The trouble with negative emissions

By Kevin Anderson and Glen Peters (Science)

Reliance on negative-emission concepts locks in humankind’s carbon addiction

In December 2015, member states of the United Nations Framework Convention on Climate Change (UNFCCC) adopted the Paris Agreement, which aims to hold the increase in the global average temperature to below 2°C and to pursue efforts to limit the temperature increase to 1.5°C.

The Paris Agreement requires that anthropogenic greenhouse gas emission sources and sinks are balanced by the second half of this century. Because some nonzero sources are unavoidable, this leads to the abstract concept of “negative emissions”, the removal of carbon dioxide (CO2 ) from the atmosphere through technical means. The Integrated Assessment Models (IAMs) informing policy-makers assume the large-scale use of negative-emission technologies. If we rely on these and they are not deployed or are unsuccessful at removing CO2 from the atmosphere at the levels assumed, society will be locked into a high-temperature pathway.


To understand the implications of the Paris Agreement for mitigation policy, we must translate its qualitative temperature limits into quantitative carbon budgets, specifying how much CO2 can be emitted across the remainder of the century to keep warming below a given temperature level (1). Uncertainties in the climate system mean that such budgets are specified with quantitative likelihoods.

Borrowing from the taxonomy of likelihoods used by the Intergovernmental Panel on Climate Change (IPCC), the most generous interpretation of the Paris Agreement’s requirement to keep the temperature rise well below 2°C is, at least, a likely (66 to 100%) chance of not exceeding 2°C.

The IPCC has assessed 900 mitigation scenarios from about 30 IAMs (2). Of these, 76 scenarios from five IAMs had sufficient data to estimate the carbon budget for a likely chance of not exceeding 2°C. These scenarios give a carbon budget of between 600 and 1200 billion metric tons (Gt) CO 2 (10 to 90% range) for the period from 2016 until the peak in temperature [updated from (1)]. Increasing the likelihood of keeping temperatures below 2°C (or shifting the ceiling to 1.5°C) will reduce still further the available carbon budget (3). The budget is also subject to a reduction each year, currently around 40 Gt CO 2 , due to continued fossil fuel, industry, and land-use change emissions.

It is important to keep in mind that despite their intuitive appeal, the complexity of carbon budgets make it impossible to assign a specific budget to a given temperature rise.


Because the carbon budgets represent cumulative emissions, different emission pathways can be consistent with a given budget. Using the 76 scenarios consistent with a likely chance of not exceeding 2°C (see the figure), two key features are immediately striking. First, the scenarios assume that the large-scale rollout of negative-emission technologies is technically, economically, and socially viable (2, 4). In many scenarios, the level of negative emissions is comparable in size with the remaining carbon budget (see the figure) and is sufficient to bring global emissions to at least net zero in the second half of the century.

Second, there is a large and growing deviation between actual emission trends and emission scenarios. The sum of the national emission pledges submitted to the Paris negotiations (COP21) lead to an increase in emissions, at least until 2030. They thus broaden the division between pathways consistent with the temperature goals of the Paris Agreement (5) and require either much more severe near-term mitigation (6) or additional future negative emissions.

It is not well understood by policy-makers, or indeed many academics, that IAMs assume such a massive deployment of negative-emission technologies. Yet when it comes to the more stringent Paris obligations, studies suggest that it is impossible to reach 1.5°C with a 50% chance without significant negative emissions (3). Even for 2°C, very few scenarios have explored mitigation without negative emissions (2).

Negative emissions are also prevalent in scenarios for higher stabilization targets (7). Given such a pervasive and pivotal role of negative emissions in mitigation scenarios, their almost complete absence from climate policy discussions is disturbing and needs to be addressed urgently.


Negative-emission technologies exist at various levels of development (8–11). Afforestation and reforestation, although not strictly technologies, are already claimed by countries as mitigation measures. Bioenergy, combined with carbon capture and storage (BECCS), is the most prolific negative-emission technology included in IAMs and is used widely in emission scenarios. It has the distinct feature of providing energy while also, in principle (12), removing CO2 from the atmosphere. Assuming that carbon is valued, BECCS can thus provide an economic benefit that may offset, at least in part, the additional costs of using the technology (13). Generally, carbon is assumed to be fully absorbed during biomass growth, captured before or after combustion, and then stored underground indefinitely. Despite the prevalence of BECCS in emission scenarios at a level much higher than afforestation, only one large-scale demonstration plant exists today.

Other negative-emission technologies have not moved beyond theoretical studies or small-scale demonstrations. Alternative and adjusted agricultural practices, including biochar, may increase carbon uptake in soils (9). It may also be possible to use direct air capture to remove CO2 from the atmosphere via chemical reactions, with underground storage similar to CCS. Enhancing the natural weathering of minerals (rocks) may increase the amount of carbon stored in soils, land, or oceans. Introduction of biological or chemical catalysts may increase carbon uptake by the ocean. New technologies, designs, and refinements may emerge over time.


The allure of BECCS and other negative-emission technologies stems from their promise of much-reduced political and economic challenges today, compensated by anticipated technological advances tomorrow. Yet there are huge opportunities for near-term, rapid, and deep reductions today at little to modest costs, such as improving energy efficiency, encouraging low-carbon behaviors, and continued deployment of renewable energy technologies. Why, then, is BECCS used so prolifically in emission scenarios?

The answer is simple. Integrated assessment models often assume perfect knowledge of future technologies and give less weight to future costs. In effect, they assume that the discounted cost of BECCS in future decades is less than the cost of deep mitigation today. In postponing the need for rapid and immediate mitigation, BECCS licenses the ongoing combustion of fossil fuels while ostensibly fulfilling the Paris commitments.

The idea behind BECCS is to combine bioenergy production with CCS, but both face major and perhaps insurmountable obstacles. Two decades of research and pilot plants have struggled to demonstrate the technical and economic viability of power generation with CCS, even when combusting relatively homogeneous fossil fuels (14). Substituting for heterogeneous biomass feedstock adds to the already considerable challenges.

Moreover, the scale of biomass assumed in IAMs—typically, one to two times the area of India—raises profound questions (10) about carbon neutrality, land availability, competition with food production, and competing demands for bioenergy from the transport, heating, and industrial sectors. The logistics of collating and transporting vast quantities of bioenergy—equivalent to up to half of the total global primary energy consumption—is seldom addressed. Some studies suggest that BECCS pathways are feasible, at least locally (15), but globally there are substantial limitations (10). BECCS thus remains a highly speculative technology.

Although BECCS, like all negative-emission technologies, is subject to scientific and political uncertainties, it dominates the scenario landscape. Yet, as recognition of the ubiquitous role of BECCS in mitigation scenarios has grown, so have concerns about its deployment (10, 11). Its land-use impacts could include terrestrial species losses equivalent to, at least, a 2.8°C temperature rise (11), leading to difficult trade-offs between biodiversity loss and temperature rise. There is also little robust analysis of the trade-offs between large-scale deployment of BECCS (and all negative-emission technologies) and the Sustainable Development Goals (SDGs). But such a level of caution is far removed from the technical utopia informing IAMs. Despite BECCS continuing to stumble through its infancy, many scenarios assessed by the IPCC propose its mature and large-scale rollout as soon as 2030 (see the figure).


The appropriateness or otherwise of relying, in significant part, on negative-emission technologies to realize the Paris commitments is an issue of risk (7). However, the distribution of this risk is highly inequitable. If negative-emission technologies fail to deliver at the scale enshrined in many IAMs, their failure will be felt most by low-emitting communities that are geographically and financially vulnerable to a rapidly changing climate.

The promise of future and cost-optimal negative-emission technologies is more politically appealing than the prospect of developing policies to deliver rapid and deep mitigation now. If negative-emission technologies do indeed follow the idealized, rapid, and successful deployment assumed in the models, then any reduction in near-term mitigation caused by the appeal of negative emissions will likely lead to only a small and temporary overshoot of the Paris temperature goals (3). In stark contrast, if the many reservations increasingly voiced about negative-emission technologies (particularly BECCS) turn out to be valid, the weakening of near-term mitigation and the failure of future negative-emission technologies will be a prelude to rapid temperature rises reminiscent of the 4°C “business as usual” pathway feared before the Paris Agreement (5).

Negative-emission technologies are not an insurance policy, but rather an unjust and high-stakes gamble. There is a real risk they will be unable to deliver on the scale of their promise. If the emphasis on equity and risk aversion embodied in the Paris Agreement are to have traction, negative-emission technologies should not form the basis of the mitigation agenda. This is not to say that they should be abandoned (14, 15). They could very reasonably be the subject of research, development, and potentially deployment, but the mitigation agenda should proceed on the premise that they will not work at scale.

The implications of failing to do otherwise are a moral hazard par excellence.


1. J. Rogelj et al., Nat. Clim. Change 6, 245 (2016).
2. L. Clarke et al., in Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, O. Edenhofer et al., Eds. (Cambridge Univ. Press, Cambridge/New York, 2014), pp. 413–510.
3. J. Rogelj et al., Nat. Clim. Change 5, 519 (2015).
4. H. J. Buck, Clim. Change 10.1007/s10484-016-1770-6 (2016).
5. J. Rogelj et al., Nature 534, 631 (2016).
6. K. Anderson, Nat. Geosci. 8, 898 (2015).
7. S. Fuss et al., Nat. Clim. Change 4, 850 (2014).
8. M. Tavoni, R. Socolow, Clim. Change 118, 1 (2013).
9. P. Smith, Glob. Change Biol. 22, 1315 (2016).
10. P. Smith et al., Nat. Clim. Change 6, 42 (2015).
11. P. Williamson, Nature 530, 153 (2016).
12. A. Gilbert, B. K. Sovacool, Nat. Clim. Change 5, 495 (2015).
13. D. L. Sanchez et al., Nat. Clim. Change 5, 230 (2015).
14. D. M. Reiner, Nat. Energy 1, 15011 (2016).
15. D. L. Sanchez, D. M. Kammen, Nat. Energy 1, 15002 (2016).
16. The figure shows the median of the 76 IPCC scenarios that limit the global temperature rise to 2°C with 66% likelihood (2). Realized negative emissions are estimated by converting the BECCS energy consumption [exajoules (EJ) per year], assuming an average biomass emission factor of 100 metric tons of CO 2 per terajoule (TJ) and assuming that 90% of the CO 2 is captured. The emission pledges (INDCs) in 2030 are estimated based on cumulative emissions from 2011 to 2030 (5).

Nature article confirms: IPCC assumptions about BECCS ignore environmental and wider climate impacts

Steve Slater via Flickr CC-BY

In his recent article in Nature, Dr Philip Williamson highlights how the targets set out in the Paris Agreement mask an underlying assumption that they will be met through large-scale carbon dioxide removal from the atmosphere and, in particular, through Bioenergy with Carbon Capture and Storage (BECCS) and large-scale afforestation (planting trees on land not forested in the recent past).

Williamson points to the startling fact that “the IPCC’s roughly 5,000-page Fifth Assessment Report…leaves out one crucial consideration: the environmental impacts of large-scale CO2 removal” and warns that these environmental impacts could translate into adverse rather than beneficial climate impacts: “Planting at such scale [as proposed for BECCS] could involve more release than uptake of greenhouse gases, at least initially, as a result of land clearance, soil disturbance and increased use of fertilizer.”

The article describes how such technologies, despite being included in IPCC scenarios, could carry significant, unintended risks to biodiversity and ecosystems. It concludes with the statement that “For now, action should focus on urgent emissions reductions and not on an unproven ‘emit now, remove later’ strategy.”

Williamson’s article goes in to some detail on the potential ecological (and thereby climate) implications of any possible BECCS or large-scale afforestation programme. He describes how limiting the global temperature rise to 2°C would require “crops to be planted solely for the purpose of CO2 removal on between 430 million and 580 million hectares of land — around one-third of the current total arable land on the planet”. It is important to note that another peer-reviewed article suggests that significantly more land may be needed.

Williamson adds that “the land requirements to make BECCS work would vastly accelerate the loss of primary forest and natural grassland. Thus, such dependence on BECCS could cause a loss of terrestrial species at the end of the century perhaps worse than the losses resulting from a temperature increase of about 2.8 °C above pre-industrial levels”. This would obviously be a terrible consequence of misguided climate mitigation policies.

That’s by no means all of it either – Williamson goes further: “…little is known about the effect of future climatic conditions on the yields of bioenergy crops; what the water requirements of such crops may be in a warmer world; the implications for food security if bioenergy production directly competes with food production; and the feasibility (including commercial viability) of the associated carbon capture and storage infrastructure.”

Another important point made in the article is that the optimistic claims made by the IPCC about the ability of BECCS to play a central role in mitigation come from the work of “physical scientists and modellers”, not ecologists. In fact, the IPCC Working Group 3 on climate change mitigation (which has written about BECCS and afforestation) was heavily dominated by economists, engineers and environmental managers, rather than climate scientists or ecologists, as civil society groups pointed out when their most recent Assessment Report was released. It’s no surprise therefore that the ecological impacts of large-scale CO2 removal technologies such as BECCS have not been considered.

A more contentious suggestion made by Williamson in the article is this: “One solution would be to abandon the term climate geoengineering and simply assess the various methods for mitigating climate change on a case-by-case basis.” Abandoning the term climate geoengineering could ultimately mean abandoning the de-facto moratorium on geoengineering agreed by the Convention on Biological Diversity, which would definitely be a step in the wrong direction. And abandoning the term is not a prerequisite to assessing the different proposals classed under it.

Williamson proposes: “It is time for the IPCC, governments and other research-funding agencies to invest in new, internationally coordinated studies to investigate the viability and relative safety of large-scale CO2 removal.” However, as his article confirms, it is vital that such work is not be left to modellers, economists, engineers and environmental managers. Such scientists have little academic background in understanding the vital and complex links between the climate and the biosphere, and the crucial role that biodiversity plays in maintaining all of the earth’s life support systems.

Where’s the Lorax When We Need Him?

It’s a shame that the Lorax and his message “Who Will Speak For The Trees” has been relegated to the realm of children’s cartoons and fantasy. Especially as trees, forests and ecosystems appear to be right smack in the epicenter of swirling debates about climate change. What those debates seem to boil down to (as the world burns around us) is whether it makes more sense to 1) cut down remaining forests and burn them for “renewable energy”, 2) put a fence around them, measure their carbon content and sell them to polluters as “offsets”, or 3) install vast plantations of trees — (perhapsgenetically engineered to grow faster), to suck up atmospheric carbon in hopes this will counter the ongoing gush of carbon into the atmosphere (geoengineering via “afforestation”) or this recent proposal which suggests that cutting down high latitude temperate and boreal forests — or replacing them with short rotation tree plantations, might help “fix” the climate.

Decisions, decisions! So many options. What shall we do with all the trees?

Here in the U.S>, a debate is brewing out west in light of recent legislation proposals from Oregon representatives Wyden and Defazio (among others) that would provide supports for “thinning and restoration” (climatespeak for logging) on public lands. The underlying motive is to get access to timber currently off limits to supply expanding demand for biomass to burn as “renewable energy.

Meanwhile, one of the few proclaimed “successes” coming out of Warsaw climate negotiations was towards an agreement on REDD+ (reducing emissions from deforestation and forest degradation). Yet this is hardly a success given mounting evidence that, among other concerns, REDD fails to address the underlying drivers of deforestation. What it has achieved is to create bitter divisions among indigenous communities faced with proposals that would commodify their lands and utterly distracted forest policymakers who are now so caught up in endless debates over REDD that they seem barely to notice that their forests are meanwhile being liquidated.

As if we were not confused enough already, brilliant scientists at Dartmouth have now determined that the value of high latitude temperate and boreal forests (let’s call them “Truffula” trees) — should not be measured solely in terms of their carbon content, nor the value of their timber, but also with respect to the “ecosystem service” they provide (to us that is) of absorbing or reflecting sunlight: their impact on albedo. The basic idea is that at higher latitudes, dark colored tree cover absorbs light and has net warming impact, whereas removing trees or keeping them small and immature allows light to penetrate, increasing the reflectivity of the white ground surface, and contributing to a net cooling effect.

The Dartmouth scientists say: “Our results suggest that valuing albedo can shorten optimal rotation periods significantly compared to scenarios where only timber and carbon are considered… we expect that in high latitude sites, where snowfall is common and forest productivity is low, valuing albedo may lead optimal rotation periods that approach zero.”

In other words those forests will be “more valuable” cut down or replaced with short rotation stunted tree plantations.

In their conclusions the Dartmouth authors state: “In particular, documenting relationships between forest biomass growth, the frequency of snowfall, latitude, and regional stumpage prices may help elucidate locations wherein different forest project strategies provide the maximum climatic benefits.”

Oh really? That sounds easy! Just plug those numbers into an equation and voila we can discover that “optimal climate benefits” indicate we should cut the forests down?

What would the Lorax say, I wonder? Probably that the “value” of forests should not be confined in so reductionist a manner where consideration is granted solely to carbon content, albedo, or timber harvest pricing. What about, just for example, the role of forests in regulating temperature and rainfall patterns over large areas of the earth? Or the role of compounds released into the atmosphere by trees in stimulating cloud formation (and hence influencing albedo)? What about the role of forest in creating fertile soils? Or the production of hydroxyl radicals by forests which are thought to play a key role in the breakdown of atmospheric pollutants? And what about the many many life forms that depend on healthy forest ecosystems?

The authors seem to have had some inkling that there might be other views of the “value” of forests, and offer lip service, recommending that “forest management should include biodiversity considerations when managing the flow of timber, carbon, and albedo services in mid and high latitude temperate and boreal forests.”

Managing the flow of services? If we cut them down then it seems most of the “flow” of “services” will likely come to a screeching halt!

The paper closes with a plug for funding: “Thus, as in all modeling work, we must take caution to consider that optimal forest management may vary quite drastically as the planet responds to climate change. Consequently, detailed and refined projections of these changes are critical for future work in this arena.”

Well, it is good that the authors realize that things will drastically change as climate change progresses. Like, for example, we might find that there is a lot less snow or it melts much faster. Which begs the question: After we have cut down the high latitude temperate and boreal forests to increase the reflective potential of snow cover, what happens when there is no more snow? Now we have no albedo and… no trees, no timber, no carbon, no biodiversity. This must be the Once-ler’s idea!

Perhaps they are taking their lead from another Once-ler, the Indonesian Ministry of Forestry. Recently when questioned about the horrendous doubling in Indonesia’s deforestation rate over the year following announcement of a moratorium on new concessions, responded that this was not “deforestation”, only “temporary deforestation“, (euphemism of the year).

The paleoclimate record suggests that in previous cases where earth’s climate has heated up or cooled down, stability was regained in large part by the sequestering of carbon in plants — forests and ecosystems. For example, Southeast Asian tropical peat forests (now being destroyed for oil palm and pulp and paper plantations and “temporary deforestation”) are thought to have played a key role in stabilizing climate between glacial and interglacial periods over the course of the last few million years of earth history.

The survival of many species faced with warming depends upon a steady northward and uphill climb towards cooler and more favorable conditions. So let’s see… now we are going to cut those forests down, further diminishing the remaining pool of ecosystem diversity in order to gain some supposed cooling due to albedo enhancement over the coming season, or two?

I am certain that the Lorax, and the children to whom that story so appeals, have far more common sense.