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In a significant development in climate science, researchers are now discarding both the most optimistic and the most dire predictions for global warming, deeming them no longer feasible. This shift reflects modest but meaningful progress in efforts to mitigate climate change, which has eased fears of the most extreme outcomes. However, it also confirms that achieving the international climate target set in 2015 is now out of reach.
Experts have updated their projections, presenting seven plausible carbon pollution scenarios for the future, effectively moving away from traditional extremes in climate policy.
In recent years, the likelihood of extreme scenarios has diminished, largely due to changes in energy production. The primary contributor to global warming, carbon dioxide, is emitted when fossil fuels like gas, oil, and coal are burned. The increasing adoption of renewable energy sources such as solar, wind, and geothermal has helped reduce the highest forecasts for carbon emissions. Nevertheless, the pace of this transition hasn’t been rapid enough to prevent a rise in minimum emission projections.
The 2015 Paris Agreement aimed to cap global warming at 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels, a goal encapsulated in the slogan “1.5 to stay alive.” However, scientists now acknowledge that even under their best case scenarios, this target will be surpassed. Conversely, projections no longer include the coal-dependent future that would have resulted in 4.5 degrees Celsius (8.1 degrees Fahrenheit) of warming by the end of the century, a scenario often featured in past scientific models.
According to climate scientist Detlef Van Vuuren of Utrecht University, who led a recent study on future climate scenarios, the newly proposed worst case scenario predicts a temperature rise of about 3.5 degrees Celsius (6.3 degrees Fahrenheit) by 2100. This is a full degree (1.8 degrees Fahrenheit) less than previous worst-case projections. Meanwhile, the updated best-case scenario is slightly warmer than initially hoped, exceeding the Paris Agreement’s target by a few tenths of a degree Celsius (0.36 degrees Fahrenheit).
“There is kind of a narrowing of the futures. It cannot be as bad as we thought, but it cannot be as good as we hoped,” said Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany.
The scenarios include a “middle” one where by the end of the century the world warms 3 degrees Celsius (5.4 degrees Fahrenheit) above pre-industrial times, which is roughly the path society is currently on, scientists said. The world is now about 1.3 degrees Celsius (2.3 degrees Fahrenheit) above pre-industrial times. Even tenths of a degree of warming cause problems for Earth’s ecosystems, as species die off, fresh water becomes more scarce and extreme weather events, such as flooding and heat waves, intensify.
It’s too late to keep below 1.5 degree goal
Because carbon pollution keeps rising globally and stays in the atmosphere for about century, the best case scenario is for warming to shoot past the 1.5 degree mark, peak at 1.7 degrees Celsius (3.1 degrees Fahrenheit) for maybe as long as 70 years, and eventually somehow come back down below 1.5 degrees if a technology can be designed to remove massive amounts of carbon from the air, said nine of the 10 scientists interviewed for this article. The world is warming at a pace of a tenth of a degree Celsius (nearly 0.2 degrees Fahrenheit) every five years, they said.
“This is just physics,” said climate scientist Bill Hare, CEO of Climate Analytics, a policy institute. “We’re losing the ability to limit warming even by two degrees without strong action and people need to be aware of that and be aware that it’s a political failure. It’s not an act of God or anything. It is just because politicians in many places are not acting fast enough.”
The 1.5 goal is not just a number, said Cornell University climate scientist Natalie Mahowald, co-author of a U.N. science report detailing the harms of going higher than 1.5 degrees.
“There’s a lot of implications for, you know, not being able to meet the 1.5. And, of course, the people who will suffer the most are on the small island developing states,” Mahowald said. “Some of them will go underwater.”
Highest warming scenario changes spark debate
American Enterprise Institute’s Roger Pielke Jr. said changes to the highest end scenario matter because it was presented as a likely future that could come true if nothing changed. Thousands of scientific studies have been based on that highest warming scenario, called RCP8.5, even though research had already shown it to be improbable.
“It was always presented as where we were headed absent explicit climate policy,” even though it was based on out-of-date and incorrect coal-heavy energy theories, Pielke said in an email.
Keywan Riahi, lead author of the 2011 study that introduced that scenario, said when it was designed the high-end case was not where scientists thought the world was heading.
“It was never a likely case. It was basically, given the underlying studies in the literature at that time, a plausible higher bound of what possible emissions could look like. This is very different than if you would ask the question, what is now the most likely scenario,” said Riahi who is director of the Energy, Climate and Environment Program at the International Institute for Applied Systems Analysis in Austria.
It’s a success story, said Riahi, because “in the last 10 years or the last 15 years, the cost of renewables, particularly solar and wind, have fallen by almost 90%.”
President Donald Trump jumped into the fray with a social media post saying: “GOOD RIDDANCE! After 15 years of Dumocrats promising that ‘Climate Change’ is going to destroy the Planet, the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!”
“The risks of climate change have not disappeared,” responded study author and scientist Van Vuuren. “The good news is that we did not follow the most dramatic emission pathway. However, we are still heading towards a future with significant climate impacts; a future we should avoid.”
A big asterisk looms
While the upward curve of emissions is flattening, there’s a factor that could still make the older high end temperature estimates come true, Mahowald, Rockstrom and Hare said. That’s because the newest batch of scenarios only look at emissions from the burning of fossil fuels, which is the control knob that humans can turn.
Nature has another knob of its own referred to as climate feedbacks, which humans don’t control. Scientists have had a hard time projecting climate feedbacks, and that can add another half a degree Celsius (nearly a degree Fahrenheit) of warming on top of what’s caused by emissions.
Those feedbacks include release of massive amounts of heat-trapping carbon now being stored in the world’s oceans, in forested areas and in the Amazon, along with changes to ocean currents and cloud reflectivity, Rockstrom said.
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