Time is running out to limit global warming to two degrees Celsius and avert potential disaster, UN climate experts warn.
Crowning a landmark review, the Intergovernmental Panel on Climate Change (IPCC) says emissions of three key greenhouse gases are at their highest in more than 800,000 years.
Earth, warns the paper, is on a likely trajectory for at least 4 C warming over pre-industrial times by 2100 – a recipe for worsening drought, flood, rising seas and extinctions.
Many could face hunger, homelessness and conflict in the scramble for precious resources.
UN chief Ban Ki-moon, who in September hosted a special summit on climate change, said on Sunday: “Human influence on the climate system is clear, and clearly growing.”
“We must act quickly and decisively if we want to avoid increasingly disruptive outcomes,” Ban said, describing it as a “myth” that tackling carbon emissions was costly.
US Secretary of State John Kerry said the report was a fresh warning – “another canary in the coal mine” – while France, which is to host a UN 2015 climate conference, called for “immediate, all-round mobilisation”.
IPCC chief Rajendra Pachauri said there remained “little time before the window of opportunity to stay within 2 C of warming closes”.
“To keep a good chance of staying below 2 C, and at manageable costs, our emissions should drop by 40 to 70 per cent globally between 2010 and 2050, falling to zero or below by 2100.”
Seizing on the report, green groups demanded the phase-out of coal, oil and gas that are driving the carbon problem.
“Renewables and the smart use of energy are the quickest and cleanest ways to cut emissions,” said Greenpeace’s Kaisa Kosonen.
“Any technology that ‘handles’ emissions rather than replaces fossil fuels is like smoking crack to solve an alcohol addiction.”
The first overview by the Nobel-winning organisation since 2007 – comes ahead of UN talks in Lima next month to pave the way to a pact in Paris to limit warming to a safer 2 C.
But negotiations have been hung up for years over which countries should shoulder the cost for reducing carbon emissions.
The report said switching to cleaner sources, reducing energy efficiency and implementing other emission-mitigating measures would be far cheaper than the cost of climate damage.
The bill today for doing this is affordable, but delaying beyond 2030 would cause the cost – and the climate peril – to spiral for future generations.
“Ambitious” carbon curbs would shave just 0.06 percentage points annually from global consumption this century, targeted to grow by 1.6-3.0 per cent annually, the IPCC said.
Under the lowest of four emissions scenarios, global average temperatures over this century are likely to rise by 0.3-1.7 C, leading to between 26 and 55 cm in sea-level rise.
Under the highest scenario, warming would be 2.6-4.8 C, causing sea-level rise of 45-82 cm.
The report warned bleakly that on current trends, “warming is more likely than not to exceed 4 C” over pre-industrial levels by 2100.