Uncertain Surface Changes

Though the Earth’s interior has long captivated scientists’ curiosity, the thin shell that surrounds it–the Earth’s crust–is no less important. After all, this thin layer of skin, on average a mere thirty km thick beneath the continents and around seven km under the oceans, has provided humanity with everything it needed: water, farmland, oil, metals, and all the other natural resources which have accumulated over millions of years. But there are strings attached: this crust is also buoyant, fragile, brittle, permanently in motion (due to plate tectonics), and affected by the convulsions of the mantle. The crust on which we live probably began forming around 3.8 billion years ago, at the end of the Hadean, when the top of the mantle cooled down and solidified. Today, the Earth’s surface is being subjected to tremendous change, due not only to natural phenomena, but also to overexploitation by humans, and sometimes to a combination of both.

The Rising Waters
The Earth is two thirds water and one third dry land. But the outline of the shores hasn’t always looked like it does today. Approximately 18,000 years ago, sea levels were 120 meters lower than today. At that time, huge glaciers were found at very low latitudes. And then temperatures started to rise. As the oceans warmed and expanded, and the ice melted, sea levels started to rise very rapidly, by an estimated 100 meters in 12,000 years. It was only 6000 years ago that the rise in sea level finally began to slow. “That’s when the coastlines began to take their current shape,” says Bernadette Tessier, from the M2C laboratory in Caen.¹ “As the water rise abated (10 to 15 meters in the last 6000 years), sediments built up and the coastlines stabilized.” Past coastlines, like ancient climates, are recorded in core samples drilled out of the ground: “Sediments record the landscapes,” Tessier explains. The records go back a million years, revealing that the oceans have expanded and retracted every 120,000 years, in tune with the succession of glacial and interglacial periods.

Coastlines on the Move
Today, the effects of global warming are visible: sea levels are rising by three to five, or even ten mm per year, depending on the region. In the great river deltas where sediments build up, as in Bangladesh, the land is subsiding. Combined with rising sea levels, this speeds up the erosion process. On the other hand, in Scandinavia, the land, freed from the weight of the now-vanished glaciers, is rising back up so fast that the relative sea level is actually falling. “Coastlines recede due to the action of waves, which increase sea level locally and accelerate erosion.” The erosion is made worse by the storms that occur at high tide, and such storms are likely to become more severe due to climate change. This will probably increase coastal erosion, especially in regions of dunes and sand.
Humans also directly affect coastal landscapes. Agricultural drainage, polders, dikes, or river dams all retain sediments. Similarly, harbor infrastructures sometimes alter the transport of sediment to such an extent that erosion increases to a spectacular degree. “Because of this, the coasts of Togo and Mauritania–south of Nouakchott–are retreating by 25 meters per year in some places,” explains Franck Levoy, from the M2C laboratory. “When we look at sandy coasts–which make up 20% of the planet’s coastlines–two thirds are being eroded, while the rest are stable or advancing.”
To better understand these phenomena, researchers have powerful tools at their disposal. In Jacques Malavieille’s team, in Montpellier,² researchers are simulating erosion in “sandpits” equipped with moving floor panels and layers of variously colored sand. They use vaporizers to replicate the runoff, the movement of materials, and the formation of catchment basins and rivers, in order to understand how climate change will remodel the terrain.
Elsewhere, scientists are studying beaches recreated in wave channels in the lab, examining satellite images, or using LIDARs–airborne lasers that draw up relief maps–which will soon be operational in Levoy’s unit. All this data is then fed into computer models to, among other things, determine the places where it will be essential to act. Choices will have to be made. The current data, which predicts a general sea level rise ranging from 18 to 59 cm this century, needs to be acted on locally. Dunes and beaches can be consolidated when there is sediment available, as is already the case in the Netherlands. Seawalls can be raised. And sometimes, it will be necessary to accept the idea of landward retreat, as the UK is planning for certain coastlines.

Erosion of Landforms
Of course, coasts are not the only victims of erosion. Landforms are also being severely tested. “We’re trying to understand the effect of change in precipitation conditions,” explains IPGP’s François Métivier. “Annual means haven’t changed much, but the increasing number of extreme weather phenomena is affecting erosion. Does erosion increase tenfold or a hundredfold when ten times as much rain falls during one event?” Since such events are still quite unusual, “we have to go to places where exceptional events are commonplace, like the islands of Guadeloupe and Martinique, where we are drawing up erosion budgets and studying rivers,” Métivier explains. During a tropical storm, river flow can increase a hundredfold in just half an hour–and the transport of material is spectacular. “A typhoon is like a giant flush, “ says Malavieille. “In Taiwan, an island with a very rugged landscape and mountain peaks as high as 4000 meters, each tropical storm washes away everything in the valleys.” This is why it’s so important to understand these transport mechanisms. Not only in the lab–by reconstructing landforms and rivers–but also in the field, by setting up sensors that detect suspended material in river water, and by using radars, and even drones, small remote-controlled planes, that photograph the terrain they fly over.

Agricultural Methods
Field observation is also important to find out what’s happening on cultivated land. Farming practices can both reduce and increase erosion of the thin fertile layer of soil that covers the source rock. “We’ve seen that erosion can vary by a factor of 1 to 20 depending on the farming techniques used in vineyards,” says Yves Le Bissonnais, a researcher at INRA in Montpellier.³ “Chemical weed control, for instance, leaves soils bare and highly vulnerable.”
Four factors affect erosion: the natural agents of erosion (wind, precipitation), slope, the type of soil, and of course, vegetation cover. “A large share of the planet’s soils is under threat by being used for cultivation. In France, soils are relatively well preserved, since agriculture developed gradually, forming a patchwork of small fields separated by hedges and paths. On the other hand, in the US’ great plains, the sudden and widespread use of land for cultivation led to unstoppable soil movement. Soils were extremely degraded until soil conservation practices were adopted.” Le Bissonnais is particularly worried about the rapid development of sugar cane in Brazil, and oil palms in Asia, pushed by the growing demand for biofuels: “Clearing tropical forests is often carried out by controlled burning. The fire leaves the soils bare, and destroys the organic matter which acts as a real protective barrier against erosion. This causes the destruction of soils which took thousands of years to form, an irreversible process on human time scales.”
Through experience, researchers now know which farm practices prevent runoff, protect the fertile layer, and avoid fresh water pollution by fertilizers and pesticides. In Argentina and Brazil, for instance, no-till farming, a way of growing crops with limited soil disruption, is often used–which therefore limits erosion. “In Europe, when crops are rotated, we should develop catch crops, such as mustards, to avoid leaving soils bare in winter,” Le Bissonnais explains. Elsewhere, it would be sensible to divide fields to halt erosion, and develop agroforestry, which combines food crops and forests. Yet protecting wilderness areas is going to be difficult. The world’s population is still increasing: there are over 60 million more mouths to feed every year, which means an increase of 2.5 billion people by 2050, at which time the population should, in theory, level out. “The additional need for food in Asia is estimated at a billion tons per year,” warns Ghislain de Marsily, a researcher at the Sisyphe laboratory.⁴ “The problem is that 75% of the available land in Asia is already cultivated.” In other words, it will be necessary to grow food elsewhere. “South America, for instance, has a billion hectares of land available,” de Marsily points out, citing work by Michel Griffon at CIRAD. But this land could only be used by clearing the Amazon rainforest. Elsewhere, due in particular to global warming, the deserts are likely to gain ground, which is the case today in the Sahel, China, and Australia. It’s hard to predict what our planet will look like 50 years from now. But one thing is sure: it will look very different.

Denis Delbecq