How high was sea level in the last interglacial?

Let me start with a disclaimer: I will not give a definitive answer to the above question. That being out of the way I can now freely discuss the difficulties of trying to answer a question like that.

It is, in a way, an interesting question: the last interglacial was only ~125.000 years ago, so the continents were all practically in the same location as they are now, and the oceans were equally deep. We know from ice cores and fossil plants and such things how high CO2 levels and CH4 levels in the atmosphere were. We know from ice cores, microfossils, and many other sources how warm it approximately was. So you could say that if you know how high sea level was back then, you know how high sea level is at these CO2 levels and these temperatures, and that might be useful. Additionally; if you can reproduce that in a climate model, you have reason to believe your model resolves sea level well, and may be able to predict it too. Unfortunately, it's not that simple. 

Picture of the 1953 flood

One thing is that we had already passed the CO2 levels of the last interglacial when the Mauna Loa observatory started measuring in the fifties. We may not yet have reached reached globally averaged temperatures comparable to those of 125.000 years ago, but it's hard to pinpoint that; every place on Earth has its own temperature history, and it's not easy to compare two periods that are so similar. And ice does not melt instantaneous. Just suppose we reach, this very day, the very temperatures we had in the last interglacial, the ice caps won’t have had time to adjust to that. And we don’t give them any; temperatures keep rising. We can only get an equilibrium when the situation remains stable for a while. That won’t happen in our lifetime!

So apart from the issues associated with a comparison, we still have this sea level issue to deal with. If you talk about “sea level” in a given time, without further specification, it almost automatically means “globally averaged sea level”. And the problem with that is that you can perhaps measure it today; satellites scan the entire Earth surface, and you can calculate the average of all their data. And sea level famously isn’t level; temperature differences, the rotation of the Earth, wind, gravitational pull of things such as ice sheets and so on, all make the sea surface rather bumpy. So you can’t measure past global average sea levels (further back than the satellite era), as it’s not possible to make a sea level reconstruction for every location on Earth covered by sea. So what can we measure? Local sea level, evidently.

There are many ways of reconstructing local sea level (such as this, this, this, this, and this), and every method, of course, has its own caveats. They also might not represent the same aspect of sea level; some might e.g. reflect low tide, while others are more representative for mean sea level. And tide ranges don't stay constant over time.

And if you have produced a local sea level reconstruction, you're not done; you also have to take into account that not only sea level in itself, but also the Earth’s crust might have gone up and down. Locations affected by earthquakes, volcanism, or (occasional) ice cover are prone to do a certain amount of moving vertically, and fast enough to pose a problem over the time scales discussed here. You can, of course, only use stable regions, but that leaves you with quite a small number of data points. An iconic paper by Robert Kopp and co-workers in Nature tried to extract a global average from a limited amount of data from the Last Interglacial. They, for instance, had no data points along the eastern and western shores of the Pacific; the only data from that ocean came from islands in the middle, and from its polar boundaries. They come up with an average of ~7m higher, but given the data scarcity it is hardly surprising they give rather large uncertainties. And these 7m, being a global average, of course are only a mathematical reality; at any specific location the value may be drastically different. 

The difficulty with reconstructing sea level also makes its prediction difficult.We only have good data coverage over the last few decades, and only moderate coverage over the last few centuries. If your sea level model manages to hindcast the patterns observed in that time interval, that doesn't necessarily mean it can resolve anything novel happening, like, say, the collapse of the West Antarctic Ice sheet. And that's exactly the sort of things we would like to get a handle on.

So if you headlines in the newspaper, on Twitter or wherever, that say something along the line of “sea level in period X (say, the last Interglacial) was #m higher/lower than today”, do realise it could mean all sorts of things. If one person for instance claims sea level was likely to be 8m higher than today in the Last Interglacial, and you read somewhere else it was 8m lower, it doesn’t mean one of them has to be wrong, as much as climate skeptics would like that to be the case. In this case it concerns one global average, and a local record from the Netherlands. And it's many, very many of such results, which may seem contradictory, that altogether will paint a comprehensive picture of past sea level. And in the long run, this will hopefully give us a solid grasp on future sea levels.

Conversion of a skeptic. Really?

The New York times let him write an op-ed. The Guardian blogged about him. The BBC had a piece about him on their news website. The Carbon Brief interviewed him. Twitter was buzzing with his name. Richard Muller was the talk of the day. Why; had he discovered something exceptional? Had he found the cure for cancer? Had he solved the hunger problem? No. He had found something out that had already been known for decades. So why all the excitement? I’m not so sure!

So what was the issue? Muller suspected there was something wrong with the current global average land temperature records. So he set out to investigate. Land temperatures are, of course, only a small part of the climate story, but he does have a point that most humans reside on land, and are therefore much more interested in land temperatures. And these are hardly independent of the ocean temperatures; the oceans have a much higher volume and heat capacity than the land (or rather; the atmosphere at a little bit above the surface, where land temperature is traditionally measured). But one can only do so much at any given time. And measuring temperatures is hampered by urbanisation and uneven distribution of measuring stations. So far no problem: it's always good if someone keeps a critical eye.

It took him and his co-workers a few years to trawl through the data. They’re done now; the accompanying manuscript has been submitted to JGR. His conclusions: actually, the records are fine. Temperatures are rising. And he goes further: humans are too blame. So far still so good. It had long been established, but there’s nothing wrong with corroboration.

Muller in action

Was that what all the buzz was about? No, it was about his conversion from climate skeptic to climate, well, scientist. Does it matter? Maybe! People like senator Inhofe have one person less to point at, saying that scientists have no consensus (like these people) and climate change is a hoax. And that’s good.

But does it mean Muller is a lighting example for climate science? No. His article is not yet published; it will be up to the editors of JGR whether he did a thorough enough job for his work to be published. If he does then we have another interesting article. But with Muller it seems not be about what he publishes, but what he says outside science. And that makes my hair stand up. Listen yourself to what he says here. My problems with these things are (in more or less chronological order):

1)    He sees a distinction between global warming and climate change. He only acknowledges the first. He acts as if these things are not related. But every rational person will see that atmospheric temperature is an aspect of climate, and that, for instance, temperature and ice volume are not independent. If you only look at land temperatures you’re missing most of the story. If you think climate change goes away when you disprove a rise in land temperature you’re not a very believable scientist. Just pointing out that some people attribute Hurricane Katrina to global warming, which indeed can’t be done, does not mean global warming is not part of the much wider process of (anthropogenic) climate change.

2)    He claims the US CO2 output is dropping. That’s not in the paper, that’s not a statement backed up by data. I struggled to find recent data on this. This blog gives a graph, and it indeed shows a decline, but the figures are provided by BP, and the report they came from  has a disclaimer that says "BP regrets it is unable to deal with enquiries about the data" so I won't take their word for it. I do hope Muller is right, but I would like him to point out where he got the data from. 

3)    His thoughts on the solution boil down to: change China! I’m not saying China does not pollute, or that it pollutes a lot, but it’s not helping if non-Chinese get all patronising and want to “help” China to develop clean technologies, while proclaiming that they themselves are not the problem and don’t have to cut down on their own pollution.

4)    He dismisses 90% of Al Gore’s Inconvenient Truth without getting into detail on what exactly is wrong with it. In the scientific community, this film/book is accepted as largely correct.

5)    He attributes global warming to humans, simply by fitting the temperature curve and the CO2 curve. And many of us have done such fitting exercises, but it's not the pinnacle of science. Muller, however, claims he has done a better job than anybody had before. That is blatantly untrue: lots of proper research (like this for instance) has been done on attributing climate change to various causes.

6)    He lashes out at Michael Mann, using false arguments. He for instance says Mann denies the existence of the Medieval Warm Period, while in reality, Mann is one of the main authors to discuss this feature (e.g. here). Do read Mike Mann’s Facebook page, by the way (it’s public); he has gone through the effort of dismissing all the false statements, with links to proof.

So what is my conclusion? I think Muller just likes the limelight. I would pay attention to what he manages to get published, as that will have withstood a critical eye. But I don’t believe a thing of what he says which is not backed up by solid evidence. A man like this might be more a Trojan Horse than anything else: pretend to be a bona fide scientist, and as soon as people believe that, fill their minds with rubbish. Watch out for Muller! And do to him what he did to the temperature records: first evaluate it properly, and only then decide if you believe it or not…