Sea and Sky – Complexity abounds, which is why climate change is so difficult to understand and project into the future.

Posted by on March 1, 2017
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Reclining on a tropical beach, it is easy to think of the ocean as deliciously wet, salty and refreshing, while the sky is simply a blue transparency, a near nothingness that sometimes brings a cooling breeze.  In fact, both are complex mixtures of substances that are engaged in equally complex dances as they abide by chemical and physical laws that govern how they mix, combine and change.  Nor do sea and sky keep to themselves; they interact with each other and with the other surface component of this place we call home – the land.  Many of these interactions play roles in generating our climate, and so, climate is also complex in its causation.  And herein lies a major problem for us.  We are doing things that change our climate, but we do not fully appreciate how, why, or how quickly those changes occur.  The struggle to understand the origins and control of our planet’s climate has occupied scientists from various disciplines for many years, and has been a subject of growing study over the last three decades or so.  We know vastly more about how, why and when than we used to, but there is also lots we do not yet know.  How well we manage the current effort to put climate change onto a new, milder path will depend on how well we apply the knowledge we already have to change our behavior in useful ways; it will also depend on how rapidly we expand our understanding of climate science and incorporate that new knowledge into our actions.

A mild winter with crazy polar weather

This winter has so far been uncharacteristically mild in many parts of North America, and the Arctic experienced unseasonably warm weather with temperatures 20oC warmer than usual during November.  Meanwhile, people in Siberia were experiencing record cold.  Since November, our North American weather has gone through the same sort of bitter cold/super warm cycling that occurred last year, a feature attributed to the destabilizing climatic effect of a warmer than usual Arctic.  In January, I discussed the surprising loss of Arctic sea ice last November, and I have covered the influence of a warming Arctic on weather further south in North America several times.  Fact is that both in the Arctic and Antarctic, warming is happening faster than in less polar latitudes, and we are now discovering new things about the way ice melts.  We have some learning to do.

Map of the Antarctic Peninsula showing the Larson C ice shelf (dotted line marks inshore boundary), and the rift (solid red line) that has almost cut off the outer half of it.  The shelf is about 500 m thick, and floating with about 60 m of ice above sea level.  (Small circles on rift line mark its westernmost terminus in Nov 2010, Aug 2014, June 2016, 1 Jan 2017, and about 1 Feb 2017 respectively.)  Map © New York Times & Earthstar Geographics.

If the Arctic has been going crazy this year, so has the Antarctic, where the latest news is of a crack in the Larson C ice shelf which has grown 27 km longer in the last two months.  The crack, which extends right through the half kilometer thickness of the ice is now close to completing its journey across the shelf.  Once that happens, the largest iceberg in memory will be set free.  While this will have a negligible effect on sea level, it is likely to lead to acceleration of flow in the glaciers feeding the shelf, and that will lead to overall break-up of the shelf and sea level rise.  This is the kind of break-up that few if any glaciologists were contemplating a decade ago.

Sea level rise – our estimates keep growing

In 1995, in its 2nd Assessment Report, the Intergovernmental Panel on Climate Change predicted that by 2100, sea level would likely rise 49 cm above its average level in 1990.  They gave a range from 20 to 86 cm.  By 2013, in their 5th Assessment Report, they were projecting sea level rise under a business-as-usual scenario (RCP8.5) of from 52 to 98 cm, although they also reported that the pace of sea level rise has been quickening from about 1.7 mm per year in the first half of the 20th century to about 3.2 mm per year during 1993-2010.  One factor for the only slightly increased estimate of sea level rise was their expectation that Antarctic ice would be increasing in extent because of enhanced snowfall.  Break-up of the Larson C ice shelf is not compatible with a growing mass of Antarctic ice, so their 2013 estimate for sea level rise, little changed from that in 1995, is certainly still too low!  Most climate scientists in 2013 expected the estimates of sea level rise would get even bigger, and studies of how glaciers melt help confirm this expectation.

In 2015, James Hansen and colleagues published a wide-ranging study including analyses of Antarctic glacial melt processes and evaluation of climate during the last interglacial period to argue that sea level was likely to rise several meters as our climate warmed, rather than the several centimeters being projected by IPCC.  (I discussed this paper on two occasions shortly after it appeared.)  Now, in an article published in Science on 20th January, Jeremy Hoffman, Oregon State University, and three colleagues have reported detailed estimates of sea surface temperature at various times during the last interglacial period 129 to 116 thousand years ago.  Their results show that sea surface temperatures were only ~ 0.5oC warmer than during 1880 to 1899, and indistinguishable from global mean temperatures between 1995 and 2014.  Sea level back then was 6 to 9 meters higher than today.  We don’t need to have any more warming than has already occurred to still melt a lot of ice!

Hoffman’s study shows that without temperatures any warmer than today, glaciers melted sufficiently to raise sea level several meters higher than today during that last interglacial.  It does not show how rapidly the melting occurred, and few climate scientists are yet talking about 6 to 9 meters before 2100.  But they are talking about a couple of meters or more before that date.  This has implications for real people, especially those living in low-lying coastal areas.

Graph showing relative sea level changes at Virginia Key, Florida, projected out to 2100, under three scenarios – average global sea level rise of +1 m, +1.5 m, and +2.5 m by 2100 – as solid lines, and “one in one hundred year” extreme high mean water levels for each scenario as dotted lines.  The dots and bars at year 2070 show the 95% confidence limits of these projections.  Graph courtesy NOAA.

Despite the policy of the Florida government among others) to not mention climate change, a quick google of Florida + sea level brings up plenty of news, and NOAA, just last month, released a comprehensive technical report detailing global and regional sea level rise scenarios for the United States.  Under most scenarios there is an alarming amount of red (meaning high risk) all along the eastern and southern coastline.  Miami may become New Venice.  In truth, the report, available here, uses color on its maps to show the relative sea level from place to place within the USA.  The full sea level rise at any location is equal to the mean sea level rise for that scenario plus the adjustment shown in color.  Thus, most of the US southeast is likely to get nearly a meter more sea level rise than the average global rise.  NOAA’s “extreme” scenario projects a 2.5 m increase in sea level globally by 2100, nearly 3.5 m in Florida.

Maps showing the relative sea level rise by 2100 for USA coastal locations under six different scenarios for global sea level rise.  To determine the actual projected increase in sea level at 2100, compared to today, add the mean value (e.g. 0.3 m for the “Low” scenario) to the amount indicated by the color.  Maps courtesy NOAA.

NOAA’s report, which uses average global sea level increases by 2100 ranging from 0.3 m to 2.5 m, draws attention to the fact that sea level is not uniform around the globe, and that sea level rise will not be uniform.  Some places will be luckier than others and experience less sea level rise than average while others, unlucky, will receive more.  For Florida… not so good.  Not only is it low lying, but it is going to receive a greater increase in sea level than average because of its location relative to the Atlantic ocean, and the way oceans behave in response to gravity.  While we think of sea level as flat, there are hills and valleys maintained by winds, by currents, by gravity.  Our world is a strange place.

Deeper warming of the upper ocean – what’s going on?

Remember the global warming hiatus so beloved by climate deniers?  There was a period at the end of the 1990s while the strong el Niño conditions of 1997-8 were abating, that global temperatures were not rising.  This was not because global warming had ceased, as the denialists crowed, because our economy kept on pumping CO2 into the atmosphere, and heat-trapping capacity continued to increase.  But the heat was not showing up in the lower atmosphere, the ocean surface, and the land where we expected it to be.  It was going somewhere else.  Climate scientists expected that it was getting stored in the deep oceans, but at first did not know why.  Why should processes that had been warming our atmosphere and land and ocean surfaces bit by bit start warming something else?  Why indeed?

Research over the last five years or so has established that the ‘missing’ heat was indeed being stored in the deep ocean.  On 9th February, in a paper published in Nature, Tim DeVries of University of California, Santa Barbara, and two colleagues, confirmed that this happened because of a weakening of the rate of what they call upper ocean overturning circulation – a process driven by systems of global-scale currents and winds that moves cooling surface waters down to the depths in places such as the north Atlantic while upwelling water in the Southern Ocean and in the subtropics.  With overturning slowing, the capacity of surface waters to absorb heat from the atmosphere slows (because the water warms up), and the ocean can even begin a net flow of heat back to the atmosphere.  With more rapid overturning, surface waters are continually being replenished by formerly deep, cooler water, and the capacity to absorb heat from the atmosphere increases.  The net result was that during the early part of this century the upper kilometer of the ocean was being warmed to a greater depth than it is now, but it was not any warmer at the surface.  In more recent years, with less overturning, the warmth is being kept closer to the ocean surface and the surface layer and the atmosphere are becoming warmer again.  It’s nice to have an explanation for what has happened.  This knowledge also permits refinements to the global models that attempt to project future changes in climate as we pump more CO2 into the atmosphere.

Methane, that other greenhouse gas

The usual emphasis, when we discuss causes of the warming climate, is on CO2 emissions driven by the global economy and its dependence on fossil fuels.  Occasionally, discussion pauses to recognize the other greenhouse gases, particularly methane, which have also been rising in concentration in the atmosphere.  Last December, in the journal Science, there was a short news item by Paul Voosen concerning the 3% increase in atmospheric concentration of methane since 2008.  This relatively large increase has puzzled scientists because most known sources, such as cattle burps and farts, have not been increasing since 2008 (the size of the global herd has not expanded), yet the extra methane has obviously come from somewhere.  Voosen reported that there are currently two competing hypotheses.

Concentration of methane in the atmosphere above Mauna Loa, Hawaii, as recorded by instruments that monitor continuously.  While methane has been increasing with the growth of the human enterprise, the rate mysteriously quickened following a slow-down in the early years of this century.  Graph courtesy NOAA.

One hypothesis holds that the global improvements in air pollution that have occurred have reduced availability of pollutants such as ozone and nitrous oxide which are precursors for the production of hydroxyl ions.  The resulting slow-down in hydroxyl generation in the atmosphere is slowing the removal of methane (which is rapidly broken down by hydroxyl).  As a result, even with no increase in the rate of delivery of methane to the atmosphere, its concentration is rising because each molecule gets to stay a bit longer.

The competing hypothesis is a bit simpler.  It holds that the changing climate is increasing precipitation over tropical wetlands, and that this is leading to increased biological activity there, particularly at a microbial level that would be more difficult to monitor.  As a result, there is increased release of methane to the atmosphere.

Neither hypothesis leads easily to actions to mitigate this growing methane concentration, and regardless of the cause the increase in atmospheric methane will lead to more warming than would have occurred if the methane concentration had remained stable.  We need to understand what and why if we are to correctly anticipate the likely trend in methane concentration in the future.  Stay tuned.

Dissolved oxygen in our oceans

While Science and Nature are independent, competing publishers of peer-reviewed science, they occasionally publish articles that mesh rather well together to cover a developing understanding.  That happened at the start of this year with respect to the amount of oxygen dissolved in the global ocean.

We might think simplistically that the ocean, in contact with the atmosphere, would absorb oxygen (and other gases) at its surface, keeping the surface layer in equilibrium with the atmosphere in terms of gas concentrations.  We might also expect that mixing and overturning would move surface water deeper, carrying these gases, so that deeper parts of the ocean would also contain dissolved gases.  We might also expect that biological activity – respiration, photosynthesis – might alter concentrations of the dissolved gases over time so that deeper waters would have different concentrations of each gas relative to surface waters.  That is more or less correct, simplistically.  The details are less simple as two papers by different research groups revealed.

In the 23rd December 2016 issue of Science, Andrew Watson, University of Exeter, published a short ‘perspective’ on dissolved oxygen in the ocean.  He drew attention to the fact that despite a relatively high concentration of oxygen in our atmosphere (currently 20%) over the last several hundred million years, the oceans, taken as a whole, hold remarkably little dissolved oxygen except in their surface layers, and that on a number of occasions in the deep past, the oceans have plunged into anoxia, with consequent severe reductions of biological activity.  Such large-scale anoxic events can be recognized by the deposition of dark sediments rich in organic compounds due to the demise, and relatively ineffective subsequent decomposition, of plants and animals.  Such major anoxic events are associated with many of the mass extinction events that litter our prehistory.  They seem to be promoted by warmer climates and to be associated with major environmental crises, particularly ones that alter the carbon cycle.

In recent years of human history, our agriculture and industry have transported considerable organic material to coastal waters, leading to the formation of quite large, but not global, anoxic regions – the 400 or so dead zones that cover 100’s of km2 of our coastal waters.  Our climate is also warming.  How close are we to another episode of major ocean anoxia?  Understanding the phenomenon of anoxia in the global ocean seems particularly important at the present time.

Every ocean basin contains an oxygen minimum zone a few hundred meters deep, at which the rates of respiration by organisms and decomposition of organic matter are sufficiently fast to reduce dissolved oxygen concentrations in the water sharply.  Below this depth, with relatively little in the way of organisms or organic debris remaining (most animals and plants are living in the surface layers), the consumption of oxygen is less rapid and oxygen concentrations tend to be higher again.  Watson reports that oxygen concentrations are falling in many ocean basins, and are already close to or at zero in the oxygen minimum zones of the equatorial Pacific and the Indian Ocean.

Basing his report on results from a workshop organized by the Royal Society (UK), Watson relates the availability of oxygen in ocean water to the availability of phosphate, a nutrient that is limiting to phytoplankton and other microbial growth in marine systems.  The downward movement of water which delivers oxygen to deeper layers of the ocean is compensated by upwelling of deep water that brings phosphate to surface layers stimulating phytoplanktonic activity.  That activity generates the organic material that consumes oxygen as it settles to deeper layers, resulting in the oxygen minimum zone and determining the concentration of oxygen in deeper waters.  A complicated feedback process is in place.  As Watson describes it, “demand [for oxygen] is governed by the amount of phosphate in the deep ocean, whereas the supply is set by the amount of atmospheric oxygen that dissolves in surface water.  A little more phosphate, and much more of the ocean would be hypoxic (low in oxygen).  Doubling ocean phosphate would be sufficient to bring on a full-scale ocean anoxic event.”  Watson then points to the importance of major volcanism in increasing the availability of oceanic phosphates in past times, and to anthropogenic activities, notably agriculture, forestry and some industry, that increase the transport of phosphate from the land to the ocean at the present time.  Might we be at risk of promoting the formation of an anoxic ocean today, and are observed reductions in oxygen content in recent years a sign that this is now under way?

Watson stresses that the process of turning the ocean anoxic is quite slow (timescales of 100,000 years), but also notes that once the oceans become anoxic there are positive feedbacks that can make it difficult to move back towards a more oxygenated state.  His essay shows the complexity of oxygenation in the ocean, while also suggesting that we might do well to watch our farming and other practices to limit our aid to the process of deoxygenation.

Just as readers were digesting Watson’s message, Sunke Schmidtko and two colleagues from the Helmholz Center for Ocean Research in Kiel, Germany, published their paper in Nature on 16th February.  Their concern is the decline in dissolved oxygen in the oceans over the last 50 years.  They report having performed “a quantitative assessment of the entire ocean oxygen inventory by analyzing dissolved oxygen and supporting data for the complete oceanic water column over the past 50 years.  We find that the global oceanic oxygen content of 227.4 ± 1.1 petamoles (1015 mol) has decreased by more than two per cent (4.8 ± 2.1 petamoles) since 1960, with large variations in oxygen loss in different ocean basins and at different depths.”  Translating their words from science-speak, the oceans hold about 3.6 quadrillion kilograms of oxygen, almost 77 trillion kilograms less than in 1960.  No matter how you say it, that is lots of oxygen, but distributed through the many km3 of ocean waters it is a pretty low concentration.

Schmidtko and colleagues attribute the decrease to several factors.  Warming of upper layers of the ocean has reduced the solubility of oxygen, lessening the amount that will fully saturate surface waters.  Warming also enhances metabolic processes, and thus the rate of consumption of oxygen in upper layers increases.  This in turn further reduces concentrations.  In deeper waters, reduced concentrations are likely due to slowed overturning circulation (slower rate of downward transport of oxygenated water) as well as biological activity in the deep ocean.  However, their study also reveals clearly that different ocean basins are behaving differently and rates of decline differ among depths and among basins.  This means that the factors responsible for the decline in dissolved oxygen over the past 50 years still require further investigation.  Overall, the message from Watson and from Schmidtko is that our oceans appear to be moving towards an anoxic state.  Schmidtko suggests the total reduction by 2100 could amount to about 128 trillion kilograms of oxygen, or around 3.5% of the current amount dissolved.  Not a large change, but not a desirable one given how much of the ocean is already close to anoxic.

We need more science, not less, and more use of that science in our models of the future

I started by suggesting that, from the perspective of a tropical sandy beach, the sea and the sky were rather simple and pleasant.  When you delve more deeply, it’s clear that a lot goes on in each, that there are complicated interactions and feedbacks among the processes, and that there are changes taking place at present that could pose problems for the functioning of biological systems in the future.  Some of those changes are due to our activities.

What to do?  If you are sitting on that tropical beach, enjoy being there.  But also, be aware of three things.  Nothing on this planet is a simple as it seems at first.  We need more science concerning the atmosphere and the ocean if we are going to be able to adequately understand the consequences of our planetary impacts.  And, our world is being changed, seldom for the better, in many complex ways by our activities.

If you have a house on that beach, you might also do well to think about the impossibility of holding back a rising sea.

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