Scientists, environmentalists and world leaders alike have generally agreed that capping Earth’s temperature rise at 2 degrees Celsius would prevent the worst effects of climate change — a cutoff touted again in the latest report by the Intergovernmental Panel on Climate Change (IPCC).

But many experts in the field, including former IPCC leaders, have said that even if global warming is kept to that limit, such a rise could nevertheless devastate the environment and endanger humanity — the very effects that the latest study warns will happen if the 2 C ceiling is breached.

“There is no such thing as a safe rise,” said Bob Watson, who was the chair of the IPCC from 1997 to 2002. “You will see food and water insecurity, human health problems, and sea level rise even with a 2 C rise.”

Global temperatures have risen 0.85 C on average since the Industrial Revolution — a change most scientists blame on human activities, such as the burning of fossil fuels. Scientists have said that at the current levels of emissions, the world is on track for as much as a 5 C rise by the middle of this century.

“Since the start of the Industrial Revolution, in the late 19th century, we’ve had about 1 C of warming, and even with that, we’ve already seen big changes in frequency of extreme events and big societal impacts,” said Radley Horton, a scientist at the NASA Goddard Institute for Space Studies and Columbia University professor.

"We’ve seen more frequently deadly heat wave events as temperatures rise and more frequent coastal flooding as sea levels rise," he added. "These are not just more frequent or longer lasting, but when they happen they’re more severe."

Residents of Pacific Island nations like the Marshall Islands tell Al Jazeera they are already feeling the effects of rising sea levels, citing the largest ever king tides, which swept through the capital Majuro earlier this year. At the same time, the nation’s northeastern atolls were hit with severe drought.

More frequent extreme weather is already hitting many parts of the world and in the U.S., the Pentagon has warned that climate change poses a national security threat as competition for resources increases.

If the Earth reaches a 2 C rise, positive feedback cycles — which refers to effects from warming that lead to even more warming — would likely be triggered.

“If we start to push the climate system to that stage, all bets are off. No one can tell us exactly what would happen next,” Horton said.

One example of a positive feedback cycle is the melting of Arctic ice sheets, which, scientists have said, could lead to an ice-free Arctic in the summer within a few years.

“Removing the white surface ice in the Arctic that is so effective at reflecting sunlight will mean replacing it with a dark surface, which absorbs and leads to more warming,” Horton explained.

This could have far-reaching effects. Heating is exacerbated at the poles, analysts have said. This could change the location of the jet stream, which regulates weather patterns worldwide — thereby influencing heat waves, cold air outbreaks, rain patterns, flooding and drought.

Though no one knows for sure what will happen if we cross the 2 C temperature increase, paleoclimatologists believe some of those changes can be predicted by looking at ancient climate record.

“The thing people need to appreciate is that a warmer world doesn’t just mean a warmer world,” said Rhawn Denniston, professor of Geology at Cornell College with a focus on paleoenvironmental records. “There might be this misconception that, ‘I can handle a couple more hot days in the summer’ … But we’re talking about changes in rainfall patterns, snow pack in the mountains.”

In one study, stalagmites – cave formations – found in Asia were used by an international team of scientists to research ancient monsoon patterns over 100,000 years. They found that Chinese monsoons intensified and weakened over that period, which coincided with warming and cooling cycles in Greenland during its ice age. According to Denniston's research, wetter monsoon seasons in Australia also coincided with millennial-scale cold periods in Greenland.

Scientists also use the ancient climate record to predict a 2 C rise's effect on sea level rise.

“We know from the past 20,000 years ago, during the ice age, when it was 4 to 5 degrees Celsius cooler and sea levels were 100 meters lower, you can look back and see that each one degree [Celsius] of warming contributed about 20 meters in sea level rise,” said David Spratt, an Australia-based climate blogger who analyzes the growing gap between science and politics.

“If we are restricted to 2 C rise, that means we will see sea level rise in the tens of meters — that’s not in dispute, its just a question of how fast.”

Analysts say the reason most world leaders along with the IPCC have agreed on the 2 C target is more political than scientific. The recommendations published in the IPCC climate reports are negotiated by over 100 nations, including oil-producing nations like Saudi Arabia.

Despite IPCC’s reports saying the worst effects of climate change could be avoided by capping global temperature rise at 2 C, some scientists believe the Earth’s climate has already been pushed beyond its tipping point. One reason is that most of the effects from the greenhouse gases already in the atmosphere are delayed.

Another reason for the schism between IPCC recommendations and other scientists’ beliefs is that the panel’s report is based on science that is already several years old.

“The cutoff date is three to four years before it’s published, meaning this report is the extent of climate science in 2010 — and a number of things have happened since then,” Spratt said.

He cited predictions of an ice-free Arctic within a decade or two, the “unstoppable” melting of West Antarctic ice sheets and faster melting in Greenland’s glaciers.

All of these are examples of interconnected systems that could feed off of each other — resulting in climate disruption.

“We now have evidence saying if we get to 2 C we’ll pass the tipping point to irreversible changes … which policymakers simply ignore,” Spratt said.