Wildfire responses to abrupt climate change
in North America
J. R. Marlona,1, P. J. Bartleina, M. K. Walsha, S. P. Harrisonb, K. J. Brownc,d, M. E. Edwardse,f, P. E. Higuerag, M. J. Powerh,
R. S. Andersoni, C. Brilesg, A. Brunelleh, C. Carcailletj, M. Danielsk, F. S. Hul, M. Lavoiem, C. Longn, T. Minckleyo,
P. J. H. Richardp, A. C. Scottq, D. S. Shaferr, W. Tinners, C. E. Umbanhowar, Jr.t, and C. Whitlockg
aDepartment of Geography, University of Oregon, Eugene, OR 97403; bSchool of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United
Kingdom; cDepartment of Quaternary Geology, Geological Survey, Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark; dRoyal
British Columbia Museum, Victoria, BC, Canada V8W 9W2;eSchool of Geography, University of Southampton, Southampton SO17 1BJ, United Kingdom;
fAlaska Quaternary Center, University of Alaska, Fairbanks, AK 99775; gDepartment of Earth Science, Montana State University, Bozeman, MT 59717;
hDepartment of Geography, University of Utah, Salt Lake City, UT 84112; iCenter for Sustainable Environments and kEcological Restoration Institute,
Northern Arizona University, Flagstaff, AZ 86011; jCentre for Bio-Archeology and Ecology (Unite´ Mixte de Recherche 5059, Centre National de la Recherche
Scientifique) and Paleoenvironments and Chronoecology, Institut de Botanique, Universite´ Montpellier 2, 163 Rue Broussonet, F-34090 Montpellier, France;
lDepartments of Plant Biology and Geology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; mDe´partement de Ge´ographie et Centre d’E´tudes
Nordiques, Universite´ Laval, Que´bec, QC, Canada G1V 0A6; nDepartment of Geography and Urban Planning, University of Wisconsin, Oshkosh, WI 54903;
oDepartment of Botany, University of Wyoming, Laramie, WY 82071; pDe´partement de Ge´ographie, Universite´ de Montre´al, C. P. 6128 Centre-ville,
Montre´al, QC, Canada H3C 3J7; qDepartment of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom; rDivision
of Hydrologic Sciences, Desert Research Institute, Nevada System of Higher Education, 755 East Flamingo Road, Las Vegas, NV 89119; sInstitute of Plant
Sciences and Oeschger Centre for Climate Change Research, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland; and tBiology and
Environmental Studies, St. Olaf College, Northfield, MN 55057
Edited by Christopher B. Field, Carnegie Institution of Washington, Stanford, CA, and approved December 29, 2008 (received for review August 19, 2008)
It is widely accepted, based on data from the last few decades and
on model simulations, that anthropogenic climate change will
cause increased fire activity. However, less attention has been paid
to the relationship between abrupt climate changes and height-
ened fire activity in the paleorecord. We use 35 charcoal and pollen
records to assess how fire regimes in North America changed
during the last glacial–interglacial transition (15 to 10 ka), a time of
large and rapid climate changes. We also test the hypothesis that
a comet impact initiated continental-scale wildfires at 12.9 ka; the
data do not support this idea, nor are continent-wide fires indi-
cated at any time during deglaciation. There are, however, clear
links between large climate changes and fire activity. Biomass
burning gradually increased from the glacial period to the begin-
ning of the Younger Dryas. Although there are changes in biomass
burning during the Younger Dryas, there is no systematic trend.
There is a further increase in biomass burning after the Younger
Dryas. Intervals of rapid climate change at 13.9, 13.2, and 11.7 ka
aremarked by large increases in fire activity. The timing of changes
in fire is not coincident with changes in human population density
or the timing of the extinction of the megafauna. Although these
factors could have contributed to fire-regime changes at individual
sites or at specific times, the charcoal data indicate an important
role for climate, and particularly rapid climate change, in deter-
mining broad-scale levels of fire activity.
biomass burning
charcoal
comet
Younger Dryas
I t is generally asserted that anthropogenic climate change willlead to widespread and more frequent fires (1, 2). Data from
western North America in recent decades are consistent with
this; they show that increases in the frequency of wildfire and the
duration of the fire season are linked to increased spring and
summer temperatures and earlier spring snowmelt (3). Changes
in the pattern of precipitation are likewise affecting fire activity
(4), as is the development of high fuel loads associated with
long-term fire suppression (5). The effects of climate variability
on fuels and fire regimes on multiple time scales have received
much attention (6–8), and some research has linked shifts in fire
regimes at individual sites to rapid climate changes (9). However,
the broad-scale response of wildfires to large, abrupt climate
changes in the past has received little attention (10, 11). One
period of particular interest is the last glacial–interglacial tran-
sition (LGIT, 15–10 ka), when large and sometimes abrupt (i.e.,
decades to centuries) changes in climate and biota occurred in
many parts of North America. In some regions, environmental
changes at the beginning and end of the Younger Dryas chro-
nozone (YDC) (12.9–11.7 ka) (12), in particular, were larger
than those at any subsequent time (13). Such changes are similar
in terms of the magnitude and rate of change to those projected
for the future (14–16) and thus provide an opportunity to
examine the response of fire regimes to rapidly changing envi-
ronmental conditions in a variety of settings.
Investigating wildfire activity during the LGIT also allows us
to test the recent proposal that a catastrophic extraterrestrial
impact event at
12.9 ka had ‘‘continent-wide effects, especially
biomass burning’’ (17). Firestone et al. (17) proposed that a
comet exploded over the Laurentide ice sheet, producing a shock
wave that would have traveled across North America at hundreds
of kilometers per hour, and if multiple large airbursts occurred,
could have ignited many thousands of square kilometers. Fire-
stone et al. (17) also hypothesized that the event triggered global
cooling, and that extreme wildfires destroyed forests and grass-
lands and produced charcoal, soot, toxic fumes and ash. These
impacts, in turn, ostensibly limited the food supplies of herbi-
vores, contributing to the extinction of North American
megafauna and forcing major adaptations of PaleoAmericans
(17), although this latter point has been disputed (18).
Even without invoking catastrophic events such as a comet
impact, there are still reasons to expect a broad-scale response
of fire activity in North America to the abrupt climate changes
during the LGIT (19–21). At the beginning of the YDC (12.9
ka), North Atlantic meridional overturning slowed or shut down
(21, 22). This led to abrupt cooling in the circum-North Atlantic
region and general changes in atmospheric circulation around
North America (23–25). Because atmospheric circulation affects
temperature, precipitation and the position of storm tracks (26,
Author contributions: J.R.M. and P.J.B. designed research; J.R.M., P.J.B., M.K.W., S.P.H.,
K.J.B., M.E.E., P.E.H., M.J.P., R.S.A., C.B., A.B., C.C., M.D., F.S.H., M.L., C.L., T.M., P.J.H.R.,
D.S.S., W.T., C.E.U., and C.W. performed research; J.R.M., P.J.B., M.K.W., S.P.H., K.J.B.,
M.E.E., P.E.H., and M.J.P. analyzed data; and J.R.M., P.J.B., M.K.W., S.P.H., K.J.B., M.E.E.,
P.E.H., M.J.P., R.S.A., C.B., A.B., C.C., M.D., F.S.H., M.L., C.L., T.M., P.J.H.R., A.C.S., D.S.S., W.T.,
C.E.U., and C.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. E-mail: jmarlon@uoregon.edu.
This article contains supporting information online at www.pnas.org/cgi/content/full/
0808212106/DCSupplemental.
© 2009 by The National Academy of Sciences of the USA
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27), the particularly abrupt onset of the YDC was registered
across the continent. A large, rapid climate reversal occurred in
regions adjacent to the North Atlantic, whereas more distant
regions registered changes in the progress of the LGIT (19, 28,
29). Other abrupt climate transitions focused on the North
Atlantic, such as the onset of the Bølling–Allerød interval (14.7
ka), or short climatic oscillations, such as the intra-Allerød cold
period (IACP) (
13.2 ka), may also have had continent-wide
impacts on climate.
Large-amplitude, rapid climate change affects fire regimes
directly by altering the patterns of ignition and fire weather (30)
and indirectly through vegetation composition (19, 31, 32), a
major determinant of landscape flammability (33). The nature of
the changes in ignition, fire weather, and vegetation composition
will not be homogenous at a regional scale, but any rapid climate
change, whatever its direction, imposes stress on an ecosystem
and can trigger some change in the fire regime. Stress would
result in increased mortality of the woody vegetation and a
buildup of fuel, for example, as a result of pest outbreaks or
physiological intolerance of new climate extremes (50). The rate at
which such factors affect the fire regime varies, so a broad-scale
change in fire activity would not necessarily exhibit absolute syn-
chroneity, but some change should still be evident at most sites.
Charcoal and pollen from 35 lake-sediment records across
North America [see supporting information (SI) Fig. S1 and
Table S1] were used to assess changes in fire activity (defined
here as biomass burned and fire frequency) and woody biomass
during the LGIT. Variations in charcoal abundance or influx
(particles/cm2/yr) provide a record of past trends in biomass
burning (34–37). Fifteen high-resolution macroscopic charcoal
records (i.e.,50 years per sample and particles100
m) were
further analyzed to reconstruct past fire episodes (defined as 1
or more fires occurring during the time spanned by a charcoal
peak) (36, 38) and charcoal peak magnitude, an assumed metric
of fire size, severity, or proximity (39) (SI Methods). The
proportion of arboreal pollen (AP) in the lake sediments, which
reflects the abundance of tree and shrub taxa on the landscape,
was used to estimate the levels of woody biomass in the vege-
tation at the sites. AP can overestimate tree cover and mask
shifts in trees and shrubs (40), so we consider it only a general
indicator of available woody fuels. Records of charcoal influx,
peak frequency, and AP were used to document trends in
biomass burning (35, 36), fire-episode frequency (hereafter
termed fire frequency), and woody fuel levels. These trends were
compared with ice-core records of CO2 (41) and 18O (21), the
latter clearly illustrating abrupt climate changes, to explain the
broad-scale changes in fire activity.
Results and Discussion
Trends in Fire Regimes and Woody Fuels. The general trend of
charcoal influx across all sites (as represented by a 3-segment
linear regression, Fig. 1C) indicates a significant (P  0.01)
increase in biomass burning until the beginning of the YDC, no
overall change during the YDC, and then a further increase in
biomass burning thereafter (P 0.01). A local regression curve,
which does not assume a specific form for the trend, displays a
similar pattern. The bootstrap confidence intervals around char-
coal influx indicate that these trends are not induced by any
particular record. Inspection of the records (Fig. 2 and Fig. S2),
however, shows that there can be different responses at individ-
ual sites reflecting modulation of the regional-scale response by
local factors. For example, whereas sites 4–9 in southern British
Columbia (BC) all show increasing biomass burning from 15 to
10 ka, spatial patterns are complex in the Pacific Northwest,
Sierra Nevada, and Northern U.S. Rocky Mountains (NRM).
The 3 sites in Alaska (AK) show increasing burning during the
Bølling–Allerød and stable levels during the YDC, but trends are
variable after the YDC. Almost no spatial coherence is evident
in the Southwest, Midwest, and East, although these regions have
limited data. Thus, whereas the composite record strongly
indicates broad-scale trends in biomass burning, heterogeneity is
expected and apparent at local to regional scales.
The overall trend in fire frequency increases during the
Bølling–Allerød (Fig. 1D, Fig. S3) and has no discernable trend
thereafter. Some regions show coherent patterns in fire fre-
quency, including AK (sites 1 and 2), the Pacific Northwest (sites
11, 13, and 14), and the NRM (sites 21–23, and 25) (Fig. S3),
although the nature of the changes naturally differ between
regions. Fire frequency is most variable after 11.7 ka; only sites
21 and 29 show little or no change after that time. In general,
peaks in fire frequency tend to match local maxima in biomass
burning (e.g., at 13.9, 13.1, 12.3, and 11.7 ka).
There are no empirical studies that link the absolute size of
charcoal peaks to a specific fire characteristic, such as area
burned or severity, so the peak magnitudes must be interpreted
with caution (Fig. S3). However, in previous research, unusually
large peaks have been linked to extreme fire years in the
historical record when large areas burned at the regional scale
(42, 43). For example, fires in 1910 that burned 400,000 ha in
the NRM comprised the largest peak of the last 120 years at site
20 (42). Consequently, peak-magnitude data suggest that many
large fire episodes occurred between 15 and 10 ka, and large or
severe fire episodes weremore likely after the end of the YDC than
before it, as for example in the Pacific Northwest (sites 11–13), the
NRM (sites 20, 23–25), and the Southwest (site 27) (Fig. S3). Fire
frequency was also high at most of these sites after the YDC.
The woody biomass trend increases during the Bølling–
Allerød, is stable during the YDC, and decreases thereafter (Fig.
1E). Trends at individual sites again vary regionally and with
elevation (Fig. 2 and Fig. S2). Woody biomass declines at most
sites in BC and increases in the Sierra Nevada, Southwest, and
Northeast. Other regions show mixed patterns. Fire–fuel rela-
tionships among sites also show regional similarities. For exam-
ple, trends in charcoal influx and AP are similar at mid- to
high-elevation sites in the Pacific Northwest and NRM (sites 13,
15, 23, 24, and 25), where biomass burning and woody fuel levels
generally increased together as open forests became more closed
or alpine vegetation was replaced by parkland and then forest
during the LGIT (8). In BC (i.e., at sites 5, 6, 7, 8, and 10), an
inverse relationship in fire and fuels is apparent because biomass
burning increased as closed mixed conifer forests were replaced
by more open forests (44). Charcoal influx is often opposite to
AP in the Midwest as well, where grass abundance (low woody
biomass) is a good predictor of biomass burning (45). Important
changes in woody fuel levels in AK are obscured in the AP
trends, because AP does not show changes in the relative
importance of shrubs versus trees. AP declines at site 3 at 11 ka,
for example, despite a large increase in Populus at that time (63).
Overall, the spatiotemporal variability in woody fuel levels and
biomass burning makes it difficult to generalize about fire–
climate–vegetation linkages at the continental scale, but the role
of climate in determining both woody fuel levels and fire activity
underpins the regional coherence in charcoal–AP relationships.
The AP data do indicate that availability of woody fuels was not
a limiting factor in determining levels of biomass burning at the
beginning or end of the YDC.
Evidence for Continent-WideWildfires at 12.9 ka.Firestone et al. (17)
hypothesized that a comet impact at 12.9 ka  50 y triggered
continental-scale wildfires across NA. One specific example has
been proposed by Kennett et al. (46). However, the well-
documented rapid climate changes of this time alone may have
triggered increased fire at a regional scale. To separate these
effects, we compared the response of fire during intervals of
rapid climate changes at the beginning and at the end of the
YDC. Fire-episode events that occurred during the transitions
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into and out of the YDC were identified in both the high- and
low-resolution records (see Methods) to determine whether fire
episodes, regardless of magnitude, were more likely to occur
(within 50 y) at 12.9 ka than at 11.7 ka (Figs. 1A and 2).
Because of high uncertainties in radiocarbon dating during the
YDC, both 100- and 500-y window widths were used to identify
fire episodes (Fig. 2). By using a 100-y window, 13 sites across the
continent (Fig. 2) showed a peak (or increasing charcoal if no
sample was within the window) at 12.9 ka. The peak was large
(i.e., 90th percentile based on quantile regression) in the 9
low-resolution records, but it was not present in any of the 5
high-resolution records that registered a peak at 12.9 ka (50 y)
(Fig. S3), suggesting that the relatively high magnitude of fires
at 12.9 in the low-resolution sites may be an artifact of the small
number of samples in these records. The data also indicate that
only 3 sites showed a peak only at 12.9 ka, whereas 12 sites showed
a peak only at 11.7 ka, the abrupt end of the YDC (Fig. 2 and Figs.
S1 and S3). Using a large 500-y window width greatly increased the
number of sites recording fires
12.9 ka; however it also increased
the number of fire episodes recorded at 11.7 ka (Fig. 2). It could be
argued that poor dating control on some of the records prevented
identification of fire episodes at 12.9 ka; however, when we limited
our analysis to the 14 records with dates within 300 years of 12.9
ka (Fig. 2), the results did not change. Peaks in charcoal influx were
registered throughout theLGIT, particularly associatedwith abrupt
climate changes, but there was no evidence of continent-wide
wildfires at the beginning of the YDC.
Potential Controls on Fire Regimes and Woody Fuel Levels During the
LGIT. The broad-scale trends in biomass burning, fire frequency
and magnitude, and woody fuels during deglaciation are consis-
tent with climate changes documented by ice cores, marine and
lake sediments, speleothem, and other records from North
America (21, 28, 47, 48). During the Bølling–Allerød, woody
Fig. 1. Reconstructions of biomass burned, fire frequency, and woody biomass levels in North America. (A) The CO2 ice-core record from Antarctica (41). (B)
The NGRIP 18Oice record, a proxy for North Atlantic temperatures (21). (C) Reconstruction of biomass burned based on 35 records; the straight lines are segmented
regression curves, and the smooth curves are local-regression fitted values. (D) Reconstruction of fire frequency based on 15 high-resolution records, expressed
as the density of peaks per site-year. (E) Trends in woody biomass based on 35 records. (F) Number of records contributing to the biomass burning (black) and
woody biomass (green) trends. (G) Number of dates per 50-year interval in the 35 paleo records. Confidence intervals (95%) are based on bootstrap resampling
of sites. Vertical lines mark the beginning (
12.9 ka) and ending (
11.7 ka) of the YDC.
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biomass, biomass burning, and fire frequency all increased (Fig.
1E), a likely consequence of warming and increased tree cover
(40). A stepped increase in biomass burning is evident at 13.9 ka,
coincident with a short period of warming and is matched by a
peak in fire frequency.
A particularly steep increase in charcoal influx occurred at
13.2 ka (Fig. 1C); this is the largest and most rapid change in
biomass burning during deglaciation. Burning was widespread
but not continent wide (see site details in SI Methods). Further-
more, the change in fire regime is not unique: Several sites show
similar peaks before the onset of the YDC, and many show an
even larger peak at the end of the YDC. The widespread increase
in fire activity (i.e., charcoal influx and peak frequency) at 13.2
ka appears
300 years before the hypothesized comet impact
(17). Of the sites that do show fire activity at 13.2 ka, many are
from regions distant from the proposed locus of the impact area
over the Laurentide ice sheet, as well as from the proximal
influence of the ice sheet on regional climates (e.g., in AK, the
Southwest, Pacific Northwest, and the NRM). The timing and
distribution of fire activity at 13.2 ka is consistent with the
IACP—an abrupt short-term climate reversal recorded in the
GISP 18O ice-core data (Fig. 1B). The IACP is associated with
a rapid oscillation in North Atlantic temperatures that may have
affected atmospheric circulation patterns across the continent
(21, 23, 49) and increased the likelihood of drought as well as
severe frost damage on some tree species (50). Any increase in
vegetation mortality associated with such events would have
added to the available fuels and facilitated an increase in fire.
During the YDC, ice-core 18O data indicate cool and variable
temperatures in the North Atlantic region. Cooling is also
evident in parts of western North America based on pollen and
speleothem records (25, 28, 49), but climate patterns likely
varied across the continent (27). The composite records (Fig. 1)
show that biomass burning was higher but more variable than
before 13.2 ka. Fire frequency and biomass burning had local
maxima at
12.3 ka and at the end of the YDC (11.7 ka).
Although there are fundamental and widespread changes in
vegetation at the beginning (and end) of the YDC (19), the
woody biomass trend shows little change during the YDC. This
lack of change does not preclude change in specific regions e.g.,
Alaska (48) or at individual sites.
Biomass burning and fire frequency both decline at 11.7 ka but
increase thereafter. Woody biomass, however, decreases from
11.7 to 10.0 ka. This contrast in behavior marks a shift in the
relationship between fire and vegetation. Before 11.7 ka, woody
biomass and fire activity generally change in parallel; after 11.7
ka, they change in opposite directions. Early-Holocene warming
and enhanced seasonality facilitated the emergence of new
vegetation communities and disturbance patterns (19, 32, 51).
Low-elevation sites in the western US show the biggest changes,
with declining woody biomass as forests became more open (44,
52) and more likely to burn (Fig. S2 and Fig. 2). High-elevation
sites in the Pacific Northwest and NRM also show increasing fire
activity but in association with increasing rather than decreasing
woody fuel levels. New fire–fuel patterns also evolved in the
Northeast after the YDC, with declines in biomass burning
associated with increases in woody biomass.
Factors other than climate may have contributed to observed
changes in fire regimes during the LGIT, including changes in
atmospheric CO2, the arrival of Clovis people between
13.4
and 12.8 ka (53), and the extinction of herbivorous megafauna
(54). Changes in CO2 affect vegetation productivity (55) and
potentially fuel loads. Atmospheric CO2 increased in stepwise
fashion from the Last Glacial Maximum to the beginning of the
Holocene (56) (Fig. 1A). The changes in woody biomass,
fire frequency, and biomass burning are not coincident with
changes in CO2, although increasing CO2 may have contributed
to woody biomass production during the early part of the
Bølling–Allerød. Clovis people appeared in North America
between 13.4 and 12.8 ka, broadly coincident with the sharp
increase in biomass burning at 13.2 ka, and then rapidly spread
out across the continent (18). Paleoindians may have increased
fire activity directly by setting more fires (57) or indirectly by
reducing megafaunal populations. The decline in megafaunal
populations, in turn, could have increased fuel loads and
changed soil moisture regimes, both of which could have pro-
moted fire (58, 59). There is some evidence for an association
between megafaunal declines based on Sporormiella data and
increased burning in the Northeast (58).
The 13.2 ka fire peak is registered at sites widely dispersed
across the continent; it is not consistent with the progressive
colonization of North America by Paleoindians. It also seems
unlikely that people (or megafauna) would have caused an
increase in burning across the full range of elevations repre-
sented by the sites and particularly at high-elevation sites (the
fire peak is evident at 5 sites2,000 m; see SI Methods and Table
S1). Furthermore, most fire records show discrete peaks rather
than permanent regime changes, as might be expected if humans
or megafauna exerted a major control on fire regimes. It is
possible, however, that the arrival of people and/or the extirpa-
tion of megafauna (18, 53, 54) played a role in permanently
altering fire regimes at the sites that show a fundamental
fire-regime shift prior to or at 13.2 ka. After 13.2 ka, fire-regime
changes are not coincident with periods of increase in human
populations. Thus, the spatial and temporal distribution of the
Fig. 2. Site summaries of changes in charcoal influx, arboreal pollen (AP), and charcoal–pollen relationships during the LGIT and of charcoal peaks at the
beginning and end of the YDC. High-resolution site numbers are in bold type and underlined. Regions are identified by alternate shading. A 1 (2) indicates
a general upward (downward) trend in charcoal influx. A  () indicates a positive (negative) relationship between charcoal and AP. Records that had a
radiocarbon or tephra date within 300 years of 12.9 ka are marked by an x in the bottom row.
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fire signal point toward climate as the primary cause of increased
fire activity at 13.2 ka.
In summary, fire records from North America show stepped
increases in biomass burning during the LGIT. Abrupt climate
changes are generally marked by a shift in the level of burning
as well as an increase in the incidence of fires. No continent-wide
fire response is observed at the beginning of the Younger Dryas
chronozone, the time of the hypothesized comet impact. The
results provide no evidence of synchronous continent-wide
biomass burning at any time during the LGIT. The data indicate
variability in the direction of changes in fire regimes among
paleofire records, which may be due in part to noise and local
variability (60), human activity, or megafaunal declines. The dis-
tribution of charcoal peaks across time and space, however, suggests
that such patterns are more likely a result of spatially complex
climate controls and/or vegetation changes.Although there is broad
congruence between changes in climate, fire, and human popula-
tions at the beginning of the YDC, we find no convincing evidence
that the observed changes in fire activity were caused solely by
changes in human or herbivorous megafauna populations.
Methods
We used 30 lake-sediment records in North America from the Global Charcoal
Database (GCD v. 1*†) and 5 records from authors that (i) were recording fire
activity before, during, and after the YDC; (ii) had at least 5 data points and
one date (radiocarbon or tephra) from 10 to 15 ka; and (iii) had pollen data
from the same site. We did not include charcoal data from records that only
sampled the beginning of the YDC because there is no baseline for analyzing
changes in the fire regime with such data (46). We also excluded marine
charcoal data (61) because there is no evidence that charcoal influx and peaks
in influx in such records reflect recent fire activity from a consistent source
region. Pollen data were obtained from authors or from the North American
Pollen Database‡ (Table S1). We examined the chronologies for each record to
ensure that the age–depth relationships were generally consistent through-
out the LGIT and that no age reversals occurred during that interval. Under
such conditions, age controls in lake-sediment records are sufficient to de-
scribe centennial-scale variations (see SI Methods).
For all analyses, charcoal concentration data (particles cm3) were con-
verted to influx values (particles cm2 y1) (see SI Methods). For the low-
resolution records, millennial-scale (background) trends were identified by
smoothing the data by using quantile regression (62). Any increase in charcoal
influx above background within a defined interval (i.e., either 50 or 250
years) was considered a peak. High-resolution records were smoothed by
using a decomposition technique (63) that separates peaks from background
charcoal and allows the reconstruction of peak magnitude and fire frequency.
Arboreal pollen proportions were obtained by dividing the sum of arboreal
and shrub pollen percentages (AP) by the sum of the total terrestrial pollen
percentages [AP/(AP  NAP)].
To display the general trends in the charcoal influx, the data were trans-
formed to stabilize the variance and standardized to facilitate comparisons
across a range of charcoal influx levels (37). To assess the significance in the
trend, we fit a segmented linear regression model to these data, with break-
points at the beginning and end of the YDC (see SI Methods). We also
summarized the data by using ‘‘lowess’’ or local regression curves. Confidence
intervals for the local regression curves were generated by a bootstrap ap-
proach in which individual records (not samples) were sampled with replace-
ment over 1,000 replications. The approach reveals the sensitivity of the trends
to the particular selection of charcoal and pollen records used here. Pollen
data were also transformed (64) and summarized by using local regression
curves. The peak frequency trends in the high-resolution records were sum-
marized by a local-density (kernel smoothing) procedure.
ACKNOWLEDGMENTS. We thank Jack Williams for providing pollen data,
Jake Bartruff for map assistance, D. Burney, D. Gavin, D. J. Meltzer, and D. K.
Grayson for discussions, and 2 anonymous reviewers for comments that
greatly improved the manuscript. This work was supported by U.S. National
Science Foundation Paleoclimatology and Geography and Regional Science
Program Grants ATM-0117160, ATM-0714146, and BCS-0727424. The charcoal
data are included in Global Charcoal Database Version 2 compiled by the
Global Palaeofire Working Group (GPWG) of the International Geosphere–
Biosphere Cross-Project Initiative on Fire. The GPWG is supported by the U.K.
Natural Environment Research Council’s QUEST (Quantifying Uncertainty in
the Earth System) Program. Data compilation and analysis was supported by
the QUEST-Deglaciation Project (M.P. and S.P.H.).
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