Concerns about global warming led to a widespread belief that no other biospheric cycle is subject to so much human interference as the global carbon cycle. This conclusion is doubly wrong. Human interference in the global water cycle is a source of more imminent problems and already a major cause of large-scale premature mortality. And human actions have already changed the global nitrogen cycle much more than carbon cycling, and the ultimate consequences of this multifaceted change may be even more intractable than dealing with excessive CO2. As a result, if we were a rational society, we would be paying a great deal more attention to these changing water and nitrogen cycles.
flux in Gt/year
Global water cycle. From Oki and Kanae (2006).
storage in Gt flux in Gt/year
Global water cycle. From Oki and Kanae (2006).
storage in Gt
No resource is needed for life in such quantities as water. It makes up most of the living biomass (60%-95%), and its absence limits human survival to just days. The water molecule is too heavy to escape the Earth's gravity, and juvenile water, originating in deeper layers of the crust, adds only a negligible amount to the compound's biospheric cycle. Human activities—mainly withdrawals from ancient aquifers, some chemical syntheses, and combustion of fossil fuels—also add only negligible volumes of water, but they have changed the water cycle (fig. 4.9) in three principal ways, and all of them will intensify during the first half of the twenty-first century (Revenga et al. 2000; WCD 2001; Rosegrant et al. 2002; Shiklomanov and Rodda 2003; United Nations 2003).
Increasing volumes of freshwater are diverted to irrigation, urban and industrial uses (growing a kilogram of wheat needs about as much water as does the production of a kilogram of computer hardware, about 1.5 tons), and electricity generation (there are some 45,000 large dams), and most of this water is released to streams, lakes, and ultimately to the ocean without any or only rudimentary treatment. Moreover, decades of climate change have already intensified the global water cycle, and higher CO2 levels have affected continental runoff. And yet, global water supplies and their uses and misuses remain a curiously neglected topic of public discourse. Perhaps no investment is as rewarding as spending on clean water, be it in the form of protecting forests in key watersheds or preventing pollution, but the world has failed to make outlays commensurate with the challenge.
This is particularly true for Asia, a continent with 60% of the world's population but only 27% of the Earth's freshwater, which is, moreover, unevenly distributed in time (because of the monsoon) and space (arid Middle East and Central Asia vs. humid South and Southeast). Global warming will produce more but unequally distributed precipitation, and higher CO2 levels will reduce transpiration (by inducing stomata closure). Hence overall river discharge should increase, but because of continuing population growth the total number of people living with high water stress will increase in the next 50 years (Oki and Kanae 2006).
As Asian and African populations keep growing, the number of countries with stressed water supply (usually taken as the annual rate lower than 1,700 m3 per capita) and water scarcity (less than 1,000 m3 per capita) will also increase. In 2005, 17 countries (mostly in the Middle East and North and East Africa) experienced water scarcity, and another 15 countries were in the stressed category. By 2025 (assuming the UN's medium population projections) nearly 30 countries will have water scarcity and another 20 will join the stressed group. The most populous countries in the first category will be Nigeria, Egypt, Ethiopia, and Iran, and in the second category, India and Pakistan (WRI 2006).
Africa's problems will also become serious. A remote sensing analysis concluded that 64% of Africans are already relying on water resources that are both limited and highly variable and that nearly 40% of existing irrigation is unsustainable (Vörösmarty et al. 2005). Moreover, national means hide major regional scarcities that exist in large and populous countries. The most notable example of these regional disparities is China, where acute water shortages in the northern provinces have led to the construction of a massive water transfer project from the Yangzi to the Huanghe basin (see fig. 3.18) (Smil 2004). This is one of the most expensive massive geoengineering tasks ever undertaken and, as with every mega-project, it raises many concerns about its eventual utility and enormous environmental impacts.
Water supply may worsen even in some places with little population pressure. Modeling shows that during the twenty-first century progressively larger areas in Spain, Italy, and the Rhine basin will move into the stressed category (Schröter et al. 2005). A conservative estimate for the year 2050 would put at least 60 countries, with nearly half the world's population, into the water-scarce and water-stressed categories. Only the installation of the most efficient irrigation systems as well as near-complete recycling of urban and industrial water could ease the deficits, but even so there will be a massive new need for desalination (and hence for substantial constant energy inputs).
Two compensating trends may provide relief. Paleoclimatological evidence indicates that in line with expectations, higher tropospheric temperatures have been intensifying the global water cycle and that the twentieth century brought large precipitation gains to regions including the subpolar Arctic, tropical Arabian Sea, and much of temperate Eurasia (Evans 2006). The last instance is illustrated by precipitation in northern Pakistan: the twentieth century was the region's wettest period of the last millennium (Treydte et al. 2006). Moreover, higher atmospheric CO2 concentrations result in lower evapotranspiration losses from vegetation, and this effect, already detectable in continental runoff records, has increased the amount of water flowing to the ocean (Gedney et al. 2006). Consequently, it is impossible at this time to assess the net outcome of the two countervailing trends (rising demand vs. higher availability of water).
Poor water quality is a much more common problem. In 2005 more than 1 billion people in low-income countries had no access to clean drinking water, and some 2.5 billion lived without water sanitation (United Nations 2003). About half of all beds in the world's hospitals were occupied by patients with water-borne diseases. Diarrhea in its many forms—acute dehydrating (cholera), prolonged with abdominal symptoms (typhoid fever), acute bloody (dysentery), and chronic (caused by waterborne bacteria like Vibrio, Salmonella, and Escherichia coli) is the leading killer (up to 4 billion episodes per year), and dehydration is the principal proximate cause of death. Contaminated water and poor sanitation kill about 4,000 children every day (UNICEF 2005). Deaths among adults raise this to at least 1.7 million fatalities per year. Add other waterborne diseases, and the total surpasses 5 million. In contrast, automobile accidents claim about 1.2 million lives per year (WHO 2004b), roughly equal to the combined total of all homicides and suicides, and armed conflicts kill about 300,000 people per year. The water treatment record of India and China, the world's two most populous countries, is appalling. But even the richest countries have a poor record of water management. Primary treatment is generally in place, but the removal of eutrophication-inducing nitrates and phosphates is still rare.
The natural nitrogen cycle is driven largely by bacteria (fig. 4.10) (Smil 2000). Fixation, the conversion of inert atmospheric N2 to reactive compounds, is dominated by bacteria. They convert N2 to NH3 using nitrogenase, a specialized enzyme that no other organisms carry. Most N-fixing bacteria are symbiotic with leguminous plant roots (some live inside plants stems and leaves) and free living cyano-bacteria are present in soils and water. Nitrifying bacteria present in soils and waters transform NH3 to NO-, a more soluble compound that plants prefer to assimilate. Assimilated nitrogen is embedded mostly in amino acids, which form plant proteins. Animals and people must ingest preformed amino acids in order to synthesize their tissues. Dead tissues undergo enzymatic decomposition (ammonification), which releases NH3 to be reoxidized by nitrifiers. Denitrification returns the element from NO- via NO- to atmospheric N2, but incomplete reduction results in emissions of N2O, a greenhouse gas about 200 times more potent than CO2.
Cultivation of leguminous crops (done in every traditional agriculture) was the first major human intervention in the nitrogen cycle, and it now fixes annually 30-40 Mt N/year. During the nineteenth century, guano and Chilean nitrate were the first commercial nitrogen fertilizers. The synthesis of ammonia from its elements—demonstrated for the first time by Fritz Haber in 1909 and commercialized soon afterwards by the German chemical company BASF under the leadership of Carl Bosch—opened the way for large-scale, inexpensive supply of reactive nitrogen (Smil 2001). By 2005 global NH3 synthesis surpassed 100 Mt N/year, with about 80% of it going to produce urea and other nitrogen fertilizers and the rest used in industrial processes ranging from the production of explosives to the syntheses of plastics (Ayres and Febre 1999). The third-largest source of anthropogenic reactive nitrogen is combustion of fossil fuels, which adds almost 25 Mt N/year in nitrogen oxides.
Losses of nitrogen from synthetic fertilizers and manures, nitrogen added through biofixation by leguminous crops, and nitrogen oxides released from combustion of fossil fuels are now adding about as much reactive nitrogen (~150 Mt N/year) to the biosphere as natural biofixation and lighting does (Smil 2000; Galloway and Cowling 2002). This level of interference is unequaled in any other global biogeo-chemical cycle. Carbon from fossil fuel combustion and land use changes is equal to less than 10% of annual photosynthetic fixation of the element, and sulfur from combustion and metal smelting is equal to only about one-third of the annual flux of sulfurous compounds produced by biota, volcanoes, and sea spray (Smil 2000; D.I. Stern 2005). Not surprisingly, this large anthropogenic fixation of nitrogen has
---flows affected or dominated by human actions
-flows mediated or dominated by bacteria
Global nitrogen cycle is governed by bacteria. Human inputs of reactive nitrogen compounds now roughly equal the natural contributions. Adapted from Smil (2002).
a number of undesirable biospheric impacts once the reactive compounds enter the environment.
Only 25%-40% of all fertilizer nitrogen applied to crops is taken up by plants; the rest is lost to leaching, erosion, volatilisation, and denitrification (Smil 2001). Because the photosynthesis of many aquatic ecosystems is limited by the availability of nitrogen, an excessive influx of this nutrient (eutrophication) leached from fertilizers promotes abundant growth of algae and phytoplankton. Subsequent decomposition of this phytomass deoxygenates water and reduces or kills aquatic species, particularly the bottom dwellers.
The worst affected offshore waters in North America are in the Gulf of Mexico, where every spring eutrophication creates a large hypoxic zone that kills many bottom-dwelling species and drives away fish (Rabalais 2002; Scavia and Bricker 2006). Other anoxic zones can be found in the lagoon of the Great Barrier Reef, the Baltic Sea, the Black Sea, the Mediterranean, and the North Sea. Algal blooms may also cause problems with water filtration or produce harmful toxins. Escalating worldwide use of urea (besides fertilizer also for animal feed and in industry) is increasing pollution of sensitive coastal waters (Glibert et al. 2006).
Nitrogen oxides formed during high-temperature combustion are essential ingredients for the formation of photochemical smog, a persistent feature of all major urban areas worldwide whose major impacts range from drastically reduced visibility to serious health effects (respiratory ailments) to chronic damage to crops and trees. Atmospheric oxidation of NO and NO2 also produces nitrates, which with sulfates compose acid rain. Atmospheric nitrates, together with volatilized ammonia (especially from fertilization and from large animal feedlots), also cause eutrophica-tion of forests and grasslands. In parts of eastern North America, northwestern Europe, and East Asia, rains annually bring more reactive nitrogen than fertilizers do. Both nitrification and denitrification produce N2O, making fertilization contributor to global warming.
Human interference in the global nitrogen cycle is an inherently more intractable challenge than the decarbonization of the world's energy supply. That will not be an easy transition (see chapter 3), but a carbon-free energy system is an eventual inevitability. By contrast, there can be no nitrogen-free organisms, and the larger and more affluent populations of the twenty-first century will demand better nutrition that will have to come largely (given the distribution of future population increments) from higher fertilizer applications, either to secure higher yields of more intensively cultivated land in Asia or to stop the still increasing nutrient mining in agricultural lands of Africa (Henao and Baanante 2006).
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