Last summer was the hottest in at least 2,000 years, according to new research that analyzed weather data and tree rings

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1. Last summer, marked by deadly extreme heat and devastating wildfires, was the [warmest in at least 2,000 years](https://www.cnn.com/2024/05/14/climate/2023-summer-hottest-2000-years-climate/index.html), according to new research, which analyzed weather data and tree rings to reconstruct a detailed picture of the past.

   The findings offer a stark insight into the “unparalleled” warming the world is experiencing today thanks to humans burning vast amounts of planet-heating fossil fuels, according to the authors of the [study](https://www.nature.com/articles/s41586-024-07512-y) published Tuesday in the Journal Nature. And it’s an alarming signal as some scientists warn 2024 is on track to [be even hotter](https://www.cnn.com/2024/05/07/climate/record-hot-april/index.html) still.

   Global warming is currently tracked by comparing temperatures to the “pre-industrial era,” before humans started burning large amounts of fossil fuels, widely defined as the period between 1850 to 1900.  Under the Paris Agreement in 2015, countries agreed to restrict global warming to 2 degrees above pre-industrial levels.

   Last summer, the world temporarily breached this threshold, according to the report. Using data taken from temperature instruments during this period, the scientists found the Northern Hemisphere summer in 2023 was 2.07 degrees Celsius warmer than the pre-industrial period.

   But observational data from this period is sparse, uncertain and skews warmer. So, for a fuller picture of how the climate varied naturally before the start of the pre-industrial era, the study authors looked much further into the past.