Ocean warming hotspots

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This little chap is a Sally Lightfoot crab (Grapsus grapsus).  Sally Lightfoot’s can be found on the west coast of South America, Central America, and Mexico, but this critter in particular lives on the Galápagos Islands – or as they are officially known Archipiélago de Colón.  These islands are home to 95 living endemics species (species found nowhere else), and famed for the now-deceased giant tortoise ‘Lonesome George’ and Charles Darwin’s work on Galapagos mockingbirds and finches which eventually gave rise to his seminal book On the Origin of Species as well as its biodiversity and outstanding beauty both on land and in its seas.  Unfortunately for our Sally Lightfoot, those seas are due to get warmer….a lot warmer.

The problem is this.  Climate change doesn’t just mean that the global mean surface temperature is increasing.  Because oceans absorb heat, they too increase their temperature.  The upper layers are affected first, but as warming increases, lower and lower depths also increase in temperature.  This temperature increase has a number of impacts on the ocean.  For us landlubbers, sea level rise through thermal expansion is a huge concern.  For those critters who live in the marine environment itself, there are issues with the amount of oxygen warm water holds (the warmer the water, the less oxygen it holds) and then there’s the temperature itself.  It’s not just that the water may be uncomfortably warm and critters who like it cooler can get by.  Like all species, marine critters have a thermal tolerance.  As temperatures approach either end of an animal’s (or indeed plants) thermal tolerance it may become stressed…less able to function, reproduce, or even survive.  Warmer waters also bring new species into areas as their old habitat warms up.  There’s only so far a species can shift though for two reasons.  Firstly, the Earth if finite – eventually species will hit polar areas.  Secondly, for species that live in coastal areas there becomes a point – particularly in the Southern hemisphere – where there is no coastline to live on for quite some distance.

We know that species are already on the move (check out two of my recent posts here  and here for a couple of examples).  We also know that some parts of the ocean are warming much more rapidly than others.  Identifying ‘hotspots’ – where the oceans have been warming the fastest – and crucially where they are most likely to continue warming the fastest – was the focus of a study by Alistair Hobday of CSIRO and Gretta Pecl of IMAS, University of Tasmania, both in Australia.

Using two separate monthly global sea surface temperature datasets and six General Circulation Models from the IPCC 4th assessment report (which models atmospheric and oceanic processes), Alistair and Gretta calculated both historic annual average sea surface temperatures (1950 – 1999) and into the future (2001 – 2050).  Because the future is not 100% set in stone, the authors calculated future temperatures based on two of the IPCC emission scenarios – A1B and A2^.  In total they identified 24 ‘hotspots’ – discrete areas of the ocean whose temperature is increasing at “faster than 90% of the rest of the ocean”.  These areas are spread out across the globe – I’ve attached one of the maps from the paper to this post for you to have a look at.  The authors note that although the rate of projected warming differed between the two IPCC scenarios, the hotspot locations of the hotspots did not change.  And yes – the Galapagos Islands are sitting right in one of these hotspot zones.

The importance of the identification of these hotspots goes beyond knowing which habitats for critters such as our Sally Lightfoot is likely to alter in the future.  The authors suggest that these areas can act as ideal locations for learning more about how the changing climate will impact on marine ecosystems and biodiversity redistribution and – crucially for people – what sorts of adaptations could be put in place.  This final point is very important.  Even if we were to completely stabilize emissions now, the oceans will continue to warm because of ‘thermal inertia’ – the time it takes for the oceans to heat up (you can read more about how this works).

So whichever way we look at it, some adaptation will be necessary.  Looking at these hotspots, the authors have pointed out that in many of these places adaptation will be incredibly important.  Many of these hotspots occur in areas where human populations are heavily reliant on the marine environment, particularly for food itself, but also for livelihoods.  The authors highlight one area in particular where food security may become a particular issue – the Angola Basin.  Not only does this region lie in a marine warming ‘hotspot’, but terrestrial-based food crops are also predicted to suffer major declines.  What these guys need is to understand how their resources are likely to change, and how they can adapt to the changes ahead so their food security is not completely removed.  Sure, adaptation isn’t going to be easy but it is not impossible, but the longer we leave it, the more difficult it could get.

This paper was published in the journal Reviews in Fish Biology and Fisheries.  Unfortunately it’s sitting behind a pay wall, but if you have access of fancy buying it, the link is dx.doi.org/10.1007/s11160-013-9326-6

Images:
1: Sally Lightfoot Crab at Waters Edge.  Credit Benjamin Jakabek/Flickr (CC BY-NC-ND 2.0)

2: The 24 hotspots for warming as identified by Alistair and Gretta.  The coloured squiggly lines in the circles areas indicate the location of the hotspots as predicted using historical data from two sea surface temperature datasets, known as HadSST2 and: ERSSTv3.  Taken from Hobday and Pecl (2013).

^ The official explanation for these scenarios can be found here.  There is a useful graph in this webpage which shows you how the scenario’s pan out in terms of emissions.  For temperature predictions see this graph.

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