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Investing in forests to fight climate change seems like a sure bet. Trees absorb carbon dioxide from the atmosphere, pump out oxygen, and live for decades. What could go wrong? The answer, according to a newly published paper in Science, is: a lot.

Fires, rising temperatures, disease, pests and humans all pose threats to forests, and as climate change escalates, so too do these threats. While forest-based solutions need to play an important role in addressing climate change, the risks to forests from climate change must also be considered.

“Current risks are not carefully considered and accounted for, much less these increased risks that forests are going to face in a warming climate,” William Anderegg, a biologist at the University of Utah and first author of the new paper, told Mongabay.

As societies strive to meet climate goals such as those set by the Paris Agreement — which aims to limit the global temperature rise to “well below” 2° Celsius (3.6° Fahrenheit) by 2100 — interest in planting, protecting, and managing forests (strategies referred to as forest-based natural climate solutions) has grown in recent years. A number of arenas and policies such as the Trillion Tree Campaign, supported by the United Nations, as well as individual companies have also launched tree-planting initiatives.

Up to 30% of global emissions today are pulled out of the atmosphere by land-based plants. But for forests to be good carbon-removal investments, they need to be relatively permanent, meaning that the plants and soil in a forest will absorb carbon and keep it locked away for decades or centuries. What climate change does is exacerbate many of the threats to forest permanence.

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forests climate change
Mist rising from the Amazon rainforest at dawn. Photo by Rhett A. Butler for Mongabay.

“Avoiding a 2° [Celsius] or 2.5° increase in temperature will be difficult without a very robust natural carbon solution,” Daniel Nepstad, president and founder of the Earth Innovation Institute, who was not involved in the paper, told Mongabay. “The paper helps us put in perspective the realistic expectations of a forest as a climate mitigation approach.”

An estimated 44% of forests are threatened with what is known as a stand replacing disturbance such as a high-intensity fire, hurricane, or disease outbreak that would kill most or all of the mature trees in the stand. The combined effects of multiple disturbances such as both drought and disease or drought and fire also hasten forest destruction.

“Climate change is going to supercharge the risks that forests face,” Anderegg said. “We’re going to see more fires, more droughts and more pests and pathogens in a warming climate.”

The recent fires in Australia and in the Amazon served as a global wake-up call about the increasing threat of fire on a warming planet and the impermanence of forests. Fire causes an estimated 12% of stand replacing disturbances to forests worldwide, and is a particular threat in Mediterranean climates, boreal forests, Australia, and the Western U.S. In the U.S., fire risk has already doubled over the past 30 years.

Droughts also threaten forests globally. A drought in California between 2011 and 2015 killed an estimated 140 million trees and caused the state’s ecosystem to be a net source of carbon rather than a sink. The disturbance accounted for 10% of the state’s greenhouse gas emissions from that time period.

Biotic agents such as insects and plant diseases also present a huge challenge to forests and forest management. The mountain pine beetle (Dendroctonus ponderosae), for example, is responsible for the deaths of billions of trees in temperate and boreal coniferous forests. At this point, science does not have a good way of predicting where, when, and to what extent these threats will present.

Anderegg and a group of global experts gathered in 2019 to talk about natural climate solutions. Here, they asked: How can we assess the risks to forest permanence? What can science contribute to be sure forest-based solutions are good investments for the climate? And how can we get that information to land managers and policymakers?

The newly published paper in Science was one of the outcomes of that meeting. In it, the authors provide a road map for assessing risks to forest permanence. Forest plot data, remote sensing, and mechanistic vegetation modeling are highlighted as some of the best scientific tools available.

Combining long-term satellite records with forest plot data, for instance, can provide solid estimates for future forest stress and disturbance.  Computer models in climate risks as well as models of tree growth and fire disturbance are also becoming more advanced.

However, because much of the forest plot data is collected in temperate forests, tropical forests have large gaps in data and monitoring. Also, many of these cutting-edge tools and techniques are not widely used outside of the scientific community, meaning policy decisions sometimes rely on science that is decades old.

The authors urge policymakers to be sure forest-based, natural climate solutions are done with the best available science. Likewise, scientists are urged to improve tools for sharing information across different groups outside of science.

Publicly available, easily usable and open-source tools to connect decision and policymakers to science and data are a priority, and something Anderegg and colleagues are currently working to create. The hope is that these tools will inform local decision-making based on current scientific understanding.

Beyond assessing risks to forests, the authors stress the importance of investing in forests in both an ecologically and socially responsible way.

“Planting native tree species and perhaps a diversity of tree species, involving local communities, and respecting indigenous communities and their rights in these forestry efforts are some of the ways to do this,” Anderegg said.

Another key point is to be mindful of how and where forests are planted. Across the high latitudes in Canada or Russia, for instance, the reflective nature of the snow cools the planet. So planting trees in these areas and covering the snow would actually tend to heat up the planet.

Finally, programs that offset carbon emissions by creating and protecting forests, while critical, should not distract from the simultaneously urgent matter of reducing fossil fuel emissions.

“There has been a tendency over the years, and it resurges every now and then, to put too much faith in forests or tree planting as a climate change solution,” Nepstad said. “First and foremost, we have to decarbonize the economy and move beyond fossil fuels, and that message has come through in this paper.”

“Keep in mind that there are lots of other reasons that we want to protect, conserve and perhaps restore forests,” Anderegg said, “such as biodiversity benefits, clean air, clean water, ecosystem services and tourism…Forests are about more than carbon.”

This article was originally published on Mongabay, written by Liz Kimbrough, and is republished here as part of an editorial partnership with Earth.Org.


Enhanced weathering is a carbon capture technology in which ocean alkalinity is increased through depositing rock particles into the ocean. It may sound simple, but there is still much to be examined as the risks are weighed against the benefits.

Currently, a diverse range of carbon capture methods are being used in an attempt to reach negative carbon emissions. Most of these are land-based methods and some are more controversial than others; Bioenergy with Carbon Capture and Storage (BECCS) technologies for example, require crops to be grown that will then get burned to release energy and store carbon underground. 

Enhanced weathering is a method that involves storing carbon in the ocean through a chemical reaction that removes CO2 from the atmosphere. In an effort to accelerate oceanic uptake of carbon in the least intrusive, yet most cost-effective way, scientists have concerns about the impact it will have on marine ecosystems. 

What is Enhanced Mineral Weathering?

Weathering is a natural process whereby rocks are broken down by rainwater, extreme temperatures or human activity. It is a process that takes place over millions of years, constituting an important carbon sink. 

The process begins when CO2 dissolves in droplets of water to form carbonic acid, a weak acid: rainwater has a pH of around 5 to 5.5, but because there is a lot of it available in the environment, it does a lot of weathering over time. Rocks that contain carbonates, like limestone, react quickly because the minerals they are largely made from, such as calcium carbonate, are more reactive than silicates. The dissolved calcium and bicarbonate ions formed from the carbonic acid travel in groundwater to rivers and the sea. Through the calcium carbonate-generating part of the chemical process, there is a net loss of readily-made carbon- half the amount of carbon at the end as there is at the beginning. As sediments continue to accumulate, the carbonate-rich layer will be buried under new layers of sediment that will in time turn to solid rock, like limestone. 

The carbonates that form increase the alkalinity of oceans, leading to a further uptake of atmospheric CO2. The rate of weathering is dependent on temperature, runoff (the availability of water to remove reaction products), grain size of rock or mineral and biological activity, like volcanoes. The annual potential of CO2 consumption is defined by the grain size and the weathering rate of the rocks used.

Enhanced mineral weathering is the speeding up of this natural process, whereby rocks are ground into fine particles and spread across large spans of land or the ocean. Overall, the process requires extraction, processing and the dissolution (reaction) of minerals. Through this, more atmospheric carbon dioxide can be sequestered than what would occur naturally. 

The use of enhanced rock weathering to increase ocean carbon uptake was first proposed in 1995, but was put aside due to the high energy costs of creating lime. 

Expected outcomes of enhanced weathering processes are uncertain. Current research is directed towards the function of alkalinity in the existing natural oceanic carbon cycle, what the effects on ocean chemistry imposed by artificial alkalinity could be and whether it could be maintained in a stable manner and technologies that can be used to increase alkalinity.

Enhanced Weathering Pros and Cons

Enhanced weathering may ameliorate ocean acidification. The added alkalinity also increases the saturation state of carbonate minerals which, if too low, negatively impacts carbonate-producing organisms in the ocean, such as shellfish and coral.

Enhanced weathering would not require its own land, nutrients or freshwater, with the latter only needing to be used when dust avoidance measures from rock deposition become necessary. Rock particles could be applied on open ocean regions or combined with agriculture with the additional benefit of enhancing crop yields and preventing soil erosion.

Additionally, while the range of technologies that have been proposed for increasing ocean alkalinity may pose significant engineering challenges, cost analyses suggest that they are still within the range of other negative emission technologies. 

However, speeding up or changing the course of nature can have disastrous effects. Scientists suggest that rapid uncontrolled changes in pH, carbonate saturation state, and dissolved aqueous CO2 can affect ocean ecosystems. While this process mimics a natural one, it is not natural; the substance would be delivered to ecosystems at rates far higher than normal which could create ‘dead zones’, areas where oxygen levels are too low to support life. Additionally, the amount of olivine necessary for these applications is extremely large and is comparable to present-day global coal mining, a counterintuitive proposition as the planet looks to turn away from mining. In the case of basalt, to sequester one billion tonnes of CO2, more than 3 billion tonnes of basalt would have to be spread, an amount equal to almost half of the current global coal production.

Additionally, at such a large scale, enhanced weathering could change the ecology of the water, leading to an increase in the microbial organisms that produce greenhouse gases such as methane and nitrous oxide. 

Although the addition of alkalinity is common practice in certain constrained marine environments such as in aquaria and shellfish production, scientists believe that more research is needed to understand the wider ecosystem response. 

Current Attitudes Towards Enhanced Weathering

In a 2017 UK survey, over 70% of participants expressed that they’d never heard of enhanced weathering. Further, support for research of the technique was found to be much stronger than support for the technique itself. Lack of public knowledge about this technology could be a factor hindering its large-scale deployment. 

The best suited locations are warm and humid regions, particularly in India, Brazil, South East Asia, and China, where almost three quarters of the global potential could be realised.

Increasing alkalinity in the ocean needs to be assessed more stringently. Some pressing issues regarding ocean alkalinity will have to address whether marine life will thrive or die in a new environment. While much of this work is still in the testing phase, enhanced weathering technologies could become economically and environmentally viable options to realise carbon capture and subsequent negative emissions this century. 


Featured image by: Richard Droker

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