Global warming beleivers will refute it almost religiously, but the climate is always changing and the sea levels have always been rising and falling. They expand and contract with the changes in the planet's mean temperature, but it takes hundreds of years simply because the oceans are so damn huge. In a nutshell.
The alternate theory is that this map is fake or something and global warming is all our fault and it's carbon dioxide that's causing it and, the kicker, we're responsible for it, lol.