matter content. As already discussed ( Chapter 2), this property has an enormous influence on all soil functions so any changes to its status will have a profound impact. In cultivated soils, organic matter content is fundamental to soil structural stability, and moisture and nutrient holding capacity. Increased organic matter turnover caused by increased temperatures may therefore have a detrimental effect on such properties, and problems such as erosion and surface capping which are more common in southern England at the present time may become more common in Scotland. Any decline in soil organic matter levels in arable soils will also reduce the carbon storage function, increase their sensitivity to added pollutants such as heavy metals through waste recycling and the risk of leaching of soluble pollutants may be enhanced. Soil biodiversity may also decrease given the relationship between SOM and microbial biomass. From the NSI resampling exercises in England and Wales, there is some evidence for arable of small decreases in carbon content over recent years. However for arable soils, these are not at a level or scale that need cause undue alarm (Loveland and Webb 2003). It is also unclear whether the small declines have been caused by changes in organic matter turnover rates or by land management practices, a combination of the two or neither.
Scotland has extensive areas (over half the land area), of semi-natural soils with highly organic surface horizons. Warmer temperatures may lead to these becoming sources of greenhouse gases rather than the sinks that they are currently considered to be. Organic carbon concentrations have declined dramatically in the limited sample of such soils in England and Wales (Bellamy et al., 2005). Although no clear trigger for this decline has been identified, a warming climate has been speculated as a significant contributor. If this is indeed the case and if the climate change scenarios for Scotland do actually happen, then the consequences for Scotland's soils and the implications for our greenhouse gas emissions inventory are profound. The declines seen in England and Wales have occurred under a marginal shift of around 0.5 °C, small compared with those that might occur over the next century. We urgently require some data from Scotland to quantify any similar changes in organic carbon concentrations in our organic rich semi-natural soils ( see also Chapter 2). Here, in addition, the development of the ECOSSE model will increase our understanding of C (and N) cycling in these organic soils and can be used to predict C losses under a number of climate change scenarios. For future monitoring and modelling, we require data on the depth of individual soil horizons, the C content of these horizons and the bulk density of a representative sample of soils. The current data contain the first two attributes but the third (bulk density) is estimated from a relatively small sample set.
3.5.2 Indirect effects
Soil can usually recover quickly from a 'bad year'. For example, if soil structural degradation during harvesting operations occurs during a particularly wet year, there is frequently significant recovery of structure during the next growing season under appropriate management. Similarly, adverse impacts can be remediated on grassland soils. However if climate change impacts in such a way that 'bad years' become more frequent, there would be implications for the long term health and use of certain soils for example those that prone to structural degradation. There is less information on the ease and rate of recovery of semi-natural soils, but these soils are much less trafficked than those under agricultural use.
Climatic factors interact with soils and can have a profound influence on their functional capacity. Soils with similar properties respond differently to different climatic inputs. An example of this is the decreased workability and trafficability of soils in west central Scotland compared with the east. Rainfall in lowland Ayrshire is approximately 50% higher than in East Lothian and this has a marked influence on the potential and actual farming systems in the two areas. If winter rainfall does increase as predicted then the risk of soil compaction (see Chapter 5) could also increase as the window of opportunity for land work narrows and farmers may be forced to access land in less than optimal conditions.
Threats from the greater occurrence of wet soils induced by climate change have been assessed in a limited number of case studies. Weather constrains the time available during the winter half year for carrying out operations with heavy machinery and also limits the time available for spreading slurry and other wastes. Modelling indicates that the number of available working days at sites in Scotland will decrease because of expected increases in winter rainfall (Cooper et al., 1997). This same study also indicated that higher winter temperatures would reduce the number of days when the soil is frozen, thereby further reducing workdays for machinery. Other studies point to the effect of rainfall changes on the net emissions of N 2O from wet soils (Flynn et al., 2005). They demonstrate that N 2O emissions will increase, assuming fertiliser applications and land use remain the same, as a result of climate change.
Another indirect effect of increased rainfall might be an increased risk of flooding (see Chapter 6). Soils are likely to be saturated for longer periods in winter and coupled with the fact that saturated soils have a reduced capacity to receive more water, the risk of direct run-off will be increased. This risk will be greater after heavy rainfall, events that are predicted with increased regularity under climate change scenarios.
The risk of soil erosion (see Chapter 6) may increase if the pattern, duration, intensity and location of rainfall events were to change. If more and/or heavier rain falls when the soil is bare or partially bare, soil erosion will increase. This is particularly the case with autumn sown crops when the soil is bare for a number of months and the soil is much more likely to be at field capacity. Soils are bare for a much shorter time in spring and are going through a process of drying out after winter.
If, as is predicted, Scotland's climate becomes drier in summer, there is a risk that some soils may experience drought more frequently leading to lower crop yields. Again the soil itself is not changing per se - the moisture storage capacity of the soil is not changing - but there will be lower direct inputs from rainfall. Although winter rainfall may be higher than at present, lower rainfall and higher temperatures during summer is likely to increase the risk of drought. Irrigation may increase as a management response but this option might not always be available given the predicted decreased rainfall during the growing season.
The indirect effects of climate change on semi-natural soils are likely to be seen through changes in the pattern and range of semi-natural habitats and species, rather than by evidence of soil compaction or poorer crop yields. Soil biodiversity may be vulnerable to specific stresses such as moisture deficit in the soil.
There is potential for an increased likelihood of peat erosion (see also chapters 2 and 6). There is uncertainty about what the primary trigger for peat erosion actually is and given that most extensive areas of eroded peat are on high-level plateaux, climate is considered to be at in least in part responsible for the erosion process. The climate has changed from the time when the peat was actively building to that of the present day when the building phase has changed or slowed down. Eroded peat contains many bare peat surfaces in hags and gullies. If these bare peat soils are exposed to warmer conditions than at present they will dry out and a crust will form at the surface - this process occurs after dry periods at present. If a high intensity rainfall event or strong wind occurs immediately after a prolonged warm dry period then there is huge potential for the surface crust to be removed and washed into the drainage system. There may also be more potential for larger scale events such as bog bursts and peat slides.
3.5.3 Soils and habitats at highest risk .
Some valued soils and habitats (see Chapter 4) are at risk of damage or reduction in size should climate warm up and/or sea levels rise. There may also an increased risk of invasion by non-native species as described in Chapter 4. Many of the calcareous soils of the machair in the Western Isles are near sea level, and although their complete disappearance is highly unlikely, they are likely to be increasingly subjected to inundation both as a result of higher sea levels and increased storminess causing more wind erosion and flooding events. If this were to happen, the environment, the economy and even the social and infra-structure of the Western Isles would be damaged. Such sea incursion may also lead to salinisation of the soils.
Our highest mountains sustain soils and habitats more commonly found in the Arctic at much lower altitudes than in Scotland. The relatively maritime nature of our climate and our relatively southerly latitude also makes the presence of these soils a valuable and unique outlier from the vast majority of their extent. Any warming of Scotland's climate will affect this habitat although the rate of change is open to speculation. In terms of soil processes, these soils are influenced much more by physical processes such as cryoturbation caused by periodic freezing and thawing rather than chemical or biological processes compared with soils at lower altitudes. These effects will be diminished and the very nature of the soils will change. Nevertheless, this environment is likely to remain extreme in nature and one where exposure to very windy conditions will have a profound influence on the above and below ground environment.
Our blanket peatlands are internationally recognized and, again because of the extreme maritime nature of our climate, are rare in global terms. The cool wet climatic conditions result in low evapotranspiration rates and accumulation of organic matter at the soil surface. Accumulation rates of organic matter that exceed that decay rates over several millennia has resulted in peat depths of up to 7-8 metres in the most extreme cases. Any change to the climatic regime affecting Scotland will alter this process although precisely how is open to question.
3.6 Conclusions
Given the uncertain nature of this threat, we cannot be clear about its scale or impact. Nevertheless we can make some judgements on potential changes to soil properties and whether soils might become more or less vulnerable to other threats. Climate change may:
- Change soil organic matter content, with a reduction more probable. Given the role that soil organic matter has on a number of soil functions, this is a key issue that affects the soil resource of Scotland and on our terrestrial carbon budget.
- Lead to a requirement for new soil management strategies on some soils. The risk of soil compaction on agricultural soils may increase and some soils might become more sensitive to these risks.
- Have a potential effect on biomass production through increased intensity and duration of soil drought.
- Increase the potential for soil erosion with possibly peaty soils most at risk and therefore increased loss of terrestrial carbon
The pattern of soils and land use in the Scottish countryside is the result of soil forming factors interacting with social and human influences over a number of millennia. Scotland may be facing environmental change at a rate hitherto not experienced and this poses questions about how our land resource might respond. Is it resilient enough to adapt to these changes gradually or are there unforeseen responses that are too difficult to predict? Given this uncertainty, there is a case for adopting the precautionary principle and where possible, ensuring that our soils retain as wide a range of potential functions as possible. In addition and as importantly, climate change will affect the whole world and may result in quite dramatic climate shifts to the extent that food production in some areas may become seriously compromised. We need to view our soils in the widest possible perspective; our soils might be quite resilient but those in other parts of the world may not be.
3.7 Key References.
Barnett C , J Hossell,, M Perry, C Procter and G Hughes (2006) A Handbook of climate trends across Scotland. SNIFFER project CC03, Scotland and Northern Ireland Forum for Environmental Research, 62pp.
Bellamy, P.H, Loveland, P.J., Bradley, R. I., Lark, R.M and Kirk, G.J.D. (2005) Carbon losses from all soils across England and Wales 1978-2003. Nature. 437:245-248
Black, A.R. and Burns, J.C., (2002). Re-assessing the flood risk in Scotland. Sci. Tot. Envir. 294, 169-184.
Cooper, G., McGeechan, M.B., Vinten, A.J.A., (1997). The influence of a changed climate on soil workability and available workdays in Scotland. J. Agric. Eng. Res. 68 253-269.
Ekstrom, M., Fowler, H.J., Kilsby, C.G., Jones, P.D., (2004). New estimates of future changes in extreme rainfall across the UK using regional climate model integrations. 2. Future estimates and use in impact studies. J. Hydrol. 300, 234-251.
Flynn, H.C., Smith, J., Smith, K.A., Wright, J., Smith, P., Massheder, J., (2005). Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use. Global Change Biol. 11, 1522-1536.
Fowler, H.J. and Kilsby, C.G., (2003). A regional frequency analysis of UK extreme rainfall from 1961 to 2000. Int. J. Climat. 23, 1313-1334.
Hulme M, Jenkins G J, Lu X, Turnpenny J R, Mitchell T D, Jones R G, Lowe J, Murphy J M, Hassell D, Boorman P, McDonald R and Hill S, (2002) Climate change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK 120pp
Kerr, A. and Allen, S. (2001). Climate Change: North Atlantic Comparisons http://www.scotland.gov.uk/cru/resfinds/erf8-00.asp on 8 June 2006
Loveland P and Webb J (2003) Is there a critical level of organic matter in the agricultural soils of temperate regions: A review. Soil and Tillage Research 70 (2003) 1-18.
Scottish Executive (2006) Changing our ways. Scotland's Climate Change Programme
« Previous | Contents | Next »