Oceans : The Key to Understanding

Climate Change

 

The oceans have never been monitored as closely as they are today. Lots of data are continually being gathered from oceanographic missions, satellites, networks of buoys, and sediment cores. They are archived and then synthesized using increasingly realistic simulations. Why so much effort? Simply because oceans are one of the best tools we have for understanding not only the present climate and how it is likely to change, but also variations in past climates. This is because oceans are very sensitive to alterations in the atmosphere which drive changes in temperature, pH, and salinity, and in return, they alter Earth's climate. Indeed, the first effects of global warming can already be observed in our oceans.

The severe storms which hit Europe in 1999, the unusually intense El Niño events, and hurricane Katrina's devastation of New Orleans are without doubt the most striking signs of possible ongoing climate change. What's more, for a large audience, these extreme weather phenomena are often seen as proof that climate change is already upon us. It's up to scientists to show that things may not be that clear-cut. And the data they acquire on oceans through the launch of numerous observation campaigns and data-enriched models may help them to do so. Take the case of El Niño, a climate anomaly which occurs every three to seven years and causes a reversal of winds and surface currents in the Pacific. The El Niño which showed up in 1997 was one of the most powerful ever recorded and had disastrous consequences. There were uncontrolled forest fires in Indonesia, devastating rains in Central and South America, drought in Northeastern Brazil, and so on. Yet according to Pascale Delecluse, “it isn't at all clear that global warming is having an effect on El Niño. We can't tell whether the 1982-83 and 1997 events were strengthened by global warming or triggered by it.” In climate change modeling, which uses large amounts of oceanic data, no clear trend emerges from the various simulations. The mechanisms that trigger El Niño are still too poorly understood for us to be able to model them accurately.

Current operating models have recently shown that there is no evidence that the number of tropical cyclones will increase in the Atlantic and Caribbean. “We have run simulations based on the global warming predicted for the 21st century, and nothing points to an increase in frequency,” says Jean-François Royer, a researcher at Météo France. “Cyclones are triggered by temperature differences between the lower and upper atmospheres. Future warming is unlikely to bring significant change to the temperature gradient.”

However, these simulations do predict that these devastating hurricanes might become more powerful, a prediction backed up by a recent article in Geophysical Research Letters.¹ So, is the humbling power of recent tropical cyclones Katrina, Emily, and Rita a sign that this is already happening? Other results published in 2005 stated that the number of category 4 and 5 tropical cyclones had increased by 57% between 1970 and 2004. But Royer is not convinced. “Personally, I believe that the series used in this study are still too short to draw any solid conclusions,” he says.

 

 

the rising sea levels

The oceans are also sending us signals, which, although perhaps less spectacular, are clearer indicators of climate change. One of them is sea level rise, which has the inhabitants of Tuvalu extremely worried: They think that their archipelago, north of Fiji in the Pacific, is about to be engulfed. According to Anny Cazenave, deputy director of the Laboratory for Studies in Spatial Geophysics and Oceanography (Legos)², things are more complex but no less worrisome for these low-lying atolls. “The average rise of sea levels around this group of islands is only a few millimeters per year. However, phenomena such as El Niño can cause sudden variations in sea level of as much as 20 centimeters. Eventually, these islands will no longer be inhabitable,” predicts Cazenave.

 

For the last fifty years, average sea levels have been rising by 1.8 mm per year, but this rise has increased to 3 mm per year in the last twelve or so years. According to scientists, about 60% of this rise is due to the thermal expansion of the ocean, which, just like the atmosphere, is getting warmer. The remainder is caused by the melting of mountain glaciers (0.8 mm per year), the melting of the Greenland ice sheet, and to a lesser extent that of the Antarctic (0.2 to 0.4 mm per year). “This rise is not homogenous across the world, because the warming is not uniform. It depends on the heat transferred by ocean circulation,” explains Cazenave. These findings are based on data mainly obtained by satellites (including Jason-1, Envisat, and soon Jason-2), which have revolutionized the study of oceans and climates. Their altimeters make it possible to map the level of the oceans with great accuracy. Researchers also use tidal range measurements reliable over the last fifty years, but most of them are restricted to ports and harbors, and a few coastal areas.

While there is no doubt that the soaring greenhouse effect is contributing to the increasing rate of sea level rise, it is quite difficult to understand how much of it is due to natural variation. Furthermore, we still lack altimeter readings over a sufficiently long period to gauge long-term trends.

 

 

FRESH WATER VERSUS SALT WATER

There is yet another sign of global warming visible in the oceans: the melting of ice at the poles. While the Antarctic ice sheet and sea ice appear to be standing firm, the retreat of sea ice in the Arctic (see p. 20) is already quite apparent. “We are also quite concerned about the Greenland ice sheet,”' warns Frédérique Rémy, a researcher at Legos. “While the ice layer round the edges of the island is getting thinner, the central part of the island is getting thicker due to increased precipitation. The problem is that as the slope gets steeper, the ice starts to flow faster. Many researchers believe that glacial retreat could spiral out of control.”

And a disruption of the Arctic system would have major repercussions on the salinity of surface water in the North Atlantic. “For the time being, the melting of the Greenland ice sheet isn't contributing to much fresh water compared with the Siberian rivers,” points out Gilles Reverdin, a researcher at LOCEAN. The salinity of surface water is closely studied by oceanographers. It depends on precipitation, surface currents, and exchanges with deep water layers. And, according to Reverdin, “an important change in salinity in the North Atlantic could have a negative impact on the entire thermohaline circulation, such as slowing down the Gulf Stream.” This is because water that is less saline is also less dense, whereas it is precisely the density gain that explains the sinking of the Gulf Stream at high latitudes.

Since the 1970s, scientists have observed a reduction in salinity in the Atlantic at latitudes above 45°N. But the trend has been reversed in the last five to ten years. Is this a natural variation or the first signs of climate change? In a constantly fluctuating system, it is difficult to be sure. At least the instruments required to collect this data are now up and running, thanks to a coordinated international effort. Networks of buoys such as Pirata³ between the coast of Brazil and Africa have been monitoring temperatures and salinity since 1997. In 2001, the Argo network,⁴ which uses even more sophisticated instruments, was deployed. It uses “profilers,” which float at a depth of 1000 to 2000 meters and rise to the surface every ten days to measure various physical parameters all the way up the water column. This is a highly efficient tool for oceanographers and modelers, who only have to wait for series of data on a long enough time period to measure the consequences of human activity.

 

 

AN INCREASINGLY CORROSIVE OCEAN

Another phenomenon observed by scientists is the increasing acidification of the ocean, a direct consequence of CO₂ emissions caused by relentless human activity. Although the influence of CO₂ on the climate is still hard to estimate, the acidification of the sea, which obeys basic chemical equations, offers more straightforward measurements. In 2005, an international team simulated the ocean's drop in pH levels over the current century using various scenarios for carbon dioxide emissions. The results are overwhelmingly clear. Within fifty to a hundred years, the ocean will become corrosive for many organisms. Aragonite, a type of limestone, will have become soluble in sea water, which will lead to the disappearance of animals such as pteropods, a group of planktonic molluscs whose shells are made of this material. “The increase in atmospheric concentrations of CO₂ has already led to a 0.1 fall in pH levels. In 2100, it will have fallen by 0.4,” according to James Orr, a researcher at LSCE.

This will have far-reaching consequences for biodiversity. For example, pteropods are a basic element in a large number of food chains at high latitudes, and whales, cod, and young salmon eat enormous amounts of them. “With a higher CO₂ content in water, there will be less available carbonate to make limestone. Even if the threshold where limestone dissolves isn't reached, organisms will have to invest a lot more energy to produce their exoskeleton, making them more fragile. Tropical corals, whose biodiversity is comparable to that of equatorial forests, are greatly endangered. The situation is even more serious for cold-water corals: In less than a hundred years, two-thirds of these reefs will be located in water that is corrosive to them,” warns Orr. Can this damage already be observed? “We're not sure. We don't have much data, and what we do have is mostly qualitative, but some researchers think that corals are already more fragile than they used to be.”

 

 

VOICES FROM THE PAST

Hurricanes, rising sea levels, melting ice, and acidification all provide us with direct information about ongoing climate change, but the ocean is also an excellent tool for investigating past climate change. “It's clear that if we want to understand the human impact on global warming and its consequences, we absolutely need to understand past events,” says Édouard Bard, a paleoclimatologist at the European Center for Research and Education in Environmental Geosciences (Cerege)⁵. We have much to learn from witnesses of these past climate upheavals, corals, shellfish, diatoms, or coccolithophores. Interpreting the information they hold is no easy matter. But although it is often fragmentary, sometimes ambiguous, and requires considerable effort to extract, it is the only long-term memory of the ocean, and an essential tool for paleoclimate reconstructions and accurate past-event and upheaval modeling.

An increasingly popular technique is core drillings from marine sediments. By using core samples drilled in the Indian Ocean and the East Pacific, Bard and his team have shown that in the past, when Atlantic deep water circulation was weak, there were major changes in both the salinity of surface water and the rate of oxygenation at intermediate depth in tropical oceans. During these periods, there were major changes in rainfall patterns in Central America and southern Asia, including a drying-up of the Indian subcontinent.

 

Using another core sample 40 meters long, drilled in 2003 during a campaign in the Southern Ocean, Xavier Crosta, a researcher in the Oceanic Environments and Paleoenvironments Laboratory (Epoc)⁶ in Bordeaux, is studying the siliceous skeletons of diatoms (single-celled algae abundant in cold seas). “The different species of diatoms have marked ecological preferences. It makes them very good indicators of temperatures, of the extent of sea ice, and of the nutrient content of surface water. Eventually, this core will give us information on the climate over a period of ten thousand years with a resolution of less than a decade. For the most significant and best-preserved periods, we should even be able to reach seasonal resolution,” states Crosta.

Relevant information can also be acquired through isotopic analysis of organic matter preserved in sediments. For instance, the ratio of carbon-13 to carbon-12 informs us on the amount of dissolved CO₂ in sea water and gives relevant information about primary production.

Among other organisms studied by researchers, corals are especially valuable. They hold information on sea level variations, while their growth bands provide temperature data on an annual time scale. Although poor temperature indicators, another group of single-celled algae–coccolithophores–are very good at revealing the degree of primary production in the ocean. “This group is particularly informative on carbon cycle. If primary production increases, more carbon is trapped by the oceans, which brings down the level of CO₂ in the atmosphere,” explains Luc Beaufort, a researcher at Cerege.

 

Coccolithophores in the Pacific revealed that primary production was high during glacial epochs, when the CO₂ content of the atmosphere was low. To complete their data on past climates, scientists are constantly on the lookout for new paleoclimate indicators. They are also trying to develop models to help them understand how environmental conditions are recorded in limestone produced by organisms. At the Laboratory of Sciences of the Marine Environment (Lemar)⁷ in Brest, researchers have managed to decipher the information contained in the shells of bivalves such as the scallop. Yves-Marie Paulet, a teacher and researcher at Lemar is enthusiastic: “This bivalve captures incredible amounts of data: It makes new growth lines every day. By calculating the ratio of oxygen-18 to oxygen-16 concentration present in its shell, we can obtain over 300 measurements of temperature per year, to the nearest half-degree. By using scallops found in prehistoric 'garbage dumps,' we've been able to reconstruct daily temperature curves for a time period several thousand years old.” So, with many more species to choose from (the Southern scallop found on the coast of Antarctica, for example), researchers won't run short of marine resources in their quest to decipher the variations of our climate.

 

Sebastián Escalón