Forests Cool the Earth: no More a Controversial Idea

Forests and water are the essence of how the Goddess controls Earth’s climate (image created by Dall-e3)

Earth’s climate is one of those huge hyperobjects that we have problems in understanding. Our mind is made to simplify and choose between binary alternatives. So, if we decide that CO2 is the main element that controls climate, we tend to rule out everything else. So, in climate science, forests are often seen as little more than carbon stocks. Once you take this attitude, the idea of slowing down global warming by planting trees becomes controversial, no matter how popular it is with politicians.

Bill Gates famously declared that “planting trees is complete nonsense” as a way to fight global warming. Serious scientists will tell you that cutting down boreal forests will cool the planet more than growing them. It is because snow falling on bare land will increase the albedo (fraction of reflected solar light), compensating for the effect of the released greenhouse gases.

Alas, we may have to reverse the application of the “complete nonsense” statement. Climate is the result of a vast tangle of interacting elements. Greenhouse gases are probably the most important factor in causing global warming, but there are others that may compensate, or even reverse the effect. Recent studies highlight the importance biophysical effect of evapotranspiration. (see, e.g. this recent paper by Makarieva et al.)

The story is complicated, but its essence is that trees are machines that create their own microclimate and also affect the macroclimate by actively managing the pumping of water from the roots to the leaves. Huge amounts of water evaporate from the leaves and then condense, forming low-height clouds. (image from Pinterest). T

The interplay of evaporation and condensation has cooling effects that involve not just moving heat from one place to another on Earth’s surface but actively radiating more heat to space because condensation occurs at higher heights than evaporation.

Several studies confirm this idea and a recent study by Barnes et al. quantifies these effects. The story is complex because you have different effects depending on the height at which temperatures are measured. But the end result is clear: forests cool the atmosphere, and it is not a small effect. They say:

“Reforestation has a cooling effect on surface and near-surface air temperature in the EUS, and likely contributed to the slower pace of warming in the region. Both ground- and satellite-based observations indicate that EUS forests cool the land surface by 1–2°C annually (Figure 3) compared to nearby surfaces with short-stature vegetation. During midday in the growing season, surface cooling is 2–5°C, and forests aged 25–50 years exhibit the strongest cooling effect.”

Note how the study confirms a point often raised by Anastassia Makarieva, that it is not just trees that cause these effects, and not even just forests. Old-growth forests are the key to controlling climate because they “know” what to do.

And it is not a local effect, either

reforestation can lead to increased evapotranspiration, resulting in increased cloud cover and precipitation that extend across the landscape. These effects would tend to amplify local cooling, particularly during the daytime. Localized land-cover changes can also cause shifts in atmospheric circulation, which can have continental or even global-scale consequences for temperature.

The most interesting result of this study may be the explanation of a puzzling phenomenon that was observed during the 20th century. Instead of following the global warming trend, the United States, and in particular the Eastern Regions, showed a significant cooling around mid-century. Here is the data (from NOAA). The effect is even stronger in the Eastern States

The fact that the 1930s were hotter than the 1990s was noted and sometimes used as an argument to attack climate science and, indeed, this graph undermines the oversimplified idea of a direct proportionality between CO2 concentrations and temperatures. Something must have cooled the Eastern states for a few decades when no warming was observed. Now, the study by Barnes et al. provides an explanation: the magic word is "reforestation.” The Eastern US regions were reforested after the Dust Bowl of the 1930s.

So, reforestation is a strong effect, not a dream of tree-huggers. It gives us an important tool to reduce global warming. Not enough, alone, but a big help!

________________________

Notes:

The paper by Barnes et al. is a little weak in terms of explaining the biophysical effects of evapotranspiration. They are mentioned in the text, but not discussed in great detail and they do not mention the all-important phenomenon of the “biotic pump”. But this is a minor defect, considering that the paper is focused on the correlation between reforestation and temperature.

A simplified description of the paper by Barnes et al. can be found on the Phys.org site. Unfortunately, they repeat the story that, in some cases, razing down forests is a good idea to increase albedo. See also a discussion by Hart Hagan, which also criticizes the lack of emphasis on the biophysical effects.