The global tree restoration potential: how many trees can be planted in our planet, and where?

• With all the ongoing initiatives and efforts to restore forests around the world (check our short overview here), these are important questions. A network of scientists led by the Crowther Lab at the ETH Zurich has, once again, helped to bring a global perspective into this question and shown the value of large-scale collaboration and science. Their research on the global tree restoration potential has just been published in Science and became a major news item (almost) all around the world for the last 2 weeks. This amount of visibility will – hopefully – help to push social and political change forward. A nice overview of the study and media coverage is available at the Crowther Lab’s research page.
• Their study shows that at least 900 million hectares of forest can be restored now, without affecting agricultural or urban areas, and point where it can be done. They estimate this could capture about 205 gigatonnes of carbon, about two-thirds of the total atmospheric emissions produced by man, making it one of the most cost-effective solutions to fight climate change. Adding agrofrestry (trees with pasture land or other crops) and urban greening could increase this number further. Cuts to current emissions are still a must, as they detail in their Q&A blog article, since CO₂-capture would happen over several decades, as trees grow and sequester carbon into biomass and the soil.
• Back in 2015, they were also in the news when they calculated the current number of trees in the world: about 3 trillion trees (3’000’000’000’000) or 430 trees for each person in the world. This was 1 trillion more than generally assumed, which was great news! The bad news was that, they estimated, the global number of trees had been nearly halved by humankind since we started changing ecosystems. The work, published in Nature (also available in the Lab’s page), was a great example of an international scientific effort that nicely combined extensive satellite and ground-based measurements to create a global picture.1 trillion trees became a nice round goal for global initiatives such as the Trillion Tree Campaign, which counts several ongoing efforts.
• Such a large effort will take time to unfold but other valuable and much needed benefits would come in a few years already. As highlighted in this letter, in the same issue of Science, restoration efforts have positive impacts in nature and people which cannot be replaced by short-term fixes. But how to plant all these trees? Read on.
• But it’s important to remember that restoring forests is more than fighting climate change and fighting climate change is more than just planting trees, despite all the media attention and passions involved:
  • Forests fulfill other vital functions, which we need to ensure, and young forests (or tree plantations) are not the same as stable, mature forests.
  • Climate change is not driven only by C0₂ emissions but also by other greenhouse gases such as methane, nitrous oxide (which have a “stronger” effect for the same amount).
  • The oceans absorb around 30% of human CO₂ emissions, more than all land systems combined, and other ecosystems such as grasslands also play an important role as carbon sinks, keepers of biodiversity and source of human benefits.
  • Finally, to reduce the huge excess of greenhouse gases in the atmosphere, we need to absorb what is there AND cut human emissions to lower the continuous input we make and – hopefully – use our planet in a smarter way.