In 2009, global leaders agreed to try not to let the world warm more than 2 degrees Celsius above pre-industrial times. This is sometimes seen as a rule of thumb for keeping on the right side of climate change, within “safe” territory.

But that’s not at all how scientists meant it, Professor Camille Parmesan, an expert in biodiversity at the University of Plymouth in the United Kingdom said. Climate risks don’t begin at 2C, she said; it’s more like where they go from high to intolerably high.

The planet has already warmed by about 0.8C (1.7 Fahrenheit) since the late-19th century.

Some of the world’s most iconic places are also the most vulnerable, and they are already feeling the effects.

“We’re already seeing contraction of species in the most sensitive ecosystems, such as those dependent on sea ice or those living on mountain tops,” she said. “We’re also seeing declines in some tropical systems, such as coral reefs, and the valuable services they provide for fish nurseries, tourism and protection from coastal flooding.”

And that’s just the beginning.

“At more than 2C, we wouldn’t just face losing the most sensitive species but some common ones, too,” Parmesan said. “So it wouldn’t just be the polar bear and the Mountain Pika, but other species living in lowland and temperate habitats that aren’t necessarily at risk right now.”

But against this backdrop, the world’s carbon emissions have continued to rise and the task of staying below 2C looms ever larger. Global leaders will meet again in Paris in December to agree on a plan for how to get ourselves on a pathway to achieving 2C in the long term.

But suppose that doesn’t happen.

Suppose we collectively decide the task of keeping to this target is too great, or the price of cutting emissions quickly is too high. What would it mean to resign ourselves to a post-2C world? And if not 2C, then what?

Science is helping to answer these important questions. Climate models tell us that if carbon emissions stay very high, global temperatures could reach 4C above pre-industrial temperatures by the end of the century, perhaps even rising to 5C. And unless emissions cease altogether after that, temperatures will continue to rise long past the end of the century.

And that would mean a world unlike anything we as humans have ever known.

On the path to 4 degrees Celsius

Climate change won’t treat all countries the same. Often the most serious and damaging effects will happen in the countries that are least able to cope.

A global temperature rise of 4C by the end of the century would see parts of Africa warm by up to 6C, making life near impossible for vulnerable urban populations and people working outdoors.

Drying of river basins and falling crop yields would raise the risk of food and water scarcity in many parts of the world, particularly among poorer rural populations.

Society is vulnerable to extreme weather. The United Nations body whose job it is to assess the science on climate change says the North Atlantic and Western North Pacific will see more strong storms like Typhoon Haiyan that tore through the Philippines in 2013. In Europe, heat waves like the 2003 event, which killed 70,000 people, are already 10 times more likely than a decade ago, and this pattern is set to continue. Scientists also know that warmer air will mean rainfall in heavier bursts, while higher seas will make storms more likely to breach coastal flood defenses.

As humans, we tend to focus on what we experience up here on Earth’s surface. So it’s often overlooked that more than 90 percent of the heat trapped by greenhouse gases goes into the oceans, warming them up from the surface to hundreds of meters below.

The oceans take up some of the extra carbon in the atmosphere, too, making them more acidic. Warming and acidifying oceans spell bad news for marine ecosystems, including valuable fisheries that people the world over depend on for their food and livelihoods.

As seawater warms, it expands. That’s why, throughout Earth’s history, changing temperatures and sea levels have always been closely linked. Since the turn of the 20th century, the global sea level has risen by nearly 20 centimeters, which is already enough to threaten low-lying island nations such as Kiribati, Tuvalu or the Maldives.

Even if the oceans continued this steady march, scientists expect sea levels to be at least another half a meter higher by the end of the century. But the higher temperatures rise, the greater the chances of tipping the balance into a totally altered state, which carries far more serious consequences.

At some point, the vast Greenland ice sheet will collapse. Scientists don’t know exactly when this will happen, but they say it’s likely to be with less than 4C of global warming. The collapse wouldn’t happen quickly, perhaps taking centuries or millennia. But once it starts, we’d be committed to a sea-level rise of several meters. This would inundate some of the world’s biggest cities, including New York and Shanghai.

At the other end of the globe, scientists are already seeing early signs of collapse in parts of the Antarctic ice sheet. And once that starts, it’s likely to be unstoppable.

In the meantime, almost all the world’s glaciers are losing ice. In the Arctic, temperatures are rising more than twice as fast as the global average, and if we stay on a path to 4C, scientists predict there could be no Arctic sea ice left in summer in as little as 30 or 40 years.

A question of risk

The climate system, in all its infinite complexity, is impossible to predict entirely.

There are some things happening that scientists don’t completely understand yet, such as why ice floating on the sea around Antarctica is currently growing slightly. Scientists think, perhaps counter-intuitively, that it’s down to climate change, too, as the winds encircling the continent push freezing water outward from the coastline, extending the icy platform offshore.

And the climate system could still hold some surprises. As the Arctic warms, the once-frozen ground is thawing and releasing the powerful greenhouse gas methane. Scientists are unsure yet just how much 4C of global warming could speed up this process.

Two, three and four degrees are all points along a global warming continuum. None represents a climate precipice, but it’s clear that as the temperature rises, so do the risks.

What’s left to decide is, how much of a chance are we willing to take? The science is solid enough that whatever we choose, we can’t tell future generations that we didn’t know the risks.