An EU report says climate change will have a growing impact on global security, multiplying existing threats such as shortages of food and water.
It warns that climate change could cause millions of people to migrate towards Europe as other parts of the world suffer environmental degradation.
States that are "already fragile and conflict prone" could be over-burdened, the report says.
EU proposals to tackle climate change will be discussed by leaders this week.
The stark warning from the report - drawn up by the EU's foreign affairs chief Javier Solana and the European Commission - is that climate change is not just a threat in itself - it is "a threat multiplier".
It says shortages of food and water - even radicalisation and state failure - are likely to get worse if no action is taken.
It may lead to... more forced migration, and even possibly radicalisation and state failure
