If important ocean currents such as the North Atlantic overturning circulation suddenly come to a standstill due to climate change, the conditions in different regions of the world change rapidly: rainfall increases or does not occur, temperatures become more extreme and storms more violent. According to new studies, when this tipping point is reached depends not only on how much the climate warms up, but also on how quickly it does so. If a lot of meltwater reaches the Atlantic from the poles in a short period of time, the critical point may occur sooner than would have been expected based on the amount of meltwater or the amount of temperature change, scientists warn.
The oceans store large amounts of carbon dioxide (CO2) and also buffer the rise in temperature caused by man-made climate change. In addition, powerful ocean currents ensure a climatic balance between warmer and cooler regions. Europe owes its mild climate to the North Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream. But this and other important climate subsystems such as the ice sheets of Antarctica and Greenland, the Amazon rainforest and the Asia-Australian monsoons are threatened by the progressive increase in CO2 in the atmosphere and the associated global warming. If a certain threshold is exceeded, they could irrevocably tip into a new state.
Speed also plays a role
The climate scientists Johannes Lohmann and Peter Ditlevsen from the University of Copenhagen in Denmark have now shown in a model study that the AMOC and possibly other climate subsystems could reach the critical threshold long before the expected point in time. In a simulation of the world’s oceans, the researchers simulated how the melting of the poles caused more and more fresh water to enter the North Atlantic. Depending on how fast you chose the influx of freshwater, the critical threshold at which the AMOC comes to a standstill shifted.
With slow melting, the threshold was comparatively high. If, on the other hand, the researchers simulated a rapid melting of the ice sheets over periods of between ten and 150 years, the AMOC already stopped at significantly lower absolute values. If the fresh water flows in quickly, a smaller amount is sufficient to cause the system to tip over. “Tipping points are often associated with the fact that a previously stable system state loses its stability if a system parameter is increased above a well-defined critical value,” the researchers explain. “In some cases, however, such transitions can occur before a parameter threshold is exceeded, provided the parameter change is fast enough.” This phenomenon is known as rate-induced tilting.
The model reacted very sensitively to the smallest changes in the starting conditions. From the researchers’ point of view, this makes predictions difficult. A clear threshold cannot therefore be defined. The model study shows that, due to the chaotic interplay of many factors, it is not possible to define a sharp boundary, according to the researchers. “This is disturbing news. Because if this is true, it reduces our safe room for maneuver, ”says Lohmann.
Take tipping points seriously
In an accompanying article, a research team led by Christoph Heize from the University of Bergen in Norway emphasizes that different tipping points of the oceans can occur regionally and at different times, but add up to global dimensions. “These tipping points, combined with gradual changes, must be addressed as seriously as individual catastrophic events to prevent the cumulative and often intensifying negative effects on society and the Earth system,” the authors say.
Even with moderate global warming of 1.5 to 2 degrees Celsius, the temperature, CO2 content and oxygen supply of the world’s oceans can reach critical values. Short-term extreme events such as marine heat waves can then cause the system to overturn – with serious consequences not only for the global climate, but also for the marine ecosystems. Because in the deep sea, even small but persistent changes in the environmental conditions can be harmful to deep sea organisms that are used to a very narrow range of variations in living conditions.
Minimize adverse effects
“As a CO2 and heat sink, the ocean provides a vital service to humanity in the face of advancing climate change – albeit at a high price, because it changes the environmental conditions permanently,” says Heinze. Together with his colleagues, he presents a number of suggestions for limiting damage.
It is particularly important to effectively reduce greenhouse gas emissions, to agree internationally on common goals and to implement them resolutely. Scientific research also plays an important role in identifying potentially particularly endangered systems and sounding out protective strategies. At the same time, communication between scientists and political decision-makers must be improved and trust in science increased. “The adverse effects of human-made climate change on the ocean can still be minimized,” the authors believe.
Sources: Johannes Lohmann and Peter Ditlevsen (University of Copenhagen, Denmark), Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2017989118; Christoph Heinze (University of Bergen, Norway) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2008478118