Additional info from The Climate Reality Project
The claim that “Greenland was once greener” is a variation of the more general denier argument that there have been periods in Earth’s history warmer than the present.
While recent years have been among the hottest on record, it’s true that Earth has seen hotter days, way back in history — before record keeping even began. These past changes in the climate were driven by natural forces, like the sun burning brighter, or an increase in volcanic activity. But we know that natural causes have little to do with the warming we see today. We have overwhelming scientific evidence that humans are causing most of today’s climate change by burning dirty fossil fuels and sending carbon pollution into the atmosphere at an unprecedented rate.
So does the fact that the climate changed before mean everything will be ok this time around? If Greenland was warmer, and greener, during Earth’s last interglacial period or during Medieval times when Vikings settled it, does that mean there’s no cause for concern now?
Nope! It’s faulty logic to say that because something happened in the past, it wouldn’t be a big deal if it happened again today. A lot has changed since the last interglacial period, when hippos lived in Germany (PDF), and even since the Medieval era. We’ve built our civilization — our cities, our agricultural systems, our economies, modern medicine — on the climate we have now. Altering the global temperature, rainfall patterns, and sea level can only mean trouble.
Consider, for example, what would happen nowadays if Greenland’s massive ice sheet were to melt. Given that it contains enough ice to raise global sea level by about 20 feet, there would be serious consequences for coastal cities around the world. It’s just one of the many reasons why we can’t afford to deny the reality of climate change any longer.