Today we are going to move onto another topic we hear about
all the time which NO ONE bothers to explain: Rising Sea Level (RSL). This may be because at its core RSL is pretty
simple to understand -- As the earth
heats up, glaciers melt – As glaciers melt there is more free water on the
planet – the water has to go somewhere so the oceans and seas rise. Like most of climate science, the effects of
a warming earth are not really debatable – this is happening and has happened
for a long time. Also like most climate
issues we are attempting to figure out how much humans have already and will
impact these changes. In future posts we will go into the effects of a rising sea level but for now we are going to show how scientists learn about sea levels of the past. Learning about the planet's past is our only way of accurately predicting its future.
As technology has advanced we are able to measure sea level
changes very accurately. Just like with
greenhouse gasses though we need to look into the earth to unlock past sea levels. This post will walk through how scientists have unlocked the history of sea levels throughout the last several glacial periods. This process "hopefully" will highlight just how clever scientists have to be to validate their ideas and data. There's not such thing as a free lunch - and there is no such thing as a free hypothethis!
Sea Levels of The Past
At the largest scale, the easiest way to determine past sea
levels is to look at the make-up of the land borders of large bodies of
water. Along these borders (think cliffs
along the ocean) we will see rock formations that seem to be formed by tides
coming in and out much higher along the cliff than the tide will ever reach
now. Similarly if we put our scuba masks
on and dived beneath the tidal line of today we would see similar formations
much deeper in the water. These
geological formations aren’t going to give us any exact information but they at
least give us some perspective – the earth’s sea level has been all over the
place throughout its history.
Using similar thinking as we did for learning about the
atmosphere of the past (Ice Cores) – we are looking to find out how the sea
level has changed over the last several hundred thousand years. This information will give us climate cycles
that are similar to the one we are in now; letting us make educated guesses about
our future and giving us thousands of years to test those guesses against. When scientists were looking to find out
about the past atmosphere they needed to find somewhere the earth stored this
information and came up with glaciers.
What on earth has spent its history storing information about sea level?
Globigerinoides ruber d'Orbigny
This little guy is known as Globigerinoides ruber d'Orbigny but we are just going to
call him a plankton (plankton are always rearing their heads when we talk about
the climate). Plankton like these live in the ocean and have hard outer shells,
like tiny single celled coral. To make
their shells they take calcium, carbon and oxygen in the water and combine them
to form something like limestone, typically having a chemical formula: CaCO3. These plankton then die and their skeletons
get turned into fossils and are embedded in the ocean floor. Hopefully something from the ice core blog post sticks out in
your mind (if not read up about it there J
); there are two distinct types of Oxygen in our atmosphere “heavy” and “light”
oxygen.
“But Joe we know that heavy
and light Oxygen levels change with global temperature not sea level!” This is
true, especially for areas of the ocean near the poles – but let’s think about
areas that might be home to our tiny plankton friends whose heavy to light
oxygen ratio should stay the same regarless of temperature.
Both the Mediterranean sea and the Red sea are exactly what
we are looking for. Our tiny plankton
friends thrive in their salt water bodies, their climates have stayed
relatively stagnant over much of recent (500,000+ years) past and they only
have one way to interact with the ocean both through narrow (comparative to
their size) straights. If sea levels
never changed in these seas, especially the Red sea, the ratio of heavy to
light oxygen in our plankton shells would remain the same.
We (and scientists) are going to use the Red Sea for our
example as its oxygen ratios and interaction with the ocean are more stagnant
than the Mediterranean Sea. As
temperatures rise and glaciers melt more
heavy oxygen evaporates out of the ocean.
This “less heavy” ocean then feeds into the Red sea from the Indian
Ocean (the extra water has to go somewhere) and new plankton shells are
created. As temperatures fall and sea
levels go down, the previous “lighter” ocean leaves the Red sea leaving a “heavier”
ocean than before. We then core the Red
Sea bottom like we do a glacier to extract the sea level data of the past.
Issues with our Cores
The biggest issue with the Red Sea cores for a very long
time (until 2012/2014) was there was no
great way to date them. The Red sea
plankton shells give very detailed sea level information – unlike an ice core
where the oxygen can escape and spread throughout the core – the oxygen in the
plankton fossils are stuck, basically forever.
This is exactly what we are looking for, however without knowing the
time these sea levels happened at we cannot compare them to the climate at the
time.
For a long time scientists attempted to use glacial cores to
date Red sea cores. If we assume that a
warmer climate gives us a higher sea level you can sort of line up the sea
level peaks with temperature peaks. This
gives you some date to work with however it has many issues. First, the glacial
cores are hard to date themselves; basing data off of already difficult to verify
data is not a winning combination. Next
and maybe even more problematic is this does not allow us to see the
relationship between climate, CO2 and sea level. We are kind of arbitrarily matching up
temperature and sea level; not allowing the data to tell us what the relationship
is.
Luckily the earth provided scientists what they
needed to accurately date the Red sea – “speleothems!”
Salt Monster!
A speleothem is a fancy word for Salt Castle! They are created when ocean air blows into a
cave; the air then condenses on the roof of the cave and leaves the salt behind
somewhere i.e. the cave ceiling. The
caves in question are the “Soreq” caves on the south east of the Mediterranean Sea
which you can visit as a tourist in Jerusalem.
These are basically mineral stalactites which contain – you guessed it- Oxygen!
The heavy to light oxygen in the Soreq speleothems is not nearly as “locked
in” to sea level as the plankton fossils in the Red Sea (the Med. Sea is affected
by temperature more than the Red Sea), however it is very easy to date. While we cannot use the speleothems to give
the exact changes in sea level it accurately tells us when the changes occured.
We now have a very accurate measure of sea level changes
from the Red Sea and a very accurate time line of the changes from the Soreq caves which yields graphs like this:
With this information in our pocket we can now start
predicting the sea level in response to temperature and green house gasses. The study from above was not completed until
2014 and more and more research is constantly being done in the field. For some reason
climate deniers pretend that everything we need to know about the environment
is known already – in reality the science is still being discovered, being
analyzed. We are gaining more and more
knowledge about things we could only make guesses about before.
The first 3 links in the citation are the studies most of this information is based off of- they are pretty dense reads but incredibly informational and worth at least parusing at your liesure ;)
Citations:
http://www.nature.com/scitable/knowledge/library/drip-water-hydrology-and-speleothems-26394838
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