A worrying outlook for Antarctica and beyond
By Ceridwen Fraser
For a long time, we have thought of Antarctica as isolated from the rest of the world. The continent is entirely surrounded by the Southern Ocean, which heaves with giant waves whipped up by intense winds and is home to the world’s strongest ocean current, the eastward-flowing Antarctic circumpolar current (ACC).
The Southern Ocean is associated with several circumpolar oceanic fronts, where sharp transitions in ocean temperature and salinity occur.
One of the most significant of these is the Antarctic polar front, a convergence zone where cold Antarctic water sinks under warmer sub-Antarctic water.
The polar front was considered to be a barrier blocking movement of marine plants and animals into and out of Antarctica.
Many groups of organisms show strong differences on either side of the front, suggesting northern and southern populations have been separated for a long time.
We know from genetic work that some species, such as some molluscs and crustaceans, have managed to cross the front in the past but there is little evidence that biological movement across the front can or does still occur.
Some live adults and larvae of crabs that had not previously been found south of the polar front have recently been detected in Antarctic waters but there is doubt about whether these are true invaders from the north, or have been around Antarctica for thousands of years.
Globally, many species are either moving up mountains or towards the poles as the Earth warms. This trend has been happening since the end of the last Ice Age but is accelerating as global warming speeds up due to human influences.
In the Northern Hemisphere, shallow waters and continental land span almost all latitudes from the tropics to the poles, making it straightforward for many tropical and temperate species to move north.
But in the Southern Hemisphere, the Southern Ocean gets in the way of plants and animals trying to head to higher latitudes.
Many species already on the southern tips of continents such as South America, Africa and Australia face extinction if they cannot move south as the climate warms.
Antarctic ecosystems are unique; they feature large numbers of species not found anywhere else.
Many Antarctic species are slow growing. Antarctic lichens, for example, take between 100 and 1,000 years to grow one centimetre.