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“Benefits of planting trees are grossly overestimated”

Julia Pongratz
Julia Pongratz
Professor of Physical Geography at LMU Munich
Stephen Woroniecki
Stephen Woroniecki
Post-doctoral researcher in sustainability science at Linköping University

Ear­ly in 2019, a sci­en­tif­ic paper enti­tled “The glob­al tree restora­tion poten­tial” was pub­lished by Bastin et al. 1 in the pres­ti­gious jour­nal Sci­ence, imme­di­ate­ly mak­ing waves around the world. In it, the authors present an argu­ment that mas­sive tree-plant­i­ng across the globe could be enough to mit­i­gate cli­mate change much more than had pre­vi­ous­ly been assumed. They con­clud­ed that plant­i­ng an extra 0.9 bil­lion hectares of trees would store 205 bil­lion tonnes of car­bon – thus, claim­ing that glob­al tree restora­tion is “our most effec­tive cli­mate change solu­tion to date”.

As such, the study promised to offer hope of an action­able and reli­able solu­tion to reduce atmos­pher­ic CO2 lev­els. By the end of the year, it became the sec­ond most cit­ed cli­mate change paper in the press, gen­er­at­ing near­ly 600 media arti­cles. Nev­er­the­less, this strik­ing claim has since pro­voked mas­sive push­back from sci­en­tists, includ­ing five tech­ni­cal com­ments2 in response stat­ing that the study great­ly over­es­ti­mat­ed the mit­i­ga­tion poten­tial of mass tree-plant­i­ng. We talked to two experts rep­re­sent­ing the nat­ur­al and social sci­ence per­spec­tives to under­stand why this top­ic is so crit­i­cal for net zero com­mit­ments, for pol­i­cy and for the sci­en­tif­ic community.

Much can be said around the sci­en­tif­ic con­tro­ver­sy that the 2019 paper gen­er­at­ed. Why are we still inter­est­ed in this, two years on?

Julia Pon­gratz. The researchers in the study used a large amount of pho­to inter­pre­ta­tion mea­sure­ments and com­bined those with infor­ma­tion on cli­mate and soils to cal­cu­late glob­al tree cov­er poten­tial. Such high-res­o­lu­tion esti­mates of poten­tial tree cov­er are much need­ed, in fields such as cli­mate research to assess how much veg­e­ta­tion exists in the absence of human inter­fer­ence. In this respect, the study is a laud­able achieve­ment. Based on this new esti­mate the authors went on to draw con­clu­sions con­cern­ing how much car­bon could be tak­en up if the tree cov­er were restored every­where [apart from present-day crop­lands]. In com­par­i­son to pre­vi­ous esti­mates, the paper was at the very high end of the scale for CO reduc­tion poten­tial. Most of the fol­low­ing tech­ni­cal com­ments crit­i­cised this point, claim­ing that Bastin et al. dras­ti­cal­ly over­es­ti­mat­ed the envi­ron­men­tal ben­e­fits of tree-planting.

Stephen Woroniec­ki. Sci­en­tists are wor­ried that we will con­tin­ue to dra­mat­i­cal­ly over­es­ti­mate how much car­bon can be drawn down by ter­res­tri­al ecosys­tems. And that, as a result, the glob­al econ­o­my will con­tin­ue to pump out green­house gas­es only to realise fur­ther down the line that the car­bon sink cre­at­ed by trees is less effec­tive than we thought – or that it is more vul­ner­a­ble to cli­mate change itself. Despite warn­ings, plant­i­ng trees are form­ing a cen­tral part of many pol­i­cy pro­pos­als. So, even though many sci­en­tists pushed back on the orig­i­nal paper, their con­cerns had impact on pol­i­cy or pub­lic debate. Plant­i­ng trees or let­ting them grow back nat­u­ral­ly is an impor­tant part of the cli­mate chal­lenge, but it is not a sil­ver bul­let, and can­not com­pen­sate for emis­sions else­where in the economy.

There also seems to be some con­fu­sion about the dif­fer­ent ter­mi­nol­o­gy being used, name­ly the dif­fer­ence between refor­esta­tion and afforesta­tion. Could you explain?

SW. Afforesta­tion is plant­i­ng trees where they have not pre­vi­ous­ly grown – or in any recent time sig­nif­i­cant to us, at least. This includes tree-plant­i­ng in grass­land or oth­er ecosys­tem types which his­tor­i­cal­ly have had few trees. Where­as refor­esta­tion is replant­i­ng trees where they pre­vi­ous­ly grew. This includes parts of the Ama­zon basin, for exam­ple, where refor­esta­tion aims to re-estab­lish pre-exist­ing forests.

JP. Sci­en­tists are con­cerned that afforesta­tion can dam­age ecosys­tems that are vital for both bio­di­ver­si­ty and car­bon seques­tra­tion. Plant­i­ng trees on grass­lands or drain­ing peat lands to plant for­est plan­ta­tions could have neg­a­tive con­se­quences for the cli­mate and for local fau­na and flo­ra. The esti­mates by Bastin et al. include using all cur­rent pas­tures and range­lands (grass­lands, shrub­lands etc), which would require mas­sive changes in our cur­rent agri­cul­tur­al man­age­ment and/or diets.

Sci­en­tists fear that afforesta­tion will dam­age ecosys­tems essen­tial for biodiversity.

There were five tech­ni­cal com­ments in total. Can you talk about the sci­en­tif­ic backlash?

JP. A key crit­i­cism of the orig­i­nal man­u­script was that it seemed that zero car­bon was assumed for the exist­ing ecosys­tems, which clear­ly would be a flawed assump­tion; even if grass­lands do not have a lot of bio­mass, soil car­bon stocks are sub­stan­tial. As a result, the method sec­tion was changed to cre­ate very sim­i­lar num­bers to the orig­i­nal while now account­ing for car­bon in the pre-exist­ing ecosys­tems. This is an unsat­is­fy­ing result for the process of sci­en­tif­ic pub­lish­ing, giv­en that with such a major method­olog­i­cal change the study should have gone through for­mal peer-review again, which it did not.

I think harm has been done to the trust in sci­ence. In Ger­many, one of the largest news­pa­pers report­ed on a full page about this new panacea against cli­mate change, only to acknowl­edge the very next day that exact­ly those state­ments of the study were heav­i­ly crit­i­cised by oth­er renowned researchers. This sug­gest­ed to the pub­lic that there was a dis­pute in the sci­en­tif­ic com­mu­ni­ty that does not real­ly exist. The state­ment about reforestation/afforestation being “our most effec­tive cli­mate solu­tion to date” was sub­se­quent­ly removed from the paper because it is not true – obvi­ous­ly, reduc­ing anthro­pogenic emis­sions from fos­sil fuels and car­bon­ates as well as from land use are the solu­tions. But the dam­age in pub­lic per­cep­tion of research in this field had already been done. 

Are there oth­er impor­tant concerns?

SW. Yes, there are two addi­tion­al issues to con­sid­er when plant­i­ng trees as a car­bon solu­tion. One con­cerns bio­di­ver­si­ty like: what effects will there be on bio­di­ver­si­ty of large-scale tree plant­i­ng? And the oth­er is social: how will these projects affect peo­ple who live in the area? The IPCC spe­cial report on cli­mate change and land3 is an excel­lent resource on these neg­a­tive con­se­quences. It also pro­vides analy­sis of refor­esta­tion show­ing that neg­a­tive food secu­ri­ty effects are like­ly to be less pro­nounced for refor­esta­tion than afforesta­tion. A lot depends on which trees are plant­ed, where, why, and do they pro­vide for local needs. There are some ter­ri­ble exam­ples of cut­ting down pri­ma­ry forests only to plant trees to claim the car­bon benefit.

It also trou­bles me that land­scapes can be seen as these emp­ty spaces that are ripe for restora­tion. When you make a map of restora­tion poten­tial, and you don’t take peo­ple who live there into account you run the risk of walk­ing over peo­ples’ rights – espe­cial­ly in coun­tries with poor gov­er­nance and sys­tem­at­ic neglect of human rights. The changes of restora­tion or afforesta­tion con­tribut­ing to pos­i­tive devel­op­ment out­comes in those places is much lower.

JP. We must not also for­get that when we talk about cli­mate impact of grow­ing trees, more than CO2 mat­ters. We also need to con­sid­er what we call bio-geo­phys­i­cal effects such as changes in ener­gy and water flux­es. If you plant a for­est, you may eas­i­ly change local tem­per­a­tures by sev­er­al degrees, depend­ing on the region. We are on the way to under­stand­ing which forests cre­ate an addi­tion­al cool­ing effect, which ones may lead to addi­tion­al warm­ing if it is pos­si­ble to mit­i­gate heat extremes or drought con­di­tions. Ide­al­ly, we would cre­ate win-win sit­u­a­tions of mit­i­ga­tion of with adap­ta­tion to cli­mate change when the plant­ed for­est coun­ter­acts the effect of glob­al warm­ing locally.

Some say that the part of the issue is try­ing to man­age tree-plant­i­ng at the glob­al scale. Is local man­age­ment a bet­ter way?

SW. In the research com­mu­ni­ty there is debate between those who favour a com­mu­ni­ty-based approach, and those who favour projects at larg­er scale, focused on glob­al tar­gets where the com­mu­ni­ty may be an after­thought. Find­ing a bal­ance between the two is why some organ­i­sa­tions like IUCN and Oxford University’s Nature-based Solu­tions Ini­tia­tive have devel­oped guide­lines for nature-based solu­tions to build the guardrails for large-scale imple­men­ta­tion of tree plant­i­ng. Involv­ing peo­ple from the begin­ning, lis­ten­ing to their his­to­ries and pri­or­i­ties, can get you so much fur­ther than march­ing in and fenc­ing off a new plantation. 

JP. All land-based car­bon diox­ide removal meth­ods are imple­ment­ed local­ly. It is great if estab­lish­ing new for­est cov­er sucks up CO2, but if it is cut down five years lat­er because local farm­ers have no oth­er option than move into this land, lit­tle has been gained. To assess social accept­abil­i­ty of mea­sures like afforesta­tion it is impor­tant to bring in direct­ly the stake­hold­ers on the ground. We are thus now mov­ing towards a more inter­dis­ci­pli­nary approach as well, where the nat­ur­al sci­en­tists and econ­o­mists that have always been inter­est­ed in forests’ inter­ac­tions with cli­mate change col­lab­o­rate inten­sive­ly with the social sci­ences and humanities.

Interview by Denise Young


Julia Pongratz

Julia Pongratz

Professor of Physical Geography at LMU Munich

Julia Pongratz has studied geography in Munich and at the University of Maryland, then investigated land use change as a climate driver throughout the last millennium during her PhD work at the University of Hamburg, which won several awards. After a postdoc at the Carnegie Institution’s Department of Global Ecology in Stanford, looking into food security and geoengineering, Julia Pongratz established a junior research group at the Max Planck Institute for Meteorology on “Forest Management in the Earth System”. She now seeks to connect disciplines around the interactions of humans and ecosystems, including the topics of carbon dioxide removal and the multifunctionality of land on our way to climate neutrality.

Stephen Woroniecki

Stephen Woroniecki

Post-doctoral researcher in sustainability science at Linköping University

Stephen Woroniecki is a researcher working in Sweden and with colleagues at Oxford University’s Nature-based Solutions Initiative, bringing together the social and the natural dimensions of challenges like climate change, biodiversity loss, and inequality. His research deals with the potential of 'Nature-based Solutions' to address these challenges, and he strives to ensure that communities have a voice and rights in environmental decision-making that affects them. He has a background in in ecology, conservation, and resilience – having studied in Edinburgh, Stockholm, and Lund – but is increasingly using social science and humanities to ask questions about the social power and people’s subjective experience of environmental change and response. He has conducted research in Latin America, Europe, Africa, South Asia, and the Pacific.

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