How well does tree planting work in climate change fight? It depends

Using trees as a cost-effective tool against climate change is more complicated than simply planting large numbers of them, an international collaboration that includes an Oregon State University scientist has shown.

Jacob Bukoski of the OSU College of Forestry and seven other researchers synthesized data from thousands of reforestation sites in 130 countries and found that roughly half the time it's better just to let nature take its course.

Findings of the study led by Conservation International were published today in Nature Climate Change.

"Trees can play a role in climate change mitigation, for multiple reasons," Bukoski said. "It's pretty easy to understand that forests pull carbon dioxide from the atmosphere and store it, and trees are something pretty much everyone can get behind -- we have seen multiple bipartisan acts for tree planting introduced in Congress. This study brings a nuanced perspective to the whole 'should we plant trees to solve climate change' debate."

Bukoski notes that expanding forests globally has been widely proposed as a key tactic against climate change since forests sequester atmospheric carbon dioxide in their biomass and soils. Harvested timber also stores carbon in the form of wood products.

There are two basic approaches to forest expansion, Bukoski said.

"Generally speaking, we can let forests regenerate on their own, which is slow but cheap, or take a more active approach and plant them, which speeds up growth but is more expensive," he said. "Our study compares these two approaches across reforestable landscapes in low- and middle-income countries, identifying where naturally regenerating or planting forests is likely to make more sense."

Using machine learning and regression models, the scientists found that natural regeneration would be most cost effective over a 30-year period for 46% of the areas studied, and planting would be most cost effective for 54%.

They also determined that using a combination of the two approaches across all areas would be 44% better than natural regeneration alone and 39% better than planting by itself.

"If your objective is to sequester carbon as quickly and as cheaply as possible, the best option is a mix of both naturally regenerating forests and planting forests." Bukoski said.

The study suggests that natural regeneration is especially cost effective relative to plantation forestry in much of western Mexico, the Andean region, the Southern Cone of South America, West and Central Africa, India, Southern China, Malaysia and Indonesia.

Conversely, plantations are preferable to natural regeneration in much of the Caribbean, Central America, Brazil, northern China, mainland Southeast Asia, the Philippines and North, East and Southern Africa.

"Which method is more cost effective in a given location is a function of multiple factors, including opportunity cost, relative carbon accumulation and harvest rates, and relative implementation costs," Bukoski said.

Other scientists in the collaboration were Jonah Busch and Bronson Griscom of Conservation International, Susan Cook-Patton of The Nature Conservancy, David Kaczan of the World Bank, Yuanyuan Yi of Peking University, Jeff Vincent of Duke University and Matthew Potts of the University of California, Berkeley.

The authors stress that reforestation is a complement to, not a replacement for, reducing emissions from fossil fuels. Achieving the entire mitigation potential of reforestation over 30 years would amount to less than eight months of global greenhouse gas emissions, they note.

The authors add that carbon is just one consideration when growing trees. Biodiversity, demand for wood products, support of local livelihoods, and non-carbon biophysical effects must also be considered when deciding where and how to reforest landscapes.

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38 trillion dollars in damages each year: World economy already committed to income reduction of 19 % due to climate change

Even if CO2 emissions were to be drastically cut down starting today, the world economy is already committed to an income reduction of 19 % until 2050 due to climate change, a new study published in Nature finds. These damages are six times larger than the mitigation costs needed to limit global warming to two degrees. Based on empirical data from more than 1,600 regions worldwide over the past 40 years, scientists at the Potsdam Institute for Climate Impact Research (PIK) assessed future impacts of changing climatic conditions on economic growth and their persistence.

"Strong income reductions are projected for the majority of regions, including North America and Europe, with South Asia and Africa being most strongly affected. These are caused by the impact of climate change on various aspects that are relevant for economic growth such as agricultural yields, labour productivity or infrastructure," says PIK scientist and first author of the study Maximilian Kotz. Overall, global annual damages are estimated to be at 38 trillion dollars, with a likely range of 19-59 trillion dollars in 2050. These damages mainly result from rising temperatures but also from changes in rainfall and temperature variability. Accounting for other weather extremes such as storms or wildfires could further raise them.

Huge economic costs also for the United States and European Union

"Our analysis shows that climate change will cause massive economic damages within the next 25 years in almost all countries around the world, also in highly-developed ones such as Germany, France and the United States," says PIK scientist Leonie Wenz who led the study. "These near-term damages are a result of our past emissions. We will need more adaptation efforts if we want to avoid at least some of them. And we have to cut down our emissions drastically and immediately -- if not, economic losses will become even bigger in the second half of the century, amounting to up to 60% on global average by 2100. This clearly shows that protecting our climate is much cheaper than not doing so, and that is without even considering non-economic impacts such as loss of life or biodiversity."

To date, global projections of economic damages caused by climate change typically focus on national impacts from average annual temperatures over long-time horizons. By including the latest empirical findings from climate impacts on economic growth in more than 1,600 subnational regions worldwide over the past 40 years and by focusing on the next 26 years, the researchers were able to project sub-national damages from temperature and rainfall changes in great detail across time and space all the while reducing the large uncertainties associated with long-term projections. The scientists combined empirical models with state-of-the-art climate simulations (CMIP-6). Importantly, they also assessed how persistently climate impacts have affected the economy in the past and took this into account as well.

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Large mammals can help climate change mitigation and adaptation

 When it comes to helping mitigate the effects of climate change by absorbing carbon, flora rather than fauna usually comes to mind. A new study published in Current Biology now explores the role of large wild animals in restoring ecosystems and battling climate change.

Professor Yadvinder Malhi, Environmental Change Institute at the University of Oxford, said:

'Conservation efforts usually focus on either trees and carbon or the broad conservation appeal of large mammals. This study looked at whether it was possible to align these agendas -- under what context could protecting and restoring large animal wildlife help us tackle and adapt to climate change.'

The researchers highlighted three key eco-touchpoints where large animals such as elephants, rhinoceroses, giraffes, whales, bison, and moose had the greatest potential to mitigate climate change: carbon stocks, albedo (the ability of surfaces to reflect solar radiation (energy from the sun) and fire regimes.

When they graze, large herbivores disperse seeds, clear vegetation and fertilise the soil, which helps build more complex and more resilient ecosystems. These activities can maintain and increase carbon stocks in the soil, roots and above-ground parts of plants, helping to reduce CO2 in the atmosphere.

When large animals graze and trample vegetation they can change the habitat from dense shrubs and trees to open mixes of grass and shrubs or trees, which can also reveal snow-covered ground in polar regions. These open habitats tend to be paler (with higher albedo) and reflect more solar radiation into the atmosphere, cooling the Earth's surface, rather than absorbing it and warming the Earth's surface.

In 2021, global wildfire CO2 emissions reached a record high. When wildfires burn, the carbon stored in trees and vegetation is released into the atmosphere as greenhouse gases. Elephants, rhinoceroses, zebras and other large grazing animals can lessen wildfire risk by browsing on woody vegetation that could otherwise fuel the fires, trampling paths and making other gaps in vegetation that act as firebreaks.

The research, commissioned by wildlife charity Tusk, also looked at how protecting and restoring large animal wildlife could support climate change efforts and found several animal-climate interaction points that could provide 'win-win' opportunities.

In temperate, tropical and subtropical grassland ecosystems, large animals can reduce forest and bush fires, increase albedo and help retain carbon in the vegetation and the soil. Protecting large animal wildlife and their role in these complex ecosystems supports local biodiversity and ecological resilience.

Dr Tonya Lander, Department of Plant Sciences at Oxford University said:

'Animals can also help with localised adaptation to climate change in these environments by diversifying vegetation and increasing habitat heterogeneity. Diversity of species and microhabitats can make the ecosystem as a whole more able to resist climate change, return to a stable state following a climate-related disturbance, or find a new stable state that functions within the changed and changing climate.'

When large herbivores are present in tundra ecosystems, they help to keep down woody plant encroachment which encourages local flowering plants and grasses -- and exposes more of the ground to the cold air. That exposure maintains the permafrost and prevents the carbon in the soil from getting released into the atmosphere. Programmes that rewild bison and other animals into the arctic tundra can play important roles in both conservation and climate change adaptation at a local scale.

In marine ecosystems, whales and other large animals fertilise phytoplankton. Phytoplankton is estimated to capture 37 billion tonnes of CO2 each year and may release particles into the air which can help seed clouds and reflect sunlight into the atmosphere.

Large terrestrial and marine carnivores also affect these processes through their influence on herbivore abundance and behaviour.

Read more at Science Daily