Possible irreversible changes in subsystems before reaching the tipping points in climate change

Recently, a few researchers from the University of Copenhagen published an article in the Proceedings of the National Academy of Sciences describing their work describing the potential for changes in the Atlantic Meridional Overturning Circulation (AMOC) and the conditions that could lead to such changes. In their paper, Johannes Lohmann and Peter Ditlevsen noted that climate models show that irreversible changes in subsystems like the AMOC, one of Earth’s global subsystems, can occur before a tipping point if changes occur at a rapid pace.

They further noted that fresh water flowing into the Atlantic Ocean as a result of melting ice could result in such an irreversible change. More recently, an international team of climate scientists published a Perspectives piece, also in PNAS, which outlines the damage that could occur if irreversible changes occur prior to tipping points, and also which models suggest that will occur when the Earth begins to reach certain tipping points.

In climate science, tipping points are climatic milestones that occur when changes are made to the global ecosystem that result in other permanent changes. For example, if too much carbon dioxide is injected into the atmosphere at one point, it will become too hot for animals to survive. Climate models suggest that there is no going back under such scenarios. Once a tipping point has been reached, we cannot solve the problem by stopping CO2 emissions. More recent research has suggested that there may be some pre-tipping points that can cause climate change on a smaller scale, but that are still irreversible. Fresh, cold water entering the North Atlantic Ocean, for example, can lead to permanent changes in the AMOC. The AMOC in particular is responsible for the mild temperatures in Europe.

In their paper, the authors of the Perspectives piece describe likely outcomes of various scenarios that could lead to changes in the subsystem prior to the tipping point. For example, they note that the world’s oceans comprise subsystems. In addition to rising water levels, the world’s oceans are undergoing acidification due to carbon dioxide being taken up from the atmosphere. Acidification harms marine life. In addition, previous research has shown that the upper part of the ocean absorbs heat from the atmosphere much faster than lower areas. The lower regions are only now beginning to feel the effects of warming, a long-lasting event that could change the entire underwater ecosystem – a change that would be irreversible.

The good news is that the researchers of the AMOC study and the authors of the Perspectives piece all agree that it is not too late to prevent such scenarios from unfolding. All it takes to stop greenhouse gas emissions, and then for scientists to focus their energy on those parts of the planet that have already been damaged.

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