ANY attempt to put an economic value on fresh air, clean water or tropical rainforests can offend the delicate sensibilities of those who argue that the conservation of nature is a moral duty. Yet although the best things in life appear to be free, that does not mean they are without financial value. It simply means that nobody asks you to pay when, for example, you watch a beautiful sunset over the hills.

Putting a financial value on the environment, however, may be the most important thing that people can do to help nature conservation. When governments allocate money, they do so according to where it will bring benefit. If a government is unaware of the value of a landscape to its tourism, or of a swamp to its fishing industry—and thus its foreign-exchange income—then it will invest too little in managing these resources. Worse, if the true value of a forest or swamp is hidden, governments may destroy it by subsidising the conversion of the land to agriculture. The costs are unknown for now, but may appear eventually as the price of building a filtration plant to remove the sediment from the water that the forest once took care of, or the price of importing food when fish vanish.

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Some estimates of the annual contribution of coastal and marine ecosystems to the global economy exceed $20 trillion, over a third of the total gross national product (GNP) of all the countries of the world. Even so, says Katherine Sierra of the World Bank, such ecosystems are typically much undervalued when governments made decisions about development.

Glenn-Marie Lange, also of the World Bank, attended a meeting in Washington DC organised by her employer to launch its report “Environment Matters” on April 6th. She told participants that one of the reasons why ecosystems become degraded is that their value to local people is often small. As a result, these people do not have much reason to manage their resources carefully. She estimates, for example, that only 36% of the income generated by the coastal and marine environments in Zanzibar goes to locals. Most of this comes from fishing; only a tiny fraction of the money from tourism ends up local hands.

More broadly, Dr Lange wants the value of the environment to be integrated into national and local accounting. She argues that governments should identify the contributions that marine ecosystems make to their countries' GNPs and foreign-exchange earnings. She also wants them to examine whether or not they are running down their countries' “natural capital”.

Emily Cooper of the World Resources Institute, an environmental think-tank, put some figures on the value of tourism, recreation, fisheries and shoreline protection in Belize. It was an impressive $395m to $559m. The entire economy was worth about $1.3 billion in 2007. These figures, she thinks, have allowed environmentalists to protect Belize's threatened mangrove forests better.

For too long, an absence of proper green accounting has allowed people to privatise the gains from the environment but socialise the costs, to paraphrase Carl Safina, an American scientist and environmentalist at the meeting. As Dr Safina puts it, “conservation is not a trade-off between the economy and the environment. It is a trade off between the short and long term.”