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"To say, 'Is it the warmest for the last 100 years, or 1,000, or even 10,000 years?' It's a trickier question to answer," Haustein said. "Before 1850 we didn't have these observations, at least not enough to say something meaningful about the global mean."

Researchers have documented temperatures from millions of years ago through natural sources like tree rings, ice cores, coral and lake sediments. This is the study of paleoclimatology.

Haustein said that comparing data collected through paleoclimatology to the temperatures this month suggests July could be the hottest month in 120,000 years.