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analysis showed that humanity already appropriates nearly a quarter of global terrestrial net primary productivity, and up to 80% in large regional swaths (Haberl et al., 2007).

Supplying the consumption of the next 2.6 billion people will almost certainly have a greater environmental impact than supplying the last 2.6 billion added since 1975. Our species has already plucked the lowest-hanging resources and converted the richest lands. To maintain the pace, metals will have to be won from ever-poorer ores, and oil, natural gas, and water will need to be obtained from ever-deeper wells and transported farther—all requiring accelerating energy use. So-called “marginal lands,” often the last holdouts of biodiversity, are the final frontier, awaiting conversion into more human biomass. Whenever biodiversity preservation poses a threat to human livelihood, comfort, or convenience, the politically expedient choice is usually to liquidate the natural capital. In sum, every increment in the human population accelerates competition with other organisms for Earth’s primary production. And, of course, not only do the present poor need more consumption, the present rich also demand it—as certainly will the newcomers. This is all in the face of signs that average per capita consumption is already unsustainable in developed regions (Ehrlich and Goulder, 2007), indicating a stark tradeoff between today’s consumption and the basic human rights of future generations.

A major byproduct of human consumption is the toxification of Earth’s ecosystems. Human agriculture and fossil-fuel combustion have multiplied the emission and deposition of nitrogen in recent decades, with negative consequences for biodiversity in grasslands (Stevens et al., 2004) and aquatic ecosystems (Carpenter et al., 1998). Widely used herbicides such as atrazine and glyphosphate harm amphibians (Hayes et al., 2002; Relyea, 2005), potentially contributing to global amphibian decline, and the use of antiinflammatory drugs such as diclofenac and ibuprofen to treat livestock in India has ravaged scavenging birds, for which cattle carcasses are a major food source (Oaks et al., 2004; Cuthbert et al., 2007).

Anthropogenic climate change stems from a special case of toxification: carbon pollution. Many biological impacts of global heating are evident, as animals and plants undergo changes in phenology, distribution, and local abundance (Parmesan, 2006). More alarming, anthropogenic heating has already been directly implicated in several extinctions (Pounds et al., 2006) and seems likely to precipitate others. In the oceans, heating is already reducing the extent and altering the structure of coral reefs via breakdown of the coral–algal symbiosis (Hughes et al., 2003). Moreover, rising CO2 concentrations are lowering oceanic pH, with potentially disastrous consequences for coral reefs and other marine ecosystems (Orr et al., 2005; Hoegh-Guldberg et al., 2007).



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