Climate change and environmental degradation is causing large-scale human movement, a phenomenon that needs to be taken more seriously by policy-makers, according to a report released by the International Organization for Migration (IOM) on Tuesday.

Sculptures representing the climate refugees are placed in water in Copenhagen, December 9, 2009.Climate change stands to drive as many as one billion people from their homes over the next four decades, the International Organization for Migration said in a study Tuesday.

The number of natural disasters has more than doubled over the last two decades, and more than 20 million people were displaced by sudden-onset climate-related natural disasters in 2008, said the report, titled "Migration, Environment and Climate Change: Assessing the Evidence."

Further climate change, with global temperatures expected to rise between 2 and 5 degrees centigrade by the end of this century, could have a major impact on the movement of people, according to the report.

Estimates have suggested that between 25 million to 1 billion people could be displaced by climate change over the next 40 years, it said.

Although experts have dismissed such figures as, at best, " guesswork" these statistics have helped to focus policy makers' attention on the likely implications of climate change on migration, it added.

The report also found that so far most of the human movement caused by climate change was internal or cross-border migration.

Movements are from rural to rural areas or from rural to urban areas while international migration is mainly cross-border movement as long-distance international migration would require planning and resources that those who have lost homes and livelihoods are less likely to have.

But international migration is likely to be increasingly important in the future and will necessitate policy and program responses that are currently lacking in destination countries in the developed world, the report said.

Potential future hotspots for international migration are also identified. These are countries which have high emigration rates, face enormous socio-economic challenges and which experience significant slow-onset climate-related disasters that impact on issues such as food security. They include Afghanistan, Bangladesh, most of Central America, several West African and South East Asian countries, amongst others.

The report also pointed out that human movement, despite the problems it causes, can also be considered a way for countries to adapt to climate change. But so far this side of the issue has largely been neglected by policy makers and the media.