When recalling past temperatures to make comparisons to the present, and, more importantly, inform future climate policy, officials such as Mr. Guterres and President Biden rely in part on temperature readings from the United States Historical Climatology Network (USHCN).

The network was established to provide an “accurate, unbiased, up-to-date historical climate record for the United States,” NOAA states, and it has recorded more than 100 years of daily maximum and minimum temperatures from stations across the United States.
The problem, say experts, is that an increasing number of USHCN’s stations don’t exist anymore.