Climate change-fueled heat waves swept the world this summer

About 7.95 billion people experienced summer temperatures that human-driven climate change made at least twice as likely, a new analysis suggests.

Why it matters: The rapid attribution analysis from Climate Central, a climate research and communications nonprofit, drives home that climate change is already tilting the odds in favor of increasingly hazardous heat, particularly in the developing world.

The big picture: Climate Central's analysis — which is based on the group's Climate Shift Index (CSI)— found that during June through August, about 48% of the global population saw temperatures that were substantially boosted by global warming.

• This past summer was the hottest in recorded history, and July was the planet's hottest calendar month on record.
• During the three-month timeframe, 6.2 billion people saw at least one day in which average temperatures were made at least 5 times more likely from carbon pollution tied to human activities (such as burning fossil fuels for energy).

Between the lines: The CSI, which debuted last year, compares observed or forecast temperatures to simulations of the same weather conditions, but with excess greenhouse gases in the atmosphere left out.

• According to the analysis, June through August saw nearly 2.4 billion people across 41 countries or territories experiencing more than 60 days with temperatures hitting 5 on the index, which is its highest value.

Of note: A total of 1.5 billion people saw temperatures hit a level of 3 every day this summer.

The intrigue: Extreme heat, likely many other aspects of climate change, is unequally distributed worldwide. This summer, countries with the lowest historical greenhouse gas emissions experienced three to four times more days with a CSI level of 3 or higher than the world's largest economies.

• "In every country we could analyze, including the Southern Hemisphere where this is the coolest time of year, we saw temperatures that would be difficult— and in some cases nearly impossible — without human-caused climate change," Andrew Pershing, Climate Central's vice president for science, said in a statement.

Between the lines: Like other rapid attribution studies, this one did not undergo traditional peer review (which can take months) in order to be as relevant as possible to recent events. The research does, however, rely on peer-reviewed methodology.

• "We want to be able to get this information out quickly and at a time where it's relevant," Pershing said during a press call. "Peer review for a science paper takes six months; by the time that happens, everybody's forgotten the event."

• "It's similar to the strategy for weather forecasts, your weather forecast has not been peer reviewed, but the models underlying it have been," he adds.

What they're saying: Friederike Otto, a climate scientist at Imperial College London and director of the World Weather Attribution program, said her group works to foster transparency. That includes making its results, methods and data available for other scientists to replicate.

• "If you want to provide scientific evidence when the world is listening, you can't do it via peer review," she said during the press call. "You have to do it by peer-reviewed methods and transparency and also building trust with explaining what we do and why we do it and how we do it," she said.
• Otto was not involved in conducting the new Climate Central study.

Thought bubble: This summer's record heat, which decades of carbon pollution made far more likely, will be top of mind when world leaders gather in New York for the U.N. General Assembly, particularly at the U.N. Secretary-General's Climate Ambition Summit on Sept. 20.