The UN climate panel sounded a dire warning Monday, saying the world is dangerously close to runaway warming — and that humans are “unequivocally” to blame.

Already, greenhouse gas levels in the atmosphere are high enough to guarantee climate disruption for decades if not centuries, scientists warn in a report from the Intergovernmental Panel on Climate Change (IPCC).

That’s on top of the deadly heat waves, powerful hurricanes and other weather extremes that are happening now and are likely to become more severe.

Describing the report as a “code red for humanity,” UN Secretary General António Guterres urged an immediate end to coal energy and other high-polluting fossil fuels.

“The alarm bells are deafening,” Guterres said in a statement. “This report must sound a death knell for coal and fossil fuels, before they destroy our planet.”

WATCH | Some climate change impacts ‘irreversible’ now, says UN climate panel:

The world must act to stop climate extremes from getting worse, says a new report by the Intergovernmental Panel on Climate Change. (Maggie MacPherson/CBC) 1:16

The IPCC report comes just three months before a major UN climate conference in Glasgow, Scotland, where nations will be under pressure to pledge ambitious climate action and substantial financing.

Drawing on more than 14,000 scientific studies, the report gives the most comprehensive and detailed picture yet of how climate change is altering the natural world — and what still could be ahead.

“This report tells us that recent changes in the climate are widespread, rapid and intensifying, unprecedented in thousands of years,” said IPCC vice-chair Ko Barrett, senior climate adviser for the U.S. National Oceanic and Atmospheric Administration.

Here are some key takeaways.

World set to cross 1.5 C threshold

Each of five scenarios for the future, based on how much carbon emissions are cut, passes the more stringent of two thresholds set in the 2015 Paris climate agreement. 

A rise of 1.5 C is generally seen as the most humanity could cope with without suffering widespread economic and social upheaval.

Wildfires fuelled by heat and drought are sweeping away entire towns in the U.S. West and Canada, including Lytton, B.C., which was levelled by a fire earlier this summer. (The Canadian Press)

Under each scenario, the report said, the world will cross the 1.5 C warming mark in the 2030s, earlier than some past predictions. Warming has ramped up in recent years, data shows.

“Our report shows that we need to be prepared for going into that level of warming in the coming decades. But we can avoid further levels of warming by acting on greenhouse gas emissions,” said report co-chair Valerie Masson-Delmotte, a climate scientist at France’s Laboratory of Climate and Environment Sciences at the University of Paris-Saclay.

In three scenarios, the world will also likely exceed 2 C over pre-industrial times — the other, less stringent Paris goal — with far worse heat waves, droughts and flood-inducing downpours “unless deep reductions in carbon dioxide and other greenhouse gas emissions occur in the coming decades,” the report said.

Sea level rise, some other harms are irreversible

Some harm from climate change — dwindling ice sheets, rising sea levels and changes in the oceans as they lose oxygen and become more acidic — is “irreversible for centuries to millennia,” the report said.

“We are now committed to some aspects of climate change, some of which are irreversible for hundreds to thousands of years,” said IPCC co-author Tamsin Edwards, a climate scientist at King’s College London. “But the more we limit warming, the more we can avoid or slow down those changes.”

Even if global warming were halted at 1.5 C, the average sea level would still rise about two to three metres, and maybe more.

Sea level rise has picked up speed, as polar ice sheets melt and warming ocean water expands. Already, associated flooding has nearly doubled in many coastal areas since the 1960s, with once-in-a-century coastal surges set to occur once a year by 2100.

A man walks a bike along a flooded road after record downpours in Zhengzhou city in central China’s Henan province July 20. (Chinatopix/The Associated Press)

Scientists could not rule out extreme rises of more than 15 metres by 2300, if tipping points trigger runaway warming.

“The more we push the climate system … the greater the odds we cross thresholds that we can only poorly project,” said IPCC co-author Bob Kopp, a climate scientist at Rutgers University.

Arctic summers will soon be ice free

Summertime sea ice atop the Arctic Ocean will vanish entirely at least once by 2050, under the IPCC’s most optimistic scenario. The region is the fastest-warming area of the globe — warming at least twice as fast as the global average.

An iceberg floats in a fjord near Tasiilaq, Greenland, in June 2018. Summertime sea ice atop the Arctic Ocean will vanish entirely at least once by 2050, under the IPCC’s most optimistic scenario. (Lucas Jackson/Reuters)

While Arctic sea ice levels vary throughout the year, the average lows during summer have been decreasing since the 1970s and are now at their lowest levels in a thousand years.

This melting creates a feedback loop, with reflective ice giving way to darker water that absorbs solar radiation, causing even more warming.

Good news: worst-case scenario, tipping points unlikely

In five previous reports, the world was on that final hottest path, often nicknamed “business as usual.”

But this time, the world is somewhere between the moderate path and the small pollution reductions scenario because of progress to curb climate change, said report co-author Claudia Tebaldi, a scientist at the U.S. Pacific Northwest National Lab.

Children sit by a dug-out water hole in a dry river bed in the remote village of Fenoaivo, Madagascar, in November 2020. As a consequence of three straight years of drought, along with historic neglect by the government of the region as well as the COVID-19 pandemic,1.5 million people are in need of emergency food assistance, according to the UN World Food Program. (Laetitia Bezain/The Associated Press)

The report said ultra-catastrophic disasters — commonly called “tipping points,” like ice sheet collapses and the abrupt slowdown of ocean currents — are “low likelihood” but cannot be ruled out.

The much talked-about shutdown of Atlantic ocean currents, which would trigger massive weather shifts, is something that’s unlikely to happen in this century, Kopp said.

More than 100 countries have made informal pledges to achieve “net zero” human-caused carbon dioxide emissions sometime around mid-century, which will be a key part of climate negotiations this fall in Scotland. The report said those commitments are essential.

 “It is still possible to forestall many of the most dire impacts,” Barrett said.

Value of IPCC reports debated

British Prime Minister Boris Johnson said he hoped the report would be “a wake-up call for the world to take action now, before we meet in Glasgow.”

If it isn’t, at least one scientist questioned whether the IPCC should even continue to issue reports.

“We have all the evidence we need to show we are in a climate crisis,” said three-time IPCC co-author Sonia Seneviratne, a climate scientist at ETH Zurich who doubts she will sign up for a fourth report.

“Policy makers have enough information. You can ask: Is it a meaningful use of scientists’ time, if nothing is being done?”

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