We know that our climate isn’t static.  Over the billions of years our planet has been rotating around the sun, it has undergone periods when it is very very cold (glacial periods) and periods when things are warmer (interglacial periods).  We are sitting in an interglacial period right now.  This one is called the Holocene, and is one of 5 major interglacials that scientists have identified.  Today technology is allowing us to record all sorts of data – both at a global scale and at a local scale – about the weather and climate.  As this technology improves, so does the quality and quantity of information we can capture.  

Even before technologies such as satellite remote sensing were around, people have kept long-term records of the weather they have experienced.  These records can be used to reconstruct the historical climate. But what about measuring oceanographic variables?  Today we have an array of tech to do this for us – like satellites, buoys, and autonomous vehicles providing all sorts of data on global and more fine-scale oceanographic conditions.  Before this technology?  Back in the 1700’s Benjamin Franklin (yep – one of the ‘founding fathers of the United States’ – he was a scientist too) had to use much simpler methods…dipping a thermometer into the ocean and recording the readings he got.

Franklin’s experiment is one of the earliest empirically-derived sea surface temperature datasets we have.  These readings are very much a one-shot deal – not something we can use in isolation to figure out what the average sea surface temperature was during that time, at the location he took them.  So how can we find out what historic ocean-climate was?  Turns out that there are all sorts of clever ways.  Here’s a couple of examples from science papers that have come out over the past couple of weeks.

Foraminifera are pretty awesome.  

The single-celled critters that make up the order foraminifera are found in all marine environments, and have been pootling about since the Cambrian period (that’s around 541–485 million years ago).  Whilst they vary in size and shape, foraminifera all have something quite important to paleooceanographers…a shell.  Shells are constructed from calcite, but also trap impurities from the water.  It was one of these impurities in particular that Oscar Bransonof the University of Cambridge in the UK, and his fellow researchers from the US and Japan were interested in for their study – magnesium.  

As the shells of these tiny critters grow, the magnesium forms growth rings (rather like tree rings), and these bands appear for every day of that individual’s life.  So what’s the link with ocean temperature you ask?  Well it seems that the level of magnesium in these bands is related to water temperature.  The more magnesium a band has, the warmer the ocean was where that critter happened to be.  When these foraminifera die, they end up on the sea floor (or stay there if they are benthic), handily leaving their shells behind with those magnesium bands stored.  So, take a core sample (the different layers from these cores can be dated), take out the shells and bobs-your-uncle you have a record of ocean temperature for the life of those individuals.  And because these guys have been around for quite some time, their collective remains can give us an indication of what the ocean temperature were like in the very very…very distant past.  Sounds simple right?  Well it’s not quite that easy.  Each one of these bands is tiny.  On the nanometer-scale tiny, and not really visible with standard microscopes.  This was a job for the Advanced Light Source synchrotron - a huge particle accelerator that “produces light in the x-ray region of the electromagnetic spectrum that is one billion times brighter than the sun” .

Pretty cool right!  Well so is the next paper.

Plankton aren’t the only ‘paleoproxies’ that are used for figuring out what the temperature of the oceans used to be.  Shayne McGregor of the The University of New South Wales in Australia and colleagues recently reconstructed ENSO over the past 600 years.  ENSO – the El Nino Southern Oscillation – is the variation of sea surface temperature and surface air pressure over the tropical east Pacific.  You’ll most likely have heard of El Nino and La Niña years.  ENSO has a huge impact on weather patterns.  If you live on the east coast of Australia, you may be more familiar with how this can play out – cyclone activity, flooding, droughts….  The strength and frequency of ENSO events shows multi-decadal variability, but understanding the long-term changes in frequency and magnitude of these events is a little more tricky.  That’s because the technology to measure these events in the detail needed hasn’t been around long enough (around 150 years).  

So to figure out anything past this, we need to use these paleoproxies.  For historic El Nino sea surface temperature data, these proxies tend to come in 3 flavours – tree rings, sediment cores and corals.  When scientists have tried to reconstruct ENSO patterns, they usually combine each individual proxy and then figure out what ENSO was doing.  Makes sense right?  The problem is that the reconstructions from these proxies don’t always line up perfectly, with a number of discrepancies between the dates of the measurements, causing issues with combining them together.  What Shayne and the rest of the team did is perhaps a little less intuitive.  They calculated ENSO activity based on each proxy individually, and then combined those activities to provide a reconstruction.  It seems to have worked well.  The team ran a number of simulations and compared it to more the recent measurements of ENSO.  

These measurements have been taken from oceanographic monitoring instruments, so we can safely say they are pretty robust, and a great candidate for testing historical models against.  So what has this more robust reconstruction of ENSO told us?  Well it seems that if we look over the past 600 years – from the 1400s up to 2009, the most active 30 -year period was between 1979 – 2009.  These results aren’t the result of researchers arbitrarily choosing a 30-year window just to ‘show something’ either.  The 30-year sliding window was chosen to focus on that multi-decadal variability.

Well it’s all very fascinating stuff this ‘what the ocean used to be like’ but…so what?  Well if we can reconstruct how the oceans used to be, and figure out how systems (biological, ecological, physical, chemical…) responded to those conditions then we can improve the predictions we make about our future climate.  This knowledge in turn can help us mitigate – and crucially adapt to – our ever-changing environment.

The paper by Shayne McGregor and co looking at ENSO variations was published in the journal Climate of the Past .  The paper is open access.

The paper by Oscar Branson and co looking at the foraminifera shells is published in the journal Earth and Planetary Science Letters. This paper is also open access (hurrah!).

If you fancy finding out a little more about foraminifera, UCL have produced a nifty little site which goes into all sort of details.   The Natural history Museum in the UK have also put some really cool foraminifera posts up on their Micropaleaeontology blog.