We haven't even been keeping climate data for 2,000 years, so this headline (and maybe the entire article) is not accurate.
"Drawing on a huge database of climate-recording objects from all over the world—including tree rings, cave formations, and ancient pollen trapped in lake mud—the study concludes that 98 percent of Earth’s surface experienced its hottest period of the past 2,000 years within living memory. That uniform heat spike “is unprecedented over the Common Era,” it says."
In other words, we have used a variety of scientific techniques to fill in the global climate data record.
This is my point. It's not that accurate and to use it as a basis for climate models is ridiculous.