This is from a Huffington Post article about the watery fate of a number of NYC neighborhoods. There are a lot of people there making various arguments about why we don’t need to worry about climate change or sea level rise, but this one was repeated often enough I figured I’d take some time to answer it. This is the “but the historical records show a slow, constant rate of sea level rise! The acceleration is only in the projections!” argument:

Stranger says:

Interesting that the past, measured sea level rise appears constant, rising at the same rate for the last 130 years. It is the projected sea level rise which is accelerating.

There are plenty of reasons for that.

First, the increase in planetary temperature didn’t really get going until the last quarter of the 20th century.

That means that things like sea ice melt weren’t really doing much until then as well. If you haven’t heard of the albedo effect, it’s an amplifying feedback loop that happens when ice decreases. Ice is more reflective than liquid water, so if ice melts, there’s more water to absorb heat from the sun. That means temperatures increase a little bit faster, causing more ice to melt, and more heat to be absorbed, and so on.

That means that as the sea ice in the arctic continues to melt, the temperature increase in the arctic will speed up. That, in turn, will mean faster ice melt on Greenland.

And that means faster sea level rise, in addition to what you get from warming water.

There are other amplifying feedbacks in play, like the thawing permafrost, which is mostly organic matter, and is rotting, and releasing methane and CO2. Both of those will trap more heat, causing more thawing, and so on.

In addition to that, there is at least one place in Antarctica where a small bump on the sea floor is currently holding a massive glacier in place. Just a small increase in sea level from where we’re at now, could lift that glacier above that bump, which would allow it to flow freely into the Antarctic sea. That, in turn, would cause a MASSIVE increase in sea level in a very short amount of time, which would cause that and other glaciers to flow more rapidly into the sea, causing more of an increase in sea level.

So yes, accelerated sea level rise is a “projection”, but it’s based on everything we know about the physics of how water interacts with heat, and about the current state of the oceans, the feedback loops I mentioned, and the Totten Glacier in Antarctica