Thomas Crowther (ecologist)

Crowther’s early research focused on how soil communities are connected. His PhD studied how grazing soil invertebrates can regulate the decomposition of wood by soil fungi. This work led him to Yale University, where he found that diverse communities of soil invertebrate grazers are important for regulating and limiting the increases in soil carbon loss that are expected under climate change.[23][24]

Since then, the scope of his research has expanded to the global scale, aiming to characterize the functional biogeography of global soil communities. His lab's research has explored the global biogeography of mycorrhizal fungi, earthworms, and nematodes across in soil, which are important for the health of ecosystems.[25][26]

Crowther's post-doctoral research, which mapped the distribution and diversity of trees across the world,[27] estimated that there are approximately 3.04 trillion trees on Earth, 46% fewer than at the onset of agriculture about 12,000 years ago. Previous estimates had suggested there may have been only be 400 billion trees on Earth.[28][29][30][31]

As part of this research, Crowther compiled a large inventory of tree data based on a combination of satellite observations and on-the-ground ecological research.[32] In 2016 he co-founded the Global Forest Biodiversity Initiative (GFBI) – a foundation which manages the world's largest tree-level forest inventory database with 1.2 million plots from more than 70 countries.[33][34] The database was used to identify that across the global forest system, a greater number of tree species consistently leads to increases in the carbon storage of local forests, even with the same number of trees.[35]

In 2019, his group extended this work in a large-scale study exploring the global tree restoration potential.[36] Utilizing measurements of tree cover from over 78,000 locations around the globe, the study built machine learning models to characterize which environments are capable of supporting trees.[37] This revealed that outside of urban and agricultural areas there are 0.9 billion hectares of land that could naturally support just over 1 trillion trees, which could capture up to a third of the excess carbon that humans have emitted into the atmosphere to-date.[38] Crowther cautioned that it would take decades for forests to mature to this point and that phasing out fossil fuel use is vital for slowing climate change.[39]

A key goal of Crowther's global ecological research is to understand the biological feedbacks to climate change. In particular, this research has highlighted the presence of a strong positive feedback between atmospheric warming and soil carbon loss. In 2016, Crowther’s research showed that a 1 °C increase in temperatures could release an additional 55 billion tons of soil carbon into the atmosphere by 2050, driving a feedback loop that could accelerate climate warming by 12-17%.[40][41] Crowther's research suggested that warming generally stimulates decomposition more than photosynthesis.[42][43][44] Increased activity of microbes and soil animals, such as worms, would be the source of the additional carbon emissions, which could accelerate global warming by 17%.[45]

Ivan Janssens, seen as one of the godfathers in the global change ecology field commented "the research had provided essential data to climate change models". A subsequent scientific review of research on soil carbon feedback cited Crowther's work, saying the "new finding of a statistical dependence of decomposition on the initial carbon content of the soil may prove to be a useful benchmark for [Earth System Model] simulations".[46] The study's data was since incorporated into ongoing work by the UN's Intergovernmental Panel on Climate Change.[47][48] In a more recent scientific review of research on soil carbon feedback, Crowther's work was highly cited as the "new finding of a statistical dependence of decomposition on the initial carbon content of the soil may prove to be a useful benchmark for [Earth System Model] simulations".[49]

Crowther's ongoing research employs location intelligence and mapping technologies to convey the status of Earth's terrestrial ecosystems. Through machine learning approaches they generate predictive models for a detailed understanding of the biosphere's spatial and temporal patterns.[50][51]

In October 2018 he delivered a keynote address at the 2018 conference for the Association for Forest Spatial Analysis Technologies (ForestSAT), hosted by the NASA Goddard Space Flight Center and the University of Maryland.[52] In February 2019, at the American Association for the Advancement of Science, Crowther announced that scientists had established there was room for an additional 1.2 trillion trees, noting that such a scale of reforestation could be one of the most powerful tools in the fight against climate change.[53][54]

Following the publication of the 2019 paper on the global tree restoration potential, a number of scientists criticized the idea that planting trees across the globe is a simple solution to climate change. In particular, several articles suggested that it is dangerous and misleading to propose that tree planting can be a silver bullet to stop climate change.[55][56][57]

In 2020, Crowther's presentation at a TED Countdown event on climate change attracted more than a million views.[58] In the talk, Crowther stressed that he agreed with the criticisms, stating that ecosystem restoration [59] cannot be used as an excuse to ignore the very real challenges of cutting greenhouse gas emissions and protecting existing ecosystems.[60] He has also emphasized that ecosystem restoration is not simply about planting trees.[61] It is about protecting ecosystems (including grasslands, wetlands, forests, and other ecosystems) so that natural biodiversity can return and facilitating the recovery of nature to promote the ecological and economic sustainability of local communities around the world.[62]

Crowther cautions that there are inherent risks associated with ecosystem restoration, which must be considered in order to realize a positive impact for biodiversity, human well-being, and climate change. In a letter to The Guardian newspaper, he identified four basic principles which any organization pledging commitment to the trillion-tree campaign should uphold:[63]

1. Cut emissions. Nature-Based Solutions are powerful tools to capture carbon from the atmosphere, but they are not a substitute for cutting greenhouse gas emissions. From a climate change perspective, we must rapidly cut fossil fuel emissions, decarbonize economies, and also maintain, sustainably manage, and restore ecosystems.
2. Conserve & protect existing ecosystems. Intact soils, forests, grasslands, shrub-lands, wetlands, and aquatic ecosystems are vital repositories of carbon and biodiversity. Yet, we are losing them at an alarming rate. Protecting these last remaining strongholds of nature is critical.
3. Be socially responsible. We must fully engage Indigenous peoples and local communities and respect and uphold their rights and leadership. We must also proactively contribute to fair and sustainable economic models that create new employment opportunities while avoiding competition with existing activities such as food production. Only when local communities benefit from the social, economic and ecological benefits that ecosystems provide can restoration be sustainable.
4. Be ecologically responsible. Nature-Based Solutions must be founded on rigorous ecological principles. Biodiversity is vital for healthy ecosystems which are more productive, resilient and beneficial. Diverse mixtures of native species are most likely to provide desired benefits such as carbon storage, food production, and protection from floods, drought and disease. Monocultures of exotic species or low-diversity plantations are unlikely to provide these desired benefits.” [64]