Meltwater from Greenland and the Arctic is weakening Ocean Circulation,
speeding up Warming down South
THE CONVERSATION
11/24/2024
By Laurie Menviel, UNSW Sydney and Gabriel Pontes, UNSW Sydney
"A vast network of ocean currents nicknamed the “great global ocean conveyor
belt” is slowing down. That’s a problem because this vital system redistributes
heat around the world, influencing both temperatures and rainfall.
The Atlantic Meridional Overturning Circulation funnels heat northwards through
the Atlantic Ocean and is crucial for controlling climate and marine ecosystems.
It’s weaker now than at any other time in the past 1,000 years, and global
warming could be to blame. But climate models have struggled to replicate the
changes observed to date – until now.
Our modelling suggests the recent weakening of the oceanic circulation can
potentially be explained if meltwater from the Greenland ice sheet and Canadian
glaciers is taken into account.
Our results show the Atlantic overturning circulation is likely to become a third
weaker than it was 70 years ago at 2°C of global warming. This would bring big
changes to the climate and ecosystems, including faster warming in the southern
hemisphere, harsher winters in Europe, and weakening of the northern
hemisphere’s tropical monsoons. Our simulations also show such changes are
likely to occur much sooner than others had suspected.
National Oceanography Center: “The Atlantic Meridional Overturning Circulation
(AMOC): What Is It and Why Is It So Important?”
Changes in the Atlantic Meridional Overturning Circulation
The Atlantic ocean circulation has been monitored continuously since 2004. But
a longer-term view is necessary to assess potential changes and their causes.
There are various ways to work out what was going before these measurements
began. One technique is based on sediment analyses. These estimates suggest
the Atlantic meridional circulation is the weakest it has been for the past
millennium, and about 20% weaker since the middle of the 20th century.
Evidence suggests the Earth has already warmed 1.5ºC since the industrial
revolution.
The rate of warming has been nearly four times faster over the Arctic in recent
decades.
Meltwater weakens oceanic circulation patterns
High temperatures are melting Arctic sea ice, glaciers and the Greenland ice
sheet.
Since 2002, Greenland lost 5,900 billion tonnes (gigatonnes) of ice. To put that
into perspective, imagine if the whole state of New South Wales was covered in
ice 8 metres thick.
This fresh meltwater flowing into the subarctic ocean is lighter than salty
seawater. So less water descends to the ocean depths. This reduces the
southward flow of deep and cold waters from the Atlantic. It also weakens the
Gulf Stream, which is the main pathway of the northward return flow of warm
waters at the surface.
The Gulf Stream is what gives Britain mild winters compared to other places at
the same distance from the north pole such as Saint-Pierre and Miquelon in
Canada.
Our new research shows meltwater from the Greenland ice sheet and Arctic
glaciers in Canada is the missing piece in the climate puzzle.
When we factor this into simulations, using an Earth system model and a high-
resolution ocean model, slowing of the oceanic circulation reflects reality.
Our research confirms the Atlantic overturning circulation has been slowing down
since the middle of the 20th century. It also offers a glimpse of the future.
Connectivity in the Atlantic Ocean
Our new research also shows the North and South Atlantic oceans are more
connected than previously thought.
The weakening of the overturning circulation over the past few decades has
obscured the warming effect in the North Atlantic, leading to what’s been termed
a “warming hole”.
When oceanic circulation is strong, there is a large transfer of heat to the North
Atlantic. But weakening of the oceanic circulation means the surface of the ocean
south of Greenland has warmed much less than the rest.
Reduced heat and salt transfer to the North Atlantic has meant more heat and
salt accumulated in the South Atlantic. As a result, the temperature and salinity in
the South Atlantic increased faster.
Our simulations show changes in the far North Atlantic are felt in the South
Atlantic Ocean in less than two decades. This provides new observational
evidence of the past century slow-down of the Atlantic overturning circulation.
What does the future hold?
The latest climate projections suggest the Atlantic overturning circulation will
weaken by about 30% by 2060. But these estimates do not take into account the
meltwater that runs into the subarctic ocean.
The Greenland ice sheet will continue melting over the coming century, possibly
raising global sea level by about 10 cm. If this additional meltwater is included in
climate projections, the overturning circulation will weaken faster. It could be 30%
weaker by 2040. That’s 20 years earlier than initially projected.
Such a rapid decrease in the overturning circulation over coming decades will
disrupt climate and ecosystems. Expect harsher winters in Europe, and drier
conditions in the northern tropics. The southern hemisphere, including Australia
and southern South America, may face warmer and wetter summers.
Our climate has changed dramatically over the past 20 years. More rapid melting
of the ice sheets will accelerate further disruption of the climate system.
This means we have even less time to stabilise the climate. So it is imperative that
humanity acts to reduce emissions as fast as possible.The Conversation"
Laurie Menviel, Post-doctoral Research Fellow, Climate Change Research Centre,
UNSW Sydney and Gabriel Pontes, Post-doctoral Research Fellow, Climate
Change Research Centre, UNSW Sydney
This article is republished from The Conversation under a Creative Commons
license. Read the original article.
https://theconversation.com/us
|
|