As UN says world to warm by 3 degrees, scientists explain what that means

As nations around the world prepare to sign a global climate treaty in Paris this December at the United Nations Conference of the Parties (COP21), the U.N.’s climate chief has said that the world is still on track for dangerous warming.

Most leaders and scientists have agreed that limiting the global average temperature rise to 2 degrees Celsius (3.6 Fahrenheit) over the next century could yet ward off the worst effects of climate change. But the world remains on a trajectory to experience an increase of 3 C (5.4 F) — even if the national emission reduction pledges to be codified in the Paris treaty are implemented — said Christiana Figueres, executive secretary of the U.N. Framework Convention on Climate Change.

Her comments came ahead of visiting Pope Francis’ U.N. General Assembly speech on Friday, when he is widely expected to weigh in on the dangers of climate change.

Al Jazeera asked scientists what a world warmer by 3 C would look like, in light of the U.N. official’s apparent concern that the treaty alone will not be enough to reduce warming to 2 C.

The most recent era in which the Earth was believed to have experienced temperatures of 3 C above pre-Industrial levels was the Pliocene Epoch — around 3 million years ago — according to Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies.

“At that time, there was almost no ice anywhere. The sea level was 20 meters (65 feet) or so higher, and forests went to the edge of the Arctic Ocean where there is now tundra,” Schmidt said.

“It takes a long time for those changes to manifest, but if we see 3 C … it pushes us in that direction,” he added.

A world 3 C warmer would see a significant drop in food production, an increase in urban heat waves akin to the one that killed thousands of people this year in India, and more droughts and wildfires, according to Ray Pierrehumbert, a physics professor at the University of Oxford.

“I have no doubt that humanity will survive. It’s not an existential threat at 3 C,” Pierrehumbert told Al Jazeera. “But it’s still a world where there are likely to be massive disruptions.”

A warmer world would also potentially lead to more refugees, he said, pointing to the refugee crisis currently unfolding in Europe.

“When talking about climate refugees, what if Bangladesh becomes uninhabitable? The scale of climate migration could dwarf anything we’ve seen,” Pierrehumbert said. Many areas of the densely populated and mostly low-lying country could become uninhabitable within a century if warming continues, he added.

The developing world would bear most of the initial impact, but the developed world “will not escape unscathed,” Pierrehumbert said.

Jason Funk, a senior climate scientist with the Union of Concerned Scientists, said a temperature increase of 3 C would seriously disrupt global economic systems and many people’s livelihoods.

“It could potentially lead to more conflicts because resources will be impacted, and people will be trying to capture access to those resources,” Funk said. “It’s not a pleasant scenario.”

“Right now we have an off-switch if we can reduce emissions and keep warming below a certain level — we can still turn off the climate change process,” Funk added.

But he warned that if significant and collective actions don’t result from U.N. negotiations, warming could continue — setting off potentially disastrous feedback processes that build and reinforce each other in a “snowball” effect. No one knows at exactly what threshold such feedback events could happen, he said.

One of the most significant such processes could occur if the Arctic permafrost starts to melt, potentially releasing vast stores of carbon dioxide into the atmosphere. In some areas, permafrost has already approached the 0 C (32 degrees Fahrenheit) threshold at certain times of the year, which means it could begin to thaw, Funk said.

“If the permafrost starts thawing and releasing carbon, we wouldn’t be able to have an off-switch to climate change,” Funk said. “The Earth’s system itself would continue to drive climate change.”

There is at least as much organic carbon locked in permafrost as the world has burned from fossil fuels since the Industrial age began, Pierrehumbert said.

Despite the bleak outlook for a world warmer by 3 C, some scientists said the U.N. official’s statement that the globe is on track for such warming should not cause despair.

“3 C represents progress,” Funk said. A “business-as-usual” scenario would see a world up to 5 C warmer, he said. “I don't think they’re throwing their hands up.”