Study: Climate change boosted July's heat for 81% of world's population

About 80% of humanity experienced unusually hot temperatures during July that were attributable in large part to human-caused climate change, a new report finds.

Why it matters: The analysis shows the extraordinary reach of climate change, and its influence on temperature extremes. It's made extreme heat far more likely, particularly in many developing countries.

The report uses Climate Central's daily tool for climate attribution known as the Climate Shift Index. It underscores how the human influence on, and exposure to global temperature extremes was far broader than the three main heat waves making headlines last month.

• It complements other research from a different group of scientists, which found the extreme heat in the Southwest U.S. and southern Europe was "virtually impossible" in the absence of human-caused climate change.

The big picture: The analysis covered climate change's influence on daily average temperatures across 200 countries and 4,700 cities. It found that during July, nearly 7 billion people experienced at least one day that was amplified at least threefold by climate change.

• The report, which is based on peer reviewed methods, found that at least 2 billion people felt a strong influence of climate change on each day in July. This was calculated based on CSI readings of three or higher.
• Study author Andrew Pershing, who leads Climate Central's science work, found that global exposure to climate-influenced extreme heat peaked on July 10, when 3.5 billion people were rated as living in locations with a CSI reading of 3 or higher.
• The study identified several hotspots for climate change influence, including the Caribbean, Central America, northern Africa, the Middle East, the Sahel and parts of Canada.

Zoom in: Much of the Caribbean was ranked as a five on the CSI scale, meaning climate change made the heat there at least five times more likely to occur today than in a preindustrial climate.

• Cities that were particularly prone to climate change-fueled heat during July include Alexandria, Egypt, Jeddah, Saudi Arabia, Dhaka, Bangladesh and Mexico City, the report found.

Zoom in: The nonprofit climate science research and communications group rolled out the CSI scale last year. Pershing likened it to a "screening tool" for climate change fingerprints, operating on daily timescales globally.

The intrigue: The new analysis reveals that during the record-hottest month on Earth, the people who live in places that contributed the least to the problem, such as small island developing states, had some of the most significant climate-related heat impacts.

Meanwhile: New numbers in Tuesday show that July was the hottest month on record for more than two dozen U.S. cities including Phoenix, which became the first U.S. city to have an average monthly temperature above 100°F.

• The all-time heat records were concentrated across the southern tier of the U.S., stretching from Florida to the Southern California desert.
• The monthly mean temperature is calculated by averaging each day's high and low temperature.

By the numbers: The southernmost city in the U.S., Key West, had its hottest month, as did Miami and Tampa Bay. The records there and at other Florida locations were tied in part to record-breaking sea surface temperatures, which is causing widespread coral bleaching.

• Another record was set in America's northernmost town, Utqiagvik, Alaska — formerly known as Barrow.
• Baton Rouge shattered its previous record for the warmest month, beating it by 1.5°F, a notably large margin.
• Las Vegas had its warmest month, breaking the old record by 1.1°F.
• Albuquerque saw its warmest month, as did several locations in Texas, including Austin, Brownsville and Corpus Christi.

The bottom line: Per the new report and other work from the World Weather Attribution group, the extreme heat seen in the southern tier of the U.S. is unambiguously tied to human-caused climate change.