The ocean currents that help warm the Atlantic coasts of Europe and North America have significantly slowed since the 1800s and are at their weakest in 1600 years, according to new research my colleagues and I have conducted. As we’ve set out in a new study in Nature, the weakening of this ocean circulation system may have begun naturally but is probably being continued by climate change related to greenhouse gas emissions.
This circulation is a key player in the Earth’s climate system and a large or abrupt slowdown could have global repercussions. It could cause sea levels on the US east coast to rise, alter European weather patterns or rain patterns more globally, and hurt marine wildlife.
We know that at the end of the last major ice age, rapid fluctuations in the circulation led to extreme climate shifts on a global scale. An exaggerated (but terrifying) example of such a sudden event was portrayed in the 2004 blockbuster film The Day After Tomorrow.
The recent weakening we have found was likely driven by warming in the north Atlantic and the addition of freshwater from increased rainfall and melting ice. It has been predicted many times but, until now, just how much weakening has already occurred has largely remained a mystery. The extent of the changes we have discovered comes as a surprise to many, including myself, and points to significant changes in the future.
The circulation system in question is known as the “Atlantic Meridional Overturning Circulation” (AMOC). The AMOC is like a giant conveyor belt of water. It transports warm, salty water to the north Atlantic where it gets very cold and sinks. Once in the deep ocean the water flows back southwards and then all around the world’s oceans. This conveyor belt is one of the most important transporters of heat in the climate system and includes the Gulf Stream, known for keeping western Europe warm.
Climate models have consistently predicted that the AMOC will slow down due to greenhouse gas warming and associated changes in the water cycle. Because of these predictions – and the possibility of abrupt climate changes – scientists have monitored the AMOC since 2004 with instruments strung out across the Atlantic at key locations. But to really test the model predictions and work out how climate change is affecting the conveyor we have needed much longer records.
Looking for patterns
To create these records, our research group – led by University College London’s Dr David Thornalley – used the idea that a change in the AMOC has a unique pattern of impact on the ocean. When the AMOC gets weaker, the north-eastern Atlantic Ocean cools and parts of the western Atlantic get warmer by a specific amount. We can look for this pattern in past records of ocean temperature to trace what the circulation was like in the past.
Another study in the same issue of Nature, led by researchers at the University of Potsdam in Germany, used historical observations of temperature to check the fingerprint. They found that the AMOC had reduced in strength by around 15% since 1950, pointing to the role of human-made greenhouse gas emissions as the primary cause.
In our paper, which also forms part of the EU ATLAS project, we found the same fingerprint. But instead of using historical observations we used our expertise in past climate research to go back much further in time. We did this by combining known records of the remains of tiny marine creatures found in deep-sea mud. Temperature can be worked out by looking at the amounts of different species and the chemical compositions of their skeletons.
We were also able to directly measure the past deep ocean current speeds by looking at the mud itself. Larger grains of mud imply faster currents, while smaller grains mean the currents were weaker. Both techniques point to a weakening of the AMOC since about 1850, again by about 15% to 20%. Importantly, the modern weakening is very different to anything seen over the last 1,600 years, pointing to a combination of natural and human drivers.
The difference in timing of the start of the AMOC weakening in the two studies will require more scientific attention. Despite this difference, both of the new studies raise important questions regarding whether climate models simulate the historical changes in ocean circulation, and whether we need to revisit some of our future projections.
However, each additional long record makes it easier to evaluate how well the models simulate this key element of the climate system. In fact, evaluating models against these long records may be a crucial step if we hope to accurately predict possible extreme AMOC events and their climate impacts.
The link below is to an article reporting on plans to drill the Atlantic coast of the USA for oil.
There has been no let up since Hurricane Harvey dumped record-breaking rains on the Houston area of Texas. Hurricane Irma lashed parts of the Caribbean and Cuba and is now heading onto the US mainland, having devastated the Florida Keys and the state’s west coast.
This very active season comes after a “hurricane drought” with very few major storms making landfall on the US coast over the previous decade.
So why are we seeing so many hurricanes now? Is climate change to blame?
How to make a hurricane
There are several vital ingredients needed for hurricanes to form. These include an initial disturbance in the atmosphere for the storm to form around, very warm sea surface temperatures to sustain the storm, and a lack of vertical wind shear so the storm is not torn apart during its formation.
In the Atlantic Ocean, hurricanes often form near Cape Verde off the coast of West Africa. They then track westward towards the Caribbean and the US.
Lots of factors can affect how strong these storms ultimately become, including how much time they spend gathering strength over the ocean, and the background weather patterns through which they travel.
This storm season we have seen sea temperatures persistently 1-2℃ above normal over the tropical Atlantic Ocean, which has allowed stronger storms to form and develop.
Atlantic sea temperatures have warmed over the past century, thus enhancing one of the key ingredients for hurricane formation. The climate change influence is clear for the sea temperatures, but not so much for the other ingredients required in forming hurricanes.
Harvey and Irma
While we have low confidence in the effect of human-caused climate change on hurricane formation, it is clear that climate change is enhancing some of the impacts of these storms.
Hurricane Harvey hit southern Texas hard by stalling over the Houston area and dumping huge amounts of rain. Climate change might have contributed to the stalling effect, but what’s clearer is that climate change is making intense extreme rainfall events like we saw over Houston more likely. By warming the atmosphere we’re also increasing its capacity to carry moisture.
When we have the trigger for heavy rainfall, climate change makes it rain harder.
Hurricane Irma is a very different beast to Harvey. It devastated several Caribbean islands including Anguilla and the Virgin Islands when it was a Category 5 system. It then struck Cuba before re-intensifying and moving north across the Florida Keys and onto the US mainland.
Irma’s main impacts have been through the storm surge, the strong winds and the heavy rains.
Climate change has likely worsened the effects of Irma. As described above, we know that climate change is intensifying extreme rain events. We also know that climate change is worsening storm surges by raising the background sea level on which these events occur.
Sea levels are projected to rise further over the coming century, by 50-100cm under a high greenhouse gas emissions scenario, and 20-50cm if we greatly reduce our emissions.
So while it’s likely that climate change is contributing to more extreme hurricanes, we have even more confidence that climate change is worsening the impacts of these storms, and will continue to do so over the coming decades.
Paving over the Gulf Coast
Besides the climate change influence, the widespread urban development on the US Gulf Coast is exacerbating the impacts of hurricanes.
Much like the Houston area, Florida also has a growing population. This means that not only are there more people in harm’s way when a major hurricane strikes, but there is also more concrete and other impervious surfaces that allow the water to pool in low-lying areas.
Is there any good news?
While climate change and development in hurricane-prone areas are worsening the impacts of these hurricanes, there are some glimmers of good news.
Scientists’ ability to track and forecast these major systems has improved greatly. Better forecasting of hurricanes allows for earlier planning for their impacts and should improve evacuation processes.
In theory, with the right plans in place, better hurricane forecasting should reduce death tolls from events like Irma. But it doesn’t necessarily reduce the economic costs of these storms, and for both Harvey and Irma the clean-up and recovery bills will be more than A$100 billion each.
It’s clear that climate has worsened the impacts of Atlantic hurricanes and will continue to do so. Improved forecasting provides a glimmer of hope that the death tolls from future events can be reduced, even as the economic impacts increase.