One of the most common designs for a thermometer works by measuring the volume of a fluid — one which expands when it heats and shrinks when it cools. The classic example of such a “thermometric liquid” is mercury, a liquid metal. Usually the fluid is enclosed in a container (often, a glass tube) so that when it expands it climbs to a greater height. By the height of the fluid, we can deduce the temperature.
Nature has provided us with a vast quantity of a liquid which expands when it heats and shrinks when it cools: water. Nature has also placed most of this tremendous volume of water into a container we call the ocean basins around the globe. If it heats up, it expands and the level of the water rises; if it cools down, it contracts and the level lowers. We refer to that level as sea level.
Other things can affect sea level, the most important being how much water is in the oceans. A tremendous amount of rainfall can move water from oceans to land, enough to make a noticeable change in sea level. But eventually that water makes its way back to the sea, so any consequent rise or fall is only temporaray. The water from melting of land-based ice also makes its way to the sea and raises sea level, but it takes heat to melt ice so that is yet another indicator of warming. On the whole, the height of the sea surface is an imperfect, but genuine, thermometer for gauging Earth’s temperature.
And what, you may wonder, does the ocean-thermometer tell us about the last few decades? It says this (sea level data based on satellite measurements, from the Univeristy of Colorado):
According to the oceanic thermometer, Earth has been getting hotter. Rather steadily, in fact. There is an annual cycle — highest in October, lowest in May — but we can remove that with a variety of mathematical methods:
No doubt about it — the sea is rising, rapidly, and it has continued to do so with only the occasional, very temporary, stutter.
Which raises an interesting question. I’d like to address this question to senator Ted Cruz (R-TX): you’ve said repeatedly, loudly, insistently, that there has been no warming for 18 years. If that’s true, then do tell us, senator Cruz, why has the sea kept rising rapidly?
I don’t expect senator Cruz to answer. Ever.