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Excellent student research: Calculating the course of climate change

27 Jul 2021

How great a threat do deforestation and global warming pose to the Amazonian rainforest? This is the topic of geographer Gergana Gyuleva’s Bachelor thesis, which has received an award for excellent student research.

Amazonian Rainforest

The rainforest itself “makes” about one-third of the precipitation that falls on it. | © IMAGO / agefotostock

Can a rainforest die of drought? Can an ecosystem that once covered an area of 6 million square kilometers be transformed into a very different one in a comparatively short time? Climatologists say yes. The Amazonian rainforest is now under acute threat owing to a combination of two mutually reinforcing factors – climate change and ongoing deforestation. This raises the possibility that a ‘tipping point’ may be reached, a threshold beyond which a degraded rainforest can no longer maintain itself, and inexorably turns into a vast savannah.

Gergana Gyuleva is often asked to explain what the climate debate is all about – and even whether the climate is changing at all. Her answer to the latter question is an emphatic yes, and she points out that this has long been clear to climatologists. The ever increasing threat to the Amazonian rainforest is also clearly supported by empirical research. Thus, climate scientists now estimate that the tipping point for the ecosystem – the point of no return – lies somewhere between a 20 and a 40% reduction in the area of the forest. Some 17% of its original extent has already been irrevocably lost.

Effects of climate change on future rainfall patterns in the Amazon Basin

Since levels of rainfall will play a vital role in determining its future, Gyuleva analyzed current projections of the effect of climate change on this parameter for the Amazon Basin in her Bachelor’s thesis for the Faculty of Geography at LMU. The rainforest itself “makes” about one-third of the precipitation that falls on it. Evapotranspiration from the leaves of trees returns moisture to the atmosphere, which condenses into clouds, which returns that water to the forest in the form of rain. Gyuleva calculated the effects of ongoing deforestation and climate change on this vital recycling phenomenon.

To do so, she took advantage of data from what are known as Earth System Models (ESMs) – simulations of the course of future climate change under a range of possible scenarios. The data were collected in the course of the modelling project CMIP6. – The climate system itself is so complex that it would be impossible to take all the relevant factors into account at once. So climate researchers turn to simulations to learn how it works. Even the simulations are so complicated that they must be run on supercomputers, which can handle billions of operations per second.

“The major results of my work are the potent effects of climate change on future rainfall patterns. ESM simulations predict a highly significant reduction in precipitation during the dry period, accompanied by an increase in the duration of the dry period. The latter factor is important because it decides whether or not a forest will at some point turn into a savannah,” Gyuleva explains. Her study also suggests that climate change presents a greater threat to the recycling of precipitation in the Amazon Basin than deforestation itself. “However, we believe that this result is attributable to the fact that the scenarios used in our models do not capture the impact of deforestation in the Amazon Basin very well,” she says.

At all events, it’s clear that climate change is likely to have drastic consequences for Amazonian forests. Longer periods of drought certainly constitute “a major stress factor” for trees. “In addition, there are indications that ongoing global warming is accelerating the rate of tree die-off. That in turn suggests that a tipping point could be reached. But further research will be needed to validate this. In particular, a broader comparison between many models is required,” she says. Previous studies have indeed suggested that the impact of deforestation on the rainforests is likely to be at least as important as that of climate change. Continuing deforestation increases the risk that, at some point, there are no longer enough trees to sustain the level of precipitation needed to ensure the survival of the rainforest.

Prize for excellent student research

The climate projections that Gergana Gyuleva used for her study were compiled during Phase 6 of the Coupled Modeling Intercomparison Project (CMIP6), an international collaborative venture that involves specialists in a range of disciplines and provides the basis for the annual reports prepared by the Intergovernmental Panel on Climate Change (IPCC). “I found it fascinating to have the opportunity to work with the latest data from these ESMs, especially since they have only recently begun to include data on vegetational processes.”

Indeed, she was the first scientist to analyze the CMIP6 simulations relating to rainwater cycling in the Amazon Basin. Her supervisor, Professor Julia Pongratz (Chair of Physical Geography and Land-Use Systems at LMU) therefore refers to her project as “a pioneering study”. Now her Bachelor’s thesis has won one of the prizes for excellent student research awarded on LMU’s Tag für gute Lehre (“Good Teaching Day”).

“The Earth is an enormously complex system. When you study it, you discover that crustaceans in the Southern Ocean are intimately linked to volcanic eruptions in the Northern hemisphere – and that the impact of humanity is detectable everywhere you look,“ says Gergana Gyuleva.

Gyuleva decided to focus on climate research in her second semester, when she heard the climatology lectures given by Julia Pongratz. “I was immediately carried away by the topic,” she says. – And to prove it, she began a supplementary course in Physics with Meteorology as a subsidiary subject in order to extend her knowledge of the science of climate change.

The choice of the theme for her Bachelor’s thesis also had a personal relevance for her, as she and her family had spent a year in South America in 2016. But her primary motivation as a scientist is to contribute personally to efforts to mitigate the effects of global warming. She continues to study the issue that she explored in her Bachelor’s thesis. In as a studi_forscht project at LMU, she began to analyze the results from several further ESMs in an effort to compare the relative impacts of global warming and changes in land use on the natural recycling of rainwater in the Amazonian rainforest. She is now a Master’s student in the Atmospheric and Climate Science program at the ETH in Zürich.

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Gergana Gyuleva sometimes finds it hard to take her mind off the urgent questions raised by climate research. “To a certain extent, one has to detach what one knows about ongoing and pending changes in the world’s climate from one’s private life. I can’t spend all my time thinking about them.” Nevertheless, she is convinced that the loss of the Amazonian rainforest would be a catastrophe – not only for local biodiversity and the indigenous people of the region. It would have an impact on the Earth’s climate on a global scale, because this rainforest has, up to now, functioned as an enormous carbon sink, which has made a large contribution to mitigating the rate of global warming. According to the latest research, this function is already at risk. And if that tipping point – the threshold between carbon sink and carbon source – is crossed, the Amazon Basin will itself become a factor that promotes global warming in the future.

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