How we know that climate change is happening—and that humans are causing it

Arndt and his colleagues sit atop a vast trove of current and historical climate data—data so valuable, it’s protected by multiple failsafes and even backed up on tape at an undisclosed, secure location, where it’s carried by hand in a nondescript suitcase several times a week.

The data in question isn’t bank account information, or Social Security Numbers, or nuclear codes. It’s even more important than that: their data encompasses everything we know about our changing climate.

All told, they host a mind-boggling 28.6 petabytes—Arndt and his colleagues spend their days poring over data, comparing the incoming numbers to the baseline they call “normal.” Using daily, weekly, monthly, and annual rhythms, they coordinate data collection and sharing, keeping an eye on the quality of the numbers and how they stack up compared to yesterday, last week, and longer.

For Arndt, climate is all about scale—and the information that passes through his office tells a story that’s become all too familiar. “The upper atmosphere is cooling while the lower atmosphere is warming,” he says. “You don’t get that without changing the composition of the atmosphere. We’re seeing changes you would theoretically see in a warming world. More big rain—that’s an expectation. More big heat—that’s an expectation. The frozen stuff is generally melting or going away, based on the time of year and the location on the planet.”