EVERY WEEK, dozens of metal flasks arrive at NOAA’s Earth System Research Laboratory in Boulder, Colorado, each one loaded with air from a distant corner of the world. Research chemist Ed Dlugokenckyand his colleagues in the Global Monitoring Division catalog the canisters, and then use a series of high-precision tools — a gas chromatograph, a flame ionization detector, sophisticated software — to measure how much carbon dioxide, nitrous oxide, and methane each flask contains.

These air samples — collected at observatories in Hawaii, Alaska, American Samoa, and…