Of course there were times in the “distant past” (ie. thousands, millions, billions of years ago) when the Earth was warmer and wetter than it is today. Earth’s temperature over long time scales has been all over the place in both hot and cold directions.
So yes, your statement that sea levels were higher say, 5,000 years ago, is correct! The difference between then and now is today there are trillions, if not tens of trillions, of dollars of infrastructure accomodating hundreds of millions, if not billions, of people along coastlines that are vulnerable to sea level rise and the associated storm surges that will come with it.
You have to understand that the anthropogenic climate change movement is not concerned with long time scales. It recognizes that it has no control over what happened slowly and naturally thousands of years ago or what might happen slowly and naturally thousands of years from now. The anthropogenic climate change movement is far more modest in its ambitions, and just wants to survive to the end of this century (ie. 2100). In such a short time-frame, relatively benign long-acting climate influences like Earth orbital changes or intensifications of volcanic activity don’t hold as much sway as the very dramatic human activity we are witnessing.
According to climate science, our greenhouse gas emissions are forcing the Earth’s climate in a given direction in a very short amount of time. Plus, there is a lot of inertia in the system so we need to plan what we do very carefully and well in advance. For example, even if greenhouse gas emissions were to magically stop today, there is enough inertia already in place that the world will continue to warm for decades at least and the seas will continue to rise accordingly. Granted, there are still a lot of complexities to be sorted out in the science, but they are relatively minor. For example, if you ask a researcher why he is bothering to study the dynamics of melting ice sheets at the poles, he will simply respond, “I want to know how much time we have.” He basically understands that eventually coastal cities are going to get flooded worldwide, but he wants to have a better handle on when and how it will happen.
The whole idea of the climate change movement right now is to buy time for humanity to wean off existing carbon-based energy sources, find a way to scrub carbon dioxide from its atmosphere, and hence master its climate moving forward. Granted these are all very ambitious technical goals with very long odds of success, but if we don’t achieve them within 50 years or so, for all practical purposes, and pardon my profanity, we’re f*cked!
I recognize that there is a lot of legitimate, libertarian fear of stupid, corrupt, authoritarian climate change mitigation strategies that won’t even work, which is why I am very tolerant of climate change political dissent, provided it is, and here is the caveat, scientifically literate. The Charles Krauthammer (CK) tack of labelling opponents “white-coated propagandists” who worship a “man-made religion” and “false messiahs” is very stupid and needlessly hostile (as is the adversarial counter-argument that CK-types are “spineless and sociopathic shills for big oil and big coal”).
Then you get sophisticated dissenters like Bjorn Lomberg and Matt Ridley who argue that yes the long-term view is dismal but in the intermediate-term the “cure” to climate change is worse than the disease, and it is strategically preferable to adapt. Of course this flies in the face of the inertia problem I mentioned above, but let’s give them the benefit of the doubt and hear them out a little. They argue things like “More people are living through warm winters than are dying in summer heatwaves” or “on an inflation-adjusted basis, storm damage is no more expensive today than it was a generation ago”. And when I say I’m not interested in the politics, I mean that I am not erudite enough to audit or validate these assertions, let alone question whether they will still apply 85 years into the future, let alone try to convince any constituency to endorse a given policy (which I don’t even have) based on information like this. I bow out of these discussions entirely, and find that I’m only interested in the climate and Earth science. The intersection of the science with economics and then politics is a complete mess as far as I’m concerned. But it’s not my job to sort it out.