1 World in Transition
minerals has been examined only marginally (Schüler et al., 2011). In the case of imminent, permanent price increases for certain raw materials, substitution seems the obvi ous choice. For example, neodymium scarcity, about which concerns are frequently voiced, could lead to an increase in the price of permanent magnets, which are also used in the construction of modern wind power sta tions. However, electric generators, such as those used in wind power stations or engines for electric vehicles, could also be realised without the use of permanent magnets. There are also sufficient alternatives available which could be used in photovoltaic technology to gen erate solar energy, rather than relying on materials with limited availability. Comparable alternatives also exist for the catalytic converters used in hydrogen electroly sis or fuel cells. In principle, the WBGU therefore does not consider the limited availability of rare materials to be a risk endangering the rapid conversion towards low-carbon energy systems.
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1.1.5.2 Nutrient Cycles Nitrogen (N) and phosphor (P) are – besides CO2 and water – the two vital plant nutrients whose global cycles are relevant for the Earth system. Anthropo genic change and acceleration of both cycles has been significant in order to increase agricultural production through the use of mineral fertilisers (Vitousek et al., 1997a; Mackenzie et al., 2002; Section 1.2.5). Over supply with these nutrients can change ecosystems to such an extent that threshold values are exceeded, trig gering fundamental structural changes or collapse. This pattern can be observed at all dimensional scales, from small local ecosystems (meadows, lakes) to large-scale anoxic marine environments (for example dead zones in the Gulf of Mexico or the Baltic Sea; Diaz, 2001). The anthropogenic production of reactive nitrogen (fertilisers, combustion processes and the cultivation of nitrogen-fixing legumes) has increased tenfold since industrialisation (from approx. 15 to approx. 156 Mt N per year), today exceeding natural flows. Over half of all synthetic nitrogen fertiliser ever produced has been applied post-1985 (MA, 2005a). Its use is expected to increase even further to approx. 267 Mt per year by 2050 (Galloway et al., 2004; Bouwman et al., 2009). As a planetary boundary, Rockström et al. (2009b) rec ommend limiting nitrogen input to around 35 Mt N per year, roughly the equivalent of a quarter of cur rent amounts, to prevent the slow erosion of ecosys tem resilience through eutrophication and acidification. The use of phosphor as a fertiliser has tripled between 1960 and 1990 (MA, 2005a). Today, around 20 Mt P per year are extracted for use as mineral ferti
liser; however, the easily accessible resources will soon become scarce (Box 1.1‑3; Rockström et al., 2009a). Ultimately, around half of this ends up in the oceans (8.5–9.5 Mt P per year). For comparison: prehistorical input into the oceans amounted to merely approx. 0.2 Mt P per year (Mackenzie et al., 2002). Such a huge input increase of phosphor into the oceans could, in the long term, lead to expansive anoxic zones, or dead zones, in the oceanic deep seas, as has been the case before in the history of the Earth (Handoh and Len ton, 2003). Despite the massively increased anthropo genic phosphor flow, Rockström et al. (2009a) believe that a planetary boundary of 11 Mt P per year should be sufficient to prevent critical load limits from being reached. In view of the increasing demand for agricul tural products (Section 1.2.5), this boundary has almost been reached.
1.1.5.3 Depletion of the Stratospheric Ozone Layer Over the past few years, the annual seasonal strat ospheric ozone hole over Antarctica has continued to reach record dimensions, fluctuating slightly from year to year. Recovery can not be expected as yet, despite the successes of the Montreal Protocol that led to the reduction of emissions of ozone-depleting substances, as the processes are still in saturation. This also applies more or less to the Arctic stratospheric ozone deple tion in the spring; again there has been no measura ble trend reversal. In the global mean, the ozone layer has also not yet recovered to pre-1980 levels, however, over the past few years, total column ozone has stabi lised at around 3.5 % (northern hemisphere), and 6 % (southern hemisphere) below pre-1980 levels. Accord ingly, clear-sky UV radiation levels in mid- and higher latitudes are still higher than they were pre-1980. The atmospheric concentration of the ozonedepleting substances regulated by the Montreal Pro tocol (measured in terms of their stratospheric ozone depletion potential) is expected to fall to 1980 levels by the middle of the 21st century. Nevertheless, the ozone layer is not expected to fully return to its pre1980 condition but to remain permanently altered due to the impact climate change has on atmospheric circu lations. Total ozone columns in the tropics are expected to remain lower than they were in 1980, whilst out side the tropics, they are expected to increase (Li et al., 2009). Without the regulation of ozone-depleting substance emissions as stipulated by the Montreal Pro tocol, the globally-averaged column ozone could have been expected to decrease by 17 % by 2020, and by 67 % by 2065, which by then would have led to a dou bling of skin damaging UV radiation levels during the