The Venetians did not build the Piazza San Marco so that it would flood on high tides. Relative to the piazza, sea level is higher now than when it was built. At Venice, sea level is higher partly because the land on which the city sits has been sinking, and it’s higher partly because sea level has been rising globally. The rate of change has been gradual enough that people have continued their lives, adjusting to the imperceptible increase in frequency of flooding. But long term, this wonderful piazza will become unusable except for mooring gondolas.
Piazza San Marco during flooding in December 2008. © Sebastiano Casellati, AFP, Getty.
We fail to notice slow changes in our environment because we continually shift our baselines – readjust our sense of what is ‘normal’ – from year to year, decade to decade. Given that we live only a few decades, we can be quite unaware of the very great degree of change that a slow process can achieve over several centuries, or over millennia. This phenomenon of shifting baselines is as true of scientists who study the environment as it is of ‘normal’ people who simply live in it, and this phenomenon explains why it is so common that profound changes, such as the total collapse of a once prolific fishery, the disappearance of a forest, or the replacement of a native fauna by a suite of invader species, can take place before we are aware that change is happening. If the pace is gradual, relative to the scale of our own lives, we tend not to notice it.
Daniel Pauly first identified this process and gave it its name, and Jeremy Jackson extended the concept from failure to notice collapses in fisheries (Daniel’s concern) to failure to notice all sorts of environmental change. The shifting of baselines is particularly relevant to global climate change and helps explain why we are not mobilizing to solve this critical problem. In Our Dying Planet, I discussed shifting baselines, and tried to explain the unanswered, but surely absolutely critical question – why do we shift our baselines, even after its been explained to us that we do tend to do this? Naming the problem is a first important step, but surely the essential step is to understand why it happens and if possible take steps to minimize or avoid the shifting of baselines in the future. Only then will people come to understand the true magnitude of what is happening as we warm the planet, rearrange where, when, and how much it rains, acidify the ocean, increase the intensity of storms, and generally push our biosphere well away from its comfort zone – that place of relatively stable environmental conditions. Anyway, my chapter on shifting baselines was perhaps the most speculative of any in the book as I argued that it could well be something to do with how we have evolved and how we are built that favors attention to rapid, urgent, immediate changes in our environment while the background drift is not even seen. I speculated about the way in which visual and auditory information travels through space, and the way in which such stimuli are detected by our sensory systems. And in doing this I had much to say about logarithmic changes in perception and the possibility that our sensory systems just have trouble detecting slow or linear patterns of change. Turns out I may not have been so far beyond reality as I thought I was.
This week, my son, who seems to find time to discover the most arcane items on the web, sent me an article in the Huffington Post reporting on recent psychological studies of young children that show that we start out thinking logarithmically, not linearly, and suggesting that the way our sensory systems function has a lot to do with it. Apparently, if you ask a six-year-old what number is halfway between 1 and 9, he or she will answer 3 rather than 5. Three is exactly halfway between 1 and 9 if you measure the distance logarithmically. What this all means for shifting baselines and our environmental crisis is that we are probably going to have to experience more than the very beginnings of the sharp upturn in temperature, in polar ice melt, in extinctions of species, or in shortage of food before we actually turn our collective attention to the environmental crisis. This is definitely not good news, given that once rates really start to accelerate it is going to be exceedingly difficult to move back from the brink of disaster. Yes, I fear tipping points are in our future because we are not capable of acting well in advance of the problems.
All of which brings me back to Piazza San Marco and sea level rise. I was thinking about sea level rise because we have just witnessed Hurricane Sandy’s impact on New York City, and much of the US North East. I wondered how bad the flooding would have been if Sandy had arrived in 1912 instead of 2012, or if Sandy II arrives in 2112. So I went off to find out what is known about sea level rise.
Turns out that sea level has been remarkably constant over the last 5,000 years, although it was rising fairly rapidly and sometimes abruptly during the period from 10,000 to 5,000 years ago. In a 2012 paper in the journal Nature Geoscience, Torbjörn Törnqvist and Marc Hijma of Tulane University look at the relationship between continental ice mass melting and changes in sea level during the period between 12,000 and 7,000 years ago when sea level was changing rapidly as enormous Pleistocene glaciers were melted back. They provide graphs for sea level at sites in the Mississippi delta and the Rhine delta between 6,500 and 9,000 years ago. During this period, sea level rose 10 meters at each site, mostly in a continuous curve rising at about 1 cm per year. At both sites, there were much more rapid increases in sea level about 8,250 years ago when Lake Agassiz (which filled the entire Great Lakes basin) abruptly emptied out when an ice dam gave way. At this time sea level jumped between 0.4 and 2.0 m in the space of 250 years. That rapid rise might have captured attention of any humans living near the coast.
In contrast, a 2011 article in Proceedings of the National Academy of Sciences, by Andrew Kemp of University of Pennsylvania, and colleagues in several other US and European universities, provides a graph of sea level for a North Carolina site showing a total change of 0.4 m over the past 2,000 years, half of which occurred in the years since 1900. From 1 AD to 950 AD, sea level at this site was constant. And in the period prior to 1900, total sea level rise was less than 20 cm. Human civilization has matured in a world where sea level does not vary (except in places such as Venice, where land subsidence is occurring).
Sea level in North Carolina since 1 AD, showing how rate of change has varied with the most rapid rise commencing around 1900. From Kemp et al PNAS 2011.
The increased rate of sea level rise since 1900 (2.1 mm per year at Kemp’s North Carolina site) is directly attributable to our warming of the climate. Some glacial melt, and the expansion of warmed surface waters are the primary cause. Robert Nicholls, of University of Southampton, and a similarly international set of collaborators, have published a 2011 article in Philosophical Transactions of the Royal Society which looks at current and likely future rates of sea level rise. While there remains considerable uncertainty concerning the upper bounds on possible rates of sea level rise over the next 100 years or so, scientists expect the rate to be greater than that in the past 100 years. Indeed, the rate since 1990 has been measurably greater than during earlier periods of the 20th century. The Nicholls group list eight separate recent estimates (made since 2007) of likely sea level rise this century if we assume a world which moves towards a 4oC increase in global temperature by 2100. (This is a world I do not want to see, but one that becomes increasingly likely as we continue to ramp up use of fossil fuels.) The estimates range from 0.5 to 1.4 meters at the low end to 0.8 to 2.0 meters at the high end. A 2.0 meter rise between now and 2100 is 20 mm per year a ten-fold increase over the rate during the 20th century. Nicholls and colleagues talk about the need for and the cost of adaptation to sea level rise of such magnitudes, given that about 50% of the human population lives near coasts. They conclude that reducing emissions to avoid a 4oC temperature rise makes eminent sense as the less expensive approach by far. Building dykes and similar structures is far too expensive.
The final paper I looked at, published in Climate Change in 2011, by Jeremy Weiss, Jonathan Overpeck and Ben Strauss, all of University of Arizona, dealt with the risk of flooding in selected coastal locations in the US, due to sea level rise. In their view a 1.0 meter rise by 2100 is very likely and will be followed by similarly large increases in at least the next couple of centuries, so they look at flooding risk by examining the percentage of area of selected cities that lies <1, <2, … <6 meters above current sea level.
Their figure showing these percentages for a number of cities of 300,000 or more people is definitely sobering. If we focus on the 2.0 meter elevation, New Orleans is virtually completely submerged, Virginia Beach is nearly 50% submerged, Miami is 35%, and Tampa is 20% submerged. New York City is 10% under water. Yes, if Sandy II comes by in 2112, New York will be flooded far worse than it was this week.
The point of all this however, is not how much sea level rise we should expect, nor whether our favorite coastal city is going to be largely spared or totally flooded. The point is that we are in the middle of an episode of relatively rapid, and rapidly increasing rates of sea level rise, and this single aspect of climate change is going to cause immense disruption to our activities and require us to spend considerable sums. If we can wake up to this fact sooner rather than later, the cost of dealing with it, by reducing our emissions of greenhouse gases and preventing/abating climate change, is going to be far less. Let’s stop shifting baselines and start shifting our own behavior. Above all, let us not do what the State legislature of North Carolina recently attempted. Seems the state’s Coastal Resources Commission had requested a study of likely future sea level rise by a science panel. The panel’s report recommended that the state should plan for a rise of 39 inches (1 meter for those of us in the real world) by 2100. However, this recommendation did not sit well with powerful real estate interests and others that had influence on the Senators. The legislature passed a law that would require the state planners to ignore estimates of sea level rise that were higher than in the past. “The science panel used one model, the most extreme in the world,” Republican Representative Pat McElraft said. “They need to use some science that we can all trust when we start making laws in North Carolina that affect property values on the coast.” Yes, indeed! Don’t buy coastal land in North Carolina!