evidence: After years of research on the Greenland ice sheet – which CNN visited when the cores were drilled – scientists reported Wednesday in the journal Nature that temperatures there have been the warmest in at least the last 1,000 years – the longest amount of time their ice cores could be analyzed to. And they found that between 2001 and 2011, it was on average 1.5 degrees Celsius warmer than it was during the 20th century. [...] During pre-industrial times, there were no weather stations in Greenland that gathered temperature data like today. That’s why the scientists relied on paleoclimate data, such as ice cores, to study the region’s warming patterns. The last robust ice core analysis in Greenland ended in 1995, and that data didn’t detect warming despite climate change already being apparent elsewhere, Hörhold said. [...] Significant warming in Greenland’s ice sheet is nearing a tipping point, scientists say, which could trigger catastrophic melting. Greenland holds enough ice that if it all melted, it could lift global sea levels by roughly 24 feet, according to NASA.