Climate Change Impacts Thermohaline Circulation
Colin Jago
Chair of GeoMôn
New research* published in March 2023 has confirmed that Climate Change Impacts Thermohaline Circulation. Climate change is slowing down the thermohaline circulation in the oceans. This circulation determines how ocean currents move water around the oceans, which in turn impacts marine life and climate. The research by an Australian team shows that enhanced freshwater input from ice melting off Antarctica is disrupting the flows of cold, salty, oxygen-rich water in deep ocean currents. These currents carry huge amounts of heat and carbon and support marine life by bringing nutrient-rich deep water to the ocean surface. Changes to the circulation will have major impacts on climate and marine ecosystems.
What is the thermohaline circulation?
While wind is the primary driver of ocean currents in the upper 100 metres of the ocean, deep sea currents that flow thousands of metres below the surface are driven by differences in the density of the seawater. Density is controlled by the water’s temperature (thermo) and saltiness or salinity (haline) so the process is known as the thermohaline circulation. While wind-driven currents at the surface of the ocean are much more vigorous than thermohaline currents – which typically move at a speed of only 1 centimetre per second – the thermohaline circulation extends to the seafloor and forms circulation patterns that envelop the global ocean.
Density differences between ocean water masses are the key to the thermohaline circulation. Cold seawater contracts and is denser than warmer seawater. Saltier water is denser than fresher water because the dissolved salts result in more mass per unit volume. Cold, salty seawater is therefore denser than warm, fresher seawater. Lighter water masses float over denser ones and dense water masses sink to greater depths.
Density differences between surface and deep waters give rise to sinking of water masses (termed ‘overturning’) of the ocean in the polar regions. The thermohaline circulation is initiated by this sinking of cold, dense water, chiefly in the northern North Atlantic and near Antarctica. The dense water forms because of heat loss (which lowers its temperature) and evaporation (which increases its salinity). In addition, the formation of sea ice makes the surrounding ocean waters saltier. These factors increase the density of the water so that it sinks to greater depths. In the North Atlantic, the cold, salty water sinks by 2 or 3 kilometres by the time it reaches the Norwegian and Greenland seas, and near Antarctica, water sinks to depths of more than 4 kilometres. The dense deep water masses spread into the full extent of the ocean and then gradually upwell (i.e. move towards the surface of the ocean). Upwelling occurs when winds move surface water and deep water moves upwards to replace it. For example, trade winds at the equator blow surface water both north and south away from the equator, allowing upwelling of deeper water. This feeds a slow return flow of surface water to the sinking regions to replace the water that sinks. This surface water cools and gets saltier as it moves towards the polar regions until it too sinks. And so the thermohaline circulation continues. Disruption of one part of the circulation has repercussions throughout the world’s oceans.
The interconnectivity of the oceans and the variable morphology of the ocean basins gives rise to the complex thermohaline circulation that we observe today. In the diagrams, surface currents are in red and bottom currents are in blue.
Surface currents are shown in red, deep waters in light blue and bottom waters in dark blue. The main deep water formation sites are shown in orange. (Rahmstorf, Nature 2002)**.
Effects on global climate and marine life
The thermohaline circulation is a conveyor belt that carries oxygen from the surface to the deep ocean. Deep sea currents suspend nutrient-rich organic detritus from the seabed and upwelling brings these nutrients to the surface where it can be utilised by marine life. The thermohaline circulation plays an important role in supplying heat to higher latitudes and polar regions. The Atlantic Ocean conveyor belt, known as the Atlantic meridional overturning circulation (AMOC), heats Northern Europe – the temperature in the British Isles is about 5°C warmer as a result. It extends to the Greenland and Norwegian Seas and pushes back the winter margin of sea ice.
How does climate change impact thermohaline circulation?
Measurements of the AMOC by oceanographers over the past 20 years have shown that it is subject to considerable variability, seasonally and from year to year. But analysis of sea surface temperatures from 1901 onwards suggests that there has been a long term weakening of the AMOC**. Other research using data on bottom sediments in the Labrador Sea shows that the bottom current which carries the cold waters back south has been slowing down since around 1750. So there is good evidence that the AMOC is getting weaker. Comparison of AMOC and weather data over recent decades suggests that the AMOC influences weather in Europe and eastern North America such that a weakened AMOC is associated with increased winter storms and summer heatwaves in Europe and extremely cold winters and intense blizzards in eastern North America.
The new research* shows that there are alarming signs that the overturning thermohaline circulation near Antarctica is also slowing because increasing volumes of meltwater are making the surface waters less salty and so less dense and less able to sink to greater depths. Global warming is speeding up this disruption of the thermohaline regime. New modelling shows that if global carbon emissions continue at the current rate, then the Antarctic overturning will slow by more than 40% in the coming 30 years. If the deep ocean current collapses, the oceans below 4 kilometres will stagnate so that nutrients are trapped in the deep ocean rather than transported to the surface waters, with significant impacts on marine ecosystems.
In the North Atlantic, where the thermohaline circulation contributes about 20% of the energy of the Gulf Stream (80% is wind-driven), the circulation is projected to slow by 20% in a few decades. Since this circulation keeps Europe mild, there would be serious cooling – so global warming would give rise to regional cooling. Additional impacts from a slowdown of the global thermohaline circulation would be a reduced capacity for the oceans to store carbon and heat, thus driving more rapid climate change. However, there are considerable uncertainties about what constitute a tipping point and how extreme the consequences would be.
Has this happened before?
Everyone knows that the AMOC stopped completely in 2004 creating an instantaneous ice age across Eurasia and North America. Except that was in the disaster movie The Day After Tomorrow. However, it possibly did occur in the not-so-distant past. It has been known for some time that a significant climate event occurred at about 12,900-11,700 years ago, primarily in the Northern Hemisphere, when the gradual climate warming after the last Ice Age was reversed. This is known as the Younger Dryas event. The reversal was sudden, taking place in decades (not overnight as in The Day After Tomorrow). The sudden cooling produced a temperature drop of 4-10°C in Greenland and glaciers advanced over much of the temperate Northern Hemisphere. Global climate conditions became unstable with changes to rainfall, including monsoon rainfall.
The most likely explanation for the sudden cooling is that a large influx of freshwater into the North Atlantic weakened the overturning thermohaline circulation. At the peak of the last Ice Age much of North America was covered by the Laurentide Ice Sheet which was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the northern United States, This melted as the climate warmed and the ice sheet retreated. Lake Agassiz was a large glacial lake in central North America, fed by glacial meltwater; its area was larger than all of the modern Great Lakes combined. A sudden draining of the lake, or a series of draining events, released an enormous quantity of freshwater into the Arctic Ocean. With a dramatic freshening of the surface waters, the overturning regime was slowed or stopped as it became impossible for low density surface waters to sink. When the thermohaline circulation slowed down, less warm water could be moved from the equatorial Atlantic to higher latitudes, so the regional climate cooled, thus giving rise to the Younger Dryas event.
The impacts of the event were severe and widespread. There was a wave of animal extinctions and substantial changes to vegetation distributions. The rapid onset of cooling occurred during the lifetime of humans who experienced it. There is evidence of substantial contractions in human populations in Asia, Europe and North America. In the Middle East, small human settlements were established which may have been a strategy to optimise resources at a time of diminishing food resources.
*Li, Q., England, M.H., Hogg, A.M. et al. Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature 615, 841–847 (2023). https://doi.org/10.1038/s41586-023-05762-w
** For a more detailed but accessible account of the thermohaline circulation check out this article by Stefan Rahmstorf. Stefan has a strong connection with Anglesey as he did his MSc in Physical Oceanography in the School of Ocean Sciences, Bangor University in Menai Bridge. He is now a leading authority on ocean circulation, the AMOC, and global warming, and he was a lead author of the 4th Assessment Report of the IPCC (Intergovernmental Panel on Climate Change). https://www.pik-potsdam.de/~stefan/thc_fact_sheet.html