New study on the differences of local climate effects of deforestation across scenarios

In a study published recently in the Geophysical Research Letters, Johannes Winckler, Christian Reick and Julia Pongratz, scientists in the department "The Land in the Earth System" at the Max Planck Institute for Meteorology (MPI-M), investigate how local temperature responds to deforestation in different scenarios of climate and land use. They show that the local temperature response to deforestation may be greatly enhanced in the future due to effects related to a warmer climate.



Natural changes in land cover, such as the northward expansion of boreal forests, might influence temperature locally. Photo: Victor Brovkin and Sergey Goryachkin.


Deforestation influences surface temperature locally by altering the exchange of water, heat, and momentum between the atmosphere and the land surface. Several aspects are known to be relevant to understand these biogeophysical impacts of past and future changes in forest cover on local temperature: First, the deforestation due to land use affects different regions across scenarios. For instance, in the tropics forests are known to evaporate a lot of water and thus cool the surface strongly, so their clearance often leads to a warming. By contrast, replacing the dark coniferous forest prevailing in boreal regions by typically brighter croplands often leads to a cooling, in particular if snow is covering the croplands that would otherwise have fallen through the tree canopy. Second, the background climate might influence the temperature response to deforestation. For instance, in a warmer world less snow might be present, and thus the effect of boreal deforestation might be less cooling. Third, forest cover changes not only because of deforestation, but also because a warming background climate causes natural biogeographic shifts. For instance, a northward expansion of the boreal forests can affect climate in those regions. Previous studies were investigating these three factors separately, so the relative contribution of these factors was unknown. In the new study, the authors close this gap by analyzing the local consequences of deforestation consistently within a single climate model.

The authors find that historically the local temperature response is dominated by the deforestation due to land use. In contrast, in future scenarios a warming background climate and subsequent natural biogeographic shifts together become similarly important as land-use change for the effects of deforestation on surface temperature. An additional, previously little studied, factor is the dependence on the extent of forests before deforestation happens: The Earth system model applied in this study (MPI-ESM) shows that in most locations worldwide, the temperature response to removing the last 25% of the forest is larger than the response to deforesting the first 75% of the forest. Thus, future deforestation - starting from today, where much of the forest cover is already reduced - is simulated to be more potent in altering surface temperature than historical deforestation. All the investigated factors differ not only across different scenarios of land use and background climate, but also across different models. This study therefore highlights that all factors have to be taken into account when comparing the climate effects of deforestation across scenarios and climate models.

Original publication:
Winckler, J., Reick, C. and Pongratz, J. (2017) Why does the locally induced temperature response to land cover change differ across scenarios? Geophysical Research Letters, doi: 10.1002/2017GL072519

Contact:

Johannes Winckler
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 215
Email: johannes.winckler@we dont want spammpimet.mpg.de

Dr. Julia Pongratz
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 255
Email: julia.pongratz@we dont want spammpimet.mpg.de