DOI: 10.18739/a2db7vq6n
title: Expert estimates of change in arctic and boreal biomass, wildfire, and hydrologic carbon flux

description: In the fall of 2013 we administered a series of expert elicitation surveys to quantify uncertainty in the scientific community surrounding the response of biomass, wildfire, and hydrologic C flux to warming. Three to five lead experts for each system refined survey questions and developed a system summary document providing relevant literature and background information. System summaries included regional and pan-arctic estimates of C pools and fluxes, a brief treatment of historical trends, and a summary of model projections where available. We identified potential participants by querying Thomas Reuters Web of Science with applicable search terms (e.g. arctic, boreal, biomass, dissolved organic carbon, fire, permafrost). To reach researchers with applicable expertise who were underrepresented in the literature, we supplemented the list with personal referrals from lead experts and all participants. In total 256 experts were invited to participate. We distributed the surveys and system summaries via email with a two-week deadline. After sending out three reminders and accepting responses for three months after initial invitation, we received 115 responses from 98 experts (38% response rate), with 15 experts participating in more than one survey (Supplementary information; list of experts). Experts provided quantitative estimates of change in biomass, hydrologic flux, or wildfire for three time points (2040, 2100, and 2300) and four regional warming scenarios based on representative concentration pathway (RCP) scenarios from the IPCC Fifth Assessment Report. Warming scenarios ranged from cessation of human emissions before 2100 (RCP2.6) to sustained human emissions (RCP8.5) and correspond to permafrost-region mean annual warming of 2 to 7.5°C by 2100. The regional warming scenarios were generated from RCP2.6, 4.5, 6.0, and 8.5 with the National Center for Atmospheric Research's Community Climate System Model 4. Experts were encouraged to consider all available information when generating their estimates including published and unpublished modeled and empirical data as well as professional judgment. Participants self-rated their confidence and expertise for each question, described rationale for their estimates, and provided background information. For the purposes of this survey, warming was assumed to stabilize at 2100 levels for all scenarios so that responses through 2300 accounted for lags in ecosystem responses to climate drivers. While we only provided temperature scenarios with the surveys, we asked experts to consider all accompanying direct climate effects (e.g. temperature, precipitation, and atmospheric CO2) and indirect effects (e.g. vegetation shifts, permafrost degradation, invasive species, and disturbance). The biomass survey consisted of a single question asking for cumulative change in tundra and boreal non-soil biomass including above and belowground living biomass, standing deadwood, and litter. The wildfire survey asked for estimates of change in wildfire extent and CO2 emissions for the boreal and tundra regions to assess changes in both fire extent and severity. The hydrologic flux survey asked for estimates of DOC and POC delivery to freshwater ecosystems in the pan-arctic watershed and delivery to the Arctic Ocean and surrounding seas via riverine flux and coastal erosion, allowing the calculation of losses during transport due to burial or mineralization. Dissolved inorganic C fluxes were not included in this survey. Questions from the three surveys: Arctic and boreal biomass Question 1. How much change in boreal forest and arctic tundra non-soil biomass would result from the following increases in pan-arctic mean annual surface air temperature? (Positive numbers represent % increase, negative represent % decrease). Note: This question addresses changes in non-soil biomass for the circumpolar tundra and boreal forest due to direct climate forcing (temperature, precipitation, atmospheric CO2, seasonality etc.) as well as indirect effects (changes in primary productivity, vegetation shifts, nutrient availability, insects, pathogens, wildfire, etc.). The table below provides estimates of current biome area and biomass. While the tundra and boreal biomes may shift over time, we are asking you to estimate biomass change for the current distribution of these biomes. For example, if biomass increased for a patch of land which currently is tundra but which becomes boreal forest, that increment would be included in your % change in biomass of arctic tundra. Arctic and boreal wildfire Question 1. How much change in the annual extent of boreal and arctic wildland fire would result from the following increases in the mean annual surface air temperature in the pan-arctic? (Positive numbers represent % increase, negative represent % decrease). Note: This question addresses changes in wildfire extent in the circumpolar boreal forest and tundra due to direct climate effects (temperature, precipitation, atmospheric CO2, seasonality etc.) as well as indirect effects (vegetation shifts, insects, pathogens etc.). Question 2. How much change in CO2 release due to boreal and arctic wildland fire would result from the following increases in the mean annual surface air temperature in the pan-arctic? Note: This question addresses changes in carbon emissions due directly to boreal and arctic wildfire. It excludes indirect carbon release due to changes in permafrost extent, net ecosystem production, biome shift, etc. Refer to Question 1 table for estimates of current emissions from wildfire. Circumarctic hydrologic organic carbon flux Question 1. How much change in the amount of organic carbon delivered to freshwater ecosystems in the pan-Arctic watershed would result from the following increases in the mean annual surface air temperature in the pan-arctic? (Positive numbers represent % increase, negative represent % decrease). Note: Questions 1 and 2 address changes in dissolved and particulate organic carbon (DOC and POC) flux in the pan-Arctic watershed (20.5 x 106 km2 (Holmes et al. 2012)) due to direct climate perturbation (temperature, precipitation, etc.) as well as indirect disturbance (permafrost degradation, vegetation shift, etc.). The table below provides estimates of current DOC and POC delivery to freshwater ecosystems (lakes, rivers, and streams) and the Arctic Ocean and surrounding seas. Question 2. How much change in the amount of organic carbon delivered to the Arctic Ocean and surrounding seas would result from the following increases in the mean annual surface air temperature in the pan-arctic? (Positive numbers represent % increase, negative represent % decrease). Note: This question addresses changes in riverine DOC and POC flux to the ocean as well as changes in POC release from coastal erosion. The difference between the riverine to marine fluxes reported in this question and the terrestrial to freshwater fluxes reported in Question 1 represent the amount of carbon lost in transit due to mineralization and storage in sediment. Refer to the Question 1 table for estimates of current DOC and POC delivery to the Arctic Ocean and surrounding seas.