The fall-out from the Chernobyl disaster propelled Phillipe Sands into international environmental law. This week he will make legal history representing Hungary against Slovakia. Graham Lawton reports

When barrister Phillipe Sands stands up in the International Court of Justice on Monday he will make legal history. Facing him in the dock will be the state of Slovakia. Sands, representing Hungary, will accuse Slovakia of unlawfully diverting the course of the River Danube. This action, he will claim, has damaged the environment and broken international agreements about preserving the natural world. The case is the most important test yet of international environmental law.

Sands's presence in the courtroom will add extra gravitas to the occasion. Ten years ago international environmental law was a little-known backwater. Now it is a thriving area of theory and practice that offers genuine solutions to the world's environmental problems. Sands, perhaps more than anybody, is responsible for the transformation.

But the 36-year-old, a reader in international law at the University of London's School of Oriental and African Studies, is not a green activist. His interest in environmental law is primarily academic. "I don't think of myself as an environmentalist," he says. He is an occasional barrister who sees legal sparring mainly as an opportunity to take ideas into the courtroom and try them out. He even claims that his expertise in international environmental law is down to luck.

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"I got into the area of the environment completely by accident," he says. In 1986 Sands was at Cambridge University working on international law relating to agreements between governments and international corporations. He had no special interest in environmental law and did not think of himself as environmentally aware. Then on April 26 the Chernobyl nuclear power station exploded.

The disaster spread radioactive waste over the globe. Suddenly, international lawyers were being asked searching questions about cross-border pollution - questions to which they had no answers. The issue had never arisen before and no legal provisions existed that could deal with it.

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A conference was quickly convened in Washington. Out of the blue, Sands was asked to give a paper on the legal aspects of the catastrophe. He protested his ignorance but agreed to have a stab. After all, he was no more ignorant than anybody else.

Sands's paper was a roaring success. He was quickly commissioned to write a follow-up book. Chernobyl: Law and Communication, the first text to deal exclusively with international environmental law, came out in 1988. It transformed him into a fully fledged expert. Suddenly, the distinctly un-green Sands had Friends of the Earth and Greenpeace on the phone asking him for advice.

Sands, by now at King's College London, asked his colleague James Cameron for help with his new-found advisory role. "We decided that instead of doing it as individuals the sensible thing to do would be to create a university-based institution," he says. In 1989, Sands and Cameron cofounded the Centre for International Environmental Law at King's.

The centre broke the mould of international law by offering research, advice and practice under one institutional roof. It expanded its role and began providing assistance to developing countries. In 1992, to reflect its growing stature, it became Field, the Foundation for International Environmental Law and Development. When Sands moved to London's School of Oriental and African Studies in 1993, Field followed. It is now a highly respected international organisation, attracting millions of pounds in research funding.

"I try to understand how international law seeks to remake the world and how the world makes international law," says Sands. "Law is an artificial construct; it provides insights into the nature of human relations and an opportunity to shape those relations. My interest in it is to gain an understanding of how international actors interact, why they act as they do and how they might be required or forced to act differently."

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Academic law is a broad church. At one end of the spectrum it is a social science. At the other are people who believe its role is to prepare people for practice. "I come somewhere between the two," says Sands. Seven years ago he was lucky to get 15 students. Now he gets classes of 70 and international environmental law is a permanent fixture in the ten most popular options of the University of London's law masters (LLM) course.

Sands thinks it is popular because it captures the mood of a generation. "The traditional view was that international law is about relations between states only," he says. "In the modern world view, that is too simplistic. The international community comprises states, international organisations and a gamut of nonstate actors - individuals, corporations and so on. International law, in particular in the environmental field, has now begun to take account of this."

The new approach has led to a drive for interconnection in what has been a fragmentary discipline. International law is being united under an umbrella term that encapsulates the environmental bent of the new generation - international law in the field of sustainable development.

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After Hungary vs. Slovakia, international sustainable development law faces another monumental challenge. In December the final round of the climate-change negotiations begins in Japan. Field will be there, as adviser to the Alliance of Small Island States.

Small islands have an enormous stake in the proceedings - they will be the first to go under when global warming pushes up sea levels. The alliance was the only group to table concrete proposals for cutting greenhouse gases at the last round of talks. Tokyo is the watershed for the alliance.

Climate-change scientists are gloomy about the future of small islands. But Sands sees that as a cause for hope. The negotiations, he says, are science-driven. "I think there will be a protocol adopted with targets and timetables. At the end of the day, states are reasonable. They don't ignore strong science and a small minority cannot prevent the sensible majority from moving along."

"But," he warns, "international environmental law is led by disasters; it is reactive rather than proactive. It requires a Chernobyl or an ozone hole to lead to a legislative response. I think with climate change until you get real impacts you're unlikely to get real action."

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If an agreement emerges in Japan then Sands can legitimately claim hero status in the battle to save the planet. But he will still be reluctant to call himself an environmentalist. "It's not the be-all and end-all," he says. "If it means putting the environment above everything else then I'm not. But if it means taking account of the environment across the range of human activities then, yes, I am an environmentalist."