Terrestrial carbon dioxide removal (tCDR) potentials (GtC) for rain-fed (a), sustainably (b) and unrestrictedly irrigated (c) biomass-producing plantations in combination with conversion efficiencies (CEff) of 50 percent, 75 percent, and 90 percent (shading) and for different climate models input for LPJmL (colors). The gray horizontal bar denotes the required tCDR of Representative Concentration Pathway 2.6 of 160–190 GtC. Graphic: Boysen, et al., 2017 / Earth's Future

By Chelsea Harvey
22 May 2017

(The Washington Post) – Scientists are expressing increasing skepticism that we’re going to be able to get out of the climate change mess by relying on a variety of large-scale land-use and technical solutions that have been not only proposed but often relied upon in scientific calculations.
Two papers published last week debunk the idea of planting large volumes of trees to pull carbon dioxide out of the air — saying there just isn’t enough land available to pull it off — and also various other strategies for “carbon dioxide removal,” some of which also include massive tree plantings combined with burning their biomass and storing it below the ground.

“Biomass plantations are always seen as a green kind of climate engineering because, you know, everybody likes trees,” said Lena Boysen, a climate researcher at the Max Planck Institute for Meteorology in Germany, who led one of the new studies while a researcher at the Potsdam Institute for Climate Impact Research. “But we just want to show that that’s not the complete story. They cannot do that much.”

Forests have long been recognized as one of the world’s most important natural carbon sinks, capable of storing large amounts of carbon that would otherwise end up in the atmosphere. Simply preserving the world’s forest resources — and replanting areas that have already been deforested — is viewed as an important step in protecting the climate.

But for years, scientists have discussed the idea of going further by using large plantations full of fast-growing, carbon-storing trees to pull extra carbon emissions out of the atmosphere, a strategy sometimes called “afforestation.” But the amount of land and other resources this strategy would require to actually help us meet our global climate goals — namely, keeping global temperatures within at least two degrees of their pre-industrial levels — is completely impractical, according to Boysen’s new study in the journal Earth’s Future, and would require the destruction of huge amounts of natural ecosystems or productive agricultural land.

Considering different scenarios for planetary emissions, Boysen and her colleagues find the land space that would be required for the amount of trees necessary to keep temperatures within a 2-degree threshold under our current climate trajectory could have “dire consequences for food production or the biosphere.” And even under more optimistic scenarios, where future carbon emissions are lower and fewer trees would be necessary, they conclude that “high inputs of managed water and fertilizers would be needed in order to avoid fierce competition for land — with potentially negative side-effects for climate and society.” [more]

Stop hoping we can fix climate change by pulling carbon out of the air, scientists warn

18 May 2017 (Potsdam Institute for Climate Impact Research) – Growing plants and then storing the CO2 they have taken up from the atmosphere is no viable option to counteract unmitigated emissions from fossil fuel burning, a new study shows. The plantations would need to be so large, they would eliminate most natural ecosystems or reduce food production if implemented as a late-regret option in the case of substantial failure to reduce emissions. However, growing biomass soon in well-selected places with increased irrigation or fertilization could support climate policies of rapid and strong emission cuts to achieve climate stabilization below 2 degrees Celsius.

“If we continue burning coal and oil the way we do today and regret our inaction later, the amounts of greenhouse gas we would need to take out of the atmosphere in order to stabilize the climate would be too huge to manage,” says Lena Boysen from the Potsdam Institute for Climate Impact Research (PIK), Germany, lead-author of the study to be published in a journal of the American Geophysical Union, Earth’s Future. Plants suck CO2 out of the atmosphere to build their woody roots, stems and leaves. This is low-tech terrestrial carbon dioxide removal that could be combined with high-tech carbon storage mechanisms, for example underground.

Three scenarios: Business as usual, Paris pledges, or ambitious CO2 reductions

“Even if we were able to use productive plants such as poplar trees or switchgrass and store 50 percent of the carbon contained in their biomass,” says Boysen, “in the business-as-usual scenario of continued, unconstrained fossil fuel use the sheer size of the plantations for staying at or below 2°C of warming would cause devastating environmental consequences.” The scientists calculate that the hypothetically required plantations would in fact replace natural ecosystems around the world almost completely.

If CO2 emissions reductions are moderately reduced in line with current national pledges under the Paris Climate Agreement, biomass plantations implemented by mid-century to extract remaining excess CO2 from the air still would have to be enormous. In this scenario, they would replace natural ecosystems on fertile land the size of more than one third of all forests we have today on our planet. Alternatively, more than a quarter of land used for agriculture at present would have to be converted into biomass plantations – putting at risk global food security.

Only ambitious emissions reductions and advancements in land management techniques between 2005-2100 could possibly avoid fierce competition for land. But even in this scenario of aggressive climate stabilization policy, only high inputs of water, fertilizers and a globally applied high-tech carbon-storage-machinery that captures more than 75 percent of extracted CO2 could likely limit warming to around 2°C by 2100. To this end, technologies minimizing carbon emissions from cultivation, harvest, transport and conversion of biomass and, especially, long-term Carbon Capture and Storage (CCS) would need to improve worldwide.

Drawing upon all possible measures instead of waiting for first-best solutions

“As scientists we are looking at all possible futures, not just the positive ones,” says co-author Wolfgang Lucht from PIK. “What happens in the worst case, a widespread disruption and failure of mitigation policies? Would plants allow us to still stabilize climate in emergency mode? The answer is: no. There is no alternative for successful mitigation. In that scenario plants can potentially play a limited, but important role, if managed well.” The scientists investigated the feasibility of biomass plantations and CO2 removal from a biosphere point of view. To this end, they used global dynamic vegetation computer simulations.

So far, biomass plantations as a means for CO2 removal have often been considered as a comparatively safe, affordable and effective approach. “Our work shows that carbon removal via the biosphere cannot be used as a late-regret option to tackle climate change. Instead we have to act now using all possible measures instead of waiting for first-best solutions,” says co-author Tim Lenton of the University of Exeter, UK. “Reducing fossil fuel use is a precondition for stabilizing the climate, but we also need to make use of a range of options from reforestation on degraded land to low-till agriculture and from efficient irrigation systems to limiting food waste.”

“In the climate drama currently unfolding on that big stage we call Earth, CO2 removal is not the hero who finally saves the day after everything else has failed. It is rather a supporting actor that has to come into play right from the beginning, while the major part is up to the mitigation protagonist,” says co-author Hans Joachim Schellnhuber, Director of PIK. “So this is a positive message: We know what to do – rapidly ending fossil fuel use complemented by a great variety of CO2 removal techniques. We know when to do it – now. And if we do it, we find it is still possible to avoid the bulk of climate risks by limiting temperature rise to below 2 degrees Celsius.”

Article:  Lena R. Boysen, Wolfgang Lucht, Dieter Gerten, Vera Heck, Timothy M. Lenton, Hans Joachim Schellnhuber (2017): “The limits to global-warming mitigation by terrestrial carbon removal”, Earth's Future (open access AGU journal).


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Climate stabilization: Planting trees cannot replace cutting CO2 emissions

ABSTRACT: Massive near-term greenhouse gas emissions reduction is a precondition for staying “well below 2°C” global warming as envisaged by the Paris Agreement. Furthermore, extensive terrestrial carbon dioxide removal (tCDR) through managed biomass growth and subsequent carbon capture and storage is required to avoid temperature “overshoot” in most pertinent scenarios. Here, we address two major issues: First, we calculate the extent of tCDR required to “repair” delayed or insufficient emissions reduction policies unable to prevent global mean temperature rise of 2.5°C or even 4.5°C above pre-industrial level. Our results show that those tCDR measures are unable to counteract “business-as-usual” emissions without eliminating virtually all natural ecosystems. Even if considerable (Representative Concentration Pathway 4.5 [RCP4.5]) emissions reductions are assumed, tCDR with 50% storage efficiency requires >1.1 Gha of the most productive agricultural areas or the elimination of >50% of natural forests. In addition, >100 MtN/yr fertilizers would be needed to remove the roughly 320 GtC foreseen in these scenarios. Such interventions would severely compromise food production and/or biosphere functioning. Second, we reanalyze the requirements for achieving the 160–190 GtC tCDR that would complement strong mitigation action (RCP2.6) in order to avoid 2°C overshoot anytime. We find that a combination of high irrigation water input and/or more efficient conversion to stored carbon is necessary. In the face of severe trade-offs with society and the biosphere, we conclude that large-scale tCDR is not a viable alternative to aggressive emissions reduction. However, we argue that tCDR might serve as a valuable “supporting actor” for strong mitigation if sustainable schemes are established immediately.

The limits to global-warming mitigation by terrestrial carbon removal

By Devon Ryan
18 May 2017

(Stanford Woods Institute for the Environment) – With the current pace of renewable energy deployment and emissions reductions efforts, the world is unlikely to achieve the Paris Climate Agreement’s goal of limiting global warming to 2 degrees C above pre-industrial levels. This trend puts in doubt efforts to keep climate change damages from sea level rise, heat waves, drought and flooding in check. A potential solution being widely discussed is removing carbon dioxide from the atmosphere, also known as “negative emissions.”

However, in a new perspective published in the journal Science, researchers at Stanford explain the risks of assuming carbon removal technologies can be deployed at a massive scale relatively quickly with low costs and limited side effects – with the future of the planet at stake.

“For any temperature limit, we’ve got a finite budget of how much heat-trapping gases we can put into the atmosphere. Relying on big future deployments of carbon removal technologies is like eating lots of dessert today, with great hopes for liposuction tomorrow,” said Chris Field, a professor of biology and of Earth system science and director of the Stanford Woods Institute for the Environment.

Reforestation and little tested technologies

Some strategies for carbon dioxide removal are well understood, such as planting trees that will store carbon from the atmosphere. Others involve immature, little tested technologies, such as bioenergy with carbon capture and storage. In that strategy, carbon dioxide produced from biomass energy is stored deep underground. In another technology called direct air capture, chemical processes extract carbon dioxide from the atmosphere.

“The models generating possible trajectories of climate change mitigation bet on planetary-scale carbon removal in the second half of the century,” said Katharine Mach, a senior research scientist at Stanford’s School of Earth, Energy & Environmental Sciences. “For policymakers trying to limit the worst damages from climate change, that bet is reckless.”

The researchers don’t reject carbon capture, instead arguing that there are important near-term opportunities for carbon removal at modest scale, often with other benefits for nature and people, and critical needs now for developing the technologies of the future. But heavy reliance on biomass energy with carbon capture and storage could require tremendous land areas. For example, relying on the technology to achieve a temperature increase of 2 C or less could require an amount of productive land equivalent to about 25 to 80 percent of total global cropland, up to about 8 percent of all of the land on Earth.

“This puts climate change mitigation, global food security and biodiversity protection on a collision course with no easy off-ramps,” says Field.

Peak and decline

Many of the climate policy discussions supporting reliance on atmospheric carbon removal focus on the idea of “peak and decline,” which involves global temperatures peaking and then dropping as carbon removal technologies surpass emissions. However, the scientists argue that peak and decline may ignore climate impacts that won’t disappear even if the planet starts to cool. For example, if warming triggers collapse of the Antarctic ice sheet, the resulting sea level rise would continue for hundreds of years.

Further, Field and Mach warn that hoping carbon removal technologies will kick in may delay concrete actions that could be taken now.

“At the right scale, carbon dioxide removal approaches are a key tool in the climate solutions kit,” Mach said. “Avoiding can-kicking ethics, however, means putting aside assumptions that massive deployments will easily materialize decades into the future. Instead, we need to embrace whole-hearted mitigation today.”

Ultimately, the scientists support a balanced approach that includes research and development of carbon removal technologies but also makes use of available means to limit and reduce carbon emissions, such as investing in renewable energy sources.

“In managing the risks of a changing climate, we need a diversified game plan. An appealing long shot is not a plan and it is not a good way to protect the planet on which we depend,” said Field.

Chris Field is also the Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies, the Perry L. McCarty Director of the Stanford Woods Institute for the Environment and senior fellow at the Precourt Institute for Energy. Katharine Mach is also an adjunct assistant professor at Carnegie Mellon University, visiting investigator at the Carnegie Institution for Science and director of the Stanford Environment Assessment Facility at the Stanford Woods Institute for the Environment.

Assuming easy carbon removal from the atmosphere is a high-stakes gamble

ABSTRACT: Proven approaches for limiting climate change include enhancing energy efficiency, capturing wind and solar energy, decreasing deforestation, and reducing industrial and agricultural emissions. These approaches are increasingly cost-competitive, consistent with large-scale use, and largely supported by public sentiment. Yet, the current pace of their deployment is far from sufficient for holding warming well below 2°C above preindustrial levels with high probability, the goal of the Paris Agreement. Two approaches for bridging this gap are widely discussed. First, the rate of decarbonization could be accelerated based on the above approaches. Second, continuing emissions could be compensated by removing carbon dioxide from the atmosphere (1, 2). Technologies for carbon removal are mostly in their infancy, yet are increasingly asserted as key to climate policy. Here, we focus on rightsizing the expectations from carbon dioxide removal (CDR).

Rightsizing carbon dioxide removal


  1. Susan Butler said...

    You can't fix a problem using the same methods that got you into trouble. Yes, vast "plantations" would be a very bad idea. Especially taking out intact ecosystems to do so. It is exactly intact ecosystems that can embody enough carbon in their soils, plants and animals, permanently, to pull enough carbon out of the air to make the difference. Agro-ecology, or permaculture, is a way to provide for human needs while imitating whole ecosystems and the many services they provide, including rebalancing the carbon cycle. We need to begin now restoring vast expanses of lands, degraded by poor agricultural practices, to rich abundant ecosystems which are the source of primary productivity on the planet. We know how to do this. Google Loess Plateau. We can employ millions, redefine the meaning of work, and create new healthy human settlements while we rescue the planet over the course of about 50 years. The over-arching purpose of human life must become the building up of deep rich soils, the intelligent partnering with ecological flows for mutual benefit.  


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