Fly on holiday and plant a few trees for little money to offset the emitted carbon dioxide. There are plenty of offers for this. Forestation is considered an important measure against global warming because forests store large amounts of carbon. But what is often forgotten, aside from many other critical aspects of carbon offseting through forestation, is that forests are darker than grass surfaces, which means that a forest absorbs more solar radiation and releases it back into the atmosphere as heat. This is particularly the case in North America and Eurasia. Understanding the consequences of these changes is important to ensure that reforestation activities are not counterproductive.

Therefore, we, a team of scientists from ETH Zurich, the University of Bern, and the Vrije University Brussels, wanted to understand how idealized global forestation and deforestation scenarios could affect the climate system and change weather patterns far beyond the forested or deforested regions. In other words, we addressed the question “What might happen to the global climate and circulation if we plant or remove trees wherever possible?”. It is important to note that in this study, we did not account for the carbon storage of forests but only consider effects related to changes in darkness, evaporation and roughness of the land surface. The results, based on a state-of-the-art climate model, show that this could have far-reaching consequences, that are likely not negligible in more realistic forestation or deforestation scenarios.

One of these consequences concerns the jet stream, the zonal band of strong winds at an altitude of about ten kilometres. The jet stream over the North Atlantic influences weather patterns in Europe and is driven by the temperature difference between the tropics and regions further north. Through forestation, this temperature difference is decreasing due to the warming of the atmosphere over North America and Eurasia. As a result, the jet stream slows down and shifts northwards. This leads, for example, to 5-10 per cent less annual precipitation in large parts of Europe.

Another consequence is the change in strength of the Atlantic meridional overturning circulation, the ocean circulation, which transports heat from the equator to the northern middle latitudes and polar regions. In response to the warming in Northern middle and high latitudes following forestation, less heat is transported poleward by the ocean. This results in a pronounced local cooling in the North Atlantic, known as the North Atlantic warming hole, affecting regional temperatures, clouds, and precipitation. The roughly reversed pattern occurs for a deforested planet.

Finally, the Northern mid- and high-latitude warming leads to changes the tropical circulation. Because most land masses beyond the tropics are situated in the Northern Hemisphere, the warming through forestation and cooling through deforestation occurs asymmetrically between the hemispheres. This leads to changes in intensity of the Hadley cell, the atmospheric circulation feature which transports heat from the equator to the subtropics and across hemispheres. As its rising branch in the tropics is strongly linked to rainfall, this results in substantial changes of tropical rainfall patterns.

As these consequences are far-reaching, considerable impacts must be expected even with regionally limited forestation. Of course, these are results from one global climate model only but there is indication that at least some other models show similar responses. It is therefore important to analyse these effects carefully for large forestation projects, especially over North America and Eurasia.

In general, there is substantial uncertainty on how forestation affects clouds, which reflect solar radiation back to space, and hence, influence the net climate effect of forestation. Further, the amount of carbon (potentially) stored in forests, and its permanence (due to e.g. wildfires), are also uncertain.  There are many very good reasons for forestation projects (e.g., biodiversity, livelihoods, recreation) but, due to the immediate urgency of the climate crisis, such projects should never replace a rapid reduction of anthropogenic greenhouse gas emissions. Given the uncertainty and potentially far-reaching consequences of the climate response to large-scale forestation in middle and high latitudes, such projects have limited, if any, suitability to offset unavoidable carbon emissions.