Category Archives: Interpreting ancient environments

It looks like sea level rise is accelerating; the era of satellite altimetry

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Sea level. It’s the most common starting point for any kind of elevation measurement, a datum, that for centuries was understood to be an invariant surface. Then some geologists came along and showed that, for eons past, sea level has risen and fallen countless times; coasts were flooded, sea floors exposed. And sea level is still changing, going up in some places, down in others, but on average it is rising; it has been doing this for the last few 1000 years.

The current globally averaged rate of sea level rise is 3.0 +/- 0.4 millimetres per year, based on satellite altimetry. Satellite measurement of sea level is now about 25 years old. Earlier measurements, some dating back to 1700, were made by tide gauges, basically glorified measuring sticks (the initial technology was just that) which over the years, have become sophisticated, automated measuring systems.  Tide gauges measure water levels on a local scale, and in order to make sense of the data in the context of global sea level, all manner of local variables need to be considered: for example, is the location open to the ocean or a sheltered harbour, storm surges, seasonal changes in currents and water mass temperatures, changes in air pressure (sea levels rise during passage of low pressure systems – this is part of the storm surge), and whether the land is rising or subsiding (i.e. local tectonics). In contrast, satellite altimetry gathers data from a much broader swath of the ocean surface. Both Jason 2 and the more recent Jason 3 satellites can cover 95% of the ice-free ocean surface in 10 days. The accuracy of the Jason 3 radar altimeter is currently an impressive 3.3 cm.

Rising sea levels and changing climate are inextricably linked because ocean water mass volumes increase or decrease in concert with changes in the volume of land-based ice (primarily the Antarctic and Greenland ice sheets), plus changes in ocean temperature (this is the steric effect) and salinity.  Thus, if there is an acceleration in atmospheric warming or cooling, there will be a reasonably sympathetic acceleration in ocean volume change and therefore, sea level change.  This is the scenario posited by many climate-change model projections – that increased warming will produce an acceleration in sea level rise. A recent publication that analyses the 25 years of satellite altimetry data (Proceedings of the National Academy of Sciences, 2018), concludes that the (global) average sea level rise is accelerating at 0.084 +/- 0.025 mm/year2, which means that the current speed of sea level rise (about 3 mm/year), will increase year upon year.

The possibility of accelerating sea level rise during the 20th century, based mainly on tidal gauge data, has been debated although most analyses indicated a degree of ambiguity in the data. In fact, a 1990 ICCP report (page 266) concluded there was little concrete evidence at that time for an acceleration, although re-analysis of 20th century tide gauge data, published in Nature (2015) did show a possible accelerating trend. If the present analysis is correct, this is the first time such an acceleration has been demonstrated with reasonable confidence from a single data set.

As the published analysis shows, teasing accurate sea level numbers from the satellite data is not a simple task.  As is the case for any kind of remote sensing or monitoring, there are data corrections and filters.  Some of the corrections include:

  • Terrestrial water storage (rivers, lakes, and groundwater); this is necessary because of natural variability in the exchange between land-based water and the oceans,
  • Natural variability in land-based ice storage and melting, that adds to, or subtracts water from ocean masses,
  • Natural variability in heat exchange between the atmosphere and oceans (the steric contribution to sea level),
  • Multi-year cycles such as ENSO (El Niño Southern Oscillation)
  • One-off events such as volcanic eruptions that affect regional temperatures because of ash and aerosols; in this case the Pinatubo eruption influence was incorporated into the analysis.
  • And the drift in satellite orbits; this variability is much less than that of tide gauges.

The sum of these errors gives the plus (+) and minus (–) value (0.025 mm/year2) that is attached to the overall result – 0.084 mm/year2 (check the open access publication for details). So, if our current rate of sea level rise is 3mm/year, then in 10 years the rate will have increased (accelerated) to 3.84 mm/year, and after 50 years to 7.2 mm/year (almost double the 2017 rate).

Data correction may seem like a bit of a fudge, but it is a critical part of almost every measurement we take, no matter where or what it is; it is part of the process in science that makes data intelligible and coherent.  Correcting data is part and parcel of any attempt to isolate causes and effects, as well as determining the kinds of error that are inherent in all measurements. The bottom line in this example, and one that is pointed to by the authors, is that the analysis is preliminary, and that some of the corrections might change as our knowledge of climate and other global systems improves.  However, there is confidence that this kind of analysis is on the right track.

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The fractured lives of ice shelves; destined to collapse

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Under the influence of gravity, ice will flow or creep, albeit glacially. Stand in front of a glacier or the edge of an ice sheet, and if you’re patient enough, you will see it creep, inexorably. It may take a while (days, months) but, like I said, be patient. Bits of ice may fall off the front (calve) but that’s more the product of gravitational instability and weakness at the exposed ice edge. If it wasn’t for the propensity to flow, there would be no glaciers, and ice sheets would stand still.

Antarctic ice shelves, those thick, floating wedges and platforms of ice, are a direct consequence of ice flow. One of them, Larsen C, has been in the news of late because a very large chunk (5800 sq. km), broke off and floated away as an iceberg; the inevitable comparisons have the new iceberg (imaginatively named A68) as twice the size of Luxembourg, or about the size of Delaware.  The Larsen C collapse took place in July 2017, during the polar winter, thus requiring thermal images; scientists had to wait for the summer sun to rise before getting a first-hand view of the new iceberg. Continue reading

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Visualizing Mars landscapes in 3 dimensions; stunning images from HiRISE

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For my 10th birthday my grandparents gave me a massive Collins encyclopedia – a 1960 update of the known universe. Among the collection of images were pictures of various planetary bits and pieces, the moon and Halley’s Comet, with the clarity of the Mt. Palomar Observatory telescope; images that set the imagination reeling. Technology back then was firmly attached to terra firma. Only three years earlier Sputnik had entered the history books.  Six decades later and I still have that sense of excitement, but now there’s a constant pictorial stream, with amazing clarity and detail of a comet’s surface, close encounters with Jupiter, vapour plumes erupting from Enceladus, Saturnian rings, and Mars rovers. We can observe sand grains entrained in dunes that move across the Martian surface. A barrage of images and videos, almost in real-time (not counting the 13 minutes it takes for the signal to reach us). Continue reading

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In the field: Mountain storms and surprise encounters, northern British Columbia

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The interior of northern British Columbia is rugged, mountainous country. Roads, that tend to be quite rough were frequently opened to provide access to mines and small settlements. It is an isolated part of the world, beautiful, even majestic, but also unforgiving.  East of the Coast Mountains and about 200km south of Yukon, is a huge swath of sedimentary rocks, referred to collectively as Bowser Basin.  The rocks are Jurassic to Cretaceous, recording a history of about 70 million years duration. Humungous volumes of sediment were eroded from older rocks to the north, that were uplifted and deformed as tectonic plates, or terranes, collided with the ancient margin of North America. Gravel, sand and mud were carried by braided rivers, supplying coarse sand and gravel to the coast and beyond, and to large deltas that supported lush forests (later converted to coal). Continue reading

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Erupting mud volcanoes; We have ignition

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Mud
It supports geological processes.
It flows, subsides, and leads to failure, sometimes catastrophically.
It can be beneficial, forming fertile river floodplains.
It can be a pain in the neck, clogging infrastructure.
It oozes when soft; dries brick-hard
People bathe in it. Pigs love it.

And it erupts, as volcanoes.

Not the magmatic kind, with 1000oC lavas or explosive ash columns, but eruptions nonetheless. Most mud volcanoes are much smaller than their magmatic counterparts; some only a metre high, others 10s of metres. Eruptions may be the quiet, oozy kind where mud flows, slithers and slides down slope, or more violent, shooting sticky stuff 10s of metres into the air (or water); some even ignite in a cascade of fireballs. And yes, they do form on the sea floor.  One example in 2015 along the Sea of Azov coast (land-locked between Russia and Ukraine), sent mud and water several metres into the air; you can see the muddy jetsam gradually expanding across the sea surface.  Continue reading

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In the field: Yukon Wolves, Moose, and Diamond Tooth Gerties

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The best field projects are those that last several seasons; the ones you kind of own or share with any number of co-conspirators. These are the projects for which there can be scientifically productive tangents, and where there is usually regional or universal context. Then there are those odd, short-term projects that, at the time, seem a bit ad hoc but still present the opportunity for good science, and new adventures; 1982 was such a season. I was tasked with sorting out a group of sedimentary rocks in west Yukon, a hop, skip and a jump from the Alaskan border. This was the first and last time I worked in an area with substantial bush cover. The best exposure was on ridges above the tree-line, but to get from one ridge to another required crashing through northern boreal forest and the odd, insect-infested swamp. The black flies, deer flies, mosquitos and no-see-ums were something else. Continue reading

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In the field: Windows into two billion year-old rocks

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My early geological education was very much New Zealand centered; the gamut of sedimentary, igneous and metamorphic rocks (there are no Precambrian rocks in New Zealand), in the context of a landmass (and attached submerged bits) still rent by active faults and erupting volcanoes. The timing was fortuitous. We learned at the cusp of the ‘new tectonics’, sea-floor spreading, and the morphing of continental drift into plate tectonics.  The fixists were a disappearing breed; now everything was on the move, attached in some way to one tectonic plate or another, rifted, drifted, and eventually subducted. Now, the rock formations, faults (particularly the Alpine Fault), and the volcanoes, were all connected in one, all-encompassing global, plate tectonic system.  Geologically active New Zealand had a place in this grand scheme.

Admittedly, not all our professors found it easy to teach these revolutionary ideas. We would be exhorted to go and read the latest journal papers, and come back with questions – I guess this gave the teachers time to read the articles themselves. But it was an exciting time, reading the claims and counterclaims. It really was a (Thomas Kuhn) paradigm shift.

Landing on the shores of Belcher Islands (Hudson Bay) was also something of a mind warp; from a country that straddles a plate boundary, has a volcanic rift zone in central North Island, and faces a subduction zone within a stone’s throw of the east coast, to a part of the Canadian Shield where not much has happened over the last two billion years.  Perhaps that’s a bit of an exaggeration, but this prolonged period of stasis had its advantages.  The rocks, despite being about 2000 million years old, are loaded with beautifully preserved structures and fossils.  They were not cooked by metamorphism during the time they spent being buried, nor fractured beyond recognition by tectonic forces. Basically, everything was intact. Stunning.

For someone interested in deciphering ancient sedimentary environments, being parachuted into the Belchers and being told to take the rocks apart, layer by layer, sequence by sequence, was initially a tad scary; an emotional response that quickly dissipated once the measuring, observation, and interpretations had begun. On finishing the work on one set of exposures, we couldn’t wait to get to the next, and the next.

If you were to stand all the Belcher strata in a single pile, it would be almost 9 km thick. But this pile was subsequently tipped on its side. Over the eons, the rocks were eroded by rivers and scraped by ice, fortuitous levellers that provided windows into each layer. Geologists are enticed to enter these portals, at least in their mind’s eye; the rewards are huge.  We can envisage times when there were broad platforms of limestone (now all converted to the mineral dolomite), that harboured a massive biomass of primitive algae, stromatolites of all shapes and sizes; layers as thin as a fingernail, and reefs 10s of metres high. The platforms were covered by warm, seas that shoaled into tidal flats and (deserted) beaches. Some areas infrequently inundated by high tides, became desiccated; there are remnants of minerals like gypsum and halite (common salt) that attest to salty seas. Walking over rocks like these kindles the imagination; a beach stroll, waves rolling in like they have done for billions of years, or parched landscapes exposed to the full effects of sunlight uninhibited by oxygen and the UV dampening effects of ozone (the incidence of UV light must have been intense). The experience is humbling.

However, idylls have a tendency to dissipate in the fog of time or, as was the case here, a smothering by erupting ash columns and lava flows. Now we get to walk across the tops of really ancient lava flows, around piles of pillow lavas, or along catastrophic pyroclastic flows of ash and pumice.  The earlier tropical paradise had been obliterated, but even in this volcanic brutality there is wonder.

Other strata tell of deep seas fed by turbulent mud flows cascading down an ancient submarine slope, and of sandy rivers turned red by iron oxidized by the gradually increasing levels of oxygen in the ancient atmosphere (deposits like this are commonly referred to as red beds). In every layer, every rock we looked at, there were mysteries waiting to be unravelled. A geologist cannot hope to solve all such questions, but finding a solution to even one of them is incredibly satisfying.

I spent a total of 5 months in the field during the 1976-77 summers. This was not the kind of location where, if I’d forgotten to do something, I could whip back for a couple of days to sort things out. Several of my student colleagues were doing similar kinds of research in remote parts of the country – field seasons were long. Once you had arrived, you were there for the duration. And despite the sense of excitement and discovery, it was always good to get back home.

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