He had been researching how forests cooled urban areas, and now turned to understand how forest affected rain in Europe. Edouard Davin was a professor at Wyss Academy of Nature where he was a well-respected researcher into the connection of land use and climate. He was the lead author of the IPCC (Intergovernmental Panel on Climate Change) document on climate change and land.
Davin and Meier working together, wrote “The effect of forests on the amount and pattern of precipitation has gained less attention, despite its high socio-economic relevance. Previous work has demonstrated that the high precipitation amounts in tropical rain forests are self-sustained by the abundance of those forests itself. Yet, the impact of forests on precipitation in extra-tropical [note: extra-tropic = outside the tropics] regions has gained only little attention. This study attempts to identify a relationship between the amount of precipitation and the abundance of forests over the European continent.” (For a fuller statement from this conference presentation see appendix)
Whether forests impact rain or not is a causal question. How does one go about showing a causal link between different variables?
Global warming affects rainfall. When air is warmer it holds water for longer, so droughts are longer. The air is like a bigger sponge. When the water vapor condenses out, there are bigger rains, being rung out like a sponge. Also, global warming causes the tropics to get more rain, and areas of Europe to get less rain (due to Hadley circulation related reasons). If we want to show the causal link between forests and rain, we have to filter out global warming effects on rainfall in our analysis of data.
There are some commonly used methods to show causal links (usually more than one is needed): i) find a physical mechanism for the causal link, ii) measure the effect happening, iii) have a control group and another group with value of variable changed; this is a common methodology in medicine, eg comparing those who smoked and those who did not, iv) do a theoretical calculation that shows the effect, v) run simulation model with variables changed and compare with the real world and other simulations
Meier and Davin, started their research with method iii), by taking pairs of land, some that were forests and some that were agriculture land, and comparing the precipitation on them. They studied rainfall data from different stations. (This weather data is open to anyone who wants to use it for their own research- its called the Global Sub-Daily Rainfall Dataset and Global Historical Climatology Network). They found 1512 station pairs to compare the effects of land on rain. And then clustered them into groups of around 5 to get a better average of the precipitation in each area. [ Meier 2021]
This was a useful method to filter out the effects of precipitation change due to global warming, since both the agriculture land and the forests, being near each other would probably experience roughly the same global warming rainfall impact.
Using this, they showed observational data for a causal link. They found that the precipitation increase for the forested areas over farms were more pronounced near the ocean. A lot further inland the precipitation differences were not so great. During the winter there were greater precipitation differences than in summer. In diagram below, the red bar (ignore the other colors) shows the precipitation difference between forests and farms for each month. In January the difference ranges from about .10mm to .39 mm per day, depending on which pair of lands one is comparing. In June the precipitation difference ranges from about .03mm to .17mm a day. Forests are better ‘makers’ of rain than farms.
Increasing drought puts the resilience of the Amazon rainforest to the test
Meier and Davin, along with their colleagues, were interested in seeing what would happen to the rain if Europe was reforested on a large scale. The map below shows the 14% of Europe that some researchers have thought most amenable to being reforested. It includes allocating a decent amount of farmland for reforestation.
Reforesting has been happening in Europe here and there. An example of this kind of reforestation happened in areas surrounding Donana National park in Andalusia, Spain. The wetlands of the National Park were disappearing because farmers were taking too much water from the aquifers that fed the wetland. So Spain launched a 1.4 billion (!) Euro project to get farmers in neighboring areas to turn their farms to forest, or convert them to rain-fed farming methods.
Reforesting can also happen through dehesas. The dehesas are are an ancient form of agroforestry practice in Spain, where trees are integrated into the farmland. One reader of this newsletter is an olive oil businessman interested in finding and getting more farms that use regenerative/agroforestry methods to grow olives, and then buying from them. There are potential economic incentives for farmers to switch to more agroforestry methods.
Meier and Davin ran a simulation reforesting 14% of the land. The figures below show what subsequent impact it had on the rain. In a. you can see a reasonable local increase in rain in the winter. The bluer the area the more the rain. In b. you see more of a minimal summer rain increase locally. (I think in their study local rain means they did not add evapotranspiration to the incoming ocean moistue.) In d you can see the winter downwind increases in the rain. Downwind rain comes from a combination of ocean moisture and land cover evapotranspiration. Winter cyclones bring rain. Coastal areas have more rain. Further downwind and inland, there is less rain as cyclones get stopped by forests. In e you can see the downwind increases in the summer rain. Most of Iberia shows an increase of downwind summer rain when land gets reforested.
One reason for why the rain increases when farms turn into forests, Meier and Davin write in a popular science article, is because “Forests typically have a higher surface ‘roughness’ than agricultural land. This creates more turbulence over the trees and slows the movement of heavy clouds causing them to rain over and downwind of the forests.”
They continue with other reasons for why the rain increases in their research paper : “Second, observations indicate that forests typically sustain higher evapotranspiration than agricultural land in particular during the summer season. We hypothesize that this is an important driver behind the downwind summer precipitation increase from forestation in most locations of Europe… Higher evapotranspiration of forests was also linked to increased precipitation in the tropics and the Sahel region.”
The researchers calculate with the reforestation program, that summer rain will increase by about 7.6 ± 6.7% on average over Europe (0.13 ± 0.11 mm per day). They say this could offset the loss of rain from global warming effects, and write “we therefore conclude that land-cover-induced alterations of precipitation should be considered when developing land-management strategies for climate change adaptation and mitigation.” Replacing modern farming methods which do not use trees with agroforestry would thus help increase the summer rain.
“Numerous studies have demonstrated that forests considerably alter temperatures at the land surface. These alterations vary in space and time due to a complex interplay of several modified energy fluxes at the land surface. The effect of forests on the amount and pattern of precipitation has gained less attention, despite its high socio-economic relevance. Previous work has demonstrated that the high precipitation amounts in tropical rain forests are self-sustained by the abundance of those forests itself. Yet, the impact of forests on precipitation in extra-tropical regions has gained only little attention. This study attempts to identify a relationship between the amount of precipitation and the abundance of forests over the European continent. Such a relationship can originate from two kinds of interactions: (1) The amount of precipitation can drive the abundance of forests, as water is a crucial resource for forests ecosystems. (2) The energy and water redistribution at the land surface associated with forests can alter processes in the atmospheric air column, which in term could affect the amount of precipitation at the location of the forest. Here, we aim to isolate the second kind of effects, as those are more relevant for human decision making.
Establishing a causal relationship between the abundance of forests and the amount of precipitation is complex due this two-way interaction. Hence, three different data sources are employed to advance our understanding of how forests influence precipitation patterns. Firstly, a geographically weighted regression is applied to the spatially-continuous, observation-based precipitation data set MSWEP2.2 (Beck et al., 2017). Besides the forest fraction, a number of topographical variables are considered as predictor variables to account for potential confounding factors (i.e, to assure that interactions of the first kind are not misinterpreted as interactions of the second kind). Secondly, closely-located, paired sites that resemble in topography, but differ in forest fraction are identified in the GHCN (Menne et al., 2012) and the GSDR (Lewis et al., 2019) rain gauge data sets. This allows to evaluate the results based on MSWEP2.2. Thirdly, the same geographically weighted regression is applied to convection-resolving regional climate simulations. By artificially defining the forest fraction distribution in model simulations, interactions of the first kind can be disabled, further fostering the understanding about the causality on the relations identified using the observations. Further sensitivity experiments could be conducted, to improve the process understanding on interactions of the second kind. Overall, our results indicate, that the abundance of a forest increases the amount of precipitation in the order of 100 mm/yr in many locations of Europe.”
