By Tim Radford
The tropics are on the march and US and German scientists think they know why: hotter oceans have taken control.
The parched, arid fringes of the hot, moist conditions that nourish the equatorial forest band around the middle of the globe are moving, unevenly, further north and south in response to climate change.
And the role of the ocean is made even more dramatic in the southern hemisphere: because the ocean south of the equator is so much bigger than in the north, the southward shift of the parched zone is even more pronounced.
Across the globe, things don’t look good for places like California, which has already suffered some of its worst droughts and fires on record, and Australia, where drought and fire if possible have been even worse.
In the past century or so, carbon dioxide levels in the atmosphere have risen from what was once a stable average of 285 parts per million to more than 400 ppm, and global average temperatures are now at least 1°C higher than they have been for most of human history.
“We demonstrate that the enhanced subtropical ocean warming is independent from the natural climate oscillations. This is a result of global warming”
And although the fastest and most dramatic changes in the world have been in the coldest zones – and particularly the Arctic – the tropics, too, have begun to feel the heat.
Researchers have observed tropical fish moving into cooler waters; they have warned that some tropical plant species may soon find temperatures too high for germination; they have mapped tropical cyclones hitting further north and south with time, and doing more damage; and they have seen evidence that tropical diseases could soon advance even into temperate Europe.
But although satellite observations have revealed that the tropical climate zone has expanded beyond the formal limits known as the Tropics of Capricorn and Cancer, and is doing so at somewhere between a quarter and half a degree of latitude each decade, no one has been able to work out why the shift is more pronounced in the southern half of the globe.
And if that expansion is more obvious in the southern hemisphere, it is because there is more sea to have more impact.
Researchers analysed water temperature patterns in the great ocean gyres, those giant circular currents that take warm waters to the poles and return cold water to the equatorial regions.
They matched satellite readings from 1982 – the first year in the series of measurements – with data from 2018, and compared these to measurements of tropical zone expansion.
The connection was clear: excess heat that had been building up in the subtropical oceans ever since global warming began had driven both tropical edges and ocean gyres towards the poles.
That is, the shift in the tropics wasn’t just one of those slow pulses of expansion and retraction, of cyclic change, that happen in a complex world. And more precisely, the tropics were expanding more clearly in those places where the gyres moved poleward.
“We demonstrate that the enhanced subtropical ocean warming is independent from the natural climate oscillations,” said Hu Yang of the Alfred Wegener Institute in Bremerhaven, Germany, who led the research. “This is a result of global warming.” – Climate News Network