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The importance of precipitation for ecosystems on Earth

A river in blue is circling its way through the greeneries of the Amazon. Photo.
How much drier can the Amazon forest become before the forest cover is impacted? Photo: iStockPhoto

Where, how often, and how much it is going to rain or snow in the future is difficult to predict. Anders Ahlström studies the importance of precipitation for ecosystems on Earth – and contributes with new knowledge in a complex and fraught area.

Across the world, researchers are in agreement on future warming patterns. However, future precipitation is more difficult to predict, and climate models show significant differences. There is a great need for more research on precipitation changes because climate change is not limited to temperature, it also includes changes to precipitation that will affect life on Earth.

How much precipitation is needed?

Anders Ahlström, researcher at the Department of Physical Geography and Ecosystem Science, is interested in past, present and future precipitation patterns. In a new study, he and his colleagues have looked at how much drier the Amazon forest can become before the forest cover is impacted. By linking satellite data and field measurements to precipitation, they have identified an important breakpoint for how much annual precipitation is required in the Amazon. The models were also used in a project in which researchers recreated what the climate in the region looked like 6 000 years ago to obtain more information about the Holocene Epoch’s vegetation and precipitation patterns. This epoch began approximately 11 700 years ago.

“If the annual precipitation exceeds 2 000 mm, not a lot happens in the forest area. Growth and other functions do not change much if the precipitation increases or decreases as long as it remains above this limit. However, if it falls below 2 000 mm of precipitation, the ecosystem quickly shifts toward a savannah-like system with more grass and fewer trees”, says Anders Ahlström.

If it falls below 2 000 mm of precipitation, the ecosystem quickly shifts toward a savannah-like system with more grass and fewer trees

This connection also seems to apply to the dry period that took place approximately 6 000 years ago. Compilations of pollen-based estimates of the forest cover and modelling of the rain that fell during this period are consistent with the data-based models that Anders and his colleagues produced for the present-day climate. This indicates that the Amazon may see only limited changes if precipitation is reduced,and that it can become quite a bit drier before there is any significant change. 

Human impact

However, the future of the Amazon depends on more than annual precipitation. A warmer climate means that plants will need more water to grow, as a warm atmosphere can hold more water vapour, increasing its demand for water . Human impact in the form of deforestation also plays a role. This is because the logging of trees has an impact on the amount of precipitation over the central Amazon region. Coastal precipitation is taken up by the plants and returned to the atmosphere in the form of water vapour that forms clouds. The clouds blow further and further in over the forest, rain falls and evaporates again and again and finally it reaches the most remote parts of the Amazon. However, if the coastal forest is cut and transformed to croplands and pastures, more water is lost through runoff in streams and never reaches the atmosphere, resulting in less rain in the central regions of the rainforest. However, distinguishing between how ecosystems are impacted by climate change, and how they are affected by factors such as logging, drainage and forest management, is generally very difficult. 

“It is fundamental that we understand how and why our ecosystems have changed over time to be able to understand how they can change in the future. If we believe a change is due to climate change when it is actually due to changes in land use, or vice versa, there is a great risk of inaccurate projections and incorrect conclusions on causes and solutions”, says Anders Ahlström.

Many puzzle pieces needed

Nevertheless, it is important to understand the relative roles of climate and human land use in the Amazon. This is emphasised by the predicted novel climates in the regions’ future, climates that are not seen on Earth today, meaning there is no analogue or region that can be studied to understand the future states and conditions of tropical forests. In such cases process-based models are commonly used in research to try to understand how ecosystems may respond to climate change or other environmental factors that are currently difficult to observe. The development of one such model, LPJ-GUESS, is led by researchers at Lund University and used for this specific purpose.

“We do not have anything to compare with when it comes to the Amazon. Being able to study the impact of a reduction in precipitation, as we have done in this study, therefore contributes an important piece of the puzzle despite other important pieces still missing, particularly the predicted high temperatures and how these will affect the forest. The combined effect is still very unclear and uncertain.”

“However, if we know more about how much precipitation an ecosystem requires, past climates and ecosystem states, and become better at accounting for and represent the human impact in models, we will be in a position to say more about the risks of climate change. Our models are getting better and better, and hopefully the uncertainties will be reduced”, concludes Anders Ahlström.