More than 6.5 billion people, or 81 per cent of the world's population, sweated through at least one day where climate change had a significant effect on the average daily temperature, according to a report issued on Wednesday by Climate Central, a science non-profit that has figured a way to calculate how much climate change has affected daily weather.
"We really are experiencing climate change just about everywhere," Climate Central vice president for science Andrew Pershing said.
Researchers looked at 4711 cities and found climate change fingerprints in 4019 of them for July, which other scientists said is the hottest month on record.
The new study calculated that the burning of coal, oil and natural gas had made it three times more likely to be hotter on at least one day in those cities.
In the United States, where the climate effect was largest in Florida, more than 244 million people felt greater heat due to climate change during July.
For two billion people, in a mostly tropical belt across the globe, climate change made it three times more likely to be hotter every single day of July.
Those include the million-person cities of Mecca, Saudi Arabia and San Pedro Sula, Honduras.
The day with the most widespread climate-change effect was July 10, when 3.5 billion people experienced extreme heat that had global warming's fingerprints, according to the report.
That is different than the hottest day globally, which was July 7, according to the University of Maine's Climate Reanalyzer.
The study is not peer-reviewed, the gold standard for science, because the month just ended.
It is based on peer-reviewed climate fingerprinting methods that are used by other groups and are considered technically valid by the US National Academy of Sciences.
Two outside climate scientists told the Associated Press that they found the study to be credible.
More than a year ago Climate Central developed a measurement tool called the Climate Shift Index.
It calculates the effect, if any, of climate change on temperatures across the globe in real time, using European and US forecasts, observations and computer simulations.
To find if there is an effect, the scientists compare recorded temperatures to a simulated world with no warming from climate change and it is about 1.2C cooler to find out the chances that the heat was natural.
"By now, we should all be used to individual heat waves being connected to global warming," Princeton University climate scientist Gabriel Vecchi, who was not part of the study, said.
"Unfortunately, this month, as this study elegantly shows, has given the vast majority of people on this planet a taste of global warming's impact on extreme heat."
In the US, 22 cities had at least 20 days when climate change tripled the likelihood of extra heat, including Miami, Houston, Phoenix, Tampa, Las Vegas and Austin.
The US city most affected by climate change in July was Cape Coral, Florida, where the analysis suggested fossil fuels made hotter temperatures 4.6 times more likely for the month and had 29 out of 31 days where there was a significant climate change fingerprint.
The further north in the United States, the less of a climate effect was registered in July.
Researchers found no significant effect in places like North Dakota and South Dakota, Wyoming, northern California, upstate New York and parts of Ohio, Michigan, Minnesota and Wisconsin.
