The Greenland ice sheet

Greenland lost 3.8 trillion tons of ice between 1992 and 2018.

These numbers may soon start accelerating, according to Niklas Boers from the Potsdam Institute for Climate Impact Research. The scientist explains, "our results suggest there will be substantially enhanced melting in the future – which is quite worrying".

It's due to a vicious cycle, which in the science world is called a positive feedback loop.

On top of the Greenland ice sheet you might be standing at an altitude of around 2 kilometres (1.2 miles). At this height the surrounding air is much colder and ice is unlikely to melt. But at sea level temperatures are warmer, and ice soon melts away. This is then replaced with ice from higher levels, which in turn melts in the warmer air, and so on. Eventually, the whole ice sheet melts away.

Streams and rivers forming on top of the Greenland ice sheet. NASA's Goddard Space Flight Center/Maria-José Viñas, CC BY 2.0

It might take a thousand years for the Greenland ice sheet to completely melt, but in that time, the sea level would rise about 7 metres (24 feet).

Dr Boers adds that "practically, the current and near-future mass loss will be largely irreversible".

See how that affects where you live. Climate Central — Land below 7.0 metres of water

The AMOC

With so much cold freshwater running off the Greenland icesheet and into the Atlantic Ocean, something's got to give. In this case it's the main ocean current circulating around the Atlantic Ocean, known as the AMOC or Atlantic Meridional Overturning Current.

The AMOC transports the waters of the Atlantic Ocean on a far-reaching journey from Africa to the Americas, then along the Gulf Stream north to Europe. At this point the cold water sinks, and the AMOC becomes a deep ocean current transporting cold water to the South Atlantic.

What's changed in the last few years? Trillions of tons of ice melting off the Greenland ice sheet and pouring into the Atlantic Ocean, that's what's changed.

There's so much of it, it's visible on satellite measurements of surface temperature. The cold, blue region around Greenland is what scientists call the Cold Blob, and it's the Cold Blob that is slowing the AMOC.

When ice melts, it becomes freshwater, and it's not salty like seawater. Without the extra salt, it's not as heavy and it's less likely to sink. But around Greenland is exactly where the AMOC should sink and start on its long return journey south.

All this means that the AMOC is now at its weakest point in a thousand years and likely to weaken even more.

Researcher Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research PIK, says, “If we continue to drive global warming, the Gulf Stream System will weaken further – by 34 to 45 percent by 2100 according to the latest generation of climate models. This could bring us dangerously close to the tipping point at which the flow becomes unstable."

These climate models might already be out-of-date. Dr Michael Mann, director of the Earth System Science Center at Pennsylvania State University believes “we are 50 to a hundred years ahead of schedule with the slowdown of this ocean circulation pattern, relative to the models."

In other words, the AMOC could stall much sooner than the models predict.

If you've ever watched the movie The Day After Tomorrow, you'll know that without the Gulf Stream, which is part of the AMOC, the climate gets much colder. It may not happen overnight like it did in the movie, and it's not New York that's worst affected but Greenland, Iceland, northern Europe and Russia. But at least the movie gets the cold part right.

The West Antarctic ice sheet

If the AMOC shuts down, the North Atlantic gets colder, but the South Atlantic, and even the Antarctic is predicted to get much warmer. This is on top of the warming the West Antarctic is already experiencing.

The problem is that beneath the ice, most of the West Antarctic is actually below sea level. While vast icefields and glaciers may have held the oceans at bay in colder times, now the ice is melting. Warmer seas flooding underneath glaciers melt their foundations and lift up the ice, which then breaks off and floats away as icebergs.

This process is happening fast. One glacier, the Pine Island Glacier, is already predicted to reach a tipping point with an ocean warming of just 1.2 °C (2.16 °F) higher than normal. After that point, the disappearance of the glacier is practically irreversible.

Its neighbour, the infamous "Doomsday Glacier", or the Thwaites Glacier, is looking at a similar fate. This vast glacier has been found to have "warm water impinging from all sides" on points holding the glacier in place. The size of this glacier, as big as Florida, means that when it weakens the rest of the West Antarctic ice sheet will soon follow.

"When" not "if". Scientists estimate that by about 2060, at current rates of warming, "the Antarctic ice sheet will have crossed a critical threshold." At that point, the fate of the West Antarctic ice sheet is sealed, and so is a sea level rise of about 3 metres (10 feet).

The Amazon rainforest

What happens to the Amazon if the AMOC stalls? The latest research may actually be good news - the Amazon climate might become much wetter if the AMOC shuts down. But can the Amazon survive long enough to see the rain?

The Amazon rainforest is dying a death by a thousand cuts. Logging, burning and landclearing by locals have shrunk the size of the Amazon by 15% from the 1970s.

Climate change is also making the Amazon drier. So much drier in fact, that in about 40% of the Amazon there is only enough rain to support an open savannah, hardly enough for a tropical rainforest.

Everything seems to be against the Amazon surviving, except maybe the collapse of that major Atlantic current, the AMOC. If that happens, the Amazon could get so much rain, it could survive global warming of up to 11 °C (19.8 °F). That's a lot of global warming.

So it may be a race against time for the Amazon. Can it last long enough to see the AMOC rain? Or will it reach a tipping point first, after which the Amazon falls and turns into grassland?

Ecologist Paulo Brando explains, “we know there’s a cliff out there, and so even if we’re not exactly sure where it is, we need to slow down."

"Instead, we’re rushing towards it with our eyes closed.”