A United Nations report raised the threat of climate change to a whole new level on Monday, warning of sweeping consequences to life and livelihood.

The report from the UN’s Intergovernmental Panel on Climate Change concluded that climate change was already having effects in real time — melting sea ice and thawing permafrost in the Arctic, killing off coral reefs in the oceans, and leading to heat waves, heavy rains and mega-disasters.

And the worst was yet to come. Climate change posed a threat to global food stocks, and to human security, the blockbuster report said.

“Nobody on this planet is going to be untouched by the impacts of climate change,” said Rajendra Pachauri, chair of the IPCC.

Monday’s report was the most sobering so far from the UN climate panel and, scientists said, the most definitive. The report — a three year joint effort by more than 300 scientists — grew to 2,600 pages and 32 volumes.

The volume of scientific literature on the effects of climate change has doubled since the last report, and the findings make an increasingly detailed picture of how climate change — in tandem with existing fault lines such as poverty and inequality — poses a much more direct threat to life and livelihood.

This was reflected in the language. The summary mentioned the word “risk” more than 230 times, compared to just over 40 mentions seven years ago, according to a count by the Red Cross.

At the forefront of those risks was the potential for humanitarian crisis. The report catalogued some of the disasters that have been visited around the planet since 2000: killer heat waves in Europe, wildfires in Australia, and deadly floods in Pakistan.

“We are now in an era where climate change isn’t some kind of future hypothetical,” said Chris Field, one of the two main authors of the report.

Those extreme weather events would take a disproportionate toll on poor, weak and elderly people. The scientists said governments did not have systems in place to protect those populations. “This would really be a severe challenge for some of the poorest communities and poorest countries in the world,” said Maggie Opondo, a geographer from the University of Nairobi and one of the authors.

The warning signs about climate change and extreme weather events have been accumulating over time. But this report struck out on relatively new ground by drawing a clear line connecting climate change to food scarcity, and conflict.

The report said climate change had already cut into the global food supply. Global crop yields were beginning to decline — especially for wheat — raising doubts as to whether production could keep up with population growth.

“It has now become evident in some parts of the world that the green revolution has reached a plateau,” Pachauri said.

The future looks even more grim. Under some scenarios, climate change could lead to dramatic drops in global wheat production as well as reductions in maize.

“Climate change is acting as a brake. We need yields to grow to meet growing demand, but already climate change is slowing those yields,” said Michael Oppenheimer, a Princeton professor and an author of the report.

Other food sources are also under threat. Fish catches in some areas of the tropics are projected to fall by between 40 percent and 60 percent, according to the report.

The report also connected climate change to rising food prices and political instability, for instance the riots in Asia and Africa after food price shocks in 2008.

“The impacts are already evident in many places in the world. It is not something that is [only] going to happen in the future,” said David Lobell, a professor at Stanford University’s centre for food security, who devised the models.

“Almost everywhere you see the warming effects have a negative affect on wheat and there is a similar story for corn as well. These are not yet enormous effects but they show clearly that the trends are big enough to be important,” Lobell said.

The report acknowledged that there were a few isolated areas where a longer growing season had been good for farming. But it played down the idea that there may be advantages to climate change as far as food production is concerned.

Overall, the report said, “Negative impacts of climate change on crop yields have been more common than positive impacts.” Scientists and campaigners pointed to the finding as a defining feature of the report.

The report also warned for the first time that climate change, combined with poverty and economic shocks, could lead to war and drive people to leave their homes.

With the catalogue of risks, the scientists said they hoped to persuade governments and the public that it was past time to cut greenhouse gas emissions and to plan for sea walls and other infrastructure that offer some protection for climate change.

“The one message that comes out of this is the world has to adapt and the world has to mitigate,” said Pachauri.