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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We reviewed existing and planned adaptation activities of federal, tribal, state, and local governments and the private sector in the United States (U.S.) to understand what types of adaptation activities are underway across different sectors and scales throughout the country. Primary sources of review included material officially submitted for consideration in the upcoming 2013 U.S. National Climate Assessment and supplemental peer-reviewed and grey literature. Although substantial adaptation planning is occurring in various sectors, levels of government, and the private sector, few measures have been implemented and even fewer have been evaluated. Most adaptation actions to date appear to be incremental changes, not the transformational changes that may be needed in certain cases to adapt to significant changes in climate. While there appear to be no one-size-fits-all adaptations, there Mitig Adapt Strateg Glob Change (2013) are similarities in approaches across scales and sectors, including mainstreaming climate considerations into existing policies and plans, and pursuing no-and low-regrets strategies. Despite the positive momentum in recent years, barriers to implementation still impede action in all sectors and across scales. The most significant barriers include lack of funding, policy and institutional constraints, and difficulty in anticipating climate change given the current state of information on change. However, the practice of adaptation can advance through learning by doing, stakeholder engagements (including "listening sessions"), and sharing of best practices. Efforts to advance adaptation across the U.S. and globally will necessitate the reduction or elimination of barriers, the enhancement of information and best practice sharing mechanisms, and the creation of comprehensive adaptation evaluation metrics.

Debris flows generate seismic waves as they travel downslope and can become more dangerous as they entrain sediment along their path. We present field observations that show a systematic relation between the magnitude of seismic waves and the amount of erodible sediment beneath the flow. Specifically, we observe that a debris flow traveling along a channel filled initially with sediment 0.34 m thick generates about 2 orders of magnitude less spectral power than a similar‐sized flow over the same channel without sediment fill. We adapt a model from fluvial seismology to explain this observation and then invert it to estimate the level of bed sediment (and rate of entrainment) beneath a passing series of surges. Our estimates compare favorably with previous direct measurements of entrainment rates at the site, suggesting the approach may be a new indirect way to obtain rare field constraints needed to test models of debris flow entrainment.

Adaptation requires science that analyzes decisions, identifies vulnerabilities, improves foresight, and develops options.

Wildfire alters vegetation cover and soil hydrologic properties, substantially increasing the likelihood of debris flows in steep watersheds. Our understanding of initiation mechanisms of postwildfire debris flows is limited, in part, by a lack of direct observations and measurements. In particular, there is a need to understand temporal variations in debris‐flow likelihood following wildfire and how those variations relate to wildfire‐induced hydrologic and geomorphic changes. In this study, we use a combination of in situ measurements, hydrologic monitoring equipment, and numerical modeling to assess the impact of wildfire‐induced hydrologic and geomorphic changes on debris‐flow initiation during seven postwildfire rainstorms. We predict the impact of hillslope erosion on debris‐flow initiation by combining terrestrial laser scanning surveys of a hillslope burned during the 2016 Fish Fire with numerical modeling of sediment transport throughout a 0.12‐km2 basin in southern California. We use measurements of sediment thickness within the channel to constrain numerical experiments and to assess the role of channel sediment supply on debris‐flow initiation. Results demonstrate that debris flows initiated during rainstorms where hillslopes contributed minimally to the event sediment yield and suggest that large inputs of sediment from rill and gully networks are not essential for runoff‐generated debris flows. Simulations suggest that both the gradual entrainment of sediment and the mass failure of channel bed sediment can increase sediment concentration to levels associated with debris flows. Finally, postwildfire debris‐flow initiation appears closely linked to the same rainfall intensity‐duration threshold despite temporal changes in the sediment source, initiation processes, and hydraulic roughness.

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