The system of ocean currents that transports heat from the tropics to the North Atlantic and Europe is approaching a “cliff-like” tipping point, according to a new study published in Science Advances. The study, led by Dutch scientists, used a complex computer model to simulate the effects of a collapse of the Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream. The results show that such a collapse would have drastic consequences for the global climate, especially in Europe.
What is the AMOC, and why is it important?
The AMOC is a large-scale system of ocean currents that circulates warm water from the Gulf of Mexico and the Caribbean Sea to the North Atlantic, where it cools down and sinks. This process drives a conveyor belt of water that returns to the south, carrying cold water and nutrients. The AMOC is one of the main regulators of the Earth’s climate, as it influences the temperature, precipitation, sea level, and weather patterns of many regions.
The AMOC is also responsible for keeping Europe warmer than it would otherwise be, as it delivers about 10% of the heat received by the continent from the sun. Without the AMOC, the average temperature in Europe could drop by 5 to 15 degrees Celsius, depending on the location and season.
How is the AMOC affected by climate change?
The AMOC is sensitive to changes in the density and salinity of the ocean water, which are affected by the melting of ice sheets and glaciers, the increase of rainfall and river runoff, and the warming of the atmosphere. These factors reduce the density and salinity of the surface water in the North Atlantic, making it less likely to sink and drive the AMOC. This leads to a weakening of the current system, which has been observed by several studies in the past decades.
The new study, however, goes a step further and suggests that the AMOC could reach a critical threshold, beyond which it would abruptly collapse or shut down. The study used a state-of-the-art climate model that included multiple feedbacks and interactions between the ocean, the atmosphere, the land, and the ice. The model showed that the AMOC could undergo a “tipping event” that would trigger a rapid and irreversible transition to a new state, with a much weaker or even absent circulation.
What would be the consequences of an AMOC collapse?
The study simulated the climate impacts of an AMOC collapse, using the year 2023 as a reference point. The results showed that an AMOC collapse would have dramatic and widespread effects on the global climate, such as:
- A significant cooling of the North Atlantic and Europe, with temperature drops of up to 27 degrees Celsius in some areas, such as Bergen, Norway. The cooling would be more pronounced in winter and spring and would extend the Arctic sea ice cover southward.
- warming of the Southern Hemisphere, especially in the Southern Ocean and Antarctica, where the temperature could rise by up to 4 degrees Celsius. The warming would be more pronounced in summer and autumn and would accelerate the melting of the Antarctic ice sheet and sea level rise.
- A change in the global precipitation patterns, with more droughts in some regions, such as the Amazon basin, and more floods in others, such as the Sahel and India. The change would be more pronounced in summer and autumn and would affect the water availability and ecosystems of many regions.
- disruption of marine ecosystems and fisheries, as the AMOC transports nutrients and oxygen to the deep ocean and supports the productivity and diversity of marine life. A collapse of the AMOC would reduce the nutrient supply and the oxygen levels in the ocean, affecting the food web and the fish stocks.
How likely and how soon is an AMOC collapse?
The study did not provide a precise estimate of the likelihood or timing of an AMOC collapse, as it depends on many factors and uncertainties, such as the rate of greenhouse gas emissions, the sensitivity of the climate system, and the natural variability of the ocean. However, the study did identify a physics-based and observable early warning signal of an AMOC tipping point: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. This indicator measures how much freshwater is carried by the AMOC from the south to the north and how much it affects the density and salinity of the water. The study found that this indicator has been decreasing over the past decades and that it could reach zero or negative values shortly, signaling an imminent collapse of the AMOC.
The study also compared its results with other sources of data, such as ocean observations, reanalysis products, and paleoclimate records. The study concluded that the present-day AMOC is en route to tipping and that it could collapse within this century, or even within this decade, if the freshwater forcing is strong enough.
The study’s authors warned that the AMOC collapse is a serious threat to the global climate and human society and that it should not be ignored or underestimated. They called for more research and monitoring of the AMOC, as well as more efforts to reduce greenhouse gas emissions and adapt to the potential impacts of an AMOC collapse.
