The number of weather-related disasters has quadrupled in the past 20 years and more should be done to prepare for them, according to Oxfam.

Severe flooding ravaged Mexico this month

The international aid agency blames rising greenhouse gas emissions for the jump in severe storms, flooding and drought.

In a report released today, Oxfam says there are now as many as 500 weather-related disasters a year, compared to an average of 120 in the early 1980s.

The number of floods has increased six-fold over the same period, while the number of people affected by weather-related disasters has also risen by 68 per cent.

Oxfam's director, Barbara Stocking, said floods in South Asia had affected more than 250 million people this year alone.

"This is no freak year. It follows a pattern of more frequent, more erratic, more unpredictable and more extreme weather events that are affecting more people," she said.

"Action is needed now to prepare for more disasters otherwise humanitarian assistance will be overwhelmed and recent advances in human development will go into reverse."

Although crises such as the African famines of the early 1980s caused a huge number of deaths, Oxfam says a worrying new trend is the increase in small to medium-sized disasters.

These affect great numbers of people but do not attract as much international aid as well-publicised events.

The charity said repeated small disasters could push poor communities into a downward spiral from which it was difficult to recover.

Oxfam has called on governments to negotiate a global deal to reduce greenhouse gas emissions when they meet at a UN conference in Bali next month.

It also called on the governments of the world's richest nations and the UN to make humanitarian aid faster, fairer and more flexible, while finding better ways to prepare for disasters.