We must respond to by addressing both population and consumption growth.
As we approach , dire warnings about are becoming increasingly common:
Climate change is "clearly a factor" in the period of stormy weather the UK has been experiencing, the defence secretary, Philip Hammond has said.
Dame Julia Slingo of the UK Met Office said the variable UK climate meant there was "no definitive answer" to what caused the storms. "But all the evidence suggests there is a link to climate change," she added.
The US Secretary of State John Kerry has said that climate change is the world's "most fearsome" weapon of mass destruction.
Reducing carbon emissions won't be easy. Low carbon energy sources, such as wind farms, biofuels and nuclear, bring their own problems, while much of the world has yet to industrialise. Improving energy use and production efficiency, while welcome, has definite limits.
Simon Ross, chief executive of Population Matters, commented, "Current climate change is almost certainly man made. The surest and safest way of reducing future emissions is to have smaller families. Most people already do so.
Simply improving education about, and the provision of, effective contraception would reduce the significant level of unplanned pregnancies worldwide. Improving the status of women and promoting the benefits of smaller families would also have a significant impact.
Smaller families bring many other benefits, from increased household disposable income and improved women's participation to reduced pressure on ecosystems and resource use of all kinds.
Together with using low-carbon technologies and reducing personal consumption, countries should seek to move to subreplacement birth rates in order to limit consumption and bring emissions down to sustainable levels."