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Two leading experts in the field of carbon capture and sequestration (CCS) — Howard J. Herzog, a senior research engineer in the MIT Energy Initiative, and Niall Mac Dowell, a professor in energy systems engineering at Imperial College London — explore methods for removing carbon dioxide already in the atmosphere in their new book, “Carbon Removal.” Published in October, the book is part of the Essential Knowledge series from the MIT Press, which consists of volumes “synthesizing specialized subject matter for nonspecialists” and includes Herzog’s 2018 book, “Carbon Capture.”

Burning fossil fuels, as well as other human activities, cause the release of carbon dioxide (CO₂) into the atmosphere, where it acts like a blanket that warms the Earth, resulting in climate change. Much attention has focused on mitigation technologies that reduce emissions, but in their book, Herzog and Mac Dowell have turned their attention to “carbon dioxide removal” (CDR), an approach that removes carbon already present in the atmosphere.

In this new volume, the authors explain how CO₂ naturally moves into and out of the atmosphere and present a brief history of carbon removal as a concept for dealing with climate change. They also describe the full range of “pathways” that have been proposed for removing CO₂ from the atmosphere. Those pathways include engineered systems designed for “direct air capture” (DAC), as well as various “nature-based” approaches that call for planting trees or taking steps to enhance removal by biomass or the oceans. The book offers easily accessible explanations of the fundamental science and engineering behind each approach.

The authors compare the “quality” of the different pathways based on the following metrics:

Accounting. For public acceptance of any carbon-removal strategy, the authors note, the developers need to get the accounting right — and that’s not always easy. “If you’re going to spend money to get CO₂ out of the atmosphere, you want to get paid for doing it,” notes Herzog. It can be tricky to measure how much you have removed, because there’s a lot of CO₂ going in and out of the atmosphere all the time. Also, if your approach involves, say, burning fossil fuels, you must subtract the amount of CO₂ that’s emitted from the total amount you claim to have removed. Then there’s the timing of the removal. With a DAC device, the removal happens right now, and the removed CO₂ can be measured. “But if I plant a tree, it’s going to remove CO₂ for decades. Is that equivalent to removing it right now?” Herzog queries. How to take that factor into account hasn’t yet been resolved.

Permanence. Different approaches keep the CO₂ out of the atmosphere for different durations of time. How long is long enough? As the authors explain, this is one of the biggest issues, especially with nature-based solutions, where events such as wildfires or pestilence or land-use changes can release the stored CO₂ back into the atmosphere. How do we deal with that?

Cost. Cost is another key factor. Using a DAC device to remove CO₂ costs far more than planting trees, but it yields immediate removal of a measurable amount of CO₂ that can then be locked away forever. How does one monetize that trade-off?

Additionality. “You’re doing this project, but would what you’re doing have been done anyway?” asks Herzog. “Is your effort additional to business as usual?” This question comes into play with many of the nature-based approaches involving trees, soils, and so on.

Permitting and governance. These issues are especially important — and complicated — with approaches that involve doing things in the ocean. In addition, Herzog points out that some CCS projects could also achieve carbon removal, but they would have a hard time getting permits to build the pipelines and other needed infrastructure.

The authors conclude that none of the CDR strategies now being proposed is a clear winner on all the metrics. However, they stress that carbon removal has the potential to play an important role in meeting our climate change goals — not by replacing our emissions-reduction efforts, but rather by supplementing them. However, as Herzog and Mac Dowell make clear in their book, many challenges must be addressed to move CDR from today’s speculation to deployment at scale, and the book supports the wider discussion about how to move forward. Indeed, the authors have fulfilled their stated goal: “to provide an objective analysis of the opportunities and challenges for CDR and to separate myth from reality.”

A global research effort led by Colorado State University, including The University of New Mexico, shows that extreme, prolonged drought conditions in grasslands and shrublands would greatly limit the long-term health of crucial ecosystems that cover nearly half the planet.

The findings are particularly relevant as climate change increases the possibility of more severe droughts in the future – potentially leading to a situation that echoes the Dust Bowl of the 1930s.

The new research, published in Science, shows that losses in plant productivity – the creation of new organic matter through photosynthesis – were more than twice as high after four years of continued extreme drought when compared to losses from droughts of moderate intensity. The work shows that these grassland and shrubland ecosystems lose their ability to recover over time under prolonged dry conditions.

“We show that – when combined – extreme, multi-year droughts have even more profound effects than a single year of extreme drought or multi-year moderate droughts,” said CSU Biology Professor Melinda Smith, who led the study with Timothy Ohlert, a former CSU postdoctoral researcher.

“The Dust Bowl is a good example of this,” she continued. “Although it spanned nearly a decade it was only when there were consecutive extremely dry years that those effects, such as soil erosion and dust storms, occurred. Now with our changing climate, Dust Bowl-type droughts are expected to occur more frequently.”

“We found that drought had highly variable effects on productivity, the growth of grassland plants, among global grasslands. However, productivity declined dramatically after multiple years of extreme drought, especially in drylands like ours, which is the likely scenario for our climate in the future.”

– UNM Distinguished Professor Scott Collins

Smith designed and led the International Drought Experiment with more than 170 researchers around the world. For the project, researchers built rainfall manipulation structures that reduced each rainfall event by a target amount over a four-year period in grassland and shrubland ecosystems across six continents.

In addition to the CSU research team, researchers at The University of New Mexico included Distinguished Professor of Biology Scott Collins. Ohlert was also a Ph.D. student at UNM under the mentorship of Collins and conducted some of the research while he was at UNM.

“We organized a coordinated, distributed drought experiment across 70 global grasslands, including sites in New Mexico,” said Collins. By simulating 1-in-100-year extreme drought conditions, the team was able to study the long- and short-term effects on grasslands and shrublands, which store more than 30% of global carbon and support key industries, such as livestock production. Variations in precipitation, as well as soil and vegetation across continents, meant different sites experienced different combinations of moderate and extreme drought years – providing unique experimental conditions that informed this study.

“Droughts are a natural part of climate variability, especially here in New Mexico. Climate change is predicted to increase the frequency, intensity and duration of droughts in our region and elsewhere,” continued Collins. “We found that drought had highly variable effects on productivity, the growth of grassland plants, among global grasslands. However, productivity declined dramatically after multiple years of extreme drought, especially in drylands like ours, which is the likely scenario for our climate in the future.”

Smith said the paper highlights the interaction between extremity and duration in drought conditions and that this interaction has rarely been systematically studied using experiments.

She added that the research suggests that the negative impacts on plant productivity are also likely to be much larger than previously expected under both extreme and prolonged drought conditions. 

Plant growth is a fundamental component of the global carbon cycle. That is because plant photosynthesis is the main way carbon dioxide enters ecosystems, where animals consume it and plants store it as biomass. Because grasslands and shrublands cover roughly 50% of the Earth’s surface, they play a large role in balancing and facilitating carbon uptake and sequestration globally. That means changes to these ecosystems caused by drought could have wide-ranging impacts.

“Grasslands are globally important ecosystems because they are economically important and also harbor high biodiversity, including many charismatic grazing animals like bison in North America and wildebeest in Africa,” said Collins. “Many grasslands are highly sensitive to changes in precipitation, which is especially true for our grasslands where boom and bust years have big impacts on these ecosystems.”

For more than two decades, Collins and his colleagues have worked on the effects of global environmental change on grassland ecosystems. The International Drought Experiment is a key example of this work. Last year, the team published findings in PNAS from the same multi-site research network that quantified the impact of extreme short-term (one year) drought on grasslands and shrublands. Smith said the pair of papers now form an important foundation for further research into this topic.

“Because of the historic rarity of extreme droughts, researchers have struggled to estimate the actual consequences of these conditions in both the near and long-term,” Smith said. “This large, distributed research effort is truly a team effort and provides a platform to quantify and further study how intensified drought impacts may play out.”