With hunting for ivory once more a concern, conservation efforts have switched to trying to influence public opinion in Asia

No one could be entirely sure that the tusks – great curves of hard dentine coated in enamel – would actually burn. To be on the safe side, park wardens doused them in petrol and arranged them in a pyre. When set alight, the ivory burned fiercely, watched by Kenyan cabinet ministers, diplomats, conservationists and, most importantly, television crews and photographers, who relayed the images all over the world.

That was the morning of 18 July 1989, when a bonfire was made of 12 tonnes – or nearly £2m worth – of ivory in a spectacle designed to send out the message that the killing of elephants and the trade in their tusks had to be stopped. In the preceding decade numbers of Africa's largest land mammal had fallen from 65,000 to 17,000.

"To stop the poacher, the trader must also be stopped; and to stop the trader, the final buyer must be convinced not to buy ivory," said Daniel Arap Moi, Kenya's then president. "I appeal to people all over the world to stop buying ivory."

Remarkably, they did. And soon after that, the Convention on International Trade in Endangered Species (Cites) placed ivory in its Appendix I, effectively creating an outright ban on the trade.

The site where the pyre was lit, two miles inside the gates of the Nairobi national park, is now marked by a grandiose stone tomb. It sits in a neglected picnic site where a scattering of concrete benches surrounds a faded sign that recounts the historic day and concludes: "As you picnic here, reflect and join Kenyans in saying never again." But for all the imploring, it has happened again. In the past five years the poaching crisis has returned in response to rising demand from newly wealthy markets in Asia.

In the Selous game reserve in Tanzania, home to one of the two largest populations of African elephants, numbers had fallen from 50,000 in 2007 to 13,000 by the end of last year. Globally, up to 100 elephants a day are thought to be poached for their tusks in a worse slaughter than that of the 1980s, according to conservationist Allan Thornton, president of the Environmental Investigation Agency, the international organisation that provided much of the evidence on which the original ban was based.

The rhino – prized for its horn, which is spuriously claimed in some cultures to have medicinal properties – is in similarly dire straits. It is in this context that up to 50 world leaders will converge on London on Thursday for a conference on the illegal wildlife trade that may represent the best hope for reversing the trend.

"Hopefully we are building to a moment similar to 1989, although the current situation is far more damaging to Africa's elephant populations," said Thornton. "Poaching and the illegal ivory trade appear to still be gaining ever greater momentum to supply the markets in China and Japan."

The 1989 ban and the accompanying publicity turned western public opinion against ivory. Within a year, the price of ivory had plummeted from tens of thousands of pounds for a carved tusk to just £1 a kilo.

Among the "range states" – countries where elephants live in the wild – there was disagreement over the 1989 ban. Several southern African nations, including Botswana and Zimbabwe, argued that stockpiled ivory, which they said had been collected from pachyderms that died of natural causes, should be sold at auction. The first of these legal auctions took place in 2002. A second "one-off sale" was sanctioned in 2007, despite fierce opposition from conservationists, who objected both to the auction and to Cites's approval of China as a bidding nation. They warned that China was in no position to control the illegal trade and that the 70 tonnes it bought would spur demand.

Peter Knights, head of the pressure group WildAid, watched what happened next. The presence of legal and illegal ivory in the market created ambiguity for consumers as well as providing cover for criminal enterprises looking to launder their supply from poachers.

"We thought we had saved the elephant and then we found ourselves at square one again," said Knights. His organisation is calling for an end to ivory sales and the destruction of existing stockpiles.

Knights believes that the 21st century equivalent of the Nairobi bonfire began in the unlikely setting of Denver, Colorado, in a warehouse on the edge of the Great Plains. It was here, at the offices of National Wildlife Property Repository, that six tonnes of ivory, seized over time from smugglers entering the US, was destroyed. It is a sign of the times that ivory is now crushed rather than burned – environmentalists had been concerned about carbon dioxide emissions.

Similar stockpiles have been crushed in the Philippines and Kenya, while Hong Kong has also agreed to destroy much of its reserve. China followed last month, feeding seven tonnes of seized ivory into a tarmac-crushing machine. But even this still represents only a fraction of existing stockpiles.

The London conference is expected to see rich nations pledge hundreds of millions of pounds to an emergency fund to finance anti-poaching efforts in the range states, but seasoned campaigners will be watching for commitments from the ivory-consuming countries.

Knights, who compares attempts to stop poaching in Africa with failed efforts to strangle the illegal drug trade in producing countries, warns that only a "demand-side" approach will work. WildAid has persuaded famous Chinese athletes such as the basketball star Yao Ming to lead a public awareness campaign to persuade Chinese consumers not to purchase ivory.

"We have tried the supply side and it clearly isn't working," he said.

Meanwhile the killing continues. There are no elephants in Nairobi national park, but a fortnight ago, only a rifle shot away from the site where the ivory was burnt, a female rhino was slaughtered – in what is supposed to be among the most heavily protected parks in east Africa.