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rejecting other candidate threats such as climate change and environmental toxics. This fungal pathogen, known as chytridiomycosis, now “poses the greatest threat to biodiversity of any known disease,” according to a recent report in a U.S. science journal.27 It certainly seems to be extraordinarily virulent: In May 2010 one group of researchers working in California’s Sierra Nevada mountains reported that a chytrid outbreak sweeping through the native amphibian population reduced the number of adult frogs in one valley site from 1,680 to a mere 22, with total extinctions noted elsewhere.28 No one knows how the fungus spreads: One theory posits that it may even be coming in on the boots of visiting amphibian researchers. If this is true, the usual conclusion that “more research is needed” may not in fact be justified.

NO BOUNDARY, NO PROBLEM?

There is no quantified planetary boundary proposed for toxic pollution. Partly this reflects the poor state of our knowledge about the issue. We simply do not know how many chemicals are circulating in the environment or what all their effects might be. The best known and documented cases of chronic toxic poisoning, from India’s vultures to the effects of TBT on marine mollusks, came as a surprise, while the real-world harmful effects of the thousands of potential toxics depends strongly on their dose and differing effects in different species. As the planetary boundaries expert group concludes: “It is impossible to measure all possible chemicals in the environment, which makes it very difficult to define a single planetary boundary derived from the aggregated effects of tens of thousands of chemicals.” Ultimately, “a chemical pollution boundary may require setting a range of sub-boundaries based on the effects of many individual chemicals combined with identifying specific effects on sensitive organisms,” the expert group suggests.

The toxics issue actually provides a good case study for the sensible use of regulation under conditions of profound scientific uncertainty. Most of those chemicals that are known beyond reasonable doubt to have toxic effects on both humans and wildlife are already strongly regulated at the international level. The Stockholm Convention on Persistent Organic Pollutants was adopted in 2001 and came into force in 2004, banning the production and use of some of the most damaging endocrine-disrupting and long-lived chemical pollutants. The initial “dirty dozen” covered by the Convention included nine pesticides (aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, and toxaphene); two industrial chemicals (PCBs as well as hexachlorobenzene, also used as a pesticide); and unintentional by-products, most importantly dioxins and furans. At a Convention of the Parties to this international agreement in May 2009 in Geneva, nine other chemicals were listed for eventual elimination. These include various flame retardants, by-products of the hazardous pesticide lindane (itself due to be eliminated), another agricultural pesticide called chlordecone, and perfluorooctanesulfonic acid, or PFOS, which is found in products such as electrical and electronic parts, firefighting foam, photo imaging, hydraulic fluids, and textiles.29 This all represents good progress, and the agreements and efforts of campaigners and policymakers should be applauded.

Where less is known, the approach to toxics regulation provides a reasonably good model of the precautionary principle in action. In Europe the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation, passed in 2007 and supervised by the new European Chemicals Agency, requires all companies producing more than one tonne of any novel chemical per year to safety-test and register the substance—and all chemicals sold in the EU must be covered by 2018.30 The EU agency expects 30,000 chemicals to be tested and registered by the 2018 deadline, a number so large that many experts warn that testing facilities will be stretched to the limit as a result.31 In April 2010 lawmakers