Ahira Sanchez-Lugo is a scientist at NOAA's National Centers for Environmental Information. Part of her job is to put climate data from the 21st century into historical perspective. She's also interested in the ways that climate and weather patterns have affected human societies.
NOAA publishes monthly Global Climate Reports — the most recent of which was about the trends and temperatures observed in April 2018. To make sense of these findings, we've got to understand the frame of reference. What exactly do climatologists mean when they say that a given month is higher or lower than "average?"
In an email, Sanchez-Lugo says "Higher than average means that the observed temperature ... was warmer than the reference temperature (typically computed by averaging 30 years of temperature data)." She then provided us with a helpful hypothetical. "For example, if the observed temperature for April 2018 was 72 degrees Fahrenheit [22.2 degrees Celsius], but the 30-year average April reference temperature for the place of interest was 60 degrees Fahrenheit [15.5 degrees Celsius], we would say that the April 2018 temperature was higher than average."
Please note — once again — that the temperatures mentioned above are purely hypothetical. Now let's dive into the real data, shall we?
The reference temperature used in the new NOAA report is 13.7 degrees Celsius, or 56.7 degrees Fahrenheit. That was the average April surface temperature on Earth during the 20th century. By comparison, the average recorded surface temperature in April 2018 was 0.83 degrees Celsius (1.49 degrees Fahrenheit) warmer.
That makes April 2018 the 400th month in a row to exceed the 20th century's global averages. (Likewise, March 2018 was hotter than the average 20th-century March; February 2018 was hotter than the average 20th-century February, and so on.) The streak goes all the way back to January 1985 — when Madonna's "Like A Virgin" topped the Billboard Charts and Ronald Reagan was being sworn in for his second term as president.