Scotland’s Land Cover

Contents

• introduction
• Origins of Land Cover Mapping
• Scottish Biogeography
• Climate
• Geology and Soils
• Time
• Land Cover Mapping
• The Land Cover Map of Scotland
• Future Landscapes of Scotland
• Support Materials
• About

3. Climate

The past million years¹³

The British Isles lie adjacent to the north-east Atlantic, one of the world’s most temperature-sensitive ocean surfaces. Scotland lies close to the Ocean Polar Front, at approximately the limit of iceberg survival just to the south of Iceland. The Polar Front, which in turn is linked to the path of the North Atlantic Drift, determines the weather experienced in the British Isles. Southerly shifts in the Polar Front have in the past brought Arctic conditions to Scotland. Deep sea cores provide evidence of a series of glaciations (dramatic increases in the volume of the world’s ice sheets) punctuated by warm episodes during the past million years.

The present global climatic regime extends back about 10,000 years, coinciding with the most recent retreat of glacier ice from the Scottish mainland. During this period, known as the ‘Holocene’, temperatures rose very rapidly to a thermal maximum of around 8,000 - 5,000 years ago. Pioneer colonising vegetation evolved into an extensive woodland cover, which reached its maximum extent around 5,000 years ago. Since then, a combination of cooler, wetter conditions (bringing about the formation of extensive peat bogs) and human disturbance, has resulted in the deforestation of much of Scotland.

Even within recent millennia there have been relatively warm episodes, such as during the Roman period. There have also been relatively wet phases and cold periods (notably during the ‘Little Ice Age’ between AD 1300 - 1800), as well as extreme weather events. Historically, the Scottish climate has been highly variable.

The climate today¹⁴

Scotland lies on the maritime fringe of continental Europe and the much larger land mass of Eurasia. Its climate is dominated by the combined effects of the warm North Atlantic Drift and the rain-bearing westerly air flow. There is a considerable difference in temperature and rainfall between east and west, and between coast and mountain top.

Global Warming¹⁵

Global changes in temperature occur over at least four time-scales:

1. over glacial and interglacial periods, due to shifts in the earth's rotation;
2. within interglacial periods;
3. over centuries, giving rise to the ‘Little Ice Age’ centred around 1600 AD;
4. in recent times, exhibiting an intermittent warming.

Like a blanket, the earth’s atmosphere keeps the earth's surface warm (about 15°C on average, in contrast to -18°C for the moon). This effect occurs because certain trace gases in the atmosphere absorb and re-emit (i.e. 'trap') a fraction of the infrared radiation which the earth's surface emits in response to solar heating. The downward component of this re-emitted radiation warms the earth’s surface and lower levels of the atmosphere. This is known as the greenhouse effect.

Emissions resulting from human activities (such as burning fossil fuels and land use change) have substantially increased the atmospheric concentrations of the main greenhouse gases: carbon dioxide (C02), methane (CH4), chlorofluorocarbons (CFCs) and nitrous oxide (N2O).

The amounts of greenhouse gasses which have been added to the atmosphere through human activities are small relative to the bulk of the atmosphere, but large relative to the amount of naturally occurring greenhouse gasses prior to the Industrial Revolution.

Small amounts of greenhouse gasses have a big effects on the temperature of the Earth’s surface. Global mean temperatures have already increased by 0.3-0.6°C over the last 100 years. Recent projections indicate a probable global warming of 2°C by 2100 (0.2°C per decade), with a corresponding rise in sea level of just under 0.5 metres (5 cm per decade)¹⁶.

The implications for Scotland are uncertain, located as it is in an especially sensitive climatic position and in the path of the North Atlantic Drift. An increase in mean temperatures of 0.2°C per decade is small compared with the variation between years in mean UK temperatures of up to 1.0°C. Over the next 2-3 decades the most noticeable signs of enhanced greenhouse warming may be a decrease in the incidence of weather events such as: i) fewer severe frosts and less snow; ii) more warm summers; and iii) mild spells in winter. An increase in storminess may occur. Increased precipitation in winter may occur, with fewer snow falls. There is evidence that the west has become wetter and the east has become drier.

Possible effects on land cover

An increase in mean temperature in Scotland is likely to become important once the increase exceeds about 0.5°C and has persisted for a decade or so, which probably means around 2020. The main effects on flora and fauna may result from: i) less snow cover and reductions in periods of critical freezing or chill temperatures; ii) an increase in length of the growing season of about 14 days for a 0.5°C increase in temperature; and iii) an increase in maximum summer temperatures which will favour flowering, reproduction and dispersal of many warmth-loving species.

A large proportion of the flora have part of their northern limits in the UK. A shift of 1°C mean annual temperature in the UK is equivalent to a latitudinal shift of 200-300 km or an altitudinal shift of 150-200 m. Consequently, there is great potential for a northward and upward movement of mobile plant and animal species [Figure 3.3]. The first plants to move may be the weedy ephemeral species with high fecundity and rapid dispersal. Movement may occur in spurts following droughts, fires or storms.

A significant element of the rare and protected flora and fauna in the UK has a northerly and montane distribution. Species confined to locations from which they cannot escape (such as isolated cliffs or north-facing slopes) may be lost, at least locally. Especially vulnerable are plant species now confined to the highest hills.

An increase in the length of growing season could improve the viability of grassland, animal production and forestry in the uplands, creating opportunities to convert moorland to grass pasture, or to introduce new tree species and crops that are currently grown further south. In the longer term, climatic warming may change the nature of soils, particularly decreasing their content of organic matter and water-holding capacity.