World leaders are under pressure to reach a historic deal to curb emissions as they arrive in Paris amid unprecedented security, Sky News has reported.
Prince Charles has told historic talks in Paris that humanity faces many threats “but none greater than climate change”.
Prime Minister David Cameron and US President Barack Obama were among around 150 leaders at the first day of the two-week United Nations summit, known as COP21.
Expectations are high - more than 600,000 people in 175 countries marched at the weekend to demand a strong deal to curb greenhouse gases.
The talks take place amid unprecedented security in the French capital, where protesters clashed with riot police after a climate march was banned following the attacks.
The gathering of so many heads of state is intended as a clear signal to negotiators that they must reach a significant agreement.
French President Francois Hollande said leaders were meeting in Paris “to reaffirm their solidarity with France” and to “assume their responsibilities in the face of the warming of the planet”.
The last major climate conference five years ago in Copenhagen failed to secure sign-up to a universal target for reducing emissions.
But in advance of the Paris talks 183 nations have submitted individual commitments, large or small, to slow global warming.
The EU has pledged to cut greenhouse gases by at least 40% by 2030 while China has vowed that its emissions will peak by 2030.
Lead negotiator for the UN, Christiana Figueres, said the pledges covered 95% of carbon emissions and would limit the rise in global temperature this century.
Mr Cameron said: “We must include a five-yearly review mechanism to increase ambition in the future.
“Whilst emissions reductions should always be pledged country by country, we must review our ambition regularly if we are to hit our final two degree goal.”