When world leaders from 195 countries gathered in Paris almost six years ago, they agreed to try to cut greenhouse-gas emissions enough to keep global temperatures from rising more than 1.5 degrees Celsius.
According to a new climate report, however, Earth's temperature is set to blow past that mark in the next 20 years — under any conceivable scenario of future emissions.
The findings, released on Monday, come from the sixth climate assessment by the Intergovernmental Panel on Climate Change (IPCC) — a United Nations body that recruits hundreds of scientists from across the globe to synthesize years of climate research and modeling.
The report found that human-driven emissions have already caused the planet to warm by 1.1 degrees in the last 170 years, and that warming trend will continue until over the next two to three decades to some degree, regardless of how much emissions drop.
What climate change will look like in the short term
Global temperatures have risen faster in the last 50 years than any other 50-year period in the last 2,000 years. That's because humanity emitted about 2.4 trillion tons of carbon dioxide between 1850 and 2019. Every trillion tons causes the world's average temperature to increase roughly 0.45 degrees Celsius (0.9 degrees Fahrenheit).
The IPCC report outlined five potential future scenarios, each of which assumes a different quantity of carbon emissions between now and the year 2100. So the scenarios all result in different levels of warming.
Even if emissions drop to net zero in the next 30 years — the IPCC authors' best-case scenario — the global temperature will rise at least 1.5 degrees between now and 2040, the report found. In the worst-case scenario, in which emissions double by 2050, temperatures would rise 2.4 degrees above pre-industrial levels between 2041 and 2060. Then that increase would nearly double by 2100.
If the best-case situation played out, though, temperatures would eventually dip back down, dropping below the 1.5-degree mark by the end of the century.
Some changes in ocean heat and sea-level rise are locked in until 2100
Even in the best-case scenario, the authors found, the ocean warming observed between 1971 and 2018 will double. Waters will also get more acidic and lose oxygen, which can devastate marine life and alter currents that are critical to seasonal weather patterns in the Northern Hemisphere. These changes will be irreversible for the next hundreds, if not thousands, of years.
The report shows as well that glaciers will keep melting for decades or even centuries, and the Greenland Ice Sheet will continue losing ice until 2100 (the Antarctic Ice Sheet will most likely do the same). Because this contributes to sea-level rise, it is virtually certain that oceans will continue rising through the end of this century.
In the best case scenario, the IPCC authors said, oceans will rise by nearly a foot over the next 80 years.
Avoiding 1.5 degrees of warming was the Paris agreement's ideal scenario, though it set 2 degrees as the threshold never to cross. To make sure we stay under that, the new report says, we have about 900 billion tons of carbon left in our budget. In 2019, emissions reached about 37 billion tons — so if that rate continues and no carbon gets removed from the atmosphere, we'd have about 25 years of emissions left.
Still, the impacts a 2-degree temperature increase will have on weather — including extreme heat and heavy precipitation — will be dramatically more severe. For every half-degree of warming, the frequency and intensity of heat waves and droughts increase. At the same time, the planet's permafrost, snow cover, glaciers, ice sheets, and Arctic sea ice shrinks.
7 fast facts from the IPCC report
The UN created the IPCC in 1988 to inform policymakers about how the climate is changing. This is its sixth assessment of existing scientific research.
For these reports, hundreds of scientists from across the globe comb through thousands of scientific papers. They assess how the climate is changing, the impacts of those changes, risks for the future, and what can be done. Almost all the observations and predictions in the report are assigned a level of likelihood or certainty.
Monday's report is just the first part of the IPCC's sixth assessment. The second and third will be released in early 2022.
Other key findings from the new report include:
- The global temperature between 2001 and 2020 was about 1 degree Celsius higher than it was between 1850 and 1900.
- The world's average sea level rose by about half a foot (0.2 meters) between 1901 and 2018. The rate of annual sea-level rise nearly tripled during that time.
- In 2019, the concentration of carbon in the atmosphere was higher than at any time in at least 2 million years. Concentrations of methane and nitrous oxide — more potent greenhouse gases than CO2 — were higher than at any time in at least 800,000 years.
- Average yearly Arctic sea-ice levels between 2001 and 2020 were their lowest since 1850. The Arctic is likely to have a sea-ice free September at least once before 2050.
- Major tropical cyclones, heat waves, and heavy precipitation events have increased in frequency around the globe over the last four decades.
- Combinations of extreme events like heavy rainfall and hurricane-caused storm surge, paired with rising sea levels, will continue to make flooding more likely in coming decades.
- The Atlantic Meridional Overturning Circulation, an ocean current that carries warm water north and cold water south, is weakening. If the current slows enough, Europe and the US East Coast would be hit by freezing temperatures.
