Saturday, May 14, 2016

The Canary in the Coal Mine

In days of yore, coal miners would take a caged canary into the mine with them as the birds were more sensitive to poisonous gases than humans; if the canary died then the miners got out – alive.

‘Climate sceptics’ have long accused ‘climate activists’ of (to continue the metaphor) breeding highly sensitive canaries and looking for dangerous coal mines. Up to now I’ve studiously respected this site's motto as being a place where ‘numbers count’ and stayed out of debate. After a recent paper on ‘vanishing islands’ in the Solomon Islands archipelago I felt I had to comment. I was partly spurred on to do this by a guest post by David Middleton on the web site.
The paper in question is “Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands” (Albert et al, Environmental Research Letters, Volume 11, Number 5). The headline message of the paper was

“..we present the first analysis of coastal dynamics from a sea-level rise hotspot in the Solomon Islands [and] have identified five vegetated reef islands that have vanished.”

That message got widespread coverage. At breakfast this morning in my hotel in Dhaka (Bangladesh) a fellow guest (a curriculum development specialist – nothing to do with climate) asked me if I had heard of the seven (sic) islands which had disappeared.

The total land area of the Solomon Islands is 27,990 km2 (World Bank figure). The area of 5 islands which have disappeared is given in the paper as 160,310 m2. Why did the authors use square metres? Why not hectares or square kilometres? More usual surely for an island? Perhaps it was because 160,310 m2 is 0.16 km2; that is 0.0006 % of the total area of the Solomon Islands.

OK. We are talking about canaries in coal mines so perhaps they are justified in a little sleight of hand. Let’s look further.

Their introduction starts:

“How islands and the communities that inhabit them respond to climate change and particularly sea-level rise is a critical issue for the coming century. Small remote islands are viewed as particularly vulnerable.” The authors do acknowledge a role for wave action but this is seen as secondary to sea level rise.

The following table is taken from the paper.

Area (m2) of the 5 islands which have disappeared

What the table shows is that there was significant loss of area between 1947 and 1962.  The loss was 41% in that period. Expressed in m2 per year the rate was 4100 m2/year for the period 1947 to 1962 and 1800 m2/year for the remainder of the period. I recognise that defining two years just because they have data might bias the answers but when the rate in the second period is less than half that in the first period it is hard to accept that loss of island area is due to increasing sea levels.

Let’s now take a look at sea level rise. The following chart shows sea levels from two sources. The first is from the Permanent Service for Mean Sea Level and covers the period 1975 to 2015. Levels were measured at two locations with a short, 5-month, overlap. The second record from 1992 to the present is from the University of Colorado Sea Level Research Group and is based on satellite altimetry. 

The PSMSL record has a rate of rise of 2.7 mm/year. The University of Colorado gives a rate of 5.9 mm/year much less than 7 mm/year quoted in the paper; the difference in rate is in part probably linked to the recent drop in sea levels due to the El Nino effect. One drawback of these data is that they do not cover the whole period 1947 to the present used for analysis of the area of the islands.

In January of this year I was in Samoa – looking at the impact of climate change on roads. There it is something to be concerned about. On both of the two main islands there are few inland roads but they do have roads all the way round each of the islands. In places these roads are on a narrow coastal band and barely above the current high tide level. So, a modest increase over the next decade or so could have serious consequences. While there I prepared estimates of sea levels from 1948 to 2014. These, together with the PSMSL figure for Solomon Islands are shown on the next chart.

The amplitude of the sea level estimates is higher for the Solomon Islands than for Samoa but they show a similar trend. I’ve also plotted a quadratic trend line through the Samoa data when shows that for the early period sea levels were more-or-less constant but in recent decades have been rising more rapidly.

In other words, if sea levels in the Solomon Islands have followed a similar trend to Samoa, the most rapid loss of area coincided with the least change in sea level.

I mentioned above that I am working in Bangladesh. At the northern end of the Bay of Bengal the 2 metre contour is 100 km from the coast. A typical spring tide has a range of 4 metres. In that part of the country most agricultural land is behind embanked polders and when they are overtopped the land becomes saline. So creeping sea level rise has a real impact there.

The paper that is the basis of this posting has, of course, succeeded in the author’s terms; it has got wide publicity for the potential impact of climate change. But whether describing the disappearance of five small islands, whose total area is that of 20 soccer pitches, has advanced climate science is a moot point.

Saturday, April 9, 2016

Glabal Sea Level Rise

  • The average rate of sea level rise from 1880 to 2013 is 1.6 mm/year
  • The rate of sea level rise is not constant. It is increasing at 0.014 mm/year/year.
  • Superimposed on the rising sea levels is a cyclical component with a periodicity of about 50 years which is synchronous with the Atlantic Multidecadal Oscillation.
CSIRO Estimate
Sea levels have risen more than 100 m since the end of the last ice age and they are still rising. This post looks at the rate of rise over the last century or so and, based on sea level data, and answers the question "Is the rate of level rise increasing?",

The CSIRO provide one of the main estimate of global mean sea levels (Church, J. A. and N.J. White (2011), Sea-level rise from the late 19th to the early 21st Century. Surveys in Geophysics, doi:10.1007/s10712-011-9119-1.). The data run from 1880 to 2013. They are available as monthly or annual values. The annual values have been analysed here.

This chart shows the CSIRO sea level data. The data are in millimeters relative to an arbitrary datum. The data show that global sea levels have risen by just over 200 mm in the period 1880 to 2013. Plotting a trend line through the graph gives an average rate of rise of 1.6 mm/year. This is 160 mm  century, much less than most of the climate change projections.

The above chart gives just one rate of sea level rise - the one for the whole period. So what if we look at year-on-year sea level change.

Year-on-year rate of sea level rise

The next chart plots the difference between the value of sea level in the given year and the value in previous year, for each year from 1981 to 2013. So, the first value is the difference between sea level in 1881 and in 1880, and so on. Looking at the chart there is a lot of year-to-year variation, from minus 17 mm/year to plus 21 mm/year. A trend line through the data shows that the rate of sea level rise has increased, by 0.0141 mm/year/year. That means the underlying rate of sea level rise was 1.9 mm/year higher in 2015 than it was in 1880. In other words, the rate of sea level rise is increasing.

Smoothed rate of sea level rise

One way of observing underlying trends more clearly when the data have a lot of year-on-year variation is to use a moving average. This takes the average of the values of the data for a number of years before and after each point plotted. As the data are so variable a long period has been used for averaging, 31 years. The first point plotted is for 1896 and is the average of the sea level from 1881 to 1911, the next is the average from 1882 to 1912 and so on.

This chart confirms that the rate of sea level rise is increasing but not in a uniform way. From a peak of 1.66 mm/year in 1900 to fell to minimum of 0.51 mm/year in 1920. It then rose to another peak of 2.2 mm/year in 1946 before falling to to a minimum of 1.33 mm/year in 1978. The rate of sea level rise then increased again to another peak of 2.84 mm/year 1997.

The trend line on the chart gives a slightly different value for the rate at which the rate of sea level rise in increasing, 0.0117 mm/year/year, to that in the chart above. This is due to effect of the averaging.

Cyclical component to sea level rise

The final chart plots the difference between the rate of sea level rise and the trend line. This is described as the detrended rate of sea level rise. For example, the peak in 1946 was 2.20 mm/year, the value on the trend line for that year was 1.63 mm/year so the value plotted was the difference between them 0.57 mm/year.

This chart shows that the rate of sea level rise has two components. The first is the underlying increase in the rate of sea level rise, this is 0.0141 mm/year/year as seen in the first chart. The second is a cyclical component with an amplitude of plus or minus 0.6 mm/year and a periodicity of around 50 years.

And the orange line? Climate scientists have detected a number of cycles in observed climate data. One of these is called the Atlantic Multidecadal Oscillation (AMO). It is based on sea temperatures in the northern part of the Atlantic Ocean. When the values of this oscillation are plotted along with the detrended rate of sea level rise they show a high degree of synchronicity. It cannot be argued that the AMO causes the variation in the rate of sea level rise. On other hand, it could be argued that both phenomena share an unknown forcing agent.