Scotland's seas: towards understanding their state

Chapter 6 Assessment of Climate Change Impacts on Scottish Seas

INTRODUCTION

Research and advice on climate change in the marine environment can be divided into two areas. The first is observation of past change and research to understand how it has affected the marine environment. The second is prediction of future change which can then be used for management planning and mitigation.

In 2007, the Intergovernmental Panel on Climate Change ( IPCC) published their fourth assessment report ⁽¹⁾, presenting strong evidence that anthropogenic inputs of greenhouse gases to the atmosphere are the most likely cause of the observed average increase in global temperatures over the last 100 years. Global atmosphere and oceans have warmed at a rate of 0.07°C per decade ⁽¹⁾ over the past 100 years and the warming has also been observed to have accelerated over the last decade, a trend which is particularly apparent in sea-surface temperatures.

Although the long-term global average trend is one of warming, there is considerable regional and temporal variability and there are also changes in other parameters. For this reason the term Global Climate Change is used rather than Global Warming to describe these effects.

The recent, since approximately the early 1990s, accelerated rise in global average temperature is more rapid than predicted by climate models ⁽²⁾. At the same time there has also been a more rapid than predicted rise in global mean sea-level. The reasons for the recent more rapid warming are as yet difficult to determine. It is possible that they can be attributed to natural variability in the climate system added on top of the effects of Global Climate Change. Another possibility is that the climate models underestimated the future rate of warming.

As will be explained in this chapter, the waters around Scotland are strongly influenced by Global Climate Change and also show natural variability. The term Atlantic Multi-decadal Oscillation ( AMO) is often used to describe the long-term pattern of change evident in both the atmosphere and the North Atlantic Ocean. It is also clear that there is a complex relationship between Global Climate Change and the Meridional Overturning Circulation ( MOC; see Chapter 2) which is not yet fully understood.

OBSERVED CHANGES IN OCEAN CLIMATE IN SCOTTISH WATERS

Changes in temperature

Regional effects mean that changes observed at a local level, which includes scales equivalent to Scottish waters, are likely to be different to the global average. In fact, the observed warming in Scottish waters since the early 1990s has been more rapid than the global and northern hemisphere averages (see below).

Data on climate trends in the waters around Scotland are gathered by a number of institutes and summarised in the Scottish Ocean Climate Status Report published by FRS⁽³⁾. For the purpose of this report, data from the longest datasets (those extending further than 10 years) are summarised here.

FRS maintains a network of coastal temperature monitoring stations around the coast of Scotland (Figure 6.1) and at three of these sites (Millport, Fair Isle and Peterhead) data have been collected for more than 25 years. Researchers at SAMS maintain a mooring in the Tiree Passage recording temperature, salinity and flow variability data in waters on the west coast of Scotland (see Case Study 2.1).

Figure 6.1 Locations of FRS coastal temperature monitoring stations (orange circles) and the SAMS Tiree Passage mooring (black triangle)

FRS also surveys three offshore sections, two across the Faroe-Shetland Channel (Figure 6.2; also see Figure 2.1) and a third in the North Sea, extending eastward from Orkney (the JONSIS Line). The measurements of temperature and salinity in the Faroe-Shetland Channel date back to 1893 and are one of the longest oceanographic time-series in the world.

Since 1975, researchers at SAMS and the National Oceanography Centre Southampton ( NOCS) have measured temperature and salinity on a short section (the Ellett Line) across the Rockall Trough, a deep ocean basin to the west of Scotland (Figure 6.2, also see Figure 2.1). Since 1996 SAMS and NOCS have been occupying an extended version of the Ellett Line that runs all the way to Iceland. The Extended Ellett Line is important oceanographically because it completes the measurements of the warm salty water flowing into the Nordic Seas from the eastern North Atlantic. It also measures around half of the returning deep and cold current, the overflow water, the rest returning to the Atlantic via the Denmark Strait which is west of Iceland.

Since the end of the 1980s, the temperature of oceanic water flowing past Scotland has increased at a rate of between 0.22-0.40°C per decade, with a longer-term (1990-2006) trend of around 0.04°C (Figure 6.3a). These changes observed in Scottish waters are similar, both in their patterns of variability and long term trends, to those observed in the wider North Sea and north-eastern Atlantic ⁽⁴⁾. For all the offshore sites, temperatures observed in 2003-2004 were the warmest on record, with 2006 being the second highest on record (Figure 6.3). The temperature data from all monitoring sites show an overall warming trend since the late 1980s. The variability in coastal waters is higher than that observed in oceanic waters and more closely coupled to atmospheric effects, and the warming trend is generally higher than observed further offshore (Figure 6.3d).

Figure 6.2 Offshore long-term hydrographic sections in Scottish waters

Although the changes observed since 1980 have been marked, they must be considered in context with longer-term variability. There is evidence that across the North Atlantic a warmer period between 1930 and 1960 was followed by a cooler period between 1960 and 1980 ⁽⁵⁾. These warmer and cooler periods can be observed in the oceanic water flowing past Scotland (Figure 6.3a) and are likely to be related to the long-term pattern of variability in the North Atlantic, known as the AMO.

Figure 6.3 Variability of temperature and salinity in Scottish waters

a) long-term (1900-2006) variability in oceanic temperatures to the north of Scotland and east of Faroe ⁽³⁾. To compare the long-term variability in Scottish oceanic waters with wider-scale trends, the Northern Hemisphere ( NH) Ocean Average temperature time series, as collated by the National Ocean and Atmospheric Administration, is included. Dashed lines show the linear trend from 1980-2006 (0.24°C per decade) and from 1900-2006 (0.04°C per decade).

b) variability in oceanic temperatures (1970-2006) to the north of Scotland, east of Faroe, west of Scotland and the northern North Sea ⁽³⁾.

c) variability in salinity (1970-2006) to the north of Scotland, east of Faroe, west of Scotland and the northern North Sea ⁽³⁾.

d) variability in surface temperature of coastal waters around Scotland.

To focus on long-term variability, the mean conditions, and where necessary the seasonal cycle, have been removed. The data are therefore presented as anomalies, which are the difference from the long-term mean after the seasonal trends have been removed.

Changes in salinity

The salinity data also show clear evidence of decadal and longer-term variability. Alternating salty and fresh periods are closely associated with changes in the wider circulation patterns in the North Atlantic. The freshwater content of the whole North Atlantic has decreased over the last 50 years (1955-2006) leading to an increase in salinity in the upper layers ⁽⁶⁾. This trend to higher salinity is clearly seen in the offshore salinity data (Figure 6.3c). In the North Sea, the variability in salinity is much larger than in offshore regions. This is because at a local level salinity is affected both by influx of water from the North Atlantic and also from freshwater inputs (rivers and rainfall). Over the wider North Sea, it is more difficult to determine a clear trend in salinity ⁽³⁾ (Figure 6.3c).

All of the offshore temperature and salinity data also show strong evidence of concurrent variability over the shorter periods (decadal scale). This variability is likely to be related both to changes in oceanic circulation and atmospheric conditions over the North Atlantic. As Scotland's weather is strongly influenced by Atlantic pressure patterns, the North Atlantic Oscillation index ( NAO), which provides a simple measure of changes in Atlantic pressure differences, has in the past been used to explain this variability. However, the NAO has, since the end of the 1990s, been weak and variable as the dominant pattern of atmospheric pressure across the North Atlantic has not been that of a typical NAO pattern.

Changes in sea-level

Sea-level rise, as a result of Global Climate Change, is thought to be due to both the thermal expansion of water and the melting of land glaciers. Global sea levels have risen by between 10 and 20 cm during the 20th century, and like sea temperature, appear to have accelerated since the late 1990s ⁽²⁾. Long-term measurements at Aberdeen show a trend over the last 100 years of around 0.7 mm/year ⁽⁷⁾. All other Scottish mainland tide gauges have also recorded a sea-level rise over the same period. In contrast, a tide gauge in Lerwick (Shetland Islands) has recorded a fall in sea-level since 1957. Increases in sea-level are offset, in some areas, by the rise in the Scottish land mass which has continued since the melting of the overlaying ice sheets at the end of the last ice age. Estimates of current sea-level change for Scotland, adjusted to take account of uplift movements, are shown in Figure 6.4. It is worth noting that coastal erosion is occurring along 12% of the Scottish coastline.

Other evidence for the impact of climate change in the marine environment

Whether the recent rapid warming can be attributed to Global Climate Change alone, or a combination of Global Climate Change and natural climate variability, there is no doubt that the effects have been marked, for example see Marine Climate Change Impacts Partnership ( MCCIP) Annual Report Card ⁽⁷⁾, with observed consequences throughout the marine environment. Many organisations have collected data so that observations of past changes can be made and have also attempted to understand how climate change has affected the marine environment. Areas studied are diverse and include aquaculture, coastal flooding and harmful algal blooms (Table 6.1).

Figure 6.4 Present rates of relative sea-level change in Scotland ⁽⁸⁾

Table 6.1 Summary of data collected on the impact of climate change in the marine environment

Indicators of the impact of climate change	Organisations collecting data
Aquaculture	FRS
Built structures	Cefas, Oil & Gas industry
Coastal flooding	Met Office Hadley Centre, POL
Coastal habitats	SNH
Fisheries	FRS
Harmful Algal Blooms ( HABs)	Cefas, FRS, SAHFOS
Nutrient enrichment	FRS, SEPA, SAHFOS, Satellite Observing Systems Ltd
Species abundance and distribution (including seabed, intertidal and non-native species, fish, birds and marine mammals)	Cefas, Centre for Ecology and Hydrology ( CEH), FRS, JNCC, MBA, Sea Watch Foundation, SMRU, SAHFOS, SAMS, University of Aberdeen, University of Cambridge
Tourism	Natural England, University of Maastricht

MCCIP brings together experts throughout the UK to summarise what is currently known about the impact of climate change on the marine environment ⁽⁷⁾. The impacts of physical changes are observed at all levels within the marine ecosystem, from plankton through to fisheries, although the complex nature of the marine ecosystem means that it is difficult to isolate climate effects from other pressures. Much of the information reported by MCCIP, detailing what is already happening, relates to the UK as a whole with only a limited amount of regional information available. However, it is hoped that regional reporting will evolve such that a Scottish regional report will be integral to a future Report Card.

The changes in the climate of the marine environment around Scotland have already been described and what is happening in terms of temperature, salinity and sea-level are reported with a high level of confidence in the Report Card. It is not the purpose of this report to reproduce the Report Card, however, it is appropriate to note that it summarises:

• the climate of the marine environment;
• the impacts of climate change on the vision for a healthy and biologically diverse marine ecosystem;
• the impacts of climate change on the vision for clean and safe seas; and
• the impacts of climate change on the vision for commercially productive seas.

As outlined in the Report Card, climate change impacts on the vision for a healthy and biologically diverse marine ecosystem include:

• a decrease in biomass by 70% since the 1960s of the cold-water zooplankton species Calanus finmarchicus;
• earlier phytoplankton blooms;
• an indication of metabolic stress in cod and eelpout during warmer years;
• the spread of non-native species as a result of climate change as well as migration and human introduction; and
• some warm-water invertebrates and algae have extended their ranges around northern Scotland.

It should be noted that investigations into the effects of climate change on marine biodiversity, specifically key intertidal species whose abundance has been shown to fluctuate with changes in climatic conditions, were undertaken in the UK through the Marine Biodiversity and Climate Change (MarClim) programme ⁽⁹⁾.

There is generally less confidence in determining how climate change is impacting on the vision for clean and safe seas. There are, however, indications that harmful algal blooms are being driven largely by ocean climate forcing, with nutrient enrichment only relevant in some cases.

Finally, in terms of the vision for commercially productive seas, there is again low confidence in reporting what is already happening. This includes the possible impacts of climate change on aquaculture; the current view is that in the short-term, climate change is unlikely to have a significant effect on farmed marine fish, over 99% of which are cultivated in Scotland. What is reported for north-west Europe is that there will be an increasing frequency of months when conditions are more comfortable for tourists. What exactly this will mean for Scotland is unclear.

PREDICTIONS FOR THE FUTURE

Future climate change can be predicted using global and/or regional climate models using estimates of the atmospheric concentration of greenhouse gases. As this is uncertain and depends upon society's response to the concerns about climate change, predictions based on a number of different future scenarios are often presented.

In 2002 the UK Climate Impacts Programme ( UKCIP) published a detailed set of future scenarios for the UK⁽¹⁰⁾ based on low, medium-low, medium-high and high estimates of future greenhouse gas concentrations. The resolution of the scenarios was very coarse with a 50 km resolution throughout the UK. At this resolution many of the small Scottish islands were not well represented. To address this problem the British-Irish Council commissioned a further report that used the results of a regional model at 25 km resolution and provided improved scenarios for Scottish Islands such as Orkney, Shetland and the Western Isles ⁽¹¹⁾. Later in 2008 UKCIP will publish new and updated scenarios that will include information about changes in the marine environment ⁽¹²⁾ and provide a better basis for future assessments.

Future sea-level rise, and an increased risk of storm surges, are likely to lead to more coastal flooding with knock-on impacts on coastal erosion, coastal habitats and built structures. Wave conditions in Scottish waters are most likely to be affected through changes in weather patterns and increased storminess. In order to better understand these changing conditions four WaveNet buoys ⁽¹³⁾ are to be installed in Scottish waters in 2008; two on both the east and west coasts.

It seems that in terms of future climate change, alterations in pH will have a widespread effect, although most probably through a cumulative effect with changes in other environmental conditions. Ocean acidification will be influential not only for marine primary producers through effects on nutrient speciation and biogeochemical processes, but also for shell-forming organisms where lower pH will hinder the formation of shells and skeletons. The latter will have greatest impact on the shellfish fisheries which will also be affected indirectly by changes in food web structure attributable to acidification.

Existing predictions from regional models indicate that annual mean air temperatures in Europe are likely to increase faster than the global mean, winter temperatures are likely to rise faster and annual precipitation is likely to increase. Many climate models predict that the MOC will weaken in the future, but at the moment the effects of this weakening are thought likely to be less than the overall warming ⁽¹⁾.

CONCLUSION

There is strong evidence that anthropogenic inputs of greenhouse gases to the atmosphere are the most likely cause of the observed average increase in global temperatures over the last 100 years. The rate of change in temperature appears to have increased in recent years such that the linear trend from 1980-2006 in Scottish waters equates to 0.24°C per decade. However, for some of the indicators discussed in this chapter there is low confidence in the interpretation and predictions drawn from them; indeed the current and future impacts of climate change remain poorly understood. As such, there is a clear need to ensure that the required research and monitoring is in place to gather the appropriate data. This requires both national and international collaboration since many of the measurements are costly and cover significant geographical regions (e.g. the global Argo Buoy network ⁽¹⁴⁾). Scotland contributes actively to the assessment of climate change in the North Atlantic as summarised in the ICES Report on Ocean Climate⁽⁴⁾. Due to the considerable maritime influences on Scotland, it is inevitable that changes in the seas caused by climate change will have environmental and socio-economic impacts on Scotland. There is a need to improve our confidence in both understanding what is happening and in predicting the likely consequences that climate change will have on the seas around Scotland.