56 | New Scientist | 12 September 2020
Rising tide
How are sea levels measured
over time?Chris Daniel
Colwyn Bay, Conwy, UK
Sea level has only been measured
accurately since the early 1800s,
when the US first began to
systematically survey its coastline.
This was originally done with a
simple measuring stick to gauge
the height of the water relative
to a benchmark on land.
The stick was often used in a
stilling well, a tube with a small
hole placed below the water’s
surface that dampens movement
caused by waves. This surprisingly
accurate system remained
unchanged in principle –
apart from the introduction
of automated recording – until
microwave radar scanners arrived
in the 1960s. These are mounted
on coastal structures, such as
bridges or breakwaters.
The drawback of these systems
is that they only measure sea level
relative to the land at that location.
Land height may also change due
to tectonic activity or movements
associated with the end of the
last glacial period.
In recent decades, corrections
based on statistical models of
ground movement have been
used. Nowadays, though, GPS
detects changes in the height
of the land to offset against
nearby sea level measurements.
It was only in the 1990s that
satellites began to measure global
sea level independently of the
land. One current example is
the European and NASA Jason-3
satellite, which scans the whole
planet (except the poles) every
10 days and uses this information
to calculate the mean sea level.
A similar joint mission called
Sentinel-6 will begin in late 2020.
Satellites have been tracking the
annual rise in sea level associated
with global warming. From 1993
to 2018, it averaged 3.2 millimetres
per year, but in the last five years
of that period, it increased
by 4.8 millimetres per year.Mike Follows
Sutton Coldfield,
West Midlands, UK
Sea level used to be measured
through stilling wells dotted
around the world, but is now
gauged using satellite altimetry.
A radar pulse from a satellite is
bounced off Earth’s surface, and
the time it takes the echo to return
to the satellite is then used to infer
the distance to the sea surface.
Satellite altimetry provides
global coverage and eliminates
the problem of the land rising or
falling locally. Some land masses
that were overlain by ice during
the most recent glaciation,
making them sink, are still
rising to their former height
in a process called post-glacial
or isostatic rebound.
When the population of a
coastal city consumes water from
the aquifers that the city is built
on, the land can subside, giving
the illusion of sea level rise. Places
where this has occured include
Bangkok, Dhaka and Tokyo.
Bangkok is sinking at a rate
of up to 2 centimetres per year.Dhaka, too, is still sinking and the
frequency of floods has increased.
Tokyo subsided by about 2 metres
over several decades, but the
rate was greatly reduced when
regulations restricting the
extraction of groundwater
were introduced in the 1960s.Cover up
When you wash a duvet cover,
why does other washing end up
in it? (continued)Lorna Cox
Cambridge, UK
The question about duvet covers
in the wash was answered in the
Last Word some decades ago.
One answer given at the time was
that any decorative printing on
the fabric could cause a differential
stiffness of the outside and inside
surfaces, promoting inversionin the washing machine. This
inversion would then result in
items finding their way inside
the cover, especially if the
machine had been overloaded.
Since reading that answer,
I have always washed duvet covers
inside out and this has solved
the problem. In my experience,
it hasn’t been necessary to close
the poppers or other fastenings.
Occasionally, something may end
up inside, but very rarely. Life is
too precious to be spent doing
and undoing poppers and the
cover will dry faster on a washing
line if the two sides are free.James Fradgley
Wimborne, Dorset, UK
Perhaps the things inside the duvet
cover are experiencing something
akin to thermophoresis, the
movement of particles in a
temperature gradient. As items
are thrown about in a washing
machine, smaller articles of
clothing like socks are likely
to have higher velocities than
larger ones, such as shirts.
You might suppose that if a
sock goes into the duvet cover,
the odds are good that it will zoom
back out again. But when it comes
to a larger item, the chances may
not be so good, as friction could
play more of a role. On the other
hand, by the same logic, it is
harder for a shirt to get stuck
in a duvet cover than a sock.
Have any studies been
done to estimate if there is a
size discrepancy at the end of
the wash between what is in
the duvet cover and what isn’t?John van Someren
London, UK
Because it is a lobster pot
made of cotton. ❚This week’s new questions
Birdily functions Are there any species of bird that hiccup? If
so, why would they do this? Freya Smith, Mosgiel, New ZealandPatchy cabbage Why do cabbages exist? What is the point
of having a tight bundle of leaves that don’t attract pollinators
and shield each other from the sun? Does its structure affect
its ability to photosynthesise? Tim Stevenson, Great Missenden,
Buckinghamshire, UKWant to send us a question or answer?
Email us at [email protected]
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BENCE^ MATE/NATUREPLIs it possible for birds
to hiccup, and if so,
why would they?