is taken to be a fixed, indestructible factor of production. The problem with the assumption is that it is wrong: nature consists of degradable resources. Agricultural land, forests, watersheds, fisheries, fresh water sources, river estuaries and the atmosphere are capital assets that are self-regenerative, but suffer from depletion or deterioration when they are over-used. (I am excluding oil and natural gas, which are at the limiting end of self-regenerative resources.) To assume away the physical depreciation of capital assets is to draw a wrong picture of future production and consumption possibilities that are open to a society.‖ (Dasgupta 2010:6) Moreover, ―property rights to natural capital are frequently unprotected or ill-specified....(which) typically leads to their overexploitation, and so to waste and inequity‖ (Dasgupta 2010: 6).

Arrow et al (2004: Table 2) estimate how much ―genuine wealth per capita‖ (including natural capital, human capital, and manufactured capital) changed during 1970-2000. The estimates are necessarily approximate, but they have been made carefully and the results are instructive. They find that while GDP per capita grew quite rapidly in all regions except in Sub-Saharan Africa, rates of growth in ―genuine wealth per capita‖ were far lower. They _declined_ sharply in Sub-Saharan Africa and in the Middle East and North Africa (by -2.6 and -3.8 percent per year respectively). They grew very slowly (well below 1 percent per year) in South Asia and the United States. They grew rapidly only in China, due to its low population growth and heavy investment in productivity. Revising the method to include more information on growth in human capital and institutional change, Dasgupta (2010: 9-10) derives far lower estimates of growth in genuine wealth per capita for China 1970-2000, and for South Asia he estimates a _decline_ of between -0.4 percent per year (India) and -1.4 percent (Pakistan).

Rates of internal and international labor migration will rise with the pressures of environmental degradation and climate change (Laczko and Aghazarm 2009, World Bank 2010a: ch2). Even current rates of migration have proved difficult to manage politically in many countries, as local residents resent encroachment on their resources. It is likely to prove very daunting to accommodate accelerated inmigration, especially when receiving areas are under greater pressure themselves. This is just one of the challenges that increasing rates of population growth pose to societies‘ capacity to adapt their social, political, and economic institutions to respond to the new needs (Kuznets 1973:91, McNicoll 1984).

Human ingenuity has faced an uphill task at devising ways of managing common property resources ─ given the institutional and political challenges posed in aligning divergent interests. Markets are very poor at incentivizing people not to overuse resources that are un-priced or under-priced relative to social cost (Arrow 1969, Dasgupta 2001, Stern 2006), especially in the case of transnational common resources (Dasgupta et al 1997). The consequent negative externalities need to be addressed through collective action, but in the absence of strong mechanisms for mutual coercion it is very difficult to align the interests of different stakeholders to this end. Ostrom (1990) has argued that common property can be successfully managed by user associations in small communities if eight ―design principles‖ are met, including the ability to effectively exclude unentitled parties. Such conditions clearly do not apply to global common resources. As Lee (1990:317) points out ―each birth inflicts costs on all others by reducing the value of their environmental birthright‖.

The juxtaposition of all these daunting scientific, executive, and political challenges suggest that even countries which have so far adapted to growing needs may find it difficult to respond to these new kinds of demands (Demeny 2011). They place high demands on national and global institutional capacity, as evidenced by the slow progress made in decades of efforts to regulate carbon emissions.

Reducing the number of births may be one of the simplest ways of reducing pressure on common property resources. For instance, it is estimated that the effect of a 40% reduction in CO2 emissions per head in developed countries between 2000 and 2050 would be entirely offset by the increase in emissions attributable to expected population growth in poorer countries over this period, even assuming no change in emissions per head in these countries (Dyson 2005). Moreover, CO2 emissions per head have risen in 10