Role of ocean in climate change and El nino effect. Climate change and global desertification process

The crucial issues of climate change and global warming involve all the components of the earth system, atmosphere, oceans, land, biosphere, and cryosphere. The oceans are among the most poorly known and understood, because of the enormous difficulty of probing the deep layers which are still basically void of observations, and the sparseness of data in the southern ocean. There are numerous ways in which the oceans affect the Earth’s climate, but we will discuss only those two that, in our opinion, are of the utmost importance.

The first crucial role played by the oceans involves the mixing of heat anomalies into the deep layers. Within the context of global warming scenarios, strong mixing of surface heat anomalies will retard surface warming rates. Mixing is primarily performed by small-scale oceanic processes, generally decomposed into multiple components: diapycnal and isopycnal diffusion (i.e., diffusion across and along constant density surfaces); mesoscale and submesoscale eddies, with scales < ~ 100 Km.; and convection. Diapycnal diffusion in particular is crucial in determining the ocean’s circulation, since it is the diapycnal mixing of heat and salinity from the ocean’s surface to its depths that gives rise to the density gradients which drive the large-scale circulation and its horizontal heat transports.

Vertical mixing by the other diffusion processes is strongest in high latitudes. There the strong cooling of surface waters favors static instability of the water column and a vertical alignment of the isopycnal surfaces. The former leads to penetrative convection; the latter to isopycnal diffusion being predominantly vertical and large amounts of potential energy becoming available for mesoscale processes. In particular, the convective cell(s) of the polar North Atlantic determine the bottom water mass that from its northern source spreads through the abyssal layers of the global ocean.

All these sub-scale mixing processes, with the possible exception of mesoscale eddies, are extremely difficult to measure in the field and, furthermore, are not resolved by the ocean general circulation models currently used for climate studies. Hence the necessity of stimulating research into process studies, both theoretical and observational, leading to improved or new parameterizations of ocean mixing in its different forms, and their related effects on the heat and, in general, water properties transports by the large-scale circulation at all depths.

The second most important role the oceans play in climate occurs in the regions where they are strongly coupled to the atmosphere. By and large, the ocean is driven by the atmosphere and the ocean circulation is a response to atmospheric forcing, i.e., wind stress, heat, and moisture fluxes. In the tropical/equatorial regions, however, the ocean exerts the strongest feedback on atmospheric motions thanks to the intense sea surface heat exchanges.

Thus in the tropical regions the interannual to decadal modes of climate variability are coupled ocean-atmosphere modes. The most famous of all is the Pacific El Niño/Southern Oscillation (ENSO) mode that has been extensively studied since the early ’80s.

El Niño is the appearance in the Pacific interior of an anomalously warm tongue of water that changes the convective atmospheric cell above it. In normal conditions, the atmospheric convective loop involves upwelling of warm air over the western Pacific which moves eastward at height and downwells over the eastern Pacific.

During El Niño, due to the warm water pool in the basin interior, two convective loops are produced, with upwelling of warm air over the pool, and two branches moving at height both westward and eastward, finally downwelling over the western Pacific and Central America. El Nino is succeeded by its opposite phase, La Niña, in which the warm pool is replaced by a cold one. This coupled oscillation of the atmosphere-ocean system has an irregular periodicity of a few years and has profound consequences on the fisheries and the economy of central/southern America. A major observational effort, the Tropical Atmosphere Ocean (TAO) array, has been in place since 1994, becoming the Triton/TAO array in 2000 with Japan participating in the western part. It comprises 11 arrays of multiple instrumentation moorings regularly distributed all along the equatorial band on both sides of the equator The TAO array has produced an incredibly rich time series of observations that have improved both the theoretical understanding and the modeling and prediction of ENSO.

Nothing analogous exists in the Atlantic and even less in the Indian Ocean. In the tropical Atlantic, in the last decade, the Prediction and Research Moored Array in the Atlantic (PIRATA) has been put in place, through cooperation mostly involving France, the U.S., and Brazil. The PIRATA array, however, is much more irregular, sparse, and limited, coarsely spanning the equatorial band and not resolving well the most important mode of variability. This is the interhemispheric dipole or meridional sea surface temperature (SST) gradient mode. The SST north-south gradient in fact controls the position of the Inter Tropical Convergence Zone (ITCZ) where the northern and southern trade winds converge. The ITCZ in its seasonal north-south migration is responsible for rainfalls or droughts over Brazil and West Africa and related epidemics of tropical diseases.

In the Indian Ocean, only in 2008 has a proposal been put forward for a multinational collaboration leading to RAMA: the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction. In the Indian Ocean the most important coupled mode of variability is the Indian Ocean Dipole (IOD), consisting of east-west anomalies of SST and precipitation/drought. The IOD is similar to ENSO in its mechanism but is much more short lived.

The paucity of observations in the tropical Atlantic and Indian oceans have considerably retarded our understanding, modeling, and prediction of these coupled modes, which are extremely important not only because of their societal consequences but because it is through them that the ocean actually drives the atmosphere. These regions and coupled mechanisms should constitute a priority of observational and theoretical research.

Desertification

Climate change is one of a number of variables that are considered to contribute towards desertification. Desertification is land degradation in dry-lands, resulting from various factors, including both climatic variations and human activities. While in previous years, dry-lands recovered easily following long droughts and dry periods, more recently they have tended to lose their biological and economic productivity quickly unless they are sustainably managed. Consequently, dry-lands on every continent are now being degraded by over-cultivation, overgrazing, deforestation, and poor irrigation practices. Such overexploitation is generally caused by economic, environmental and social pressures.

Desertification reduces the productivity of land and contributes to poverty. This is because prime natural resources - fertile topsoil, vegetation cover, and healthy crops - are the first to disappear in the face of desertification. As a result, dry-lands across the world tend to have the lowest gross domestic product (GDP) per capita and the highest infant mortality rates. Soil degradation in dry-lands exacerbates the problem even more. The decline in the fertility of land reduces crop production and additional income sources.

Land degradation can also trigger a vicious cycle of environmental degradation, impoverishment, migration and conflicts, often also putting the political stability of affected countries and regions at risk. Globally, it is estimated that over 250 million people are directly affected by desertification, and around a billion are on some level at risk of the effects of desertification.

The relationship between the two processes

It is essential to recognise that desertification is essentially a man-made phenomenon which is exacerbated by climate change. This is because an increase in weather extremes such as droughts and heavy rains as a result of climate change will lead to further land degradation. This in turn aggravates existing problems associated with poverty, forced migration and, in some areas, conflicts. While desertification is already responsible for signific ant forced migration, more than a billion people – one in seven of the current world population - could be forced from their homes between now and 2050 if climate change worsens. The Middle East and North Africa (MENA), in particular, is considered to be the region most at risk if such projections prove accurate. The relationship between the two processes does not, however, move in only one direction.

It is also possible that desertification may in turn affect climate change, due to the effects of land degradation reducing surface moisture. Because less water is available for the sun’s energy to evaporate, more energy is left over for warming the ground and, consequently, the lower atmosphere. At the same time, wind erosion in dry-lands releases dust and other particles into the atmosphere. By absorbing the sun’s rays or reflecting them back out into space, they may help to cool the Earth’s surface. However, the energy they absorb can heat the lower atmosphere and in this way reduce temperature differences between the atmosphere’s vertical layers; this can lead to fewer rain-showers and thus drier land.

Finally, the periodic burning of arid and semi-arid grasslands, often associated with unsustainable slash-and-burn agriculture, emits greenhouse gases. The unsustainable use of fuel-wood and charcoal, a major cause of land degradation, also contributes to greenhouse gas emissions.

There are several immediate consequences of desertification, both direct and indirect. They are: environmental degradation that reduces the land’s resilience to climate variability; compromised potential for food production; an increase in the incidence of famine; indirect pressures on areas outside the immediately affected areas; and socio-economic instability. In turn, these factors have the potential to exacerbate other challenges facing the region.

Desertification reduces the land’s resilience to natural climate variability. Soil, vegetation, freshwater supplies, and other dry-land resources tend to be resilient. They can eventually recover from climatic disturbances, such as drought, and even from human-induced impacts, such as overgrazing. When land is degraded, however, this resilience is greatly weakened, resulting in both physical and socio-economic consequences. Soil becomes less productive when exposed and eroded topsoil is blown away by the wind or washed away by rainstorms.

The soil’s physical structure and biochemical composition can then deteriorate as vital nutrients are removed by wind or water. If the water table rises due to inadequate drainage and poor irrigation practices, the soil can become waterlogged, and salts may build up. When soil is trampled and compacted by cattle, it can lose its ability to support plant growth and to hold moisture, resulting in increased evaporation and surface run-off. The loss of vegetation cover is both a consequence and a cause of land degradation. Loose soil can sandblast plants, bury them, or leave their roots dangerously exposed. When pastures are overgrazed by too many animals, or by inappropriate types, edible plant species may be lost, allowing inedible species to invade.

The link between desertification and food production is also strong. A nutritionally adequate diet for the world’s growing population implies tripling food production over the next 50 years. This will be difficult to achieve even under favourable circumstances. Given the high rates of population growth in recent decades across the southern and eastern Mediterranean, adequate levels of food production are essential to ensure that there is sufficient production to maintain export levels and to feed local populations. However, if the extent of desertification is not reversed in the coming years, food yields in many affected areas are likely to decline.

Malnutrition, starvation, and ultimately famine may result, although famine typically occurs in areas that also suffer from poverty, civil unrest, or war. Drought and land degradation often help to trigger a crisis, which is then made worse by poor food distribution and the inability to buy what is available. The relationship between soil degradation and crop yields, however, is seldom straightforward. Productivity is affected by many different factors, such as the weather, disease and pests, farming methods, and external markets and other economic forces.

Some of the consequences of desertification are also borne by people living outside the immediately affected area. For example, degraded land may cause downstream flooding, reduced water quality, sedimentation in rivers and lakes, and siltation of reservoirs and navigation channels. It can also cause dust storms and air pollution, resulting in damaged machinery, reduced visibility, unwanted sediment deposits, and mental stress. Wind-blown dust can also worsen health problems, including eye infections, respiratory illnesses, and allergies.

Dramatic increases in the frequency of dust storms were recorded during the Dust Bowl years in the US, in the Virgin Lands scheme area in the former USSR in the 1950s, and in the Sahel-Sahara region of Africa in the 1970s and 1980s. There are also immense social costs that are caused by the incidence of desertification, mass migration, and conflicts. In Africa, many people have become internally displaced or forced to migrate to other countries due to war, drought, and dry-land degradation.

 The environmental resources in and around the cities and camps where these people settle come under severe pressure. Difficult living conditions and the loss of cultural identity might further undermine social stability. There is little detailed data on the economic costs resulting from desertification, although an unpublished World Bank study suggested that the depletion of natural resources in one Sahelian country was equivalent to 20 per cent of its annual GDP.

Other indirect economic and social costs suffered outside the affected areas, including the influx of “environmental refugees” and losses to national food production, could possibly be even greater.

There are a number of significant longer-term risks associated with the prospect of continued desertification in the Mediterranean. They are: increased frequency of water shortages and a decline in water quality; compromised food security; public health risks; permanent ecosystem damage; losses to national economies. These risks might in turn threaten to cause or exacerbate political instability in the region.

Increased frequency of water shortages and decline in water quality

It is likely that the first impacts of climate change will be felt in the Mediterranean water resource system. Reductions in water availability would hit southern Mediterranean countries the hardest. In Egypt, Libya, Tunisia, Algeria, Morocco, Syria, and Lebanon, water availability already falls below, or approaches 1,000 m³ per person per year - the common benchmark for water scarcity. In addition, some water supplies could become unusable due to the penetration of salt water into rivers and coastal aquifers as sea level rises. Water pollution - already a major health hazard in the region - would become still worse as pollutants become more concentrated with reductions in river flow.

Food security threatened by falls in production and world price rises

Livestock production would suffer due to a deterioration in the quality of rangeland associated with higher concentrations of atmospheric carbon dioxide and to changes in areas of rangeland as climate boundaries move northwards. In the European Mediterranean, the area of unproductive shrub land is expected to expand, while in the MENA region, most of the steppe rangeland could give way to desert by 2050 or earlier.

Yields of grains and other crops could decrease substantially across the Mediterranean region due to increased frequency of drought. While losses may be partially offset by beneficial effects from carbon dioxide, crop production would be further threatened by increases in competition for water and the prevalence of pests and diseases and land losses through desertification and sea level rise. In the MENA region, changes in average climate associated with a doubling of carbon dioxide could cause yield losses of over 20 per cent for wheat, corn and other coarse grains - even before allowance is made for losses through other causes.

In coastal areas, large areas of productive land may be lost through flooding, saline intrusion and water-logging. In Egypt, for example, agricultural production may cease altogether over an area extending 20 km inland. World prices for many key commodities such as wheat, maize, soybean meal and poultry could rise significantly as a result of global climate changes.

Not only might Mediterranean countries loose substantially in economic terms, but the combination of higher prices and crop losses would lead to a deterioration in levels of food security, especially in the poorer southern rim countries.

New, widespread risks to public health

Reductions in food security could increase the risks of malnutrition and hunger for millions in the southern and eastern rim countries. The combination of heat and pollution could lead to an upsurge in respiratory illness among urban populations, while extreme weather events could increase death and injury rates. Water shortages and damaged infrastructure could increase the risk of cholera and dysentery. Higher temperatures might then increase the incidence and extent of infectious diseases, such as malaria, dengue fever, schistosmaisis and yellow fever.

Many valuable ecosystems would be lost

Many valuable ecosystems could be lost as species fail to keep up with the shift in climate boundaries and/or find their migration paths blocked by human activities. Wetland sites will face the dual threats of drying out and sea level rise. Up to 85 per cent of wetland sites in southern Europe could disappear with a 3 to 4 degrees centigrade rise in temperatures. In Tunisia, for example, rising temperatures could contribute to the loss of all food plants and breeding waterfowl and the disappearance of nationally important fisheries.

To dispel the myth, desertification is not a natural advance of the desert or the movement of sand dunes. It is a process by which fertile soils get degraded due= to climate change and human activities like overgrazing, over-exploitation by intensive farming and forest exploitation for fuel. Each year, the earth is losing 12 million hectares of land and global forest cover is shrinking by 13 million hectares. The green patches on the world map are being replaced by brown specks.

According to a UNCCD report, land degradation due to drought and desertification affects about 1.9 billion hectares of land and 1.5 billion people globally. In Africa, some 60 million people face displacement within five years as their land turns into desert. In fact, two-thirds of Africa’s fertile land could be lost by 2025 due to growing desertification.

The Sahel area, which straddles 27 countries from Senegal in the west to Djibouti in the east, is home to about 232 million people. In a first, the entire area has been mapped and about 166 million hectares of degraded land have been identified, which, decades ago, were covered with forests, crops and grasslands. The early results of the Great Green Wall initiative, a flagship programme to combat the effects of desertification, pale in comparison to enormity of the challenge that lies ahead.

 

To reverse the impact of decades of overgrazing and deforestation in Africa, about10 million hectares of land need to be restored each year, according to the Food and Agriculture Organization (FAO). "It's a battle against time, because dryland forests are disappearing and climate change is really happening - and more droughts and floods will not make the work easy," said Nora Berrahmouni, forestry officer for dry lands at FAO.

Asia and Middle East

Out of a total land area of 4.3 billion hectares, some 1.7 billion hectares of land is arid, semi-arid and dry sub-humid. Like Iran, Mongolia and Pakistan, deserts are expanding in China. The Tengger Desert, located on the southern edge of the Gobi Desert, has grown at an annual rate of more than 1,350 square miles. Many villages have been lost. In Syria, sand dunes are encroaching on agricultural lands and mountain slopes of Nepal are getting eroded. Even the highlands in Laos are deforested and overgrazed.

In India, 96.40 million hectares of land—29.32 per cent of the total geographic area—was undergoing degradation during 2011-13. Similarly, around 3.63 million hectares of arable land has degraded and 0.74 million hectares has converted from low severity to high severity degradation category.

Even the lowest point on Earth, the Dead Sea, is not spared the assault of desertification. The salty lake is shrinking at a staggering pace.  

Latin America and Caribbean nation

Latin America and the Caribbean (LAC) nations are not just about rainforests. About one-fourth of it is desert and dry lands (20,533,000 sq km). The deserts of the Pacific coast extend from southern Ecuador to northern Chile. As of March 2016, desertification and the degradation of soil affected 11 of the 27 federal districts in Brazil, affecting an area of about 1.35 million square kilometres. Bolivia’s second largest lake has disappeared due to desertification and contamination. Poopó Lake, which was larger than 1,000 sq miles in size, has now turned into a salty desert. Climate change, excessive sedimentation and mining activities have scripted its demise.

Europe

In southern, central and eastern Europe, eight per cent of the territory (about 14 million hectares), shows very high and high sensitivity to desertification. The affected regions within the continent would increase to more than 40 million hectares if moderate sensitivities are also taken into account. The situation is most serious in the areas bordering the Black Sea in Bulgaria and Romania, southern Portugal, Spain, Sicily and south-eastern Greece