One of the most startling changes in the behaviour of the world’s oceans took place in the winter of 2009–10 in the North Atlantic. Here, the circulation is driven by warm water moving northwards, the ocean’s water sinking at high latitudes and returning as dense, cold, deep water towards the equator, a phenomenon known as the overturning circulation.
That winter, the circulation dropped by about 30 per cent. ‘That was quite a dramatic dip, outside the range of variability that climate models show,’ says Professor Meric Srokosz of the National Oceanography Centre (NOC) in Southampton. The circulation recovered a year later, but even so, the pace of the circulation has slowed by 15–16 per cent during the past four years, compared to the four years before that.
The discovery that the overturning circulation current is changing quite significantly – and is much more variable than scientists had previously thought – has prompted research into whether anthropogenic climate change is behind its seemingly stuttering momentum. One theory being proposed is that melting Greenland glaciers, warmed by higher air temperatures, are adding fresh water to the ocean, which in turn means that ocean water is less dense and saline and doesn’t sink – and so the strength of the overturning current is reduced.
Srokosz stresses that he and colleagues need to be guarded in their conclusions at this stage. ‘Whether the fluctuations can be attributed to climate change is difficult to state because of the short length of the observations, but we may be seeing some indication of climate change impacts,’ he says. ‘If you start melting Greenland, seeing more rainfall at higher latitudes, putting more freshwater from rain into rivers and you warm the ocean, then you get warmer and less saline water coming back south. The fact that it happened suggests that the ocean has some surprises in store for us.
‘There’s some evidence that this dip was associated with the very cold winters we had in 2010 and 2011,’ he continues. ‘This was a statistically significant dip, but we have only been looking at this for eight-and-a-half years, so we can’t say if this is typical or atypical behaviour.’
The changes in the overturning circulation were captured by the RAPID Watch array project in the Atlantic, which uses moorings to measure the strength of the overturning current along the 26°N line of latitude. Below this latitude, the water is warming up; above it, the ocean delivers heat to the atmosphere. The moorings track from Cape Verde, just south of the Canary Islands to Florida, and measure water movement in Sverdrups – a unit equivalent to one million cubic metres of water per second.
The study of the Atlantic Ocean is complemented by another gargantuan research project, Argo – named after Jason’s mythical Greek ship – which has been returning real-time data on temperatures and salinity since 2000. Argo involves more than 30 countries and has deployed more than 6,000 floats in the ice-free deep oceans around the world.
Readings are taken by fire-extinguisher sized processors – 3,000 of them – spread across the world’s oceans. They operate on a ten-day cycle, dropping from the surface to a depth of 2,000 metres before transmitting data on temperature, salinity and circulation of the global oceans to a passing satellite. In October 2012, Argo took its one millionth reading.
Data collected from the Argo programme has proved useful for other studies. In 2012, Dean Roemmich, professor of atmospheric science and physical oceanography at the University of California, San Diego, compared Argo’s findings with those of HMS Challenger, which took about 360 ocean-temperature measurements during a 115,000-kilometre voyage between 1872 and 1876 to all the world’s oceans except the Arctic. The comparison showed a 0.59°C temperature rise at the ocean’s surface over the past 135 years.
Even though the warming amounts to only a fraction of a degree, it’s important to consider the difference between heating air and water. Srokosz points out that while air can be quickly warmed by, for example, a hair dryer, a bucket of water requires the addition of much more energy before it warms.
Indeed, according to the latest IPCC report, ocean warming dominates the increase in energy stored in the climate system, accounting for 90 per cent of the energy accumulated between 1971 and 2010. The report states that between 1971 and 2010, the heat content of the upper ocean increased at a ‘likely’ rate of 137 terawatts. The strongest warming is found ‘near the sea surface (more than 0.1°C per decade in the upper 75 metres between 1971 and 2010), decreasing to about 0.015°C per decade at 700 metres’.
‘Argo has contributed to our understanding of the impacts of anthropogenic climate change,’ says Srokosz. ‘There’s talk about the supposed “hiatus” in global warming. But much of the heat is going into the oceans rather than the atmosphere and that warming is almost certainly down to anthropogenic climate change.’
Sea levels are also, unequivocally, rising. According to the IPCC report, tide gauge records and, since 1993, satellite data indicate that between 1901 and 2010, global mean sea level rose by 0.19 metres. The IPCC also says that it’s ‘virtually certain’ that the rate of global mean sea-level rise has accelerated during the past two centuries, and ‘very likely’ that between 1901 and 2010, the mean rate was 1.7 millimetres a year, increasing to 3.2 millimetres a year between 1993 and 2010.
One of the primary sources on which the IPCC was able to draw for its conclusions was the data collected by the Permanent Service for Mean Sea Level, which was established in 1933 and is responsible for the collection, publication, analysis and interpretation of sea-level data from the global network of tide gauges. It’s based in Liverpool at the NOC, whose Dr Svetlana Jevrejeva was a lead author for the IPCC fifth assessment report’s chapter on sea-level change.
‘From our studies using instrumental records, we know that present-day sea-level rise started about 200 years ago,’ says Jevrejeva. ‘During the 19th century, sea-level rise was about six centimetres and during the 20th century it was about 20 centimetres.
‘We know that there are two main contributors: changes in the thermal expansion of the ocean and the melting of glaciers and ice sheets,’ she continues. ‘We have been able to attribute one third of sea-level rise since 1993 to the changes in ocean heat content [as water warms, it expands], and almost two thirds to the melting of glaciers and ice sheets.’
Acidification of the ocean as a result of climate change is another area of increasing concern. Dr Alex J Poulton, a research fellow at the NOC, explains: ‘Oceans absorb around 25 per cent of anthropogenic carbon. As you put carbon dioxide into the ocean, it forms carbonic acids and carbonic ions, a process that influences the pH. You are changing the concentration of hydrogen ions at a very rapid rate. If you change the content, you will dissolve calcium carbonate much faster, and that affects bivalves and phytoplankton. It’s just basic chemistry, just like the basic physics of throwing an apple in the air and it coming down.’
The IPCC report reinforces these observations. It concludes that ‘it is very likely that oceanic uptake of anthropogenic CO2 results in gradual acidification of the ocean. The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26 per cent increase in hydrogen ion concentration.’
RATE OF CHANGE
Acidification is clearly taking place, but scientists remain unsure exactly what the effects will be. ‘We know that the pH of the oceans is changing and CO2 is going into the atmosphere and that the two are connected. It’s the knock-on effects that are uncertain,’ says Poulton.
‘But the changes that are happening are pretty sharp,’ he continues. Indeed, analysis of ocean acidification over the past 300 million years published in Science in 2012, suggests that the rate of change that the oceans are currently undergoing is unprecedented over that period. The most comparable event, which took place 55 million years ago, was linked to mass extinctions of calcareous deep-sea organisms and significant changes to the surface ocean ecosystem. Although the rate of change of ocean pH during that event was rapid, it’s thought that it was ten times slower than the current rate.
Acidification is already causing problems for many creatures. A growing body of research is cataloguing the effects that increasing acidification has on fish and invertebrates. Of particular concern is the effect that it has on coral reefs.
Acidification leads to a shortage of a carbonate mineral known as aragonite, which corals use to build their skeletons. Already, about 60 per cent of coral reefs are surrounded by waters that have less than adequate aragonite concentrations. As more CO2 enters the oceans, that figure will only get worse, potentially destroying reefs and the ecosystems that depend on them.
The effects are particularly pronounced in polar regions, essentially because cold water can absorb more gas than warm water. ‘As sea ice melts, more carbon dioxide is absorbed into the water and this inhibits the ability of fish and mussels to make calcium for their bones and shells,’ says Professor Tom Arnbom, an Arctic biologist with WWF. ‘Research is now showing that this is going to become a big issue.
‘It is going to happen across the planet, but the Arctic will see extreme changes first,’ says Poulton. ‘If you’re phytoplankton, you may be able to float away. But if you’re a coral, you’ll just have to deal with it. Corals are already dealing with temperature changes, so if we add in ocean acidification, the potential for them to struggle is obvious.’
Organisms living in Antarctic waters face similar problems. A study published in Nature Geoscience in 2011 reported that in parts of the Southern Ocean, the shells of pteropods – small marine snails that are a key species in the local food web and play an important role in the oceanic carbon cycle – are already dissolving. The snails, which were collected in 2008 by a team led by Dr Nina Bednaršek of the British Antarctic Survey, showed signs of unusual corrosion in the outer layers of their hard shells.
And if all of this wasn’t enough to worry about already, acidification reduces the oceans’ capacity to act as a carbon sink.
Recent research, meanwhile, has linked the warming of the oceans to an increasing frequency of severe storms. Research by the Niels Bohr Institute in Denmark demonstrated an increasing tendency for cyclones to occur when the climate is warmer. The study, published in 2012 in the Proceedings of the National Academy of Science, showed that globally warm years have been associated with a significantly higher risk of extreme hurricane surges, such as that which engulfed the New Orleans area in 2005 during Hurricane Katrina.
The research team, led by Aslak Grinsted from the University of Copenhagen and Svetlana Jevrejeva from the NOC, calculated that extreme hurricane surges are twice as likely to occur in warm years than in cold years.
This story was published in the March 2014 edition of Geographical Magazine