Do forests cool the Earth?

When visiting offices at the LSCE, Saclay, France nothing stands out: there are the mandatory bikes, nerdy posters, leftovers from yesterday’s lunch, coffee-stained desks, trinkets from home, and piles of research papers. Dual monitors on every desk show windows of scrolling text against a black background and each office has a white board with half-erased equations overwritten by interrupted thoughts.

Like Comment

The paper in Nature is here:

Don’t be mistaken, behind this banal façade one of the most exciting endeavors in modern day ecology is taking place: searching for generalities in plant ecology and synthesizing them into a single mathematical framework known as an Earth System Model. When combined with field observations, experimental results, and remotely-sensed data, these Earth System Models become powerful tools to test our understanding of human actions on the Earth system, and which interventions could help to avoid catastrophic global change.

Forestry was initially proposed in the late 1970s as a cost-efficient tool to keep the atmospheric CO2 concentration in check and  buy time to work towards profound emission reductions. By the early 2000s a subtle shift in terminology had happened and forestry was commonly claimed – without substantial evidence – to mitigate climate change and thus cool the Earth. Against the odds, my 2009 proposal searching to answer the question whether forests cool the Earth was generously funded by the European Research Council. Nevertheless, my euphoric feelings quickly turned to stress:  it is not too difficult to ask a sticky question but now people expected me to answer it.

The making of a paper in numbers (infographic by Guillaume Marie)

I succeeded in hiring a handful of committed post-docs and together we wrote 28,000 lines of computer code, fixed 3,000 bugs including 100 non-trivial problems, convinced over 35 other researchers to share their data with us, used over 500,000 hours of computer time (around 57 years of calculations on your laptop), generated over 14 TB of model output (a drawer full of external hard disks), were deliberately avoided by colleagues during the second week of hunting down the toughest bug ever, wrote three papers for high-impact journals, emptied 17 white-board markers, accumulated an unhealthy amount of sleep deprivation, ate 48 kg of green beans (often the only vegetarian option available in the restaurant), and frequently snuck past the night guards after getting locked-in while debugging.  Surprisingly the team managed to stay mentally sane (assessment based on self-evaluation), which we assign – without substantial evidence – to daily lunch breaks during which we discussed life, the universe and everything in between seen through the eyes of six different nationalities.

The project started from a model that simulated forest as homogeneous green slime covering the land surface. By the end of the project, the green slime paradigm was abandoned and replaced by a structured canopy. This switch allowed us to answer the initial question of whether forests cool the Earth or not. The answer turned out to neither “yes” nor “no” but shows a trade-off between carbon and climate management due to compensating processes in the exchanges of water, energy and carbon between the trees and the atmosphere. The litmus test that will reveal whether the study was worth the effort will be whether we succeed in making an evidence-based impact on how the community at large thinks about forests. This may take a few years. In the meantime I will take a walk in the forest and simply enjoy the beauty.

Sebastiaan Luyssaert

Associate professor, VU University Amsterdam