Results

This section begins with views of the current climate in terms of VPD, precipitation, wildfire hazard and insured loss as captured by the Verisk Wildfire Model for the United States. Future projections of these variables follow.

Near-present risk

Present-day climate in terms of mean May-to-October VPD and total annual precipitation for all ecoregions in the continental U.S. are presented in Mean May-to-October vapor pressure deficit (VPD) for each ecoregion in the continental U.S. in the current climate (around 2018). and Total annual precipitation for each ecoregion in the continental U.S. in the current climate (around 2018). respectively. Mean May-to-October vapor pressure deficit (VPD) for each ecoregion in the continental U.S. in the current climate (around 2018). shows that during May to October the Western U.S. is most arid in south California, south Nevada, and southwest Arizona. The southwest region along the U.S.-Mexico border is highly arid in general and this lessens towards the north, northwest and northeast. Total annual precipitation for each ecoregion in the continental U.S. in the current climate (around 2018). shows that based on total annual precipitation, the northwest coastline and southeast regions tend to be the wettest. Precipitation tends to anti-correlate with VPD with south California, Nevada, Arizona, and New Mexico having the lowest annual rainfall.

Figure 1. Mean May-to-October vapor pressure deficit (VPD) for each ecoregion in the continental U.S. in the current climate (around 2018). .
Data derived from GridMET. Model domain is denoted by black line.

Figure 2. Total annual precipitation for each ecoregion in the continental U.S. in the current climate (around 2018)..
Data derived from GridMET. Model domain is denoted by black line.

Average annual burn fraction (%) for each ecoregion in the domain of the Verisk Wildfire Model for the United States . presents the average annual burn fraction (average annual burn area per ecoregion area) for each ecoregion based on the U.S. Wildfire 10,000-year catalog. Ecoregions with highest burn fraction (greater than 3%) are in California, particularly southern California, Utah, southeastern Oregon, northern Nevada, and Arizona. For most of these ecoregions, their historical burn fraction is explained by VPD changes but for ecoregion 12 precedent precipitation is the better explanatory variable, while for ecoregion 80 both VPD and precedent precipitation are required. A summary of which climate variables best explain historical burn fractions in each ecoregion is presented in Climate-fire regimes for each ecoregion in the domain of the Verisk Wildfire Model for the United States: direct (May-October VPD only), facilitative (precedent precipitation only), and mixture (both VPD and precipitation). The results are based on the regressions performed for the statistical models. Ecoregions are assigned to one of three regimes: direct if VPD best explains burn fraction, facilitative if precedent precipitation is the best predictor, and mixture if both VPD and precedent precipitation are necessary to best explain the burn fraction. The sensitivity of burn fraction to climate is illustrated by the magnitude of the highest β regression parameter, presented in Climate-fire sensitivity based on the magnitude of the highest β regression parameter in the statistical model for each ecoregion of the Verisk Wildfire Model for the United States domain., where higher values indicate greater sensitivity.

The Southern Rockies of Colorado (ecoregion 21) and Klamath Mountains of California (ecoregion 78) have relatively high burn fractions, greater than 1%, and are highly sensitive to VPD. Other ecoregions with relatively high burn fractions that are highly sensitive to VPD are the Middle Rockies (17), Canadian Rockies (41) and Idaho Batholith (16). These ecoregions are also heavily forested.

Other ecoregions that are highly sensitive to VPD include Coast Range (1), along the northwest coastline, the Cascades (4) which lies across Washington, Oregon, and northern California, and the Southern California Mountains (8). Coast Range (1) has historically had a relatively low burn fraction (less than 1%) while the Southern California Mountains (8) has the highest burn fraction in the model domain, greater than 5%.

Ecoregions where climate has a direct influence, through VPD, tend to have higher forest coverage. The converse is also true: facilitative ecoregions are either desert (ecoregions 14 and 81) or agricultural land (7 and 12). These ecoregions are weakly sensitive to precipitation changes.

Figure 3. Average annual burn fraction (%) for each ecoregion in the domain of the Verisk Wildfire Model for the United States ..
Average annual burn fraction is the average annual burn area per ecoregion area. Data derived from the 10,000-year catalog. Model domain is denoted by black line.

Figure 4. Climate-fire regimes for each ecoregion in the domain of the Verisk Wildfire Model for the United States: direct (May-October VPD only), facilitative (precedent precipitation only), and mixture (both VPD and precipitation)

Figure 5. Climate-fire sensitivity based on the magnitude of the highest β regression parameter in the statistical model for each ecoregion of the Verisk Wildfire Model for the United States domain.

How hazard risk translates into loss risk depends on exposure and its vulnerability. Average annual loss (AAL) for each county in the Verisk Wildfire Model for the United States domain based on the 10,000-year catalog and Average annual loss (AAL) for each state in the Verisk Wildfire Model for the United States domain based on the 10,000-year catalog show the industry AAL by county and state respectively based on the 10,000-year catalog. Counties with the highest AAL are in southern California in San Bernadino, Riverside (Palm Springs) and Los Angeles. AAL is moderately high in southern Arizona (Phoenix), central California (Sacramento) and western Nevada (Reno). Risk is also notable in Texas and Washington.

California contributes around two-thirds of the overall AAL estimated by the model. Arizona contributes 7% and Texas, Washington, and Colorado contribute about 4% each. In these states, some ecoregions are highly sensitive to VPD, most notably ecoregions in northern and southern California, central and western Colorado, and central and western Washington. Although Montana has the third lowest AAL of all states, its losses are concentrated in the west near ecoregions 4 and 17 that are highly sensitive to VPD. Ten ecoregions that contribute the most to the industry average annual loss (AAL), including their AAL contribution (%) and fire-climate sensitivity (highest β regression parameter in the statistical model) displays the top ten ecoregions that have the highest contributions to the industry AAL and their fire-climate sensitivity as defined by the highest β regression parameter in the statistical model. Seven of the top ten are fully or partially in California. Ecoregions 8 (California) and 21 (Colorado) have the highest fire-climate sensitivity of the top ten.

The projected impact of climate change on modeled wildfire hazard and loss risk is presented in the following section.

Table 1. Ten ecoregions that contribute the most to the industry average annual loss (AAL), including their AAL contribution (%) and fire-climate sensitivity (highest β regression parameter in the statistical model)
Rank	Ecoregion	Name	AAL (%)	Fire-climate sensitivity (-)
1	85	Southern California/Northern Baja Coast	27%	2.3
2	6	Central California Foothills and Coastal Mountains	17%	7.2
3	8	Southern California Mountains	9%	13.7
4	5	Sierra Nevada	7%	7.9
5	81	Northern Basin and Range	5%	2.5
6	10	Columbia Plateau	3%	1.1
7	13	Central Basin and Range	3%	4.8
8	21	Southern Rockies	3%	12.3
9	23	Arizona/New Mexico Mountains	2%	5.2
10	14	Mojave Basin and Range	2%	2.4

Figure 6. Average annual loss (AAL) for each county in the Verisk Wildfire Model for the United States domain based on the 10,000-year catalog

Figure 7. Average annual loss (AAL) for each state in the Verisk Wildfire Model for the United States domain based on the 10,000-year catalog