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Online Resource 9. Interactions of IGSM-CAM Variability with MC1 Thresholds

Climatic Change Article: Quantifying and Monetizing Potential Climate Change Policy Impacts on Terrestrial Ecosystem Carbon Storage and Wildfires in the United States

Authors: David Mills, Russell Jones, Karen Carney, Alexis St. Juliana, Richard Ready, Allison Crimmins, Jeremy Martinich, Kate Shouse, Benjamin DeAngelo, and Erwan Monier

Corresponding author: David Mills, Stratus Consulting Inc., [email protected]

Fire within MC1 (a dynamic global vegetation model) occurs within a cell once certain threshold conditions are satisfied. In order to provide greater insight into how variability in meteorological data generated from the Integrated Global Systems Model (IGSM) Community Atmospheric Model (CAM) is interacting with MC1 to produce such large estimates of the area burned under different emission scenarios and initializing conditions, we examine the projected value for the proportion of a cell burned for the year 2100.

Figure 9.1 shows a map of the projected results for the proportion of the cell burned produced by MC1 for the reference (REF) emission scenario in 2100 and identifies the specific cell of interest that will be the subject for subsequent graphs of information on transient meteorological values from IGSM-CAM. This cell was selected because of the extremely high value for the proportion of the cell burned projected by MC1 in 2100 in this scenario. Fig. 9.1 Example of MC1 fire output for year 2100

Figure 9.2 shows 115 years of output values from MC1 for the proportion of a cell burned for a single cell in north-central Nebraska (latitude 42.25 N, longitude 100.75 W, see Figure 9.1). This yearly graph indicates that the largest value for the fraction of the cell burned, equivalent to the greatest area burned for the cell, occurs in year 2100. Examining the climate inputs from IGSM-CAM for the REF emission scenario in year 2100 (Figures 9.3 through 9.5), it appears that the threshold conditions to initiate fire within MC1 were reached in the month of July and reflect the combination of extremely high maximum and minimum temperatures (Figures 9.3 and 9.4, respectively) and very low precipitation in July and the preceding months (Figures 9.5).

This multi-month combination of meteorological extremes in the summer of 2100 in this cell appears to be unique across the evaluated time period of 2001–2115. For example, as noted in Figures 9.3 (Tmax) and 9.4 (Tmin), July 2095 and 2100 in this cell had almost identical average monthly temperatures; in both years Tmax values exceeded 40°C (104°F) and 25°C (77°F) for Tmin. However, in 2100, July followed an extremely hot June. Specifically, the Tmax value in this cell for June 2095 was just over 30°C (86°F), while its counterpart in 2100 was ~ 37°C (~ 98°F) (see Figures 9.3 and 9.4). Similarly, while monthly precipitation values for July 2095 and 2100 were both very low (see Figure 9.5), with totals less than 50 mm (< 2 in), the 2100 season followed a series of dry months (see Figure 9.5). Specifically, in May 2095, approximately 160 mm (6.3 in.) of precipitation was projected for this cell while projected values for both May and June 2100 had less than 100 mm (3.9 in.) of precipitation. Therefore, the prolonged period of dryness combined with very high temperatures in July 2100 was enough to pass the threshold needed to initiate a substantial fire compared to other years. Fig. 9.2 Output from MC1 showing proportion of cell burned by year

Fig. 9.3 Maximum average monthly temperature input to MC1 (Tmax) Fig. 9.4 Minimum average monthly temperature input to MC1 (Tmin)

Fig. 9.5 Average monthly precipitation input to MC1