Climate change Australia
(Image: AAP/Darren Pateman)

The catastrophic impacts of human-induced climate change have perhaps never been clearer than they are this summer, as searing heatwaves, record droughts and deadly floods tear across the world.

It’s just the start of what experts forecast to be a worsening situation, according to the first new assessment in seven years by the UN-affiliated Intergovernmental Panel on Climate Change, or IPCC. The report, released on Monday, is a stark compilation of the latest climate-change research. It details how profoundly humans have altered the climate and what the future could look like if harmful carbon emissions continue on their current trajectory. But the report also outlines a brighter future, where political will to create a low-emissions future could check runaway temperatures and limit the worst of the damaging impacts.

The report “is a code red for humanity”, UN secretary-general António Guterres declared in a statement. “Global heating is affecting every region on earth, with many of the changes becoming irreversible.”

Here are some of Foreign Policy’s key takeaways from the IPCC’s latest assessment.

Human activity is the ‘unequivocal’ driver

Blistering temperatures in the Arctic, southern Europe and American west are just signs of a stark global trend: humans are warming the climate at a rate unprecedented over the last 2000 years, according to the report. Finding the last two periods of high temperatures like those of the last decade requires going back 6500 and 125,000 years.

That’s mostly a function of huge amounts of carbon dioxide in the atmosphere, beginning with the industrial revolution but really ramping up in the latter 20th century: carbon dioxide concentrations are higher than at any time in the last 2 million years, the report found. And that’s driving all sorts of impacts, and not just on land: global sea level rise since 1900 is higher than any century in the last 3000 years, and oceans are warming faster than any time in the last 11,000 years.

At current rates of greenhouse gas emissions, the world will exceed a global temperature increase of 1.5 degrees — the ostensible target of the Paris climate agreement — during this century. It will even surpass 2 degrees unless steps are quickly taken to zero-out emissions. The impacts from that half a degree are big, the IPCC notes: At 2 degrees, “compound” events, such as deadly heatwaves and killer droughts at the same time in the same location, are more likely.

Some worst-case scenarios outlined by the IPCC point to even higher temperature increases of 3 to 5 degrees. But even the middle-of-the-road projections are off the charts for human civilisation: the last time temperatures were 2.5 degrees above normal was likely to be 3 million years ago.

A lot of the worst impacts likely to be already baked in

Glaciers will continue to shrink for decades or centuries, regardless of the steps that are taken now. The thaw of arctic permafrost, which itself releases loads of carbon and methane, “is irreversible at centennial timescales”, according to the report. Sea levels will almost certainly continue to rise for the rest of the century, with a mid-range estimate of between .4 and .7 metres — bad news for coastal cities, which often bear the brunt of rising sea levels, especially with regard to infrastructure. Eight of the world’s 10 biggest cities are near the coast, and in the United States, almost 40% of people reside in coastal areas where rising sea levels can influence flooding and shoreline erosion. Weather events that in the past came once a century will now be likely to be annual episodes in many coastal locations.

And no matter what steps are taken now to rein in emissions, huge future sea level rises are all but guaranteed. “In the longer term, sea level is committed to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years,” the IPCC report found. 

There are some real black swans out there

Most of the negative effects of rising temperatures can be more or less projected, depending on how aggressive the planet is about cutting emissions. But there could be nasty surprises, too — and more of them the warmer it gets. “The probability of low-likelihood, high impact outcomes increases with higher global warming levels,” the IPCC report found. 

Those outcomes include things like massive ice sheet loss, the collapse of forests, or even the breakdown of the circulatory system in the Atlantic Ocean that regulates much of the northern hemisphere’s weather and climate. Even without disaster-movie scenarios, it’s likely that warmer temperatures will still lead to combinations of extreme events that are harsher, longer-lasting, and more widespread ​​than anything “in the observational record”, according to the report.

Despite the gloom, all is not lost

Since there’s a near-linear relationship between carbon in the atmosphere and rising temperatures, aggressive action now to cut global emissions to the bone over the next few decades could limit the temperature increase to about 1.5 degrees. That would also minimise the worst expected impacts, such as torrential rain and flooding and deadly heat waves, later in the century, according to the report.

“The IPCC report underscores the overwhelming urgency of this moment,” John Kerry, the US special presidential envoy for climate, told Politico. “The world must come together before the ability to limit global warming to 1.5 degrees Celsius is out of reach.”

And if the most advanced technologies come into widespread use — such as sucking carbon out of the atmosphere and burying it — some of that temperature increase, though not the sea level rise or ice melt, could actually be reversed by the end of the century. 

“This report is a reality check,” said Valérie Masson-Delmotte, co-chair of working group one at the IPCC. “We now have a much clearer picture of the past, present and future climate.

“If we reduce emissions to #NetZero by 2050, we can keep temperatures close to 1.5 degrees.”

Christina Lu is an intern at Foreign Policy. Twitter: @christinafei