Changing weather patterns mean experts face fresh challenges trying to forecast conditions, a leading meteorologist warned ahead of Monday's meeting of world leaders and environment experts in Geneva.

Climate change had brought about a "new situation," World Meteorological Organisation Director General Michel Jarraud warned ahead of the World Climate Conference.

This meant that basing key political and business decisions on issues such as flood defences, crops or power generation could no longer be based on past weather patterns, he said.

Climate-dependent decisions have often relied on experience of past weather patterns and sea levels, he argued.

He cited as examples the building of the dykes in the Netherlands on the basis of centennial floods and precautions against tsunami in Asia.

"Now we need to anticipate change, we can no longer base ourselves on the past to take decisions for the future," he told journalists.

The United Nations will try to start building a new global climate observation and forecasting system at the five-day conference of 150 countries, to cope with the unpredictable era brought about by climate change.

The WMO is urging countries to share the tools that will help them to better predict and cope with the local impacts of global warming.

The idea is to change the face of a weather report from a simple next-day forecast to one that also looks at months or seasons ahead.

The proposed "Global Framework for Climate Services" could shape decisions on water, agriculture, fisheries, health, forestry, transport, tourism, energy, and managing disaster risk, according to the agency.

Swiss President and Finance Minister Hans-Rudolf Merz, who is due to open the conference, stressed the economic importance of sound climate information.

"Climate change poses unprecedented challenges for our societies, economies and environment, these changes will include great costs worldwide," he said in a statement.

Officials claimed the Geneva conference could provide an "important building block" for the troubled talks on a new global climate change deal in Copenhagen in December.

The Copenhagen meeting is meant to produce measures to help countries prevent or adapt to the impact of more extreme weather conditions produced by global warming.

About 1,500 participants are expected in Geneva, including scientists, ministers and about 20 heads of state.

The proposed framework is aimed at improving both longer term forecasting and for the local or regional effects. These localised effects can vary significantly from global or national averages and be much harsher.

"Without this infrastructure we might not know how much climate is changing," said Swiss state weather agency Meteosuisse chief Daniel Keurleber.

"In addition to our existing weather predictions, the timescale reaching up to months and years is a new challenge."

Europe, where some national weather agencies already do complex, computer-powered climate modelling and long term forecasts, is setting up its own regional network.

But the head of the 182 nation WMO underlined the need for a "robust" network covering all the planet, including Africa and underdeveloped areas.

"Observation networks have improved in some parts of the world, unfortunately in other parts of the world they have degraded," Jarraud said.

While some officials described the conference objective as "vague", they also acknowledged that two previous world conferences had produced significant results.

The 1979 conference set in motion world climate research -- and ultimately the panel of international scientists that provided groundbreaking scientific evidence of climate change and of humanity's influence on global warming.