A Major Puzzle for Scientists

In early 2004, oceanographers around the world were stunned by a confidential Pentagon report leaked to the press. The report stated that ocean currents in the North Atlantic, including the Gulf Stream (which grants Europe its temperate climate) could undergo a major transformation around the year 2010. This, the report said, could lead to a new ice age on both sides of the Atlantic, and to drought and famine around the globe. Most scientists see such an apocalyptic climate scenario as highly improbable. However, the incident did have the merit of highlighting two basic facts. Firstly, that our planet's climate is controlled mainly by the oceans, which cover 70% of its surface. And secondly, that ocean conditions are changing at an astonishing speed. As the oceans continue to warm up and sea levels continue to rise, there will soon come a point where our current climatic conditions will become untenable.

Like their counterparts throughout the world, CNRS scientists are working overtime to gather data from the oceans and incorporate it into their models back in the lab. Their primary objective is to understand how the oceans are changing and the impact this will have on climate. And this works both ways. How will climate change affect the major ocean currents that convey heat around our planet? Will the oceans still be able to absorb a large part of man-made carbon emissions? Will the methane trapped in the deepest parts of the ocean floor be set free and discharged into the atmosphere? What follows is an overview of what we know, and the major questions that have yet to be answered.

 

 

what future for the GULF StrEAM?

The oceans absorb nearly two-thirds of the solar radiation that falls on the Earth. Ocean currents, such as the Kuro-Shio in the Pacific or the Gulf Stream in the Atlantic, then transmit this heat to all parts of the world. Scientists liken this heat-transfer mechanism to a giant conveyor belt. “When warm surface water gets to the North Atlantic, it cools down considerably. It therefore becomes denser, which makes it sink towards the bottom of the ocean,” explains Herlé Mercier, senior researcher at the Ocean Physics Laboratory in Plouzané (Brittany).¹ This sinking water is then replaced by more surface water, which is what drives the Gulf Stream and, at a more global level, the ocean conveyor belt, more properly known as thermohaline circulation. So could this sinking mechanism slow down or even stop due to a loss of density caused by an influx of fresh water, as predicted by the Pentagon scenario? “No,” says Mercier, “simply because water temperature is not the only factor that drives ocean currents. Nonetheless, it could slow down the Gulf Stream by about 25% by the year 2100.” But this slowing down could itself be countered by another likely consequence of warming: increased evaporation, which makes seawater saltier, and hence denser.

Another reason why the Pentagon's alarming scenario is unlikely to happen is that the Gulf Stream is also driven by wind. This wind carries water northwards, thereby replacing water that has been pushed southwards by the trade winds. So the Gulf Stream isn't about to come to a halt, unless the winds cease to blow, an unlikely scenario.

Nonetheless, the recent history of the Earth shows that there is a real possibility that the current could slow down. For instance, “around 17,000 years ago, massive melting of glaciers in the northern hemisphere, ranging from present-day Canada to the Arctic, led to major disruption of the Gulf Stream and hence of the climate,” points out Jean-Claude Duplessy, senior researcher at the Environmental and Climate Sciences Laboratory (LSCE)² at Gif-sur-Yvette. “A similar scenario occurred about 8200 years ago, though it was not as dramatic as the first example. In both cases, these events were connected to the end of the last glacial period.” Understandably, this makes it quite difficult to draw any conclusions about the future. As Duplessy explains, “global warming caused by human activity has no historical parallels. However, it's likely that if it continues at the same pace, leading to the melting of the Greenland ice sheet and with the predictable impact that this will have on oceans and currents, then the whole of the next climate cycle will bear the stamp of human activity.” In other words, we could be talking about the next 100,000 years...

 

 

What climate if the pack ice melts?

Another key factor controlling climate is sea ice, or pack ice, which refers to vast areas of ocean covered with ice. Today, 14 million km2 of the Arctic Ocean is covered with pack ice in winter. In summer, this surface is halved, but still covers an area about 13 times that of France. Pack ice has a huge impact on climate for two reasons. “First of all, it makes an excellent insulating material and curbs heat exchange between the ocean and the atmosphere,” indicates Jean-Claude Gascard, researcher at LOCEAN³ in Paris. “What's more, it has a very high reflectivity, or albedo. It reflects 90% of solar radiation back to space, compared to 30% for ice-free oceans.”

All current models agree that within a few decades, Arctic pack ice could disappear altogether in summer. Gascard is the coordinator of the European integrated project Damocles, whose mission is to gather the much-needed data currently lacking. In September, the vessel Tara was intentionally trapped in pack ice and set to drift in it for two years before being freed in summer 2008 close to Spitsbergen, northeast of Greenland. The data collected will be used to assess the interactions between different climatic parameters, in particular feedback mechanisms which make it extremely hard to make reliable global predictions. For instance, as Gascard wonders, “given ongoing climate warming, will there be an increase in evaporation, and hence in cloud cover and precipitation in the Arctic? That might partially make up for the loss of pack ice, since some clouds have very high reflectivity.” Answers to these questions will no doubt give a clearer picture of tomorrow's climate.

 

 

ARE ATMOSPHERE/OCEAN EXCHANGES GOING HAYWIRE?

Today, we are well aware that our planet's climate depends on exchanges of heat and humidity between the oceans and the atmosphere. For instance, the oceans are the main source of atmospheric water, and therefore of clouds. Futhermore, water vapor is also the most abundant greenhouse gas. “But the main point is that oceans have high thermal capacity, which means that in winter in particular, they remain warmer than land masses,” explains Laurence Eymard, director of Locean. This surplus warmth, which is especially significant in tropical regions, is partially returned to the atmosphere through evaporation from the ocean surface, thus bringing warmer winters to Europe. “The surface temperature of the ocean is the main regulator of exchanges with the atmosphere,” Eymard points out. “And the slightest anomaly can have major side effects: A localized temperature increase of 1°C at the ocean surface can cause abnormal winds over a distance of nearly 100 km!” A famous example of this is El Niño which is partly caused by the eastward movement of a large mass of warm water in the Pacific Ocean. This effect is also illustrated by the 80 or so tropical cyclones which form every year because of heat building up at the ocean surface. Or by monsoons, seasonal phenomena marked by intense rainfall in Asia, Africa, and even Australia, which, as Eymard explains, are caused by “the great difference in temperature between the ocean and the continents in early summer, with land masses that have a tendency to overheat.” However, in the last thirty years, the monsoon patterns have changed considerably, with, in particular, a lack of rainfall in Africa which has caused a severe drought in the west of the continent. The AMMA program⁴ should enable us to get a better understanding of the changes taking place in Africa. “Human activity has already led to the warming of the ocean surface,” explains Laurent Terray, deputy director of the Earth and Astronomical Sciences Unit at the European Center for Research and Advanced Training in Scientific Computation (Cerfacs)⁵ in Toulouse. “And this can alter the monsoon circulation and affect the hydrological cycle on adjacent continents.”

A final illustration of this phenomenon is provided by the Atlantic Multidecadal Oscillation (AMO). “This results in a great difference in sea surface temperatures between the North and South Atlantic,” Terray explains. “And we have shown that the AMO was responsible for 15% of the near 1°C temperature rise that has been observed in France since the 1990s.” This is a significant finding, since modeling ocean-atmosphere exchanges is no easy task. As Eymard points out, “we understand most of the processes on a small scale, but in order to generalize these processes for the entire planet, we need to improve our models and acquire more data.”

 

 

WILL OCEANS still trAP CARBON?

 

“If it wasn't for the ability of oceans to trap carbon, the concentration of carbon dioxide (CO2) in the atmosphere would be much higher,” observes Nicolas Metzl, a researcher at Locean. The situation is already critical. In January 2006, Metzl measured an atmospheric concentration of 380 parts per million (ppm) of CO₂ (one of the main greenhouse gases), as compared with the 280 ppm estimated at the dawn of the Industrial Revolution. And the models are predicting a concentration of 700 ppm by the end of the century. “Every year, humans release between six and seven billion tons of carbon into the atmosphere,” Metzl confirms. “And at least one third of it is absorbed by the oceans.” A total of 39 trillion tons of carbon is locked up in the oceans, as compared to 600 billion tons in the atmosphere and 610 billion tons in the terrestrial biosphere. Here again, exchanges between the atmosphere and the oceans take place continuously. On the one hand, there are “sources,” i.e., areas of the ocean that regularly release CO₂. And on the other, there are the “sinks,” other areas that absorb CO₂, and which are overall more substantial than the sources. These exchanges work in two ways. First, there is the “biological carbon pump,” whereby CO₂ is extracted from the air by plankton and deposited on the sea floor. But the major player in limiting the build-up of carbon in the atmosphere is the “physical carbon pump.” “When tropical surface water flows up to high latitudes, it cools down,” explains Laurent Bopp, a researcher at LSCE. “Carbon dioxide is more soluble in cold water than in warm water. So large quantities of CO₂ are absorbed by this water before it sinks to the bottom.” After a long period of time, part of this carbon is returned to the atmosphere when this deep water eventually mixes with surface water. This cycle is the subject of considerable research all over the world, with the aim of identifying and studying sinks and sources. Until now, researchers thought that the main sink for carbon released by human activity was the North Atlantic. But first results from various studies like the OISO campaigns⁶ coordinated by Metzl, show that similar quantities of carbon are locked up in certain areas of the Southern Ocean.

However, other findings are even more worrying. “We've recently discovered that during certain years, the carbon sink in the North Atlantic turns into a carbon source, mainly due to the warming of surface water,” Metzl reports. Should we conclude that the ocean carbon pump is beginning to fail? “At first, the oceans adapt to the increase in atmospheric CO₂ concentration, and absorb more and more carbon,” Bopp replies. “But our models predict that the pump will become less efficient over the next few decades.” In fact, the amount of carbon absorbed could fall by as much as 25% by 2100. This is partly due to global warming, since carbon dioxide is less soluble in warmer water. But this warming also increases the amount of fresh water at high latitudes, which causes a greater “layering” of the oceans. “There will be fewer interactions between the various layers of the ocean. As a result, the surface layers won't mix as easily with the underlying layers, which will reduce the amount of carbon that the deep ocean can potentially absorb,” Bopp explains. However, once again there are several overlapping phenomena which complicate the issue. For instance, this layering also means that there will not be as much deep carbon-rich water rising to the surface, which will reduce the amount of carbon released into the atmosphere by ocean sources. Given such conditions, it is quite difficult to make accurate predictions about the net result of these multiple climate-induced alterations.

 

 

could we RUN SHORT OF PLANKTON?

 

In our oceans, carbon is also a material used for “primary production,” the phenomenon whereby chemical elements present in water (carbon dioxide, nitrogen, and phosphorus) are transformed into living organic molecules in algae and cyanobacteria, whether planktonic or bottom-dwelling. “If this biological pump didn't exist, there would be 30% more CO₂ in the atmosphere,” points out Stéphane Blain, a researcher at the Oceanography and Biogeochemistry Laboratory (LOB)⁷ in Marseille. This is because photosynthesis, the process responsible for phytoplankton growth, uses up enormous quantities of carbon. The food chain does the rest. Phytoplankton is the favorite food of zooplankton, and part of the carbon which sinks to the bottom as organic waste ends up incorporated into sediments on the ocean floor. However, the nature of the organic pump may be about to change. “Two worrying new facts are emerging,” states Paul Tréguer, Director of the European University Institute for Marine Studies (IUEM)⁸ and Scientific Director of the Eur-Oceans network of excellence for ocean ecosystem analysis (which links together 66 European institutes working in this area). “Firstly, due to global warming, a great number of planktonic species, adapted for living in temperate waters, are migrating northwards, taking their predators with them,” which is upsetting the balance of ecosystems. “Secondly, the buildup of CO₂ in the oceans is a serious threat to the biological pump,” adds Metzl. “This is because it is making oceans more acidic and reducing the concentration of carbonate ions in seawater,” which is endangering many species of plankton. This very rapid acidification, which is already underway, could modify the biological mechanism that drives the carbon pump.

 

 

Given the seriousness of the situation, an increasing amount of research is being carried out. Together with his colleagues on the KEOPS mission, Blain is attempting to understand mechanisms of phytoplankton blooms in certain areas of the Southern Ocean, where the biological pump is highly inefficient even though the water contains abundant nutrients. According to Tréguer, “the relative absence of phytoplankton can be explained to a large extent by the fact that the water is deficient in iron, which is an essential element for efficient growth.” Which is why some industrialists, keen to continue their CO₂-producing activities, are proposing to boost the biological pump by seeding the oceans with iron. “It's a dangerous idea and it certainly wouldn't work,” disputes Blain. “First of all, the fact that biological activity also releases other greenhouse gases, including nitrous oxide (N₂O), into the atmosphere shouldn't be forgotten. So artificially speeding up the biological pump wouldn't necessarily be a positive thing. Furthermore, if it was done on a large scale, the chemical and biological equilibrium of the ocean would be completely disrupted.”

 

 

A Deeper threat?

 

 

Finally, a substance buried in ocean sediments could potentially act as the biggest threat to our climate. Methane, trapped in vast quantities in the form of icy methane hydrates, is twenty times more potent as a greenhouse gas than CO₂. “It is estimated that there are between 500 and 2500 billion tons buried in the oceans,” explains Jérôme Chappellaz, researcher at the Laboratory for Environmental Geophysics and Glaciology (LGGE)⁹ in Grenoble. “But the 5 billion tons of methane already present in the atmosphere accounts for 20% of the human-induced greenhouse effect just on its own.” So even a partial degassing of oceanic methane could disrupt the atmosphere for several decades. “In theory, warming of the oceans could cause part of the gas hydrates to melt,” says Benoît Ildefonse, researcher at the Montpellier Tectonophysics Laboratory¹⁰ and chairman of the French scientific committee in the Integrated Ocean Drilling Program (IODP). And there are two possible scenarios for this occurrence. “Either degassing takes place slowly and gradually, giving most of the methane enough time to dissolve in the ocean; or else the eruption is violent, triggering radical atmospheric changes in just a few months,” explains Chappellaz. Data from polar ice cores show that degassing occurred slowly in the last 150,000 years. However, specialists continue to blame oceanic hydrates for the massive rise in temperature that occurred 55 million years ago, and which took 200,000 years to be rectified. So what does the future hold? “There are two opposing effects,” according to Chappellaz. “Warming, of course, which could destabilize the hydrates. But also rising sea levels, which would increase pressure at depth and tend to stabilize the hydrates reservoir.”

The most recent revelation about the perilous relationship between oceans and climate concerns submarine mud volcanoes, which may also release methane into the atmosphere. A recent study¹¹ carried out by Ifremer showed that one of them, the Håkon Mosby volcano, located in the Barents sea between Norway and the island of Spitsbergen, intermittently emits large amounts of methane, much of which may be released into the atmosphere. In any case, we are far from understanding the numerous ways in which oceans and climate interact. Much more research will be needed to refine predictions and find ways of dealing with the consequences of the most catastrophic among them.

 

Matthieu Ravaud