Actually large sections of what is now North America were covered with shallow seas over 100 times in the last several millions of years, so the answer to your question is it's probable that it will happen again...more than once. On the plus side it usually takes several hundred thousands of years for this to happen and then several hundreds of thousands or even millions of years for the water in these seas to get locked up as polar ice. Human civilization started about 10,000 years ago when a significant portion of Ice Age ice had moved north and human beings began to plant crops. Since that lucky day we now have humans all over the planet burning stuff and altering the atmosphere. So far, pretty good...'warming' has been incremental though measurable and the effects small, but significant. Significant in the sense that where for millions of years there were no people, there are now literally billions of human beings still dependent on post ice age agriculture. Even a small change up or down means big troubles for everyone and everything. Even a slight change in 'climate' will change 'weather' patterns...if the rain doesn't fall people won't eat. And that, bro, is the significances of this situation...it's not the 'heat', or the cold, it's dependence that we all have on the rain and on water sources. Only a few decades or less of longer and dryer summers, as is happening NOW, will make the difference in whether a few billion people will eat or not. So don't just check the temperature, check the rain gauge. The truth is out there!