This post is in celebration of World Oceans Day. We should be at one with the oceans, they course through our blood. But we are not, and we are changing them, perhaps irrevocably. I hope not.
Image © Pixshark.com
Elementary, my dear Watson – the Warming of the Oceans
Stand a bucket of cold water in the sun and it will slowly warm up. Eventually it will become as warm as the air around it, and if the day turns cool, the water may even end up being warmer in the evening than the air. This is all due to basic physics; heat energy flows from warm to cool objects and the specific heat of water (4.186 J.g-1.degree C-1) is nearly four times that of an equivalent mass – and much larger volume – of air (1.005 J.g-1.degree C-1). (The hieroglyphics mean it takes just over 4 Joules of energy to raise the temperature of 1 gram of water by 1oC, versus just over 1 Joule for a gram of air.) As a consequence, warming up water requires a lot more heat than warming air, and the bucket of water tends to lag the temperature of the air around it, as heat flows between the two. As another consequence, a gram of water at a particular temperature will ‘hold’ a larger amount of heat energy than will a gram of air. Now consider an ocean of water in contact with an atmosphere of air. If the air is warmer than the water, heat will flow into the ocean and warm it until the surface water is at the same temperature as the air it contacts. If we warm up the atmosphere, the temperature of the ocean will lag, but will also eventually warm up too. As I said, just basic physics, and we have been warming up the air.
The relatively high capacity of water to hold heat energy, and the tendency for the surface layers of the ocean to equilibrate with the surrounding atmosphere account for the fact that the world’s oceans have absorbed 93% of the ‘excess’ heat that has been generated via climate change over the past 45 years. Sunlight brings a continuous stream of light energy to the Earth, and much of that light is transformed into heat as it is absorbed by surfaces. As we emitted CO2 and other greenhouse gases, our atmosphere became a better insulator, impeding the rate at which heat radiates away from the planet. The result is that the planet has been warming, but most of this extra heat is now stored in the warmer oceans.
Figure 1, Box 3.1, Chapter 3 of the report of WG1 for IPCC’s Fifth Assessment (2013 showing the amount of additional heat energy (in ZJ, or 1021 Joules) that has been measured to have accumulated in each of several portions of the planet. Largest by far, and together accounting in 2011 for about 25.5 ZJ, are the upper 700 m, and the deeper ocean. The recent (2014) paper by Durack and colleagues has shown the correct amount for the upper ocean should be increased by some 22-71 ZJ above that shown here. Figure © IPCC.
How much ‘excess’ heat is currently stored there? In its 2013 report, The IPCC Working Group 1 provided a figure showing the accumulation of heat in different components of the planet since 1970. It shows the upper ocean (to 700 m depth) accumulating about 170 ZJ (zeta joules or 1021 Joules) of heat over that time period while approximately 80 additional ZJ have accumulated in deeper waters. A recent (5 Oct 2014) article in Nature Climate Change by Paul Durack and three colleagues at the Lawrence Livermore National Laboratory, and Cal. Tech’s Jet Propulsion Laboratory, provided an updated estimate for the upper ocean revealing that limited data from the southern ocean had resulted in a substantial underestimate (approximately 22–71 ZJ more heat), and helping explain where the heat had been going during the period in the early 2000s when air temperatures seemed to suggest global warming was slowing down. The revised total, some 290 or so ZJ for the entire global ocean, is an enormous quantity of heat, and while it means that our climate has warmed far less than it otherwise would (if all that heat had stayed in and warmed the atmosphere), it has several very important consequences for the oceans. In honor of World Ocean Day, June 8th, let’s examine what our greenhouse gas emissions have been doing to our oceans. (I’ve used the IPCC Fifth Assessment, completed in 2014, for much of the information here.)
How the Ocean Warms
Continuing with basic physics, all that extra heat has entered the ocean from its surface, and most of it still remains in the uppermost layers because there has not been time for the slow diffusive processes of the ocean to mix the heat into deeper waters. To be specific, while the upper 75 m of water are warming at a rate of about 0.11oC per decade, at 200 m depth the warming rate is about 0.04oC per decade, and at 500 m the rate is only about 0.02oC per decade.
This extra warming of the upper layers of the ocean is creating a stronger depth gradient for temperature. This has the effect of increasing the density gradient between light, warm surface and denser, cooler deeper waters making vertical mixing more difficult. One result of this is that upper ocean concentrations of dissolved oxygen, which diffuses down from the surface, and concentrations of nutrients such as nitrates or phosphates, which diffuse up from deeper waters, are falling. But it does not end here.
To begin with, warming by the sun is not uniform across the planet, and the differential warming plays a critical role in driving the global ocean circulation and in transporting heat from the tropics towards the poles. The increasingly warmer surface waters may in future begin to slow down the so-called ocean conveyor system by impeding the meridional overturning circulation (MOC) whereby surface waters in polar seas cool, become denser, and sink to become part of the deeper, or bottom water, moving back towards the equator in the deep ocean basins. Sinking of surface waters in northern seas draws surface water up from more equatorial regions.
This global conveyor system delivers heat from the tropics to the poles, and oxygen to the deep ocean. With surface waters now becoming warmer and less dense, there is the possibility that cooling in polar seas will become insufficient to make them dense enough to sink below the deeper water. (This argument is more complex than I have suggested here, because changes in salinity also affect the density of water, and surface waters of the polar oceans are becoming supplied with more low salinity water due to melting of ice caps. Low salinity water is less dense, warmer water is less dense; will the cooling taking place be sufficient to drive the sinking, and therefore the circulation via the ocean conveyor system? (I’ve written previously about this system and the risks of a slow-down.)
In addition, as water warms it expands very slightly and becomes less dense. This fact means that the warming of the oceans is contributing to sea level rise through the very slight expansion of that enormous quantity of water. Melting of glaciers on land (Greenland, Antarctica, and continental montane glaciers elsewhere), also being enhanced by global climate change, adds to the supply of liquid water and also contributes to sea level rise. Sea level has risen at about 1.7 mm per year between 1900 and 2010, however the actual rate has been increasing during that time, and averages 2.1 mm per year if we consider only years since 1970. The thermal expansion caused by warming has been responsible for about 30% of the total sea level rise; the remainder is due to melting of glaciers, and other redistribution of water between the land and the ocean. Overall, while estimates of sea level rise are imprecise, IPCC anticipates an increase of 0.5 to 1.0 m by 2100, and significant further increases beyond that point. The real significance of sea level rise is cultural; human civilization developed and has prospered under a regime of near stasis in sea level over the past several thousand years. It’s also worth noting that in past geological periods, such as the mid Pliocene, 3.3 to 3.0 million years ago, when temperatures were 2o to 3.5oC warmer than now, sea level was up to 20 m higher than today. It is not going to happen quickly, but if substantial melting of glaciers occurs, sea level could reach such heights over the next couple of centuries.
Changes in the Arctic
While it does not alter sea level, the melting of Arctic sea ice is now closely monitored, and is another consequence of warming of the oceans. In recent years the seasonal melt has grown more extensive, and it is clear that in a few more years, we may come to have brief summer ice-free periods in that ocean.
A graph of the seasonal trend in sea ice extent in the Arctic for the period through February and June. The mean trend over all years since 1981 is shown as solid black line, with a gray zone denoting ± 2 SD around this mean. Colored lines show the trends for each of the last five years; they fall consistently below the mean, meaning a more rapid and more extensive loss of ice cover than average. At present, 2015 looks set to be lower than any of the most recent years.
Figure courtesy NOAA NSIDC
While plenty of corporations and national governments are eyeing these trends hungrily because they will greatly expand access to the Arctic and its resources, the full environmental impacts of the changes to Arctic seas have yet to be assessed. As of May, 2015 is turning out to be a year in which the Arctic sea ice melt is more rapid and more extensive than any recent year.
Turning the Oceans more Acid
Our emissions of CO2 do not only lead to addition of heat to the ocean. Any gas in our atmosphere tends to equilibrate at the ocean surface, and as a consequence, the increasing concentration of CO2 in the atmosphere has led to increasing amounts of CO2 dissolving in surface ocean waters. About 28% of all anthropogenic emissions of CO2, or about 155 Pg (that’s 155 x 1015 grams CO2) have now dissolved into the oceans, and climate warming has been far less than it would have been if that CO2 had remained in the atmosphere. But dissolving CO2 in the ocean leads to the phenomenon called ocean acidification, a progressive lowering of pH of ocean waters. I discussed this phenomenon in some detail back in March of 2012, and some of its likely environmental impacts in July 2013.
The CO2 is concentrated in surface waters for the same reason that heat and O2 are concentrated there – slow diffusive mixing processes take time. Thus acidification is a problem of the upper levels of the ocean. Indeed, mixing will eventually distribute the increased H+ throughout the water column and problematic low pH surface waters will be a thing of the past. Only problem is that this mixing will take many centuries and surface waters will suffer low pH in the meantime.
In many ways, ocean acidification is a misnomer. The most acid ocean waters on the planet still have a pH greater than 7, the neutral value. The ocean is not turning into lemon juice. But the changes that have occurred over the past century are greater than at any time in the recent past.
Another Time in Another World
It was a very long time ago, in a very different world, but the Permian seas were rich in life including creatures we would quickly recognize as fish, as sharks, as molluscs, as corals. They also included creatures such as the trilobites and eurypterids, which, while they shouted out ‘arthropod’ being clearly related to all those other well-armored, jointed-leg creatures like crabs, spiders, and insects, were unlike anything we have ever seen. And then the seas were nearly dead. The end-Permian mass extinction was the most severe of all mass extinction events that have occurred since the Cambrian period began 550 million years ago. It happened over a span of some 60,000 years, 252 million years ago. Life nearly disappeared; in the oceans, 51% of all families, 82% of all genera, and somewhere between 92 and 97% of all species disappeared permanently from the planet. On land, the extent of the extinctions was not quite so bad – many of the early groups of insects disappeared entirely, and overall, about 70% of species of animal or plant were lost. Just try to imagine what our world would look like if 70-90% of all living species became extinct.
Permian seas were rich in abundant and diverse life, much like tropical seas today, and yet, so very different. In the space of 60,000 years, 90% of the creatures present became extinct.
Image © University of Michigan Museum
This mass extinction, which occurred in two phases over its 60,000 year course, had major effects on the evolution of life on Earth because it winnowed biodiversity so extremely. Indeed, it came close to terminating the grand experiment that eventually produced, among many other wonderful species, a naked ape now capable of severely jostling his planet. It’s fun to speculate on what might have happened if the winnowing had been less extensive, or had eliminated all amphibia instead of all trilobites – what would a world that had never evolved primates be like today?
While the cause or causes of the end Permian mass extinction are not yet fully understood, one primary culprit is the massive series of volcanic eruptions that took place in a region called the Siberian Traps – some two million square kilometers of central Siberia built of basaltic rock. This was the largest episode of volcanism known on the planet, and occurred during the time of the mass extinction. A report in Science this April, by Edinburgh University’s Matthew Clarkson and 9 colleagues from UK and German schools, (see also here) has revealed that the final, 10,000 year-long extinction pulse coincided with a rapid collapse of ocean pH that seems best explained as due to eruptions of massive amounts of carbon into the atmosphere, leading to rapid ocean acidification. They based this hypothesis on a detailed analysis of specific isotopes of carbon and boron in rock formed as sediments in a shallow part of the Tethys Sea, a major ocean of the Permian world. Shifts in proportional abundance of the isotopes, 13C and 11B, together can signal delivery of carbon, and associated shifts in pH in the ocean where the rock is being deposited. The analyses showed a rapid shift of 0.6 – 0.7 pH units at the time of the second phase of extinctions. The rate of change of ocean pH was apparently slower than the rate today (although it went on for a very long time). Earlier phases of the volcanism may have been at least partially responsible for ocean warming, increased anoxia, and other changes that contributed to the earlier pulse of the mass extinction.
The rocks near Musandam, UAE, that revealed a story to Matthew Clarkson of rapid ocean acidification at the close of the Permian, the likely cause of the second of two pulses of extinction that together nearly wiped out life on Earth. Photo © Science.
So Why Does All This Matter?
The changes we are bringing to the upper levels of the ocean – the portion where the vast majority of life resides – are multiple and profound. Just how profound? Well, when I took a course in introductory oceanography a few decades ago, we were taught that features like pH were essentially non-varying characteristics of ocean water. The pH of surface waters (upper several hundred meters) is changing perhaps 70 times more rapidly than in any of the Pleistocene deglaciations and is rapidly approaching values not seen since the Pliocene. Changes in temperature, and concentrations of Oxygen and nutrients are also pronounced compared to any time during the Holocene (last 10,000 years). These changes could be sufficient, if they are allowed to continue, to bring the oceans to a point of rapid, dangerous change, as has happened several times before. I think we should be profoundly concerned.
Taken together, the changes occurring in the ocean are having a variety of impacts on marine life. Geographic distributions and timing of seasonal patterns of abundance are being altered for many marine species. Physiological processes are being altered, and capacity to complete reproduction and early embryonic or larval development is being impeded. As a consequence of these ‘species-level’ changes, ecosystems are being altered as different species do well or less well under the changed circumstances.
We have always depended on the ocean for food. Can we continue to depend on it while we change it so substantially? Photo © JA Bruson, FFI
Ultimately, of course, it matters to us. Our civilizations developed at a time of stable sea level and mostly centered on productive estuaries and deltas at sea level. Those places are now being submerged. We have always used the oceans as a major source of food, particularly for its animal protein, but even beyond the effects of our overfishing, failures by fishery species to cope with the changing environment, are impacting our ability to continue to extract food from the sea.