An international conference on global warming has warned time is running out to avoid "abrupt and irreversible" climate change.
A major scientific report has shown the Earth is heading for a warmer age at a quickening pace because of human activity.
The evidence is in the warming of air and ocean temperatures, widespread melting snow and ice and rising sea levels.
United Nations secretary general Ban Ki-moon has warned the world must work together to fight the menace of climate change.
Mr Ban said "Concentrated and sustained action can still avoid some of the most catastrophic scenarios."
Responding to the information contained in the report, Prime Minister Gordon Brown said developed countries "must show leadership and take the first and largest responsibility".
Mr Brown will set out in the next few days various ways in which he believes emissions can be cut.
The report by a Nobel-winning UN scientific panel, will be used by political negotiators to begin talks on a successor to the Kyoto Protocol, the action plan for controlling greenhouse gas emissions.
It is the result of six years of research compiled by the Intergovernmental Panel on Climate Change, and declares that climate systems have already unquestionably begun to change.
The report warns that hunger and disease will be more common, droughts, floods and heat waves will afflict the world's poorest regions, and more animal and plant species will vanish, unless action is taken.