"We have the means to limit climate change," said IPCC chair Raj Pachauri. "The solutions are many and allow for continued economic and human development. All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change."

According to the IPCC, the report finds that human influence on the climate system is clear and growing, with impacts observed on all continents. If left unchecked, climate change will increase the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. There are, however, options for adapting to and mitigating climate change that would keep the impacts manageable.

"Our assessment finds that the atmosphere and oceans have warmed, the amount of snow and ice has diminished, sea level has risen and the concentration of carbon dioxide has increased to a level unprecedented in at least the last 800,000 years," said Thomas Stocker, co-chair of IPCC Working Group I, which examined the physical science basis.

The Synthesis Report, said the IPCC, makes a clear case that many risks constitute particular challenges for the least-developed countries and vulnerable communities, given their limited ability to cope.

"Many of those most vulnerable to climate change have contributed and contribute little to greenhouse-gas emissions," Pachauri said. "Addressing climate change will not be possible if individual agents advance their own interests independently; it can only be achieved through cooperative responses, including international co-operation."

Adaptation can help prepare for the risks of climate change to which the world is already committed by past emissions and existing infrastructure, while being integrated with the pursuit of development, according to Vicente Barros, co-chair of IPCC Working Group II, which examined impacts, adaptation and vulnerability.

But substantial reductions of greenhouse-gas emissions are also needed. Because this mitigation reduces the rate as well as the magnitude of warming, it also lengthens the time available for adaptation to a particular level of climate change, potentially by several decades, said the IPCC.

There are multiple routes to achieving the emissions reductions needed across the next few decades to have a greater than 66% chance of limiting warming to 2°C this century, the report found. But delaying additional mitigation to 2030 will substantially increase the technological, economic, social and institutional challenges associated with attempting to keep below this limit.

"It is technically feasible to transition to a low-carbon economy," said Youba Sokona, co-chair of IPCC Working Group III on mitigation. "But what is lacking are appropriate policies and institutions. The longer we wait to take action, the more it will cost to adapt and mitigate climate change."

Cost estimates for mitigation vary but indicate that global economic growth "would not be strongly affected". In business-as-usual scenarios, consumption – a proxy for economic growth – grows by 1.6 to 3% per year across the 21st century, said the IPCC press release. Ambitious mitigation would reduce this by about 0.06%. "Compared to the imminent risk of irreversible climate-change impacts, the risks of mitigation are manageable" said Sokona.

The last word goes to Pachauri. "To keep a good chance of staying below 2 °C, and at manageable costs, our emissions should drop by 40 to 70% globally between 2010 and 2050, falling to zero or below by 2100," he said. "We have that opportunity, and the choice is in our hands."

Related links