World leaders must now respond to an "unequivocal" message from climate scientists and act with policies to cut greenhouse gas emissions, the United Nations secretary-general urged on Friday.

Introducing a major report from a high level UN panel of climate scientists, Ban Ki-moon said, "The heat is on. We must act."

The world's leading climate scientists, who have been meeting in all-night sessions this week in the Swedish capital, said there was no longer room for doubt that climate change was occurring, and the dominant cause has been human actions in pouring greenhouse gases into the atmosphere.

In their starkest warning yet, following nearly seven years of new research on the climate, the Intergovernmental Panel on Climate Change (IPCC) said it was "unequivocal" and that even if the world begins to moderate greenhouse gas emissions, warming is likely to cross the critical threshold of 2C by the end of this century. That would have serious consequences, including sea level rises, heatwaves and changes to rainfall meaning dry regions get less and already wet areas receive more.

In response to the report, the US secretary of state, John Kerry, said in a statement: "This is yet another wakeup call: those who deny the science or choose excuses over action are playing with fire."

"Once again, the science grows clearer, the case grows more compelling, and the costs of inaction grow beyond anything that anyone with conscience or commonsense should be willing to even contemplate," he said.

He said that livelihoods around the world would be impacted. "With those stakes, the response must be all hands on deck. It's not about one country making a demand of another. It's the science itself, demanding action from all of us. The United States is deeply committed to leading on climate change."

In a crucial reinforcement of their message – included starkly in this report for the first time – the IPCC warned that the world cannot afford to keep emitting carbon dioxide as it has been doing in recent years. To avoid dangerous levels of climate change, beyond 2C, the world can only emit a total of between 800 and 880 gigatonnes of carbon. Of this, about 530 gigatonnes had already been emitted by 2011.

That has a clear implication for our fossil fuel consumption, meaning that humans cannot burn all of the coal, oil and gas reserves that countries and companies possess. As the former UN commissioner Mary Robinson told the Guardian last week, that will have "huge implications for social and economic development." It will also be difficult for business interests to accept.

The central estimate is that warming is likely to exceed 2C, the threshold beyond which scientists think global warming will start to wreak serious changes to the planet. That threshold is likely to be reached even if we begin to cut global greenhouse gas emissions, which so far has not happened, according to the report.

Other key points from the report are:

• Atmospheric concentrations of carbon dioxide, methane and nitrous oxide are now at levels "unprecedented in at least the last 800,000 years."

• Since the 1950's it's "extremely likely" that human activities have been the dominant cause of the temperature rise.

• Concentrations of CO2 and other greenhouse gases in the atmosphere have increased to levels that are unprecedented in at least 800,000 years. The burning of fossil fuels is the main reason behind a 40% increase in C02 concentrations since the industrial revolution.

• Global temperatures are likely to rise by 0.3C to 4.8C, by the end of the century depending on how much governments control carbon emissions.

• Sea levels are expected to rise a further 26-82cm by the end of the century.

• The oceans have acidified as they have absorbed about a third of the carbon dioxide emitted.

Thomas Stocker, co-chair of the working group on physical science, said the message that greenhouse gases must be reduced was clear. "We give very relevant guidance on the total amount of carbon that can't be emitted to stay to 1.5 or 2C. We are not on the path that would lead us to respect that warming target [which has been agreed by world governments]."

He said: "Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions."

Though governments around the world have agreed to curb emissions, and at numerous international meetings have reaffirmed their commitment to holding warming to below 2C by the end of the century, greenhouse gas concentrations are still rising at record rates.

Rajendra Pachauri, chair of the IPCC, said it was for governments to take action based on the science produced by the panel, consisting of thousands of pages of detail, drawing on the work of more than 800 scientists and hundreds of scientific papers.

The scientists also put paid to claims that global warming has "stopped" because global temperatures in the past 15 years have not continued the strong upward march of the preceding years, which is a key argument put forward by sceptics to cast doubt on climate science. But the IPCC said the longer term trends were clear: "Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850 in the northern hemisphere [the earliest date for reliable temperature records for the whole hemisphere]."

The past 15 years were not such an unusual case, said Stocker. "People always pick 1998 but [that was] a very special year, because a strong El Niño made it unusually hot, and since then there have been some medium-sized volcanic eruptions that have cooled the climate."

But he said that further research was needed on the role of the oceans, which are thought to have absorbed more than 90% of the warming so far.

The scientists have faced sustained attacks from so-called sceptics, often funded by "vested interests" according to the UN, who try to pick holes in each item of evidence for climate change. The experts have always known they must make their work watertight against such an onslaught, and every conclusion made by the IPCC must pass scrutiny by all of the world's governments before it can be published.

Their warning on Friday was sent out to governments around the globe, who convene and fund the IPCC.

It was 1988 when scientists were first convened for this task, and in the five landmark reports since then the research has become ever clearer. Now, scientists say they are certain that "warming in the climate system is unequivocal and since 1950 many changes have been observed throughout the climate system that are unprecedented over decades to millennia." That warning, from such a sober body, hemmed in by the need to submit every statement to extraordinary levels of scrutiny, is the starkest yet.

"Heatwaves are very likely to occur more frequently and last longer. As the earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions," Stocker said.

Qin Dahe, also co-chair of the working group, said: "As the ocean warm, and glaciers and ice sheets reduce, global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years."

Prof David Mackay, chief scientific adviser to the Department of Energy and Climate Change, said: "The far-reaching consequences of this warming are becoming understood, although some uncertainties remain. The most significant uncertainty, however, is how much carbon humanity will choose to put into the atmosphere in the future. It is the total sum of all our carbon emissions that will determine the impacts. We need to take action now, to maximise our chances of being faced with impacts that we, and our children, can deal with. Waiting a decade or two before taking climate change action will certainly lead to greater harm than acting now."