Global warming is increasing the intensity and frequency of extreme weather phenomena, such as the recent floods in Spain. The cause is “cold drops” made more intense and more frequent by the destabilization of the polar jet stream. In recent days, a weather phenomenon called “DANA” (acronym for depresion aislada in niveles alto, or isolated high-level depression in French) has left heavy rain and flooding in its wake in Spain. The Mediterranean regions and Andalusia, particularly around Valencia, Castilla-La Mancha and in the Balearic Islands. The storm left dozens dead and missing and caused immense damage in the affected municipalities.
Fifty years ago, DANAs, which were then called “cold drops”, occurred three to four times a year, mainly in November. Today, they occur throughout the year. For what?
■ How are DANAs formed?
These storms arise in the same way as hurricanes in the Atlantic or typhoons in the China Sea, with the difference that in the Mediterranean, their trajectory is necessarily reduced. They therefore store less energy and water vapor. A few decades ago, the surface of the Mediterranean Sea was only warm enough to cause increased evaporation into the atmosphere in late summer. Today it is warm throughout the year: a huge amount of water vapor is constantly generated and rises to the upper layers of the atmosphere.
At the same time, polar areas are also much warmer than 50 years ago. As a result, the polar jet stream, the air current that surrounds the Earth at an altitude of around 11,000 meters, is weakened. Like any river that flows slowly, it has larger meanders. It is these meanders which help to introduce cold air above Spain from Greenland, at high altitude. So the water vapor evaporated on the sea surface encounters very cold air and condenses. The Earth’s rotation then spins the rising air counterclockwise, and then the condensation of water vapor releases enormous amounts of energy.
Due to this combination of factors, the torrential rains are concentrated in Spain, on the Balearic Islands and the reliefs of the Mediterranean coast, sometimes reaching the Sierra del Segura and the Cuenca mountain range. The phenomenon is rapid and very violent. Sometimes this moisture-laden air even travels towards and crosses the Alps, dumping downpours of water in central Europe.
■ Warming of oceans and polar zones
With fossil fuels, human beings have discovered a gigantic source of energy which actually derives from the energy of the sun collected by plants and small animals 30 to 300 million years ago, including the atoms of carbon form today’s hydrocarbons. However, we will exhaust this stock built up over tens of millions of years in approximately 300 years. This energy source is made up of carbon compounds: coal, hydrocarbons and natural gas. To extract energy from them, they must be burned, which produces molecules such as carbon dioxide, methane, nitrogen oxides and other compounds with more than two atoms. All of these molecules trap part of the infrared rays coming from the Earth’s soil and sea surface and reflect them back to the planet’s surface. The planet is thus heated both by solar energy and by this greenhouse effect.
This process is the cause of climate change. When these carbon molecules, particularly methane, accumulate underground, the water cools and the carbon dioxide captured by the waves is trapped inside. When sea levels drop on a cold planet (glacial stage), methane eventually rises into the atmosphere. The atmosphere heats up, warms the sea, and the sea, like a heated soft drink, releases CO2 which in turn amplifies the effect of methane. Result: the planet is warming more and more, glaciers are melting and sea levels are rising. This succession from hot to cold has occurred about eight times over the last few million years: these are the ice ages.
Let’s continue to burn carbon?
Today we are forcing this process by emitting enormous amounts of greenhouse gases. Can we limit these emissions? Until now, this has been impossible, and we understand why: every human being wants to be richer tomorrow than today. By 2050, there will be around two billion more human beings on the planet, who will need food, housing and transport, that is, chemical fertilizers, cement and gasoline or diesel or natural gas.
Measures are proposed to replace these components with others which limit the combustion of carbon compounds. But these measures are insufficient and are developing very slowly. Hopes for electric cars have been sharply reduced. In Europe, we may be making progress in the field of electricity from photovoltaics and wind power. But electricity represents only about a third of the energy consumed, and the development of electricity from renewable sources is only really happening in Europe. China is progressing, but is not stopping building coal-fired power stations. The most realistic prospect, despite grandiose plans, is that we will continue to burn carbon compounds for many decades and that greenhouse gas concentrations will increase during this century. And with them, the temperature of the planet, and with it, the DANA and the floods. All we have to do now is adapt, which is entirely possible, because it does not require international agreements. In Spain, flooding could be controlled through massive reforestation of the mountains and water capture systems at the source, by building microcanals on these slopes. The latter would represent brakes on the arrival of water and, at the same time, would allow water to be gradually returned to rivers and marshes. Not only is it possible, but it would be inexpensive and generate many jobs.
The original version of this article was published on The Conversation
