Humanity will experience more extreme weather in the coming years and it will suffer the consequences of rising sea levels and melting Arctic ice, scientists working from across the globe said in a crucial UN climate report.
The alarming report by a UN scientific panel, released on Monday, called changes to the climate “unprecedented”, added that it is “unequivocal” that humans are to blame, and laid out the case for drastic cuts to emissions in order to hold the global temperature to under the limits set by the 2015 Paris Agreement.
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“The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years,” said the report issued by the Geneva-based Intergovernmental Panel on Climate Change (IPCC).
Earth’s average surface temperature is projected to hit 1.5C (2.7F) or 1.6C (2.9F) above pre-industrial levels around 2030 in all five of the greenhouse gas emissions scenarios – ranging from highly optimistic to reckless – considered by the report.
That is a full 10 years earlier than the IPCC predicted just three years ago.
By mid-century, the 1.5C (2.7F) threshold will have been breached across the board, by a tenth of a degree along the most ambitious pathway and by nearly a full degree at the opposite extreme.
The report was compiled by 234 experts from 66 countries and is the most comprehensive to be released by the UN panel since 2013.
UN Secretary-General Antonio Guterres called the IPCC’s assessment “code red for humanity”.
“This report must sound a death knell for coal and fossil fuels, before they destroy our planet,” he said in a statement. “Countries should also end all new fossil fuel exploration and production and shift fossil fuel subsidies into renewable energy.”
Since about 1960, forests, soil and oceans have absorbed 56 percent of all the carbon dioxide humanity has chucked into the atmosphere – even as those emissions have increased by half.
Without nature’s help, Earth would already be a much hotter and less hospitable place.
Global oceans have risen about 20cm (eight inches) since 1900 and the rate of increase has nearly tripled in the last 10 years.
Crumbling and melting ice sheets atop Antarctica, especially Greenland, have replaced glacier melt as the main driver.
If global warming is capped at 2C (3.6F), the ocean watermark will go up about half a metre over the 21st century. It will continue rising to nearly two metres by 2300 – twice the amount predicted by the IPCC in 2019.
Because of uncertainty over ice sheets, scientists cannot rule out a total rise of two metres by 2100 in a worst-case emissions scenario.
Significant advances in palaeoclimatology – the science of natural climate in Earth’s past – have delivered sobering warnings.
For example, the last time the planet’s atmosphere was as warm as today, about 125,000 years ago, global sea levels were likely five to 10 metres (16 to 32 feet) higher – a level that would put many big coastal cities under water.
Three million years ago, when atmospheric carbon dioxide concentrations matched today’s levels and temperatures were 2.5C (4.5F) to 4C (7.2F) higher, sea levels were up to 25 metres (82 feet) higher.
The report includes more data than ever before on methane (CH4), the second most important greenhouse gas after CO2, and warns that failure to curb emissions could undermine Paris Agreement goals.
Human-induced sources are roughly divided between leaks from natural gas production, coal mining and landfills on one side, and livestock and manure handling on the other.
Methane lingers in the atmosphere only a fraction as long as carbon dioxide but is far more efficient at trapping heat.
Methane levels are their highest in at least 800,000 years.
The IPCC warns against abrupt, “low likelihood, high impact” shifts in the climate system that, when irreversible, are called tipping points.
“Abrupt responses and tipping points of the climate system … cannot be ruled out,” the report says.