Ocean-atmosphere linkages maintain desert climate

The intense solar radiation hitting the earth of the tropical belt (between latitudes 23o South and 23o North) sets off air currents bringing dry winds to sub-tropical areas (within latitude 25o and latitude 35o, either North or South), thus denying them precipitation and making them deserts. The western seaboard sections of the African and American continents are deserts due to dry inland conditions induced by upwelling of deep cold seawater, driven by ocean coastal currents. Deserts also occur on the leeward sides of mountain ranges that are deprived of oceangenerated moisture. It is rather paradoxical that the forces which induce highly productive conditions next to deserts - the high solar radiation in the tropical rainforests, the cold and nutrient-rich upwelling of western coastal seawaters, and the moisture-laden tropical trade winds reaching tropical continental mountains - are also all factors maintaining the aridity of deserts.

Rainfall patterns within deserts also depend on climatic processes outside deserts. When the desert surface is cold, moisture blown from the sea condenses and generates winter rains; when the surface is warm, the moisture drawn into the atmosphere from various sources condenses to generate summer rains. Rainfall can be also augmented by of fog, formed when water droplets kept in suspension over the ocean are blown into the desert.

The great year-to-year variations in desert rainfall are modulated by processes away from deserts, such as the Southern Oscillation - a global weather cycle associated with a fluctuation of atmospheric pressure between the South Pacific and tropical Indian Oceans, and expressed in alternating 3-7 year cycles of El Niño Southern Oscillation (or ENSO) and La Niña events. Within this cycle, El Niño develops when the warm water that accumulates in the eastern equatorial Pacific Ocean decreases the upwelling of cold water along the coasts of North and South America; the sea surface then warms-up, resulting in an increase in coastal winter rainfall. After a few months, El Niño conditions begin to recede, paving the way for La Niña: a strong westward flow of surface currents forces the upwelling of cold waters along the coasts of the American continent, thus inducing drought along the coastal deserts. This cycle affects the coastal deserts of Atacama and Baja California, Namibia, Western Australia, and Atlantic Morocco. Paradoxically, since deep seawater upwelling also modulates the productivity of coastal water, the pulse of rainfall-induced high desert productivity coincides with a pulse of low coastal ocean productivity, and vice versa (see also Chapter 1).

Because the El Niño-induced coastal rainfall occurs when moist air moves from warm oceans onto cooler land, the increase in precipitation occurs mostly in coastal fog deserts such as the southern Namib or the Atacama deserts. Inland, summer-rain deserts such as central Australia, the Thar Desert in India and Pakistan, or the Brazilian Caatinga, however, tend to become drier during El Niño years but enjoy increased precipitation during La Niña years, due to the differential in temperatures between these desert lands and the surrounding seas. Because deserts are so limited by scanty rainfall, these year-to-year climatic oscillations produce pulses of abundance and scarcity of resources with immense ecological repercussions. This highlights the importance of tele-connections in global ecology: a gradient of air pressures developing over the sea critically affects the ecological dynamics of the driest of lands.

Global climate change affects desert climate

Global climate change, the directional change induced by anthropogenic emissions of greenhouse gases (to be distinguished from longterm or short-term climate variations not caused by global-scale human impact on the climate system) also affects deserts. Deserts warmed-up between 1976 to 2000 at an average rate of 0.2-0.8ºC/ decade - an overall increase of 0.5-2ºC (Table 3.1), much higher than the average global temperature increase of 0.45ºC, which has been attributed to the increase in atmospheric concentrations of greenhouse gases (IPCC 2001). Global warming is expected to induce an overall increase in rainfall; but high latitudes are projected to warm more than the mid- and low-latitudes, resulting in more rainfall in higher latitudes linked to reduced rainfall in subtropical ones. Indeed, in most deserts within the subtropical belt, rainfall has already been decreasing in the last two decades (Table 3.1).

As for the future, using two global emissions scenarios developed by the IPCC (2001), projected deviations of temperature and precipitation for the period 2071 to 2100 relative to the period 1961-1990, are expected as follows: temperature increases of 2-7ºC or 1-5ºC (scenarios A2 and B2, respectively), and 5-15 and 5-10 per cent precipitation decrease for most deserts, but also no change, or 2-15 and 2-10 per cent precipitation increase for a few deserts. Moreover, global climate change is expected to increase year-to-year variations in desert rainfall, including an increase in desert droughts induced by a negative feedback between the ENSO cycle and the global atmosphere (Tsonis and others 2005). hence as the global atmosphere continues to warm, the frequency of El Niño events is expected to increase, bringing more rainy pulses to winter-rain deserts and more drought pulses to summer-rain deserts.

Global change (climate and atmospheric CO2) will impact the ecology of deserts

The global climate change-induced desert warming is bound to increase evapotranspiration. This water loss may be greater than the water gain in those deserts where global climate change also increases precipitation. In most deserts, the combined effect of higher evapotranspiration, lower precipitation, and more severe and protracted droughts will reduce soil moisture - the limiting resource on which desert productivity depends - resulting in an overall reduction of desert vegetation cover (Lioubimtseva and Adams 2004). Since all other life-forms directly or indirectly depend on plants, desert biodiversity is expected to be impacted too. While animal species of low mobility and plant species of low dispersal ability may decline in numbers or become locally extinct, those with dispersal rates faster than the expected rate of warming advance will extend their ranges into areas that had been cooler previously. Thus, for each local desert area a turnover of species will take place. For example, nearly half of the Chihuahua Desert bird, mammal and butterfly species are projected to be replaced by other species by the year 2055 (Peterson and others 2002). Increasing aridity will also lead to the loss of grasses in favour of shrubs, reinforcing a new stable ecosystem state with "islands" of fertility beneath shrubs and relatively nutrient-poor soils in the barren areas in-between (Schlesinger and others 1990). The increase in global atmospheric CO2 concentration, the major driver of global warming, is also the major plant resource, absorbed through the stomata (microscopic pores in the leaf surface of plants) for manufacturing plant carbohydrates through the process of photosynthesis. Interestingly, the direct effect of global CO2 on desert plants may counteract its indirect effect on them as a driver of desert aridity. This is because increased atmospheric CO2 enables plants to open their stomata less, thus reducing the amount of water lost during photosynthesis and hence increasing their water-use efficiency. Indeed, most experiments demonstrate that water-use efficiency of desert vegetation increases at high CO2 concentrations. Depending on differences in the biochemical pathways of photosynthesis, some species within a desert plant community are more responsive to elevated CO2 than others in the same community, and this would lead to changes in plant community structure (Morgan and others 2001). Furthermore, it was found that increased growth under elevated CO2 is most likely in wet rather than in dry years (Naumberg and others 2003), which suggests that the effect of the predicted increased droughts will be further amplified by this differential plant response to elevated CO2. In addition, since the increased productivity is carbon-based, it is associated with an increased carbohydrate-toprotein ratio, making plants less nutritious and less digestible. Also, increasing CO2 encourages alien invasive species such as annual grasses that are prone to wildfire. This would eliminate shrubs and leave barren desert soils (Smith and others 2000).

Deserts affect non-desert climate

It is rather paradoxical that while anthropogenic warming of the global atmosphere already warms and dries deserts, deserts habitually cool the adjacent global atmosphere - a state that is also projected to further intensify, due to global warming. This is due to the desert albedo (the direct reflection of solar radiation by the earth's surface back to outer space). In contrast to the intuition that views the long hours of intense solar radiation reaching the bright desert surface through the dry atmosphere as a cause of enhanced warming, the actual effect of deserts is that of cooling the global atmosphere (Charney 1975). The typical desert albedo is 20-35 per cent of solar radiation reflected back to space (much higher than the 15% of the savannah and the 5% of the rain forest; Pinty and others 2000). With little water available for evaporation from the dry desert surface, most of the remaining solar radiation heats the desert surface, and the generated thermal radiation escapes to space through a dry atmosphere. The heated dry air cools at the rate of 10°C km-1 as it rises to 3-4 km, such that the tropospheric air column above deserts is cooled. This high-altitude cooled air is dispersed by the winds over great distances away from deserts, at least as far as the adjacent non-desert drylands, which become cooler and drier.

Global warming is projected to increase desert albedo, through reducing desert vegetation cover, which will further amplify the effect of cooling the non-desert atmosphere and drying adjacent nondesert drylands. Thus, whereas global climate change makes the desert drier, deserts make the global atmosphere cooler, and the drier the desert becomes, the more its cooling effect will increase. The same logic applies also in the opposite direction. "Greening the desert" by restricting grazing or by irrigation would reduce the albedo of the Sahara desert and enhance precipitation over the Sahel; it will also decrease the cooling effect of the desert on the global temperature, thus contributing to anthropogenic global warming.

Will deserts mitigate global warming?

On average, more CO2 is absorbed than is released by global vegetation, which thus functions as a "carbon sink" by permanently binding this net amount, through the process of photosynthesis, into organic compounds that make up the plants' tissues and organs. This makes the overall mass of the global vegetation a "carbon reserve". After generations and millennia, much of the carbon stored in plants moves to the soil, such that currently the global soil carbon sink is about three times larger than that of global standing live vegetation. Carbon sequestration in vegetation and soils counteracts the currently much faster process of carbon emissions, responsible for global warming. But under the current influence of deforestation, land use change, and climate change, part of the soil and vegetation reserve can function as a source, and further exacerbate global warming. The uptake of atmospheric CO2 stored in desert plant biomass is among the lowest of the world's biomes. Moreover, in most desert areas where there has been no recent change in vegetation, no further net storage of carbon in vegetation takes place. Desert soils are relatively poor in organic carbon and much richer in soil inorganic carbon, in the form of "secondary carbonates" - carbonate minerals precipitated from the calcareous soil solution rather than inherited from the soil parent material (Monger and Martínez-Ríos 2000). Lack of soil water for most of the time means slow weathering of the carbonates and little organic carbon accumulation.

Yet, since deserts cover a quarter of the earth's land surface, it is worth assessing the carbon storage and sequestration potential of all deserts combined (Table 3.2). Soils sequester carbon in inorganic and organic compounds. It is the organic carbon that is most readily sequestered - rainfall allowing - with rates of accumulation of 5-10 g C m-2 y-1 under best-practice rain-fed farming in arid-semiarid regions (Lal 2002). The average rate of accumulation of the inorganic carbon is considerably slower - some 0.1-0.6 g C m-2 y-1 (Schlesinger 1985). Yet, though the rate at which atmospheric CO2 is precipitated as soil carbonate is slow, and the rate at which it can be sequestered as soil organic matter is relatively fast, the global desert ecosystem has accumulated much inorganic carbon but only little organic carbon. Thus, only 9-10 per cent of global soil organic carbon is held in deserts. Therefore, in spite of their large extent, deserts do not play a significant role in the global carbon cycle.

However, if some desert regions do become significantly moister under global warming, they have the potential to function as a globally significant sink that could tangibly mitigate global warming (Lioubimtseva and Adams 2004). On the other hand, those deserts that become drier, with their vegetation only weakly responding to CO2 enrichment, will not become a significant sink. These deserts are also not likely to act as a significant source driven by land degradation, because the turnover rate of the large desert sink of inorganic soil carbon is too slow to generate significant CO2 emissions. Also, although the turnover of soil organic carbon is fast and land degradation in deserts might increase CO2 emissions (as the carbon in eroded soil is oxidized), the pool of soil organic carbon that might be affected by land degradation is too small to make this a significant contribution to global atmospheric CO2. Between-ecosystem comparison (Batjes and Sombroek 1997) suggests that under the scenario of further desert warming and reduced precipitation, the ratio of soil organic carbon to soil inorganic carbon in deserts will be reduced. This will further reduce the role of deserts in the modulation of global climate change, unless drastic human interventions for increasing desert productivity take place.