From the article:

"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.

Looking further in the past doesn't really help. It's been a really long time since we had CO2 concentrations this high.

I'm no expert, but I'm pretty sure we have a good understanding of historical weather based on various natural indicators in the earth's crust. You can read about some of the techniques on NOAA.gov (https://www.ncdc.noaa.gov/news/how-do-scientists-study-ancie...)

"keeping climate data"?!? Have you heard of inference?

Yes. Yes we have. Not only for the last 2000 years. But the 5000 years as well. It might not be in a format we can easily understand. It may not even be accurate compared to today. Agricultural records go back really, really far.

" It may not even be accurate compared to today"

This is my point. It's not that accurate and to use it as a basis for climate models is ridiculous.

And the trend of this current warming period has been happening since ~ 1840.