One of the expected consequences of global warming is global sea level rise. There are a few mechanisms responsible for this, but at present the main one is thermal expansion of the oceans. Another, currently slightly less important mechanism is the melting of mountain glaciers and the glaciers of Greenland and Antarctica.
Lubos likes the sea level rise as a measure of global temperature increase, apparently because he has found a set of tide gauge measurements that appear to show a nearly linear increase. It's typical of his climate science naivete that he thinks that measuring sea level change in one place is good enough. Of course it isn't, for all sorts of reasons* well documented in the literature, and more systematic and scientific analyses show a different pattern of sea level increase - slower at first, and more rapid recently, just as temperature change patterns would lead us to expect.
Most of his acolytes remain faithful, though.
* To name just a few potential confounding factors: major changes in land use, tectonic effects, and post glacial rebound.