So, sea level rise has been in the news, recently, in part because of recent warnings from Dr. James Hansen that we could see as much as ten feet of rise in the coming decades. The story is a little more complicated than that and has caused significant controversy. In the interests of clarity, I’ll summarize what I’ve been able to learn about Dr. Hansen’s announcement and also explore the overall topic of sea level rise.
According to Ars Technica, Dr. Hansen and his colleagues have completed a research project that involved using a computer model to explore how ocean currents would respond to various speeds of sea level rise. They could set the model to “ten feet in fifty years,” and the computer would show them ocean currents for that scenario. The study did not look at how much the sea will actually rise how quickly, only at the consequences of different rise scenarios. Then, the team submitted a paper on their research to a peer-reviewed journal, but the review process can take a long time. The team evidently wanted their results to support meaningful climate action at the Paris conference in December, and worried that the paper might not be published in time. So, they chose a journal that has an unusual public peer-review process, enabling the researchers to speak publicly about their results before the review is complete. As part of getting the word out, they also released a short summary of their research, which included speculation that the sea could in fact rise ten feet in the next several decades–something this particular research project did not address, but Dr. Hansen is entitled to make educated guesses in his area of expertise.
The problem is that the wording of the summary leaves it unclear that he is speculating, and the public media have generally reported that the study actually predicted a rapid ten-foot rise–something that is well beyond scientific consensus at this point. To be clear, that doesn’t mean the rapid rise won’t happen. It probably could, because Dr. Hansen is very much an expert on the subject, and he could turn out to be right. He is not in any way misrepresenting his research, he is just talking about something else besides his research and doing it in a way that leaves the distinction between the two unclear.
Here is an analogy:
Say that an auto-safety researcher conducts a series of crash-tests and concludes that a given model of car has a design flaw such that a particular type of crash is lethal at an unusually low speed. She then calls a press conference, presents her findings, and says “given my results, I am especially concerned about young drivers–the traffic mortality of drivers in the 16-to-21 age group may sky-rocket if the manufacturer does not correct the flaw.” The media then respond by saying that safety test predicts teen driver mortality to sky-rocket, even though that isn’t at all what the test was about.
It is possible that Dr. Hansen created the current media buzz deliberately in order to get attention for his cause–but if he’s being alarmist, it’s only because we are in an emergency that deserves the sounding of alarms. It’s not wrong to shout “FIRE!” if the crowded theater is, in fact on fire.
I expect climate denialists will pounce on this one and paint it as an episode of deliberate dishonesty, but they would probably find a way to do that no matter what Dr. Hansen decided to say.
So, here is the overall situation with sea-level rise as we know it so far:
The world is warming, and has been for some time, now. That’s not a prediction, it’s simply historical fact. It can be difficult to measure the rise in any one location because not only does the sea go up and down, but so does the land. In much of New England, for example, the land is very gradually rising because it is still rebounding from the weight of the glaciers of the last ice-age. That makes it harder to notice sea-level rise on New England coasts. In contrast, my home area, in Maryland, is sinking, making sea-level rise seem faster than it really is. But by comparing multiple sites, measuring from satellites, and other techniques, scientists can work out how fast the seas are actually getting bigger; the water has risen about eight inches since the Industrial Revolution. Most of that rise is due to thermal expansion–warm water takes up more space than cold water. The rest is due to meltwater from glaciers.
Glaciers anywhere in the world, even those that are nowhere near the sea, raise the sea level as they melt because the meltwater eventually flows into the sea. We know how much ice is currently locked up in glaciers, so we know how much the sea could rise if all of it melted. And because all that ice takes a long time to melt, we know that even if the global temperature stabilized tomorrow, the ice would continue to melt and the sea would continue to rise until the world caught up to its new temperature. What we don’t know is how fast the ice will melt or exactly how much melting we have already committed itself to.
Not all ice has an effect on sea level, however. Floating ice–either sea ice, which forms when the ocean surface freezes, or icebergs, which form when chunks of glacier break off and land in the sea, can melt without changing the sea level at all. To demonstrate this, fill a glass with tap-water, drop a few ice cubes in, and carefully mark the water level. Allow the ice cubes to melt, and you’ll see that the water level remains unchanged. This is because when ice melts it shrinks and the volume it displaces when it floats is precisely equal to the volume of water it turns into. The melting of the arctic sea ice is a terrible catastrophe, but it’s irrelevant to sea level rise. The fact that sea ice around Antarctica is growing (remember that warmed-up ice remains ice until it reaches 32 degrees, and Antarctica is very cold) is also irrelevant.
What does matter is how much ice is floating in the sea, so if a glacier starts calving off icebergs faster (as many glaciers are), that raises the sea-level, even if those floating icebergs don’t melt. Also, much of the ice surrounding Antarctica is actually sitting on the sea-bed. That is, the glaciers rest on solid rock, and that rock is below sea level. If those glaciers thin to the point that they begin to float, then not only does the water that melted off them raise the sea, so does the fact that they are floating. One of the scary things about the science here is that it’s not always obvious from the surface which ice has begun to float–there are tests scientists can do, but those tests sometimes give surprisingly bad news.
We’ve had a lot of bad news from glaciers recently, some of which have moved very quickly, broken apart, or melted away quite unexpectedly because of reactions below the surface that scientists did not anticipate. We’ve never seen the world warm this quickly before, so we don’t know what ice does in situations like this. That is one reason why Dr. Hansen could be right–although the speeded-up melting he warns about has not happened yet, and nothing we know about ice suggests it is going to happen, there is a lot we still don’t know about ice.
Dr. Hansen is guessing that what we don’t know will hurt us.