Back when CO2 was at safe levels in 1896, August in New York City was so pleasantly cool that only 1,300 people and more than 1,000 horses died from the heat

Heat Waves Throughout History — HISTORY Lists

The Great New York Heat Wave of 1896
At the end of the 19th century, New York City was home to some 3 million people, many occupying the notoriously cramped and stifling tenements of the Lower East Side and other low-income neighborhoods. When 10 days of relentless heat baked the Big Apple in August 1896, these abysmal living conditions went from an uncomfortable reality to a death sentence for an estimated 1,300 New Yorkers...More than 1,000 horses also died during the crisis.
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The North American Heat Wave of 1936
...temperatures soared to all-time highs in 12 states, clearing the 120-degree mark in some regions. (The Canadian provinces of Ontario and Manitoba also saw record heat that summer.) Like the blistering summer of 2010, the 1936 heat wave started early and followed an unusually cold winter, leaving Americans unprepared for such a drastic change in weather...By summer’s end, upward of 5,000 Americans and 1,100 Canadians had died from heat-related causes or drowned while trying to cool off in rivers and lakes.