The sea around Shetland has warmed to such a degree that it is now at a temperature once considered typical for the south of England.
During almost the whole of the 20th century the waters close to Shetland rarely crept above 13°C but from 2000 to 2008 it happened five times. Similarly, in winter the temperature rarely rose above 8°C – happening only six times between 1900 and 1985. Now it rarely falls below that level, occurring just once between 2000 and 2009.
The statistical picture of warming waters comes from scientists engaged by the Shetland Oil Terminal Environmental Advisory Group, known as SOTEAG, which has as its core purpose monitoring for any effects of oil pollution from Sullom Voe.
As a by-product, the 35 years of study have built up a mass of data on environmental trends thought to be the only record of its kind in the UK, including analysis of the sea temperature at the surface.
As well as waters close to Shetland, including around Fair Isle, SOTEAG has data for a larger area of sea, taking in Orkney and the north coast of Scotland. The figures for that area show that in the decade from 1993 to 2003 alone the average sea temperature in the summer rose by 2.7°C.
From 1980 – which was the coldest year for sea temperatures in recent decades – to 2006 (the warmest year), the average winter sea temperature increased by 2°C, or almost 0.8°C per decade.
While warmer waters could present new economic opportunities, such as farming high-value turbot, they could prove short-lived because nobody can be sure the cycle won’t go into reverse. The 1930s and 1940s were relatively warm around Shetland too before a colder period from the 1950s to the mid-1980s prior to the warming seen since.
SOTEAG’s Climate Change Report 2010 states: “It is clearly apparent that sea-surface temperatures in the waters around Shetland have increased over the last century. There has not, however, been a continuous increase, with two periods of warming having taken place separated by a period of cooling.”
It continued: “The overall trend has been upwards and the last decade or two have witnessed the warmest sea surface temperatures of any time since 1900.”
Around the UK coast, sea temperatures have increased by 0.5 to 1°C since the 1870s compared with about 1°C around Shetland.
One of the scientists involved with SOTEAG, Professor Alistair Dawson of the Aberdeen Institute for Coastal Science and Management, warned against branding the phenomenon “climate change” because the climate changes all the time.
He said: “If climate change was the cause of the rise then it is also the cause of the fall. Are we therefore to say that the climate change that people are worried about is only the climate change that has been taking place in the last two decades?
“It is known that around about the time of the Second World War it was also quite warm then. But then again 100 years ago it was exceptionally cold.”
He warned that the consequences of these complex changes, influenced by currents and pathways in the sea and air pressure and jetstreams, could be “wide-ranging”.
Rising sea temperatures have been implicated in a number of changes in fish behaviour around Shetland, including the shortage of sandeels which hit seabird colonies. It may also account for poor cod breeding, the appearance of John Dory in local boats’ catches in recent years and the unprecedented abundance of small pipefish in coastal waters.
As Professor Dawson said: “Fish are no mugs – they don’t just drift around aimlessly. They respond to changes in the quality and temperature of the water.”
SOTEAG observed a dramatic increase in the abundance of warm-water plankton species around Shetland after the mid-1980s, which affects other sea life.
On top of the long-term trend of the sun warming the waters in the North Atlantic, the seas are affected by cycles of the Atlantic Multidecadal Oscillation, which swings between warmer and cooler cycles lasting between 20 and 40 years. The AMO is thought to be a natural phenomenon driven by changes in the circulation of the North Atlantic Ocean.
Another factor affecting a huge area including Shetland is the North Atlantic Oscillation (NAO), which relates to atmospheric pressure at sea level. It has been changing over the last 30 years from a period which tended to mean mild winters with increased rain and storminess during the 1980s to reduced storminess but colder during the last decade.