After last week’s amble through the record of 2018’s earthquakes, the week of 10-16 January 2019 is back in the realms of serendipity. There’s a mix of geoscience topics on offer this week. True, there are earthquakes, but we’re also looking at research into the oceans’ absorption of heat (and what we can learn from studying it) and also what’s happening in the Antarctic.
Filling in Some of the Gaps: Global Warming and Ocean Heat Content
I can’t stress too often that the Earth system is complex and its various parts are interconnected. Every change to one element has impacts elsewhere. One of those is in the context of global warming: as the atmosphere warms, where does the heat go?
You might expect that most of it stays in the atmosphere but it doesn’t. Most of it is absorbed by water (think about it: it you leave a glass of water out in the sun it will warm up) and that means it’s taken up by the oceans.
In order to understand this relationship, it’s important to understand how and to what extent the heat is absorbed and, because the oceans are anything but uniform in terms of temperature, where in the ocean the heat goes. New research from Oxford University, published this month in the Proceedings of the National Academy of Sciences, offers data which extend our understanding of the processes back over a period of 150 years.
The report found that warming took place over the period between 1871 and 2017, but wasn’t constant. There were notable increases between 1921–1946 and 1990–2015. The figures are consistent with others over shorter time periods, showing how much heat the ocean is taking up.
This new methodology is important because, as the report notes: “Observational constraints on future anthropogenic warming critically depend on accurate estimates of past ocean heat content (OHC) change”. This approach relies on mathematical modelling and is not without its limitations, but it provides a potential method for extending our understanding of how the ocean takes up excess anthropogenic atmospheric heat.
Estimates of Antarctic Ice Mass Balance
I said in the introduction that today’s items were serendipitous, but the first two are, nonetheless connected. Above I talked about estimates of change in ocean heat over a period of time. This week a report was published which looks at changes in the mass balance (i.e. overall amount) of Antarctic ice.
There are obvious differences, apart from the subject matter. The Antarctic study uses a different methodology — satellite observations and computer modelling — and covers a much shorter time period, from 1979-2017.
The findings were that: “The total mass loss increased from 40 ± 9 Gt/y in 1979–1990 to 50 ± 14 Gt/y in 1989–2000, 166 ± 18 Gt/y in 1999–2009, and 252 ± 26 Gt/y in 2009–2017”. In other words, the rate of loss of the Antarctic ice has increased by over six times during the period. The research further concluded that: “The contribution to sea-level rise from Antarctica averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ± 2.0 mm since 1979”.
This study and that discussed above aren’t directly related, although you don’t have to be a geoscientist to see how the subject matter is connected — ocean warming is a significant factor in melting of ice. But both of them are potentially significant contributors to our increased understanding of the exact extent and impact of a warming atmosphere.
Earthquake Chat: The Northridge Earthquake of 1994
I couldn’t let today slip by without a mention of the 25th anniversary of California’s M6.7 Northridge earthquake, which occurred on 17 January 1994. It’s a reminder, if we need it, of the impacts of a large — though not enormously so — earthquake on a major urban area.
The ‘quake occurred in the San Andreas Fault Zone, though no on the San Andreas Fault itself (which runs some distance to the north east). The Caltech summary of the event places it on the: “Northridge Thrust (also known as Pico Thrust)… several other faults experienced minor rupture, rupture during large aftershocks, or triggered slip”.
The Northridge earthquake wasn’t by any means the largest to strike California: there are a dozen on record of at least the same magnitude since 1812. Nor was it the most deadly, although its death toll of 60, very low in a global context, places it on a par with other California earthquakes of the second half of the twentieth century such as San Fernando in 1971 and Loma Pieta in 1989. It does, however, have the dubious distinction of being the most economically costly earthquake in US history (between $18-27,000m in 2006 prices).
The Northridge earthquake wasn’t ‘The Big One’. Its magnitude of M6.7 is less than tenth of that of the 1906 San Francisco earthquake (M7.8) and the death toll is just a fraction of that caused by smaller earthquakes in less well-prepared locations. But it’s a reminder of how vulnerable our society is to earthquakes whose timing and magnitude we cannot predict.