Climate change ecosystems: more robust through patterns

Dthe devil is in the details, or, as the physicist Wolfgang Pauli is said to have said: God created the volume, the devil the surface. The modeling of earth systems such as the Greenland Ice Sheet becomes more complicated if one considers the systems in spatial extent: with specific geography, heat transport, water flow. For the authors a paper published in the Journal Science these spatial effects are decisive for the question of whether a system is facing a tipping point. The researchers develop this idea on the basis of savanna systems, but suspect that the validity extends over many earth systems where tipping points have previously been assumed.

The idea of ​​the tilting elements in the context of climate change goes back to the physicist Hans Joachim Schellnhuber. In the meantime, complex models for different earth systems are being used to research at which global temperature rise a tipping point could be reached. For example, at what global warming the Greenland ice sheet melts and under what conditions the Gulf Stream could collapse.

Spatial patterns, such as bare spots in the vegetation, were often interpreted as indicators of an impending tipping point. On the contrary, say the science authors. The formation of spots and stripes makes a system more stable. Instead of suddenly switching from one state to the other, the system could exist in an intermediate state, i.e. in a mixed pattern of two states, for example of desert and savanna. The system could be more resilient than assumed, even in the face of climate change.

The science paper considers a simple model for tilting behavior, which shows what is known as bistability. Under certain external conditions, two different states of the system can be stable, in a sense they are both on offer. A tropical region could, for example, exist either as a savannah with vegetation – or as a desert – with the same precipitation and the same temperature.

Pattern formation as a sign of stability

The previous history is decisive. Climate changes can then cause such a system to tip over from one state to the other. Take the Sahara as an example: the vegetation there dried up a good 5500 years ago. Model simulations show that anthropogenic climate change could turn parts of the Sahara green again. This tilting behavior is based on feedback. Plants store water and improve the penetration of water into the soil. When rainfall decreases, fewer plants grow, which means less water is stored. This can lead to self-reinforcing changes.

However, the feedback also has a spatial dependency. Although vegetation stores water, it draws it from the surrounding area, which means that there is no water there. If one takes into account the scale dependence of the feedback, according to the Science authors, it becomes apparent that different states can coexist. In the transition zone from savannah to rainforest, these patterns appear in the form of finger-like foothills of the forest or as a labyrinth of trees.

“We have to go back to the beginning,” says lead author Max Rietkerk from the University of Utrecht, “we have to find out which systems are endangered by a tipping point and which are not.” The Utrecht ecologist generally sees pattern formation as a sign of stability and believes that this consideration is in principle also valid for systems such as ice sheets and glaciers. The question is open where the theory of pattern formation plays a role and where the theory of tipping behavior applies. The climate researcher Schellnhuber, on the other hand, sees the relevance of mixed patterns for some ecosystems, but this cannot be generalized.

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