Introduction
Proxy records of biomass burning are important for understanding the
relationship between climate variability and fire on long timescales. A
diverse range of paleo-proxies has been used to reconstruct biomass burning,
including fire scars on tree rings, sedimentary charcoal records, and ice core
records of gases and aerosol-borne chemicals . These records reflect a wide
range of different aspects of fire location, frequency, distribution, and
intensity. The proxies also integrate over a wide span of spatial and
temporal scales. Despite these efforts, there is not yet a coherent global
picture of historical variations in biomass burning and their relationship to
climate over the past few millennia.
Northern Hemisphere burning trends on centennial timescales have been
developed using composites of terrestrial sedimentary charcoal records
. These suggest that Northern Hemisphere burning declined
during the Common Era, from about 1 to 1750 CE, then increased until 1870 CE.
During the industrial period, burning rose from 1900 to 1950, declined from
1950 to 2000, and rose again after 2000 CE . This composite
record reflects conditions in Europe and North America, with few records from
northern Asia. Greenland ice core ammonium, black carbon, and levoglucosan
provide evidence for elevated North American burning prior to 500 CE and from
1000 to 1200 CE, although these records are not entirely consistent with each
other .
A range of ice core proxies has been used to reconstruct biomass burning
. Stable isotope ratios of ice core methane
(δ13CH4) have been used to infer global biomass burning, using
end member isotopic compositions of methane sources . Elevated ammonium concurrent with elevated levels of
other chemicals has been used to reconstruct regional biomass burning. The
difficulty of using ice core ammonium is that it is derived from several
sources . Ice core black carbon has been used as a tracer for
pre-industrial burning. Differences between these records could be due to
variability in combustion conditions and transport . The
incomplete combustion of biomass produces organic aerosols. A large
percentage of the biomass-burning-derived organic aerosol is composed of
levoglucosan, which is emitted by all plant matter containing cellulose
. However, levoglucosan has the potential for rapid
degradation during atmospheric transport .
Aromatic acids in ice cores have also been used as ice core proxies for
regional variability in biomass burning . These compounds
are produced from the pyrolysis of lignin and they retain the basic structure
of the lignin backbone . These compounds have been
extensively studied in laboratory burning studies and atmospheric aerosols
. These studies
have shown that North American and European conifer and deciduous tree
species produce vanillic acid (VA) . VA is observed in atmospheric aerosols in the Arctic and Antarctic
after long-distance transport . The lifetime
of aromatic acids in the gas phase is on the order of a day due to oxidation
by the hydroxyl radical. However, modeling studies suggest that in aerosols
these compounds may be shielded from oxidation, resulting in atmospheric
lifetimes of several days . Laboratory and field studies
have also shown decreased volatility of low molecular weight organic acids in
aerosol form as a result of interactions with sea salt and other cations.
Such studies have not yet been carried out on aromatic acids
.
Until recently, relatively few measurements of these types of compounds have
been made on ice cores. Recent studies include measurements of vanillic and
para-hydroxybenzoic acid (p-HBA) on cores from Greenland, Switzerland, the Kamchatka
Peninsula, and Svalbard . The first
continuous, millennial timescale ice core records of VA and p-HBA from
Siberia and Svalbard have recently been published .
In this study, VA was analyzed in an ice core from the Tunu region in
northeastern Greenland (Fig. ). The ice core samples range in age
over the past 1700 years (268–2013 CE) and this study presents the first
continuous multi-centennial timescale ice core record of VA from Greenland.
The Tunu record is compared to previous records of ammonium and black carbon
from Greenland, to sedimentary charcoal records from likely source regions,
and to patterns of historical climate change.
Tunu (78.04∘ N, 33.88∘ W; this study), the North Greenland
Eemian Ice Drilling (NEEM) (77.49∘ N, 51.2∘ W),
GISP2 (72.2∘ N, 37.8∘ W), 20D (65.01∘ N, 44.87∘ W), Eclipse Icefield
(60.51∘ N, 139.47∘ W), and Mt. Logan (60.58∘ N, 140.58∘ W) ice core drilling
sites .
Methods
Ice core dating and sample collection
The Tunu ice core was drilled in 2013 to a depth of 212 m (Fig. ;
78.04∘ N, 33.88∘ W; 2000 m above sea level). The mean annual
temperature at the Tunu-N weather station is -27.5 ∘C
78.02∘ N, 33.99∘ W; 2113 m a.s.l.;. The ice core site has an average accumulation rate of
0.100 m w.e. yr-1 . The Tunu ice core age
scale is based on the annual layer counting of non-sea-salt sulfur and sodium
signals Fig. S1 in the Supplement;. The Tunu timescale agrees to within
±2 years with the North Greenland Eemian Ice Drilling (NEEM) project (NEEM-2011-S1) ice core based
on comparison of the ages of major volcanic events .
Samples for this study were obtained from melting of a 33×33 mm cross
section of the core . Samples of the
melt stream were collected continuously via fraction collector. Each sample
has a time resolution of roughly 2.5 years.
Ice core sample analysis and data processing
VA and p-HBA were analyzed by
anion exchange chromatographic separation and tandem mass spectrometric
detection with electrospray ionization in negative ion mode (IC-ESI-MS/MS)
. The experimental method is described in detail
in . The experimental system is a Dionex AS-AP autosampler,
ICS-2100 integrated reagent-free ion chromatograph, and ThermoFinnigan TSQ
Quantum triple quadrupole mass spectrometer. VA and p-HBA were detected at
mass transitions of m/z167→108 and m/z137→93,
respectively. Synthetic external standards, ranging in concentration from
0.1 to 2 ppb, were prepared using reagent-grade VA and p-HBA in Milli-Q water.
These standards were analyzed in sequence with ice core samples. The
retention times of VA and p-HBA were 11.1 and 11.8 min,
respectively, with peak width half heights of 0.4 min. Detection limits
for VA and p-HBA were 0.005 and 0.034 ppb, defined as 3 times the standard
deviation of Milli-Q water blanks (n=58). Overall, 546 ice core samples from the Tunu
ice core were analyzed in this study.
Tunu ice core VA ranges from below detection to 0.080 ppb, which is about
15 times the detection limit (Fig. ). The distribution of the measurements
was skewed strongly towards lower concentrations, with VA levels below
detection in 40 % of the samples. The VA measurements below detection were
replaced in the data set with a value of one-half the detection limit, or
0.0025 ppb. The geometric mean VA level for the whole data set was
0.005-0.003+0.006 ppb.
Tunu ice core VA record. The horizontal lines show the Roman Warm Period (RWP),
Late Antique Little Ice Age (LALIA), Medieval Climate Anomaly (MCA), and the Little Ice Age (LIA)
. The arrow indicates the limit of detection. All data below the limit of
detection are treated as half of the limit of detection. Six measurements above 0.03 ppb not shown in
this figure are shown in Fig. .
In analyzing the Tunu vanillic acid data set, a number of different methods
were examined, including (1) bin averaging of the log-transformed data
, (2) LOESS smoothing , and (3) peak
detection using singular spectrum analysis . The
trends discussed here are robust in the sense that they are evident
regardless of the data analysis method used. For example, similar patterns of
centennial variability emerge from all of the methods (Fig. S2).
Although p-HBA is present in many samples, the levels are not reported due to
interference at the m/z137→93 mass transition eluting at nearly
the same retention time (Fig. S3). This peak was present in most of the ice
core samples, several of the 58 Milli-Q water blanks, and standards. The
detection limit for p-HBA was determined using blanks that did not show this
contamination. The presence of this peak in blanks and standards suggests
that it is a contaminant that was introduced locally during sample handling.
This peak was not observed in previous analyses of p-HBA in other Arctic ice
cores .
Air mass back trajectory analysis
Air mass back trajectory studies indicate that North America is the major
source region for aerosols transported to the central Greenland ice sheet
. Here, we examine transport from potential biomass burning
source regions to the Tunu ice core site (78∘ N, 34∘ W)
during the boreal burning season. The 10-day air mass back trajectories were run
from the Tunu ice core site at 00:00 LT and 12:00 LT (UTC–2)
beginning 100 m above ground level. Back trajectories were computed each day
for spring (1 March–31 May), summer (8 June–31 August), and fall (1 September–30 November).
The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to compute the trajectories using
National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) meteorological data for the 10-year period (2006–2015)
. Possible source regions were
partitioned into ecofloristic zones in North America, Europe (defined as west
of 42∘ E), and Siberia (defined as east of 42∘ E). Food and
Agriculture Organization classifications were used to define ecofloristic
zones in each of these regions Fig. S6;
http://cdiac.ornl.gov/epubs/ndp/global_carbon/carbon_documentation.html, last access: 8 November 2016;.
Results and discussion
Tunu vanillic acid time series
Tunu VA levels do not exhibit a significant linear trend over
the past 1700 years. They do exhibit pronounced variability on centennial
timescales (Fig. ). This variability is observable in the raw
data as multidecadal to century-long periods in which all of the
measurements were above the detection limit. The variability is more clearly
shown in the 40-year bin averages of the log-transformed data (Fig. ). There is a maximum early in the record (280–400 CE) during the
Roman Warm Period (RWP; 550 BCE–350 CE), followed by a period of generally
lower levels from 500 to 1000 CE. A second maximum occurred during the Medieval
Climate Anomaly (MCA; 1080–1240 CE) followed by generally declining levels
through the Little Ice Age (LIA) and into the 20th century. There is
shorter-term (decadal or multidecadal) variability throughout the record. A
peak during the latter half of the 20th century (1955–1985) stands out
as the largest example of such variability in the record (Figs. , ).
Tunu ice core VA record. The gray-filled lines are ±1 standard error of
the 40-year bin averages of the data. The horizontal lines show the Roman Warm Period (RWP), Late
Antique Little Ice Age (LALIA), Medieval Climate Anomaly (MCA), and the Little Ice Age (LIA)
. Light gray shaded areas are elevated periods in the record.
The 20th century Tunu ice core VA record.
Burned area in the boreal forest of North America was high in the early
1900s, declined into the middle of the 20th century, and began to
increase again beginning in the 1960s . The short-term
period of elevated Tunu VA from 1955 to 1985 does not mirror the trend in this
burned area record. A record of large boreal wildfires in Canada, with burned
areas exceeding 200 ha, shows periods of large wildfires around 1960, the
late 1970s to the early 1980s, around 1990, and the mid-1990s
. The similarity between the Tunu VA record and large boreal
fire record from Canada suggests that 20th century VA record may be
showing large fires. These large fires represent 3.1 % of the number of
Canadian fires from 1959 to 1997, but 97 % of the area burned .
We also examined variability in the Tunu VA record by analyzing the frequency
of occurrence of highly elevated VA levels. This “peak detection” approach
has been used to extract fire proxy signals from sediment charcoal and ice
core ammonium records . To apply this
method, we used singular spectrum analysis (SSA) to decompose the Tunu VA
record into 30 principle components (PCs). The PCs were converted to
reconstructed components (RCs). RC 1 contains most of the low-frequency
content of the Tunu VA record. This component shows similar centennial-scale
variability to the 40-year bin-averaged data, with elevated VA early in the
record and around the MCA. It also emphasizes the decreasing trend from
1200 to 1900 CE and the increase during the 20th century (Fig. a).
Tunu ice core vanillic acid analyzed by singular spectrum analysis (SSA) and peak detection
showing centennial-scale variability. (a) Low-frequency variability in Tunu vanillic acid reconstructed
using component 1 of the SSA; (b) higher-frequency variability in Tunu vanillic acid reconstructed
using SSA components 2–30, excluding component 1; (c) peaks detected in the higher-frequency Tunu
vanillic reconstruction (components 2–30), shown as the fraction of ice core samples exceeding a peak
threshold (75th percentile) in a 40-year running window. The horizontal lines at the top show the
Roman Warm Period (RWP), Late Antique Little Ice Age (LALIA), Medieval Climate Anomaly (MCA), and
the Little Ice Age (LIA) .
The higher-frequency content of the VA record was reconstituted by summing
RCs 2–30 (Fig. b). Individual peaks in the
reconstituted record were detected as samples with VA levels exceeding a
specified threshold. The threshold was defined as a percentile of the
reconstituted data and peak frequency (peaks yr-1) computed in a
moving 40-year window. Peak thresholds of the 65th, 70th, and 75th
percentiles were used. The results are insensitive to the choice of threshold
in this range (Fig. c). The peak detection algorithm
shows strong centennial-scale variability, in general agreement with the
40-year bin averaging. The amplitudes of the centennial-scale features are
relatively constant until the onset of the Little Ice Age, after which they
appear to decline. At face value, the peak detection algorithm suggests that
these centennial-scale changes in Tunu VA involved not only variations in the
abundance of VA but also changes in the frequency of large fire events. In
theory, comparing peak detection and bin-averaging signals could
differentiate between biomass burning aerosols from distant fires that might
be well mixed throughout the Arctic atmosphere, as compared to large episodic
inputs from major fire plumes in the immediate source regions impacting the
site.
Relationship between Tunu vanillic acid levels and accumulation rate
Interpretation of ice core vanillic acid levels in terms of atmospheric
aerosol concentrations or fluxes requires consideration of the processes by
which aerosols are incorporated into the polar ice sheet. The concentrations
of ions in polar ice cores likely reflect changes in the composition and
abundance of the overlying aerosols but may also be influenced by
depositional and post-depositional processes that affect the air–snow
transfer function . We examined the relationship between VA
and snow accumulation rate in order to assess the influence of variations in
local conditions on Tunu ice core VA levels. VA flux was computed as the
product of accumulation rate and VA concentration (F=CiceAH2O
ρice). VA flux exhibits the same major features observed in the
concentration record, indicating that the variations in VA likely reflect
changes in atmospheric composition and are not caused primarily by changes in
snow accumulation (Fig. ). Excluding the 20th century, there is a
positive correlation between VA flux and accumulation rate (r2=0.25, p<0.001), with a slope of 0.011±0.001×10-9 kg m-2 yr-1 VA/kg m-2 yr-1 water flux (Fig. ). This positive
relationship is typical of ice core impurities and may be interpreted in the
context of a simple model of dry/wet deposition, such as
Ftotal=Fdry+Fwet=CairVd+CairRscavPH2O,
where Ftotal is the total VA flux, Fdry is the VA dry deposition
flux, Fwet is the VA wet deposition flux, Cair is the concentration
of VA in the atmosphere, Vd is a dry deposition velocity for the
VA-containing aerosols, Rscav is the wet deposition scavenging ratio,
and PH2O is the snow precipitation rate .
Tunu ice core vanillic acid concentration (a), water accumulation flux (b), and
vanillic acid depositional flux (c) computed as the product of vanillic acid concentration
and water accumulation flux. The horizontal lines at the top show the Roman Warm Period (RWP),
Late Antique Little Ice Age (LALIA), Medieval Climate Anomaly (MCA), and the Little Ice Age (LIA) .
For a chemically stable, non-volatile species with variations in both
atmospheric concentration and snow precipitation (that are not highly
correlated), one would expect to observe (1) a positive correlation between
the depositional flux and water accumulation, and (2) a positive y intercept
reflecting the dry deposition component. A negative intercept could be
interpreted as evidence of re-volatilization of VA from the snowpack, but the
magnitude of this effect is evidently small.
During the 20th century, there is little change in accumulation rate
associated with the very large increase in VA. This confirms that the 20th
century increase is likely a real atmospheric feature and not an artifact
related to local depositional conditions. If re-volatilization does occur
after deposition, one would expect that loss of methoxy aromatic acids may
increase with increased acidity. In this record, the opposite appears to be
the case. Elevated levels of VA generally coincide with elevated levels of
acidity, sulfate, and, to a lesser degree, nitrate during the 20th century
(Figs. S4; S5). Increased acidity therefore does not appear to have induced
significant loss of vanillic acid from the Tunu ice core.
Relationship between Tunu vanillic acid depositional flux and water accumulation flux
with linear regression to samples with ages older than 1900 CE (black points and line). Data
younger than 1900 are shown in orange. Gray dashed lines show the uncertainty in the slope and
intercept. Data below the limit of detection are not included.
Percentages of air mass back trajectories crossing over or reaching various geographic regions and ecofloristic
zones beginning at the Tunu drilling site (percent rounded to the nearest integer). Ecofloristic zones are defined using Food and
Agriculture Organization classifications Fig. S2; http://cdiac.ornl.gov/epubs/ndp/global_carbon/carbon_documentation.html,
last access: 8 November 2016;.
Season
Geographic region
Ecofloristic zone
Spring
Summer
Fall
North America
Boreal tundra woodland
10
3
14
Boreal coniferous forests
6
2
8
Other
16
12
15
Total
32
17
37
Europe
Total
4
2
4
Siberia
Total
3
1
4
Potential source regions for Tunu vanillic acid
The back trajectories confirm that aerosols reaching the Tunu site
predominantly originated in North America. North American trajectories
comprised 32 %, 17 %, and 37 % of the total for spring, summer, and fall,
respectively (Table ; Fig. S7). The most commonly transected
North American ecofloristic zones were boreal tundra woodland and boreal
coniferous forests. Trajectories from North American ecofloristic zones were
more common in the fall and spring than in the summer. The percentages of
trajectories from European and Siberian ecofloristic zones were all lower
than 3 % and 2 %, respectively. The remainder of the trajectories
originated over the oceans and did not transect North American, European, or Siberian
ecofloristic zones. These results do not preclude wildfire emissions from
Europe or Siberia reaching the Tunu ice core site but suggest that the
frequency of such events is much lower than that for North American events.
This result is consistent with , an earlier study of air mass
transport to Summit Station, Greenland, over a 40-year period (1946–1989).
Comparison to the NEEM Greenland ice core record of ammonium and black carbon
In order to develop confidence that ice core proxies of biomass burning are
broadly representative of large-scale regional paleofire trends, it is
important to demonstrate similar historical patterns from ice cores in the
same geographic region and influenced by similar air mass trajectories.
Levoglucosan and black carbon in the NEEM ice core from northern Greenland
are elevated from 200 to 600 and 100 to 700 CE, respectively (Fig. S8)
. These periods overlap the period of elevated Tunu VA from
280 to 400 CE. They are still elevated 200–300 years after the Tunu VA record
has declined. There is also an overlapping peak in the GISP2 ice core
ammonium record from 320 to 330 CE . NEEM levoglucosan, black
carbon, and ammonium are also elevated from 1000 to 1200, 1000 to 1600, and
1200 to 1500 CE, respectively, at about the same time as the peak in the Tunu VA
record from 1080 to 1240 CE . This peak is slightly
earlier than a period of elevated ammonium, oxalate, and potassium in an ice
core from the Eclipse Icefield in western Canada from 1240 to 1410 CE
. Periods of elevated burning in Greenland and Canadian ice
core records after 1240 CE are not pronounced in the Tunu VA record. These
periods include elevated NEEM levoglucosan from 1500 to 1700 CE, 20D and GISP2
ammonium from 1790 to 1810 and 1830 to 1910 CE, and periods of elevated burning
in the Mt. Logan and Eclipse ice cores from western Canada in the 18th–20th
centuries .
To further examine these relationships, we compare Tunu VA with ammonium and
black carbon measurements on the NEEM ice core from North Greenland
. Air mass trajectories indicate that the NEEM site should
experience similar transport to Tunu, with eastern Canada as the major fire
source region, followed by western Canada . We first compare
Tunu VA and NEEM black carbon using the first principle component of a
singular spectrum analysis of each record (RC 1; Fig. ). NEEM
ammonium was not included in this analysis, as it is believed that long-term
variations in ammonium levels reflect biogenic rather than pyrogenic
emissions .
Relationships between biomass burning proxies in two north Greenland ice cores for the
past 1700 years: Tunu ice core VA, NEEM ammonium (NH4), and NEEM black carbon
(BC). (a) First component from the singular spectrum analysis of the three ice core signals
(RC 1),
(b) frequency of peaks in the ice core signals reconstructed using singular spectrum components 2–30
and peak threshold of the 75th percentile, smoothed with a 40-year running window, and (c) 95 % confidence
intervals of correlation coefficients for the ice core peak frequencies using a 200-year running
window (p<0.001). The horizontal lines at the top show the Roman Warm Period (RWP), Late Antique
Little Ice Age (LALIA), Medieval Climate Anomaly (MCA), and the Little Ice Age (LIA) .
There are some similarities between Tunu VA and NEEM black carbon. Both
records are low prior to the MCA and during the LIA, and both records are
elevated during the RWP and MCA. There are also notable differences. For
example, vanillic acid declines sharply into the Late Antique Little Ice Age
(LALIA) around 450 CE, while black carbon and ammonium remain relatively high
until 700 CE. Another difference is that black carbon reaches its LIA minimum
around 1750 and begins to rise thereafter. By contrast, VA continues its
decline, reaching its minimum around 1900 CE. The divergence of these proxies
during the industrial period is not surprising, as black carbon is emitted
from fossil fuel combustion while VA is not. The divergence between VA and
black carbon during the industrial period was previously observed in a
shallow central Greenland ice core .
We next used the peak detection method described earlier in Sect. 3.1 to
examine the relationship of Tunu VA with NEEM ammonium and black carbon. A
peak threshold of the 75th percentile was used for all records. Increasing
the threshold from the 80th to 95th percentile reduces the number of peaks but
does not significantly alter their timing. For the data set as a whole, all
three of the fire proxies are positively correlated with one another
(p<0.001). Visual inspection of the results shows that the relationships
between Tunu VA, NEEM ammonium, and NEEM black carbon are complex, with
periods of both positive and negative correlation. In order to better
illustrate the relationships, we computed the correlation coefficient between
the three pairs of proxies in a 200-year moving window (Fig. a). The results show a strong positive correlation for all of the
records from 650 to 1200 CE but not subsequently. This abrupt change in
relationship occurs around the same time as a major change in North Atlantic
climate discussed below (see Sect. 3.6).
Comparison to charcoal records
We compared the Tunu ice core VA record to sedimentary charcoal records from
North America. All charcoal records from Canada available in the Global
Charcoal Database (106 records; 40–80∘ N,
10–160∘ W) were analyzed using the paleofire R package
http://gpwg.paleofire.org, last access: 20 April 2017;. Regional Canadian
charcoal records were normalized using the Box–Cox, min–max, and z-score
transformations for 200–2000 CE, and composited using 40-year bin averages.
The Canada charcoal record exhibits neither a linear trend nor century-scale
variability that is significant at the 95 % confidence interval (Fig. S9).
Canadian charcoal records show a low near 1600–1700 CE in the LIA and a high
centered around 1000 CE early in the MCA. In the VA record, the high is
around 1100–1200 CE and there is a gradual decline that reaches a minimum
from 1800 to 1900 CE. While both records show a decline from the MCA to LIA, the
timing does not match and there seems to be offset of 100–200 years between
the maxima and minima in the two records. We also examined composite records
for a number of smaller regions within Canada in an effort to identify
variability similar to that in Tunu VA (Fig. S10). These regions are also not
significant at the 95 % confidence interval. Only the records from western
Canada (40–80∘ N, 110–180∘ W) show any
similarity to the Tunu VA record, with a slight increase around 1400 CE. The
lack of significant correlation between charcoal records and Tunu VA can in
part be due to uncertainties in chronologies as charcoal record chronologies
generally have much larger uncertainties than ice cores.
Relationship to climate
Over the past two millennia, Northern Hemispheric climate has been modulated
by several centennial-scale climate anomalies: the Roman Warm Period or Roman
Climate Optimum 550 BCE–350 CE;, the Dark Ages Cold Period
400–765 CE;, or Late Antique Little Ice Age
536–660 CE;, the Medieval Climate Anomaly 950–1250 CE;, and the Little Ice Age 1400–1700 CE;. The
regional climate variability and dynamics associated with these anomalies are
not well understood, particularly for the earlier Roman Warm Period and Late
Antique Little Ice Age. During the Roman Warm Period, climate proxies from
the North Sea, the Qinghai–Tibet Plateau, southwest Greenland, Spain,
Iceland, and other Northern Hemisphere locations show increased climatic
variability . Proxy records indicate that the
Mediterranean experienced a wet and humid climate episode during this period
. The Late Antique Little Ice Age is evident in tree ring
records from the Russian Altai and European Alps. This period of cooling
followed large volcanic eruptions and overlaps the Dark Ages Cold Period that
spanned the Northern Hemisphere . The contributing factors
for this period are under debate but may involve ice-rafting events, North
Atlantic Oscillation, and/or El Niño–Southern Oscillation ENSO;.
These climate anomalies appear to be reflected in the Tunu VA record, with
elevated VA during the warm periods and lower levels during the colder
periods (Fig. ). Visual inspection of hemispheric mean temperature
data suggests that elevated VA levels from 1080 to 1240 CE followed elevated
Northern Hemisphere temperatures from about 970 to 1090 CE. Tunu
δ18O Past Global Changes (PAGES) 2k Arctic and North American temperature
reconstructions show a similar relationship with VA levels (Figs. S11, S12)
. Elevated VA levels during the Roman Warm
Period overlap elevated Arctic temperatures. Elevated VA levels from
1080 to 1250 CE follow elevated temperatures in the Arctic from about 930 to 1230 CE and North America from about 750 to 1150 CE. This relationship could be due
to climate-driven changes in temperature or precipitation on burning extent,
frequency, or location, as well as to changes in atmospheric transport
patterns.
Comparison between Tunu VA and climate records. From top: Tunu VA, the
gray-filled lines are ±1 standards error of the 40-year bin averages of the data; Northern
Hemisphere land temperature anomaly composite before 1850 (EIV method; black line) and after 1850
(CRU instrumental record; gray line; Mann et al., 2008a, b; Wang et al., 2017a, b; Sicre et al., 2011a, b;
Trouet et al., 2009a, b); 30-year low-pass-filtered Atlantic multidecadal
variability (AMV) (, b), b; North Atlantic alkenone sea surface temperature (SST) anomaly
based on MD99-2275 sediment (, b); and North Atlantic Oscillation (NAO) index reconstruction (, b).
examined relationships between Canadian forest fire season
severity and prior winter global sea surface temperature variations from
1953 to 1999. They found three principle modes of influence: (1) the global
near-linear trend in SST (i.e., Southern Ocean warming and Atlantic and north
central Pacific cooling) positively correlated with fire severity across most
of Canada, (2) the modulation of Atlantic SST associated with the Atlantic
Multidecadal Oscillation (AMO) or variability (AMV), with high fire severity
across most of Canada associated with Atlantic cooling, and (3) variations in
Pacific SST associated with the Pacific Decadal Oscillation (PDO) and ENSO, with high fire severity in western
Canada and low severity over southern and eastern Canada associated with warm
Pacific SST. Based on these results, one might speculate that externally
forced climate variability due to orbital variations or volcanic eruptions
might modulate Canada fire emissions via the global SST mode, while internal
variability in the AMV and PDO modes might dominate on multidecadal
timescales (60–80 years). We examined several proxy reconstructions for these
modes in order to test these relationships.
The PAGES 2k ocean reconstruction shows a monotonic cooling trend for the
global ocean over the past two millennia, with the exception of warming
during the past century . A notable exception to the trend
is a period of warming in the Atlantic from 1000 to 1300 CE during the MCA
Fig. , while Pacific SST maintained
its slow cooling trend. Proxy records indicate that both the AMV and the
North Atlantic Oscillation were in the positive phase during this period,
consistent with a warm Atlantic .
Surprisingly, the observation of elevated Tunu VA during the warm MCA
suggests a relationship between North American fire and SST that is opposite
in sign to that inferred from analysis of the latter half of the 20th century
by . That could indicate that different modes of variability
influence North American fire on centennial and decadal timescales.