Category Archives: Planetary geology

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

Facebooktwitterlinkedininstagram

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.

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

Visualizing Mars landscapes in 3 dimensions; stunning images from HiRISE

Facebooktwitterlinkedininstagram

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

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

Subcutaneous oceans on distant moons; Enceladus and Europa

Facebooktwitterlinkedininstagram

Our blue Earth, rising above the lunar horizon, is an abiding image of our watery state that must evoke an emotional response in any sensible person. Cloud-swirled, Van Gogh-like. Such a blue – there’s nothing like it, at least in our own solar system.  A visitor to Mars three billion years ago might have also seen a red planet daubed blue, but all those expanses of water have since vanished, replaced by seas of sand.

Earth’s oceans are unique in our corner of the universe. Except for a thin carapace of ice at the poles, they are in a liquid state, and are in direct contact with the atmosphere to the extent that feed-back processes control weather patterns and climates.  Sufficient gravitational pull plus the damping effect of our atmosphere, prevents H2O from being stripped from our planet by solar radiation (again, unlike Mars). Our oceans exist because of this finely tuned balancing act. Continue reading

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

Near Earth Objects; the database designed to save humanity

Facebooktwitterlinkedininstagram

The media love natural disasters, even those that don’t exist. Last week (early October, 2017), dramatic footage of a (simulated) super-volcano eruption beneath Auckland city was aired by several international media outlets, with headlines announcing the city’s calamitous destruction. But there is no super-volcano beneath Auckland. The excitement was short-lived.  While Auckland smouldered (as if that wasn’t enough), it was announced that New Zealand’s North Island could experience a subduction zone earthquake that, in its aftermath, would leave 1000s dead. An interesting backdrop to New Zealand’s recent election. Having scared the local population to death, our purveyors of science moved on to the next concern; other “what ifs…”.

Asteroid impacts are no longer de rigueur; perhaps it’s the turn of super-volcanoes’, or because NASA and the European Space Agency (ESA) have stated, with some confidence, that no large impacts are expected within the next 100 years. And whereas the media may find this Continue reading

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

The origin of life: Panspermia, meteorites, and a bit of luck

Facebooktwitterlinkedininstagram

In the opening scenes of Stanley Kubrick’s 2001 Space Odyssey (1968), Neanderthal-like folk are scrounging for food, squabbling with a neighbouring tribe who are intent on competing for the meagre lickings (a reactionary condition that would not bode well for future humanity). One of them picks up a large bone.   There’s instant recognition, seemingly influenced by a black obelisk that appears mysteriously, that it can be used for something else. His neighbour lies in a crumpled heap. In what has become an enduring Sci-fi image, he triumphantly hurls his weapon into the air, whereupon Kubrick transforms it into an orbiting space station. Continue reading

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

The intriguing paradox of global warming piggybacking on global cooling

Facebooktwitterlinkedininstagram

Flood, fire, drought … We have, by luck and muddled management, thwarted pestilence, but it seems that changing weather patterns everywhere are leading us on a merry dance.  Our climate is giving us a bumpy ride; anyone living in the Caribbean and southeast US, or Bangladesh, will attest to this, given the havoc that hurricanes and tropical cyclones have wrought over the past few months (northern hemisphere summer, 2017).  The skinny, outer layers of our world (air and oceans) seem to be getting warmer. No doubt there are consequences?

It may seem paradoxical, but global warming is taking place against a backdrop of global cooling. Forcing of global climates is governed by internal (within our own skinny sphere) and external agents; the latter by solar output and earth’s changing orbit. There is now, good Continue reading

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram

Io; Zeus’s fancy and Jupiter’s moon

Facebooktwitterlinkedininstagram

Zeus, the head-honcho of assorted Greek gods, heroes, nymphs, and mortals, was chiefly the God of the Sky, or Heavens. One of his minor portfolios was the upholding of Honour, but, as mythology relates, he didn’t put much energy into that particular task; he was a philanderer, much to the annoyance of his own wife, Hera (I guess his energies were directed elsewhere).  One such misdirection was Io, a mortal woman, who had the misfortune to be turned into a heifer by Zeus, to hide the infidelity from Hera. Io’s memory now survives as planetary body; one of the Galilean moons of Jupiter is named after her (to be named a moon of the Roman God Jupiter, seems like a historical slap in the face to the Greek deity). Continue reading

Facebooktwitterlinkedin
Facebooktwitterlinkedininstagram