Extreme weather

Publication - April 1, 2008
There is strong evidence that extreme weather events – such as hurricanes, floods, droughts and heat waves – are increasing because of climate change.

The Financial Initiative of the UN Environment Programme (UNEP) has calculated that the economic costs of global warming are doubling every decade. The cumulative number of people affected by disasters rose to two billion in the 1990s, up from 740 million in the 1970s. Virtually all of these millions were concentrated in poorer countries.

It is difficult to attribute any single weather event to climate change, but there is agreement that climate change increases extreme weather events. In very broad terms, this is because climate change is putting more energy (heat) into the world's weather systems. This energy speeds up the whole system, increasing the number and intensity of storms.

Watch a slideshow on extreme weather.

Heat waves

Climate change may reduce the number of deaths from cold, but the increase in heat related deaths from climate change will likely far outweigh decreases.

The European heat wave of 2003 killed 14,800 people in France alone and more than 30,000 across the continent. According to the French National Institute of Health the death rate was 60 percent higher than normal for that time of year.Scientific models show that climate change could cause thousands more heat related deaths per year in many major cities by 2050 – independent of population growth. In a breakthrough paper on the attribution of climate change impacts, scientists from the UK concluded with greater than 90 percent likelihood that climate change doubled the risk of that heat wave, which was by far the worst in the historical record.

Heat waves don't only affect people; they can also harm crops, livestock, fish populations and wildlife.

Increased rain and flooding

As the atmosphere warms, it can hold more and more water vapour. This will very likely dry up some areas, dramatically increase precipitation in others, and cause more volatile weather systems in general. The increased precipitation will cause more flooding of homes, flash floods, landslides, erosion, crop damage, and strain on dams.

An example: In summer 2004, two-thirds of Bangladesh, along with much of Assam and Bihar in India, was under water, with more than 50 million people affected. Tens of thousands suffered from diarrhoea as sewage mingled with the flood waters. The main monsoon rice crop was also severely damaged – forcing perhaps 20 million people to seek aid. A similar devastating flood occurred only six years earlier, in 1998.


Climate change will likely cause dry areas to become drier. In general, the risk of drought will likely increase in the mid-latitude interiors of continents.

The increase in droughts will hurt rich and poor nations alike. Regions already experiencing food and water shortages will be the harder hit. A study published by scientists at the National Center for Atmospheric Research in Colorado indicates that the area of the Earth experiencing ‘very dry’ conditions has more than doubled since the 1970s.

For example, Africa already has a highly variable and unpredictable climate. Climate change is making that worse. In the Sahel, there has been on average a 25 percent decrease in annual rainfall over the past 30 years – consistent with climate change models.


Hurricanes need seawater temperatures above 27° Celsius (81°F) in order to form. Water this warm allows massive evaporation that can then condense and form the storm's "vortex".

As the seawater temperature goes even higher the likelihood of storms increases exponentially. Although there are other complex factors involved in hurricane formation, the link between warmer seawater and hurricanes is well established. It is also certain that climate change is raising ocean temperatures. Therefore, climate change is making the conditions under which hurricanes, cyclones and tropical storms form more common.