At first I thought it must be one of the great Victorian explorers who recounted their travels in that very room: Dr Livingstone, I presumed. But I glimpsed a familiar glint of spectacles - and recognised the unquiet spirit of John Major.
Exactly five years ago, the ex-PM muffed his first big entrance on the international stage after the 1992 election, at the Rio Earth Summit: this conference was discussing whether Tony Blair would do better at its follow-up in New York this month.
Incredible as it now seems, John Major - then in the short summer of his content, between his election victory and Black Wednesday - was seen as one of the most authoritative world leaders. He was eagerly awaited: we were told to expect a decisive intervention, even money to help finance green measures in the Third World.
Came the day, he had little to say, and less to offer - apart from the so-called Darwin Initiative on conservation, which showed that, having failed to provide even peanuts, he had dredged up The Beagle.
The weather gods seemed fed up too. Throughout the summit, world leaders had gone up Corcovado mountain to be photographed by their national press, in brilliant sunshine, under the outstretched arms of its statue of Christ. But when Mr Major tried, there was thick fog.
o NOW IT'S Tony Blair's turn for a honeymoon, and his chance to establish himself internationally. He could break a deadlock in implementing the agreements reached at Rio, because the rich world has broken its promise to help finance them. Until now there has seemed no chance of progress. But the British and French elections could change the political weather: Labour, for example, is committed to reversing the decline in aid.
Blair is taking along John Prescott, Robin Cook, Clare Short and Michael Meacher, but Downing Street does not yet seem to have grasped the importance of the opportunity. The proposals being discussed so far - such as a pounds 1m fund for environmental education - sound fatefully like Mr Major's cherished Darwin Initiative.
He might like to look at some of the ideas raised at last week's conference, organised by UNED-UK. I am reminded of one by Mr Eric Lamburn, a reader from Birmingham, who writes to me about the huge contribution of aircraft to global warming. He's right. A return flight to Florida pumps out as much carbon dioxide for each passenger as each of us emits, from all other sources, in 10 weeks.
Astonishingly, aircraft fuel is not taxed. Yet a levy of just 2p per litre would add onlypounds 6 to the average air ticket while reducing pollution and raising pounds 7bn a year. This way of helping finance the Rio agreements is on the New York agenda. But Mr Blair may miss the point: he's flying there by Concorde.
o JOHN PRESCOTT - who went to the conference by Tube - dons his frogsuit as readily as Gordon Brown shuns his penguin outfit. He confesses "a passion for diving" and will be at it today at the St Mary's Island marine nature reserve, at Whitley Bay.
In the early 1980s he wore it to swim, Chairman Mao-like, down the Thames from Battersea to Westminster to protest against dumping nuclear waste at sea. The then Speaker told him he could not get out at the steps at Parliament because it would be "political". Wondering how to avoid a longer swim, he noticed a painting, behind the Speaker's head, of five MPs escaping down the steps from Charles I. That seemed rather political too, he said - and got his way.