NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1 D espite two decades of effort to curb emissions of CO 2 and other greenhouse gases (GHGs), emissions grew faster during the 2000s than in the 1990s 1 , and by 2010 had reached ~50 Gt CO 2 equivalent (CO 2 eq) yr −1 (refs 2,3). The continuing rise in emissions is a growing challenge for meeting the international goal of limiting warming to less than 2 °C relative to the pre-industrial era, particularly without stringent climate policies to decrease emissions in the near future 2-4 . As negative emissions technologies (NETs) seem ever more necessary 3,[5][6][7][8][9][10] To have a >50% chance of limiting warming below 2 °C, most recent scenarios from integrated assessment models (IAMs) require large-scale deployment of negative emissions technologies (NETs). These are technologies that result in the net removal of greenhouse gases from the atmosphere. We quantify potential global impacts of the different NETs on various factors (such as land, greenhouse gas emissions, water, albedo, nutrients and energy) to determine the biophysical limits to, and economic costs of, their widespread application. Resource implications vary between technologies and need to be satisfactorily addressed if NETs are to have a significant role in achieving climate goals.options, to be able to decide which pathways are most desirable for dealing with climate change.There are distinct classes of NETs, such as: (1) bioenergy with carbon capture and storage (BECCS) 11,12 ; (2) direct air capture of CO 2 from ambient air by engineered chemical reactions (DAC) 13,14 ; (3) enhanced weathering of minerals (EW) 15 , where natural weathering to remove CO 2 from the atmosphere is accelerated and the products stored in soils, or buried in land or deep ocean [16][17][18][19] ; (4) afforestation and reforestation (AR) to fix atmospheric carbon in biomass and soils [20][21][22] ; (5) manipulation of carbon uptake by the ocean, either

Article 2 ͉ UNFCCC ͉ climate change impacts A rticle 2 of the United Nations Framework Convention on Climate Change (UNFCCC) commits signatory nations to stabilizing greenhouse gas concentrations in the atmosphere at a level that ''would prevent dangerous anthropogenic interference (DAI) with the climate system.'' The UNFCCC also highlights 3 broad metrics with which decision-makers are to assess the pace of progress toward this goal: allow ''ecosystems to adapt naturally to climate change,'' ensure that ''food production is not threatened,'' and enable ''economic development to proceed in a sustainable manner.'' In an effort to provide some insight into impacts that might be considered DAI, authors of the Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) identified 5 ''reasons for concern'' (RFCs) in (1). Each RFC categorizes impacts of a similar type, providing a set of metrics reflecting severity of risk. Relationships between various impacts reflected in each RFC and increases in global mean temperature (GMT) were portrayed in what has come to be called the ''burning embers diagram''; the image was also included in the Summary for Policy Makers of the contribution of Working Group II to the TAR and highlighted in the Synthesis Report.In presenting the ''embers'' in the TAR, IPCC authors did not assess whether any single RFC was more important than any other; nor, as they noted, did they conclude what level of impact or what atmospheric concentrations of greenhouse gases would constitute DAI, a value judgment that would be policyprescriptive. The ''embers'' were designed primarily to communicate the associations of impacts with increases in GMT and facilitate examination of the underlying evidence for use by decision-makers contemplating responses to these concerns.The IPCC Fourth Assessment Report (AR4) states that ''the 'reasons for concern' identified in the TAR remain a viable framework for assessing key vulnerabilities'' (2). In this article, we revise sensitivities of the RFCs to increases in GMT, based on our expert judgment about new findings in the growing literature since the publication of the TAR in 2001.* Furthermore, our judgments are supported by a more thorough understanding of the concept of vulnerability that has evolved over the past 8 years, † as well as a more careful articulation of the criteria by which any specific vulnerability can be labeled ''key,'' and thus contribute to a reason for concern (3). ‡ Section 1 defines and reviews the RFCs and ''burning embers'' figure as presented in the IPCC TAR. Section 2 presents the 1 To whom correspondence may be addressed. E-mail: jsmith@stratusconsulting.com or shs@stanford.edu. *These judgments were vetted by 3 rounds of IPCC review and were approved in the Summary for Policymakers of both the AR4 Working Group 2 and Synthesis Reports by the IPCC Plenary. † Vulnerability to climate change is the degree to which geophysical, biological and socioeconomic systems are susceptible to and unable to cope with adve...

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