Posts
CONFIRMED:
Methane
releases increase over winter
in the Barents & Kara Seas
&
Methane
Last winter (2018) Margo and I observed that methane levels actually went UP, something that didn't make sense to either of us. After all, in winter, there is a layer of ice that would (presumably) stop a release of methane.
A recent paper by Leonid Yurganov et. al demonstrates that satellite obervations have shown that methane emissions have been higher in winter than they are in summer.
Margo explains this extremely important information that is likely to go largely unreported very clearly in the video below.
Perhaps I can explain it this way.
There is a layer in the Arctic, called the Pycnocline which prevents methane from clathrates from rising to the surface in summer. In winter the cooling surface and the warmer water coming in from the Atlantic breaks down this barrier. This explains why methane emissions in the Kara and Laptev Seas are higher in winter than they are in summer.
You can read the paper HERE
Methane increase over the Barents and Kara seas after the autumn pycnocline breakdown: satellite observations Leonid YURGANOV1*, Frank MULLER-KARGER2& Ira LEIFER3 University of Maryland Baltimore County (ret), Baltimore, MD,
Abstract
Seven
operative thermal infrared (TIR) spectrometers launched at
sun-synchronous polar orbits supply huge amounts of information
about Arctic methane (CH4) year-round, day and night. TIR data are
unique for estimating CH4 emissions from a warming Arctic, both
terrestrial and marine. This report is based on publicly available
CH4 concentrations retrieved by NOAA and NASA from spectra of TIR
radiation delivered by EU IASI and US AIRS sounders. Data were
filtered for high thermalcontrast in the troposphere. Validation
versus aircraft measurements at three US continental sites reveal a
reduced, but still significant sensitivity to CH4 anomalies in
the troposphere below 4 km of altitude. The focus area is the Barents
and Kara seas (BKS). BKS is impacted with warm Atlantic water
and mostly free of sea ice. It is a shelf area with vast deposits of
oil andnatural gas (~90% CH4), as well as methane hydrates and
submarine permafrost. Although in summer AIRS and IASI observe no
significant difference in CH4 between BKS and N. Atlantic, a strong,
monthly positive CH4 spatial anomaly of up to 30 ppboccurs during
late autumn–winter. One of explanations of this increase is a
fall/winter pycnocline breakdown after a period ofblocked mixing
caused by a stable density seawater stratification in summer:
enhanced mixing lets CH4 to reach the sea surface and
atmosphere.
The
Arctic has experienced the fastest warming on Earth over recent
decades, termed Arctic amplification, with the Arctic Ocean
warming at nearly double the rate of the world’s oceans
(Hoegh-Guldberg and Bruno, 2010). Due to Arctic
warming, there is concern that the vast stores of the important
greenhouse gas methane (CH4) in hydrates, permafrost, and other
vulnerable reservoirs can be released to the atmosphere. The
radiative impact of CH4 is second after carbon dioxide on
century timescales, and larger on decadal timescales (Solomon et
al., 2007). Warm Atlantic currents make the Barents and Kara
seas (BKS) climatically important with respect to instability of
seabed CH4 due to the input of warming seawater. The Barents Sea
is a shallow-shelf sea (average depth 230 m) with depressions
to 400 m. It currently is close to ice-free year-round (Onarheim
and Årthun, 2017). The Kara Sea is even shallower
(average depth 100 m) and ice-free in summer.
The
BKS has extensive proven oil and natural gas deposits,the latter of
which are primarily CH4 (Shipilov and Murzin,2002). Although rather
limited, existing data show these reservoirs drive CH4 seepage
into the water column (Chand et al., 2014; Platt et al., 2018).
At depths greater than 250 mCH4 forms hydrates that are stable in the
cold BKS deep waters (~1 ). Additional seabed CH °C 4 is
sequestered in the Kara Sea and the South Barents Sea as
submarine permafrost (Osterkamp, 2010). Warming seabed water
may destabilize submarine permafrost and hydrates,
although specific mechanisms and timescales are uncertain.
Most
seep bubble CH4 dissolves in process of ebullition in deep water
where methanotrophic bacteriaoxidize it on weeks’ to years’
timescales (Reeburgh, 2007). Only a small fraction of the
seep bubble methane directly reaches the air. The seasonal
summer pycnocline prevents vertical turbulent diffusion of
methane, with concentrations near the seafloor that may be
extremely high (Gentz et al.,2014). They studied dissolved CH4
concentrations to the West of Prins Karls Forland (Svalbard) in
August 2010 (not in winter) and found that CH4 bubble plumes
dissolved below the pycnocline and were not vertically mixing to
the upper water column. They suggested that during winter
in high latitudes after the summer stratification breaks
down, vertical transport of CH4 from the bottom layer is
not limited anymore, and CH4 can reach the sea surface. The most
comprehensive CH4 measurements at the seafloor and in the
atmosphere in the region were collected to the West of Svalbard
in June–July, 2014, by Myhre et al. (2016).
They
found that summer CH4 release from seabed sediments
substantially increases CH4 concentrations above the seafloor
with a sharp decrease above the pycnocline. They suggest that
dissolved methane captured below the pycnocline may only be
released to the atmosphere when physical processes remove the
pycnocline as a dynamic barrier. Mau et al. (2017) field
measurements between Svalbard and Bear Island in
August–September, 2015, also demonstrated the pycnocline as a
barrier for CH4. All three investigations came to a conclusion
on insignificant methane flux in summer-early autumn around
Svalbard and between Svalbard and Bear Island. Meanwhile, as it
is well known (Rudels, 1993), during winter in high latitudes
convective mixing extends to the seafloor. Currently, this
seasonality ofpotential CH4 pathways has not been considered in
CH4 budgets (Fisher et al., 2011; AMAP, 2015). To our
best knowledge, direct measurements of CH4 flux in winter
have not been published for the BKS to date. Summertime
CH4 emission density near Svalbard was estimated by Myhre et
al. (2016) as 0.04 nmol·m−2·s−1 . Formal
extrapolation onto the whole year and total area of BKS
(2.317×109 km2) results in0.047 Tg CH4·a−1, i.e. negligible
flux.
Shakhova
et al. (2010) investigated summer and winter methane flux from
the East Siberian Arctic Shelf (ESAS) encompassing the Laptev,
East Siberian seas, and Russian part of the Chuckchi Sea. More
than 5000 at-sea measurements of dissolved methane have been
collected. Greater than 80% of ESAS bottom waters and greater
than 50% of surface waters were supersaturated with
methane regarding to the atmosphere. In summer diffusive
and ebullition fluxes were estimated, respectively, as 1.24
Tg CH4 and 1.68 Tg CH4 (total summer 2.92 Tg CH4), in winter
3.23 Tg CH4 and 4.49 Tg CH4 (total winter 7.72 Tg CH4), totally
10.64 Tg CH4·a−1
An
assessment by Berchetet al. (2016) based on surface NOAA network and
inverse modeling is significantly lower: 0.5–4.3 Tg CH4·a−
. Satellite observations are extremely useful
for characterization of Arctic CH4, especially over open sea,
due to their continuous and global data record. Thermal infrared
(TIR) CH4 data are available globally, day and night, whereas
SWIR
CH4
instruments are ineffective in the Arctic due to low or no sunlight,
low reflectivity from water and ice, and long atmospheric
optical path (Leifer et al., 2012). TIR orbital sensors include
AIRS/Aqua, IASI/MetOp-A, B, C, CrIS/NPP,CrIS/NOAA-20, and
GOSAT/TANSO-TIR (see a list of abbreviations at the end of the
article). Example current SWIR sensors are TROPOMI and
GOSAT-SWIR. CH4 retrievals for AIRS and IASI are
publicly available (see links in Section 2.1). Yurganov et al.
(2016) suggested a filtering technique for the Arctic data
and presented data on seasonal, spacial and
interannual variability of methane in the layer below 4 km.
They concluded,
“Seasonal increase in methane has been observed since late
October–early November. This can be associated with the
beginning of vertical convection in the ocean, caused by the
cooling of the surface layers and the simultaneous increase in
temperature of the underlying water layers. Bottom layers
saturated with methane are brought to the surface” (translated
from Russian). Preliminarily, Yurganov et al. (2016) assessed
the annual emission of methane from the Arctic Ocean in
2010–2014 as ~2/3 of land emission to the North from 60°N.
Arctic terrestrial emission is now estimated as 20–30
Tg·a−1 (AMAP, 2015). Therefore, total marine CH4 flux from
the Arctic can be expected in the range 15–20 Tg
CH4·a−1 (without the Sea of Okhotsk). The most recent
analysis ofavailable measurements of the global methane flux
from oceans (Weber at al., 2019) by training
machine-learning models gave much lower oceanic flux (6–12 Tg
CH4·a−1).
However,
their conclusion that very shallow coastal waters contribute
around 50% of the total methane emissions from the ocean agrees
with our data for the Arctic (see below, Section 3). Here
we show that CH4 spatial anomaly (SA) over BKS is seasonally
growing together with the Mixed Layer Depth (MLD). Therefore, an
increase of emission in winter, predicted by Gentz et al. (2013)
and Myhre et al. (2016) agrees with remote sensing data and
seasonal cycle of MLD. However, this agreement does not exclude
other explanations of the effect and a confirmation of a
significant wintertime flux from Arctic seas by direct
measurements is necessary.After that it may be included into the
methane bud
Conclusions
Satellite
measurements of CH4 concentration in the lower 4 km of the
troposphere reveal a significant, up to 30 ppb, monthly anomaly
referenced to North Atlantic maximized in
November–December–January over vast areas of BKS, as well as
a part of the Greenland Sea. Satellite observations. Arctic
methane: satellite data 389 show similar, but less pronounced,
anomalies in
autumn–winter
over other Arctic seas (Yurganov et al., 2016; Yurganov and
Leifer, 2016a). This CH4 anomaly may be interpreted as
intensification of CH4 flux induced by turbulent diffusion
and/or convection after a breakdown of
the
summer stratification. After a confirmation of a significant
wintertime flux from Arctic seas by direct measurements the
marine CH4 emission may be included into the methane budget.
