Winds of change blow worldwide
People sometimes like to see everything on the planet as being connected somehow with everything else.
According to this perspective, the fluttering of a butterfly’s wings on one side of the world might cause an earthquake on the other side of the globe.
It may seem fanciful, but work by scientists associated with the New York University Abu Dhabi (NYU Abu Dhabi) has indicated that changes in one part of the world can have surprisingly far-reaching geographical consequences.
Work by the university’s Centre for Global Sea-Level Change (CSCL) published in the journal Nature indicates that temperature effects in the North Atlantic have influenced ice flows around the Antarctic, thousands of miles to the south.
The CSCL’s head, Professor David Holland, a professor of maths and ocean science at NYU New York, readily admits that the conclusion is unexpected.
But he and his team, which includes the senior author of the recent paper, Xichen Li, a PhD student at NYU New York, have a wealth of observational and computer modelling data to support their ideas.
Originally, Prof Holland wondered whether temperature fluctuations in the tropical North Atlantic, where the past three decades have seen an average warming of about 1C, might be having an effect on the ice sheets around Greenland, which have undergone significant change over the past decade. So his group looked for a correlation – but didn’t find one.
What they did find was a link between the warming of the North Atlantic and increases in the temperature of the Antarctic peninsula.
Not only that, there was a similar correlation between the North Atlantic temperature and changes in the distribution of sea ice in the Antarctic.
Despite gradual average rises in global temperatures, the area around the Antarctic Peninsula covered by sea ice has not declined in area in recent years, in contrast to the declines observed in Arctic ice-sheet cover. Instead, it has shifted in distribution. It has shrunk in the Amundsen-Bellingshausen-Weddell seas, which surround West Antarctica, while in the Ross sea, which inhabits the vast bay that cuts between West and East Antarctica, it has grown. These changes, say Prof Holland and his co-authors, are “perplexing”.
So what’s going on? How are the increases in temperatures in the North Atlantic affecting the sea ice in the Antarctic?
The key factor is wind patterns, and their knock-on effects from one region of the world to the next.
Those winds are linked; a clockwise movement of weather systems above the North Atlantic tends to go hand in hand with a similar weather system above the Antarctic.
And it seems that one is causing the other, that the North Atlantic system is causing that similar weather pattern over the Antarctic by a series of knock-on effects on the atmosphere between them.
The researchers believe this is the result of a climatic phenomenon called Rossby Waves, which were discovered in the mid-20th century by Carl-Gustaf Arvid Rossby, a Swedish-born meteorologist who is also credited with identifying the jet stream.
Essentially the ‘waves’ model suggests that each weather system encircling the globe is bouncing against the one further sound in something like a domino effect. That way, a change in the weather in the North Atlantic can have influences all the way down to the Antarctic.
“It’s a tricky thing. It’s not air from the Atlantic [that is travelling to the Antarctic], but it’s a wave, where a piece of air pushes the next piece,” said Prof Holland.
This wave ultimately generates a clockwise movement of high and low-pressure weather systems over the Antarctic. And it is this that pushes ice away from the peninsula in one area, causing the Amundsen-Bellingshausen-Weddell seas to have less ice cover, while forcing ice into the Ross Sea, explaining the increase in ice cover there.
Finding a correlation between two variables is one thing. It is quite something else to prove that a change in one causes a change in the other.
Prof Holland readily admits it is difficult to be certain that the North Atlantic weather system is affecting the Antarctic.
However, the fact that computer simulations replicate the effect seen with observational data does seem to offer strong evidence in favour of the researchers’ conclusions.
What is less clear is whether the warming seen in the North Atlantic, and its knock-on effects in the Antarctic, is being influenced by a wider global warming effect.
Average global ocean temperatures do naturally oscillate. That oscillation could be enough to account for the warming, and for the observed changes in ice distribution.
There is a weather pattern that affects the tropical North Atlantic called the Atlantic Multidecadal Oscillation. It runs on a 60 to 70-year cycle and over the past 35 years or so has been causing an increase in sea surface temperatures in the region.
At the moment, the magnitude of the effects of climate change are smaller than the variations caused by this natural cycle, so teasing out the various influences on temperature is difficult.
“The ocean is warming and it will fire off these responses. I don’t think it can tell who is kicking it … I don’t think we can separate anthropogenic forcing from natural,” said Prof Holland.
So will North Atlantic temperatures eventually revert to their long-term average? Or will they be pushed by global warming into a very different and new temperature regime?
It would take, says Prof Holland, thousands of years for atmospheric warming to significantly affect the ice sheets of Greenland and Antarctica.
But if wind patterns change there would be alterations in ocean currents, potentially bringing warmer water into contact with the ice sheets, shrinking them in size much more quickly and causing potentially devastating rises in sea levels.
“There’s only one single way we could have large sea-level change: bringing warm water towards the ice sheets,” said Prof Holland.
“That’s where this research is headed: to find out if there’s … a scenario where [winds] force the ocean to bring warm water to Greenland or Antarctica.”
The answer to that question could have significant implications for planet.