Human-started wildfires expand the fire niche across the United States

May combine two claims.

The area dominated by human-started wildfires was about seven times larger than the area dominated by lightning-started wildfires, with human fires dominating 5.1 million km2 (60%) and lightning fires 0.7 million km2 (8%) of the coterminous US.

“dominated an area seven times greater than that affected by lightning fires”

• Also stated in: § Human-Related Ignitions Vastly Expanded the Extent of Wildfire · plain

  “Based on this definition, 5.1 million km2, or 60% of the total land area of the coterminous United States, was dominated by human-started wildfires, whereas only 0.7 million km2, or 8% of the area, was dominated by lightning-started fires.”

• Also stated in: § Discussion · plain

  “Sixty percent of the total land area of the coterminous United States was dominated by human-started wildfires, whereas only 8% of the area was dominated by lightning fires.”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

When lightning-started fires were rare (before May 13 or after September 16), humans ignited 842,289 wildfires, a 35-fold increase over the 24,081 lightning-ignited wildfires in those seasons.

“When lightning-started fires were rare (<5% and >95% quantile; i.e., before May 13th or after September 16th), humans ignited 842,289 wildfires, effectively increasing the number of wildfires 35-fold compared with the 24,081 lightning-ignited wildfires during these spring, fall, and winter seasons.”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Lightning-started fires were found primarily in the intermountain west and Florida and occurred predominantly in summer, reflecting national lightning strike patterns.

“In contrast, lightning-started fires were found primarily in the intermountain west and Florida and occurred predominantly in the summer, reflecting national lightning strike patterns (31) (Fig. 2C).”

• cites [31] Orville RE Huffines GR 2001 Cloud-to-ground lightning in the United States: NLDN results in the first decade, 1989–98 Mon Weather Rev 129 5 1179 1193Not checked yet

  Not checked in the free run.

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

The seasonal trends differed: lightning-caused large fires increased mainly in summer, while human-caused large fires increased mainly in spring.

“Overall trends in lightning-caused fires were primarily driven by increasing numbers of large summer fires (Fig. 5B), whereas overall trends in human-caused fires were primarily driven by increasing numbers of large spring fires (Fig. 5D).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Regions and seasons with moderate to high human-started fire density had median lightning-strike density 0.11 strikes/km2/month and median 1,000-h fuel moisture 17.8%.

“In contrast, regions and seasons of moderate to high human-started fire density (>0.4 fires per 1,000 km2 per month) had a median lightning-strike density of only 0.11 (IQR: 0.025–0.39) strikes per square kilometer per month and a median 1,000-h fuel moisture of 17.8% (IQR: 15.95–19.25%) (Fig. 4B).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Compared with lightning fires, human fires expanded most sharply in the East and in central and southern California.

“Although human-started wildfires were pervasive across the United States (Fig. 1), the expansion of human-started wildfires relative to lightning-started fires was most dramatic in the eastern United States and central and southern California (Figs. 1 and 2C).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Human ignitions dominated the eastern United States and western coastal zones, whereas lightning fires dominated the mountainous West.

“The eastern United States and western coastal areas were dominated by human-started wildfires, whereas lightning-started fires dominated the mountainous regions of the western United States (Fig. 1, Table 1 and Table S1).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Spatially, lightning-caused large fires increased most in the Northwest Forested Mountains ecoregion, while human-caused large wildfires increased most in the Great Plains ecoregion.

“Spatially, lightning-caused fires increased the most in the Northwest Forested Mountains ecoregion (Fig. S4A), whereas human-caused wildfires increased the most in the Great Plains ecoregion (Fig. S4B).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Lightning fires are mainly limited to areas with lightning-strike density above 100 strikes per grid cell per month (0.04 strikes/km2 per month) and with dry fuels below 15% fuel moisture.

“our results show that lightning fires are primarily constrained to areas with a lightning-strike density of greater than 100 strikes per grid cell per month (0.04 strikes/km2 per month) and are concurrent with drier fuels (< 15% fuel moisture) (Fig. 4).”

• AssumptionThe authors state this as an assumption: Lightning-strike density is a valid measure of natural ignition pressure.

May combine two claims.

Fires started by people reach areas that have wetter fuels and greater net primary productivity.

“Human ignitions expand fires into regions with higher fuel moisture (Fig. 4) and higher NPP (Figs. S1 and S2), suggesting that humans create sufficient ignition pressure for wetter fuels to burn.”

• AssumptionThe authors state this as an assumption: Net primary productivity is a valid proxy for fuel amount.

May combine two claims.

Fires started by humans lengthen the fire season earlier in the East and later in the West.

“Human-started fires extend the fire season earlier in the east, and later in the west (Fig. 3 and Table S2).”

May combine two claims.

78% of lightning-started fires occurred in summer months, while 76% of fires started by humans occurred in spring, fall, or winter.

“The vast majority (78%) of lightning-started fires occurred during the summer months, whereas 76% of human-started fires occurred during the spring, fall, and winter months.”

May combine two claims.

The peak day of wildfires in the United States occurs on July 4th, coinciding with Independence Day fireworks displays.

“There was also a notable mark of American culture on the distribution of wildfires, with the peak day of wildfires occurring on July 4th, concurrent with Independence Day fireworks displays (Fig. 2).”

May combine two claims.

Over the past two decades, large fires increased significantly across the United States for both human and lightning ignition sources.

“Over the past two decades, there was a significant increase across the United States for both human- and lightning-caused large fires (Fig. S3).”

The quote only partly states this claim.

The seasonal pattern of human versus lightning fires varies substantially by ecoregion, reflecting an eastern-western dichotomy in the United States.

“Moreover, this trend varies substantially by ecoregion, reflecting again the principle dichotomy between the eastern and western United States (Fig. 3).”

May combine two claims.

Most wildfires in the eastern US that required agency suppression were caused by escaped debris-burning fires, which occurred mainly in spring, or in winter in Texas and the Gulf Coast.

“The majority of the wildfires requiring agency suppression in the east can be attributed to escaped fires from debris burning occurring in the spring months (or winter in Texas and the Gulf Coast) (Fig. 3).”

May combine two claims.

July 4 was the most common day for human-started fires, with 7,762 fires starting on that day over the record, while July 22 was the most common day for lightning-started fires.

“Summed across the 21-y record, the most common day for human-started fires by far was July 4th, US Independence Day, with 7,762 fires starting that day over the course of the record (Fig. 2), whereas, the most common day for lightning-started fires was July 22nd.”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

During 1992-2012, large wildfires ignited by lightning and by humans both showed significant increasing trends (lightning n=4,312, slope=12.2, P=0.001; human n=4,143, slope=3.6, P=0.004).

“During the 21-y time period, there were significant increasing trends in large wildfires ignited by both lightning (n = 4,312; Theil-Sen estimated slope = 12.2; P = 0.001) and humans (n = 4,143; Theil-Sen estimated slope = 3.6; P = 0.004) (Fig. S3).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

The lengthening of the fire season at national scale came from spring human fires in the East and late-summer or autumn human fires in the West.

“This national-scale expansion is driven by earlier (spring) human-started fires in eastern ecoregions coupled with later (late summer or fall) human-started fires in western ecoregions (Table S2).”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Areas and months with moderate to high human-caused fire density had about 40% fewer lightning strikes and nearly 50% higher fuel moisture levels than those with moderate to high lightning-caused fire density.

“Areas and months of moderate to high human-caused fire density had approximately 40% fewer lightning strikes, and nearly 50% higher fuel moisture levels (based on median values) than for moderate to high lightning-caused fire density.”

• AssumptionRests on an assumption: The government agency wildfire records accurately distinguish human- from lightning-caused wildfires.

May combine two claims.

Humans alter fire regimes through three main pathways: changing ignition distribution and density, shifting burning seasonality, and altering available fuels.

“Humans primarily alter fire regimes in three ways: changing the distribution and density of ignitions, shifting the seasonality of burning, or altering available fuels (2, 3).”

• cites [2] Bowman DM 2011 The human dimension of fire regimes on Earth J Biogeogr 38 12 2223 2236 22279247 PMC3263421 sourceNot checked yet

  Not checked in the free run.

• cites [3] Bowman DMJS 2009 Fire in the Earth system Science 324 5926 481 484 19390038 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

In the western United States, evidence indicates that warming and drying, including from human-caused climate change, are associated with more frequent and larger fires and longer fire seasons.

“In the western United States, there is strong evidence that regional warming and drying, including that directly attributed to anthropogenic climate change, are linked to increased fire frequency and size and longer fire seasons (5–9).”

• cites [5] Abatzoglou JT Williams AP 2016 Impact of anthropogenic climate change on wildfire across western US forests Proc Natl Acad Sci USA 113 42 11770 11775 27791053 PMC5081637 sourceNot checked yet

  Not checked in the free run.

• cites [6] Westerling AL Hidalgo HG Cayan DR Swetnam TW 2006 Warming and earlier spring increase western U.S. forest wildfire activity Science 313 5789 940 943 16825536 sourceNot checked yet

  Not checked in the free run.

• cites [7] Westerling AL 2016 Increasing western US forest wildfire activity: Sensitivity to changes in the timing of spring. Philos Trans R Soc Lond B Biol Sci 371(1696): 20150178, erratum in Philos Trans R Soc Lond B Biol Sci 371(1707):20160373 sourceNot checked yet

  Not checked in the free run.

• cites [8] Williams AP Abatzoglou JT 2016 Recent advances and remaining uncertainties in resolving past and future climate effects on global fire activity Curr Clim Change Rep 2 1 1 14 sourceNot checked yet

  Not checked in the free run.

• cites [9] Jolly WM 2015 Climate-induced variations in global wildfire danger from 1979 to 2013 Nat Commun 6 7537 26172867 PMC4803474 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Higher population and housing densities, lower proportions of public land, and more extensive land use and development in the eastern US could lead to more anthropogenic ignitions.

“Some explanations for this distinction include higher population and housing densities, lower proportions of public land, and more extensive land use and development in the eastern United States (33, 34), all of which could lead to more sources of anthropogenic ignitions.”

• cites [33] Radeloff VC 2005 The wildland-urban interface in the United States Ecol Appl 15 3 799 805 35340097 sourceNot checked yet

  Not checked in the free run.

• cites [34] Riitters KH 2002 Fragmentation of continental United States forests Ecosystems 5 8 815 822 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

Recent work for California found that anthropogenic variables explained half of the variability in fire probability over the past four decades.

“Recent work for California confirms the important role of humans, with anthropogenic variables explaining half of the variability in fire probability over the past four decades (30).”

• cites [30] Mann ML 2016 Incorporating anthropogenic influences into fire probability models: Effects of human activity and climate change on fire activity in California PLoS One 11 4 e0153589 27124597 PMC4849771 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

A similar cultural signal has been shown globally, with fewer wildfires on Sundays than on other weekdays.

“A similar cultural mark has also been demonstrated globally with a marked decline in wildfires on Sunday compared with other weekdays (41).”

• cites [41] Earl N Simmonds I Tapper N 2015 Weekly cycles of global fires—Associations with religion, wealth and culture, and insights into anthropogenic influences on global climate Geophys Res Lett 42 21 9579 9589 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

The wildland-urban interface now makes up 9% of US land and is expected to double by 2030, chiefly in the intermountain West.

“Moreover, the current wildland–urban interface, where houses intermingle with natural areas, constitutes 9% of the United States total land area (33) but is projected to double by 2030, predominantly in the intermountain West (49).”

• cites [33] Radeloff VC 2005 The wildland-urban interface in the United States Ecol Appl 15 3 799 805 35340097 sourceNot checked yet

  Not checked in the free run.

• cites [49] Theobald DM Romme WH 2007 Expansion of the US wildland–urban interface Landsc Urban Plan 83 4 340 354 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Fires started by people have pushed the fire niche into places with historically sparse lightning strikes, like Mediterranean California, and into places where lightning and dry conditions rarely coincide, like Eastern Temperate Forests.

“As a consequence, human ignitions have expanded the fire niche into areas with historically low lightning-strike density, such as Mediterranean California, or low concurrence of lightning and dry conditions, such as Eastern Temperate Forests (Fig. 1).”

• AssumptionThe authors state this as an assumption: Lightning-strike density is a valid measure of natural ignition pressure.

Probably not a substantial claim: may not pass S3 (load-bearing).

The large annual swings in human-caused ignitions could indicate the extent to which human decisions shape fire regimes.

“The strong year-to-year variability in human ignitions (Fig. S3 and S4) may reflect the degree to which human choices can affect fire regimes.”

May combine two claims.

Warming trends combined with greater human ignition pressure are raising the count of large human-caused wildfires.

“Moreover, the convergence of warming trends and expanded ignition pressure from people is increasing the number of large human-caused wildfires (Fig. 5).”

May combine two claims.

Even if higher CO2 lessens drought stress via plant responses, warming probably will dry fuels faster and raise risk where people commonly ignite fires.

“Although plant physiological responses to rising CO2 may reduce some drought stress (46), climate change will likely lead to faster desiccation of fuels and increased risk in areas where human ignitions are prevalent.”

• cites [46] Swann ALS Hoffman FM Koven CD Randerson JT 2016 Plant responses to increasing CO2 reduce estimates of climate impacts on drought severity Proc Natl Acad Sci USA 113 36 10019 10024 27573831 PMC5018756 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

Climate change has lengthened the fire-weather season over most of the world, including parts of the US, by roughly two weeks in the last thirty years.

“Observations suggest that climate change has extended the duration of the fire weather season across most of the globe, including parts of the United States by a couple of weeks over the past three decades (5, 9)”

• cites [5] Abatzoglou JT Williams AP 2016 Impact of anthropogenic climate change on wildfire across western US forests Proc Natl Acad Sci USA 113 42 11770 11775 27791053 PMC5081637 sourceNot checked yet

  Not checked in the free run.

• cites [9] Jolly WM 2015 Climate-induced variations in global wildfire danger from 1979 to 2013 Nat Commun 6 7537 26172867 PMC4803474 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Human activities and cultural drivers provide ignitions that substantially alter the distribution of fire across the United States, with examples including road networks, the wildland-urban interface, and agricultural-forest boundaries.

“Increased wildfires can follow road networks (36), the wildland–urban interface (13), and boundaries between agricultural and forested areas (37), highlighting just a few examples of how human activities and cultural drivers provide ignitions that substantially change the distribution of fire across the United States (38).”

• cites [36] Cardille JA Ventura SJ Turner MG 2001 Environmental and social factors influencing wildfires in the Upper Midwest, United States Ecol Appl 11 1 111 127 sourceNot checked yet

  Not checked in the free run.

• cites [13] Syphard AD 2007 Human influence on California fire regimes Ecol Appl 17 5 1388 1402 17708216 sourceNot checked yet

  Not checked in the free run.

• cites [37] McCarty JL Korontzi S Justice CO Loboda T 2009 The spatial and temporal distribution of crop residue burning in the contiguous United States Sci Total Environ 407 21 5701 5712 19647857 sourceNot checked yet

  Not checked in the free run.

May combine two claims.

Projected development growth will raise both ignition pressure and the exposure of new infrastructure.

“This expected development expansion will increase not only ignition pressure, but also the vulnerability of new infrastructure.”

May combine two claims.

Climate warming is projected to reduce fuel moisture and make fire-conducive weather more common, while climate-linked earlier springs are advancing phenology.

“Additionally, projected climate warming is expected to lower fuel moisture and create more frequent weather conditions conducive to fire ignition and spread (45), and earlier springs attributed to climate change are leading to accelerated phenology (42).”

• cites [45] Flannigan M 2013 Global wildland fire season severity in the 21st century For Ecol Manage 294 54 61 sourceNot checked yet

  Not checked in the free run.

• cites [42] Schwartz MD Ahas R Aasa A 2006 Onset of spring starting earlier across the Northern Hemisphere Glob Change Biol 12 2 343 351 sourceNot checked yet

  Not checked in the free run.

Probably not a substantial claim: may not pass S3 (load-bearing).

Over recent decades, warmer and drier conditions beginning earlier have increased fire activity in the western United States.

“In the past few decades, early onset of warmer and drier conditions has promoted greater fire activity across the western United States (6, 7, 40).”

• cites [6] Westerling AL Hidalgo HG Cayan DR Swetnam TW 2006 Warming and earlier spring increase western U.S. forest wildfire activity Science 313 5789 940 943 16825536 sourceNot checked yet

  Not checked in the free run.

• cites [7] Westerling AL 2016 Increasing western US forest wildfire activity: Sensitivity to changes in the timing of spring. Philos Trans R Soc Lond B Biol Sci 371(1696): 20150178, erratum in Philos Trans R Soc Lond B Biol Sci 371(1707):20160373 sourceNot checked yet

  Not checked in the free run.

• cites [40] Dennison PE Brewer SC Arnold JD Moritz MA 2014 Large wildfire trends in the western United States, 1984–2011 Geophys Res Lett 41 8 2928 2933 sourceNot checked yet

  Not checked in the free run.