For a country suffering its longest drought on record, rainfall of 2.7 inches in the capital city might be a cause for satisfaction. Unfortunately, it fell in a single hour in Canberra at the end of February. Freak storms left three-foot drifts of hail, damaging hundreds of buildings, led to flash flooding in Sydney, and showed a paradox of climate change: an increase in both droughts and floods. The storms, like other "extreme weather events" such as Hurricane Katrina, cannot be attributed directly to global warming. But meteorologists say they illustrate dramatically some of its expected effects.

Storms will become more frequent, according to prediction models. But large areas of the world will be affected by severe droughts. Given these twin effects, many regions could suffer a "double whammy" – water shortages coupled with heavy rainfall that could cause mudslides, sweep away crops and topsoil and damage buildings. Research into the Indian monsoon revealed how changes to rainfall patterns may already be taking place in one of the world's biggest agricultural economies.

Scientists from the Indian Institute of Tropical Meteorology published late last year a paper in the peer-review journal Science based on rainfall data from central India from 1951 to 2000.

They found India was experiencing worse rainstorms and a greater risk of floods. Over the period, "extreme monsoon rainfall events" became more common but the "moderate rainfall events" decreased. This is a crucial change because moderate rainfall provides a steady supply of water seeping into the ground. But heavier rainfall in shorter bursts is not such good news, because even though crops receive the same amount overall, they can suffer shortages between storms and when too much falls, much is wasted.

The report's authors concluded: "A substantial increase in hazards related to heavy rain is expected." They also said climate change was the likely cause of the differences.

It is difficult to predict in detail the effects of climate change on rainfall patterns, however. Scientists will be assisted in their forecasts by the development of the Global Earth Observation System, a network of satellites and other systems being built by 33 countries, spearheaded by the US, and expected by 2015. One of its aims is to provide information for "integrated water resource management" by allowing countries a better understanding of rainfall, river flow, glacier melt, and the extraction of water from underground aquifers.

It will also improve understanding of another problem linked to climate change: sea level rises. Rising sea levels, according to a recent study from the University of Tokyo, will reduce the amount of fresh water available for human use in coastal areas.

Salination of underground water supplies in coastal areas could cause as much damage as the more widely publicised problem of flooding of coastal areas.

The well-documented melting of glaciers also has the potential to cause enormous problems. For instance, in India and the Himalayas as many as 500m people depend on glaciers for water. As they melt seasonally, rivers flow through fields providing water.

But warming temperatures are causing the glaciers to melt faster. This leads to two problems: heavier river flows and flooding; and when glaciers are smaller, the amount available from melt will reduce, leading to shortages.