Pensioners left on their own during a heatwave in industrialised countries. Single mothers in rural areas. Workers who spend most of their days outdoors. Slum dwellers in the megacities of the developing world.

These are some of the vulnerable groups who will feel the brunt of climate change as its effects become more pronounced in the coming decades, according to a game-changing report from the UN's climate panel released on Monday. Climate change is occurring on all continents and in the oceans, the authors say, driving heatwaves and other weather-related disasters.

And the changes to the Earth's climate are fuelling violent conflicts. The UN for the first time in this report has designated climate change a threat to human security.

The overriding lesson of this report, the scientists said, was that unless governments acted now to reduce greenhouse gas emissions and adopt measures to protect their people, nobody would be immune to climate change.

"There isn't a single region that thinks we can avoid all the impacts of even 2 degrees of warming by adaptation – let alone 4 degrees," said Dr Rachel Warren of the Tyndall centre for climate change research at the University of East Anglia.

"I think you can say that in order to keep global temperature rises at 2 degrees we need to reduce emissions greatly and rapidly, but even at 2 degrees there are still impacts that we can't adapt to."

"We live in an era of manmade climate change," said Dr Vicente Barros, who chaired the report. "In many cases, we are not prepared for the climate-related risks that we already face. Investments in better preparation can pay dividends both for the present and for the future."

But those who did the least to cause climate change would be the first in the line of fire: the poor and the weak, and communities that were subjected to discrimination, the report found.

Scientists went to great lengths in the report to single out people and communities who would be most at risk of climate change, with detailed descriptions of locations and demographics.

"People who are socially, economically, culturally, politically, institutionally or otherwise marginalised are especially vulnerable to climate change," it said.

One impact is through the reduction in crop yields, which leads to higher prices. "The story is that crop yields have detectably changed. As time goes on the poor countries that are in the warmer and drier parts of the planet will feel the crop yield decreases early," said Michael Oppenheimer, professor of geosciences and international affairs at Princeton University. "When you get above two degrees and into the three- and four-degree range, adaptation becomes less effective and even some of the wealthy countries that have advanced agriculture start suffering."

"People who were already disadvantaged, more of them are going to be suffering from malnutrition," he added.

In a further cruel twist, the report said climate change would also make it harder for developing countries to climb out of poverty, and would create "poverty pockets" in rich and poor countries.

It already has. Maarten van Aalst, director of the Red Cross climate centre and an author of the report, said the agency was already seeing evidence that the poor were being hit hardest in weather-related disasters.

"It's the poor suffering more during disasters, and of course the same hazard causes a much bigger disaster in poorer countries, making it even poorer," he said.

There are already more weather-related mega-disasters such as heatwaves and storm surges occurring under climate change.

And the number of natural disasters between 2000 and 2009 was around three times higher than in the 1980s, Van Aalst said. "The growth is almost entirely due to 'climate-related' events," he said.

Graphic: guardian.co.uk

Other threats are looming because of climate change. The Pentagon and the CIA have released a number of threat assessments in recent years identifying climate change as a threat to military installations, and as a potential driver of conflict – a "threat multiplier".

The UN agrees in this report, saying climate change could lead to war and increased migration.

"Climate change can indirectly increase risks of violent conflicts in the form of civil war and inter-group violence," the report said.

The authors, however, were cautious about sending the message that climate change causes war per se.

"Climate change, on its own, does not start wars," said Neil Adger, a professor of geography at Exeter University, and one of the authors of the report. "But it does have a hand in producing situations that lead to conflict.

"The things that drive conflict are sensitive to climate, particularly poverty and economic shocks," Adger said. "If there is a decrease in food supply or lots of people are pushed into poverty … it creates the environment where you are susceptible to conflict," he said.

The conflicts are also on a different scale: food riots and unrest triggered by spiralling prices; clashes between farmers and herders of livestock over land and water; competing demands on water for irrigation or for cities.

"It will be within communities or between farmers and it might not necessarily be violent," Adger said. "It's more likely to be more local and more site-specific."

And it could set back efforts to deal with climate change. "Conflict itself actually reduces the ability of places to react to climate change," Adger added. "The impact of conflict in destabilising regions, wiping out infrastructure, not allowing the state to fulfil its social contract to protect its own people … conflict itself is making people more vulnerable to climate change."