Climate change is set to inflict “severe, widespread, and irreversible impacts” on people and the natural world unless carbon emissions are cut sharply and rapidly, according to the most important assessment of global warming yet published.

The stark report states that climate change has already increased the risk of severe heatwaves and other extreme weather and warns of worse to come, including food shortages and violent conflicts. But it also found that ways to avoid dangerous global warming are both available and affordable.

“Science has spoken. There is no ambiguity in the message,” said the UN secretary general, Ban Ki-moon, attending what he described as the “historic” report launch. “Leaders must act. Time is not on our side.” He said that quick, decisive action would build a better and sustainable future, while inaction would be costly.

Ban added a message to investors, such as pension fund managers: “Please reduce your investments in the coal- and fossil fuel-based economy and [move] to renewable energy.”

The report, released in Copenhagen on Sunday by the UN’s Intergovernmental Panel on Climate Change (IPCC), is the work of thousands of scientists and was agreed after negotiations by the world’s governments. It is the first IPCC report since 2007 to bring together all aspects of tackling climate change and for the first time states: that it is economically affordable; that carbon emissions will ultimately have to fall to zero; and that global poverty can only be reduced by halting global warming. The report also makes clear that carbon emissions, mainly from burning coal, oil and gas, are currently rising to record levels, not falling.

The report comes at a critical time for international action on climate change, with the deadline for a global deal just over a year away. In September, 120 national leaders met at the UN in New York to address climate change, while hundreds of thousands of marchers around the world demanded action.

“We have the means to limit climate change,” said Rajendra Pachauri, chair of the IPCC. “The solutions are many and allow for continued economic and human development. All we need is the will to change.”

Lord Nicholas Stern, a professor at the London School of Economics and the author of an influential earlier study, said the new IPCC report was the “most important assessment of climate change ever prepared” and that it made plain that “further delays in tackling climate change would be dangerous and profoundly irrational”.

“The reality of climate change is undeniable, and cannot be simply wished away by politicians who lack the courage to confront the scientific evidence,” he said, adding that the lives and livelihoods of hundreds of millions of people were at risk.

Ed Davey, the UK energy and climate change secretary, said: “This is the most comprehensive and robust assessment ever produced. It sends a clear message: we must act on climate change now. John Kerry, the US secretary of state, said: “This is another canary in the coal mine. We can’t prevent a large scale disaster if we don’t heed this kind of hard science.”

Bill McKibben, a high-profile climate campaigner with 350.org, said: “For scientists, conservative by nature, to use ‘serious, pervasive, and irreversible’ to describe the effects of climate falls just short of announcing that climate change will produce a zombie apocalypse plus random beheadings plus Ebola.” Breaking the power of the fossil fuel industry would not be easy, McKibben said. “But, thanks to the IPCC, no one will ever be able to say they weren’t warned.”

The new overarching IPCC report builds on previous reports on the science, impacts and solutions for climate change. It concludes that global warming is “unequivocal”, that humanity’s role in causing it is “clear” and that many effects will last for hundreds to thousands of years even if the planet’s rising temperature is halted.

In terms of impacts, such as heatwaves and extreme rain storms causing floods, the report concludes that the effects are already being felt: “In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans.”

Droughts, coastal storm surges from the rising oceans and wildlife extinctions on land and in the seas will all worsen unless emissions are cut, the report states. This will have knock-on effects, according to the IPCC: “Climate change is projected to undermine food security.” The report also found the risk of wars could increase: “Climate change can indirectly increase risks of violent conflicts by amplifying well-documented drivers of these conflicts such as poverty and economic shocks.”

Two-thirds of all the emissions permissible if dangerous climate change is to be avoided have already been pumped into the atmosphere, the IPPC found. The lowest cost route to stopping dangerous warming would be for emissions to peak by 2020 – an extremely challenging goal – and then fall to zero later this century.

The report calculates that to prevent dangerous climate change, investment in low-carbon electricity and energy efficiency will have to rise by several hundred billion dollars a year before 2030. But it also found that delaying significant emission cuts to 2030 puts up the cost of reducing carbon dioxide by almost 50%, partly because dirty power stations would have to be closed early. “If you wait, you also have to do more difficult and expensive things,” said Jim Skea, a professor at Imperial College London and an IPCC working group vice-chair.

Tackling climate change need only trim economic growth rates by a tiny fraction, the IPCC states, and may actually improve growth by providing other benefits, such as cutting health-damaging air pollution.

Carbon capture and storage (CCS) – the nascent technology which aims to bury CO2 underground – is deemed extremely important by the IPPC. It estimates that the cost of the big emissions cuts required would more than double without CCS. Pachauri said: “With CCS it is entirely possible for fossil fuels to continue to be used on a large scale.”

The focus on CCS is not because the technology has advanced a great deal in recent years, said Jean-Pascal van Ypersele, a professor at the Université Catholique de Louvain in Belgium and vice-chair of the IPCC, but because emissions have continued to increase so quickly. “We have emitted so much more, so we have to clean up more later”, he said.

Linking CCS to the burning of wood and other plant fuels would reduce atmospheric CO2 levels because the carbon they contain is sucked from the air as they grow. But van Ypersele said the IPCC report also states “very honestly and fairly” that there are risks to this approach, such as conflicts with food security.

In contrast to the importance the IPCC gives to CCS, abandoning nuclear power or deploying only limited wind or solar power increases the cost of emission cuts by just 6-7%. The report also states that behavioural changes, such as dietary changes that could involve eating less meat, can have a role in cutting emissions.

As part of setting out how the world’s nations can cut emissions effectively, the IPCC report gives prominence to ethical considerations. “[Carbon emission cuts] and adaptation raise issues of equity, justice, and fairness,” says the report. “The evidence suggests that outcomes seen as equitable can lead to more effective [international] cooperation.”

These issues are central to the global climate change negotiations and their inclusion in the report was welcomed by campaigners, as was the statement that adapting countries and coastlines to cope with global warming cannot by itself avert serious impacts.

“Rich governments must stop making empty promises and come up with the cash so the poorest do not have to foot the bill for the lifestyles of the wealthy,” said Harjeet Singh, from ActionAid.

The statement that carbon emissions must fall to zero was “gamechanging”, according to Kaisa Kosonen, from Greenpeace. “We can still limit warming to 2C, or even 1.5C or less even, [but] we need to phase out emissions,” she said. Unlike CCS, which is yet to be proven commercially, she said renewable energy was falling rapidly in cost.

Sam Smith, from WWF, said: “The big change in this report is that it shows fighting climate change is not going to cripple economies and that it is essential to bringing people out of poverty. What is needed now is concerted political action.” The rapid response of politicians to the recent global financial crisis showed, according to Smith, that “they could act quickly and at scale if they are sufficiently motivated”.

Michel Jarraud, secretary general of the World Meteorological Organisation, said the much greater certainty expressed in the new IPCC report would give international climate talks a better chance than those which failed in 2009. “Ignorance can no longer be an excuse for no action,” he said.

Observers played down the moves made by some countries with large fossil fuel reserves to weaken the language of the draft IPCC report written by scientists and seen by the Guardian, saying the final report was conservative but strong.

However, the statement that “climate change is expected to lead to increases in ill-health in many regions, including greater likelihood of death” was deleted in the final report, along with criticism that politicians sometimes “engage in short-term thinking and are biased toward the status quo”.