In recent years, there is evidence that human activities are altering the planet’s climate. According to the IPCC (2013), ocean warming represents approximately 93% of the increase recorded on Earth between 1971 and 2010, with 64% of the total corresponding to the upper layer of the ocean (0 to 700 m). An increase in the salinity of mid-latitude surface waters has also been reported, as well as a decrease in oxygen concentrations in the thermocline in many regions of the ocean since the 1960s. Recent observations have strengthened the evidence of variations in the main ocean circulation systems, and have been detected by evaluating each year or longer periods (decades). In addition to this, warming of the first 700 m of ocean depth has contributed to sea-level rise by an average of 0.6 mm yr-1 since 1971, and the magnitude of extreme high-sea-level events has increased since 1970.
The effects of climate change are already visible in terrestrial and marine ecosystems around the world and are manifested as changes in the distribution of species and their phenology. Future consequences for ecosystems are predicted to be devastating, including mass extinctions and changes in ecosystem functioning.
Long-term studies have confirmed the existence of a consistent pattern of changes in many ecosystems around the world, related to climate change. Although examples can be found in almost every ecosystem on the planet, the changes in marine systems have been very evident.
Coral reefs are one of the marine ecosystems most threatened by global change due to their massive bleaching related to the increase in sea temperature. In addition, large-scale climate-related changes have occurred in pelagic ecosystems in the North Atlantic and North Pacific. In the south-west of Great Britain, catches of herring, a cold-water species, and of sardine, a warm-water species, have been shown to be related to climatic fluctuations over the last four centuries, with herring dominating in cold periods and sardine in warm periods. The abundance and distribution of cold- and temperate-water plankton species have also varied with temperature over the past seventy years (Southward et al., 1995). The occurrence of events, such as the migration of squid and flounder, have shown a correspondence with increases in water temperature, driven by the North Atlantic Oscillation that may be changing with the climate.
In Galicia, as in the other regions of the planet, the effects of climate change are already quantifiable. The first Report for Climate Change of the Xunta de Galicia (2012) indicates that there has been an increase in warm water species, both in plankton and in fish, and a continued increase in the production of “elongated” barnacles, without commercial interest. Likewise, it is noted that the growth rate and quality of the mussel have shown a decline in recent decades, while the extraction period has been reduced due to the increase in the presence of harmful microalgae. Likewise, it is noticed that the Iberian sardine population has shown a long-term decrease and that octopus catches have been significantly reduced, due to changes in the frequency and intensity of the winds in the periods of upwelling and subsidence. The report also indicates that there has been a decrease in the duration of the upwelling by 30% and by 45% in its intensity, probably contributing to doubling the renewal time of the estuaries and reducing the net production of the continental shelf by 50% and of the estuaries by 15%.
