Improper amount of CO2, can trap too much heat and cause melting of large ice-masses. Rise in sea-levels would lead all kind of problems. So in some ways those two problems are not completely independent.
There’s enough trees and ocean to filter the carbon dioxide.
Actually no (and I am a math guy). What is alleged is that too much CO2 will cause increased levels of water vapor (water vapor, is a much more effective green house gas then CO2). This increased water vapor is suppost to rise high into the atmosphere and stay there trapping heat and warming the Earth. However, it turns out that the water vapor turns into rain at mid level altitude and has a cooling effect not a warning effect!
You mentioned Viking settlements under the ice in Green land. Hmmm, why would the Viking of 1,000 years ago build settlements under the ice? Could it be that the Earth was warmer (much warmer) a 1000 years ago?