Marine dimethyl sulfide (DMS) is important to climate due to the ability of DMS
to alter Earth's radiation budget.
Knowledge of the global-scale distribution, seasonal variability, and sea-to-air ...flux of DMS is needed in order to improve understanding of atmospheric sulfur, aerosol/cloud dynamics, and albedo.
Here we examine the use of an artificial neural network (ANN) to extrapolate
available DMS measurements to the global ocean and produce a global climatology
with monthly temporal resolution.
A global database of 82 996 ship-based DMS measurements in surface waters was
used along with a suite of environmental parameters consisting of
latitude–longitude coordinates, time of day, time of year, solar radiation,
mixed layer depth, sea surface temperature, salinity, nitrate, phosphate, and
silicate.
Linear regressions of DMS against the environmental parameters show that on a
global-scale mixed layer depth and solar radiation are the strongest predictors
of DMS.
These parameters capture ∼9 % and ∼7 % of the raw DMS data variance,
respectively.
Multilinear regression can capture more of the raw data variance (∼39 %) but
strongly underestimates DMS in high-concentration regions.
In contrast, the artificial neural network captures ∼66 % of the raw data
variance in our database.
Like prior climatologies our results show a strong seasonal cycle in surface
ocean DMS with the highest concentrations and sea-to-air fluxes in the high-latitude
summertime oceans.
We estimate a lower global sea-to-air DMS flux (20.12±0.43 Tg S yr−1)
than the prior estimate based on a map interpolation method when the same gas transfer velocity parameterization is used.
Our sensitivity test results show that DMS concentration does not change unidirectionally with each of the environmental parameters, which emphasizes the interactions among these parameters.
The ANN model suggests that the flux of DMS from the ocean to the atmosphere will increase with global warming.
Given that larger DMS fluxes induce greater cloud albedo, this corresponds to a negative climate feedback.
Methyl bromide (CH3Br) is an ozone depleting trace gas that is now mainly emitted from natural sources. Roughly 80% of anthropogenic production of CH3Br was phased‐out in response to the Montreal ...Protocol on Substances that Deplete the Ozone Layer beginning in 1999 and atmospheric levels of CH3Br have declined considerably since. Here we use surface measurements of CH3Br from NOAA's global air sampling network, along with a six‐box atmosphere/ocean model to explore interannual variability in atmospheric CH3Br mole fractions. We find that CH3Br mole fractions are strongly correlated with the El Niño Southern Oscillation (ENSO) phenomenon, but variability in winds, sea surface temperature, and biological production during ENSO are unlikely to drive the observed changes in atmospheric CH3Br directly. Rather, the results indicate that ENSO‐driven changes to biomass burning are an important cause of the observed interannual CH3Br variability.
Plain Language Summary
Methyl bromide (CH3Br) is a trace gas known to destroy stratospheric ozone. Atmospheric mole fractions of CH3Br have declined substantially over time because of Montreal Protocol controls on production of this ozone‐depleting gas. What remained unexplained were the substantial temporary variations observed in the measurement record. Here we find that those variations are related to El Niño Southern Oscillation (ENSO) events, and the increased burning during the warm phase of ENSO is determined to be the most likely cause of these interannual changes.
Key Points
Interannual variability in the global atmospheric methyl bromide (CH3Br) mole fraction is strongly correlated with the El Niño Southern Oscillation (ENSO) phenomenon
Changes in the ocean (sea surface temperature and winds) during ENSO are unlikely to cause the observed variability in atmospheric CH3Br mole fraction
The majority of interannual variability in atmospheric CH3Br mole fraction is explained by ENSO‐driven fire emissions
Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical ...composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm
) and 33% (36 cm
) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm
) in late-autumn but only 4% (4 cm
) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.
This study uncovers an early 1990s change in the relationships between El Niño–Southern Oscillation (ENSO) and two leading modes of the Southern Hemisphere (SH) atmospheric variability: the southern ...annular mode (SAM) and the Pacific–South American (PSA) pattern. During austral spring, while the PSA maintained a strong correlation with ENSO throughout the period 1948–2014, the SAM–ENSO correlation changed from being weak before the early 1990s to being strong afterward. Through the ENSO connection, PSA and SAM became more in-phase correlated after the early 1990s. The early 1990s is also the time when ENSO changed from being dominated by the eastern Pacific (EP) type to being dominated by the central Pacific (CP) type. Analyses show that, while the EP ENSO can excite only the PSA, the CP ENSO can excite both the SAM and PSA through tropospheric and stratospheric pathway mechanisms. The more in-phase relationship between SAM and PSA impacted the post-1990s Antarctic climate in at least two aspects: 1) a stronger Antarctic sea ice dipole structure around the Amundsen–Bellingshausen Seas due to intensified geopotential height anomalies over the region and 2) a shift in the phase relationships of surface air temperature anomalies among East Antarctica, West Antarctica, and the Antarctic Peninsula. These findings imply that ENSO–Antarctic climate relations depend on the dominant ENSO type and that ENSO forcing has become more important to the Antarctic sea ice and surface air temperature variability in the past two decades and will in the coming decades if the dominance of CP ENSO persists.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. ...Measurements of BC, vanillic acid, and non-sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value.
We use isoprene and related field measurements from three different ocean data sets together with remotely sensed satellite data to model global marine isoprene emissions. We show that using monthly ...mean satellite-derived chl a concentrations to parameterize isoprene with a constant chl a normalized isoprene production rate underpredicts the measured oceanic isoprene concentration by a mean factor of 19 ± 12. Improving the model by using phytoplankton functional type dependent production values and by decreasing the bacterial degradation rate of isoprene in the water column results in only a slight underestimation (factor 1.7 ± 1.2). We calculate global isoprene emissions of 0.21 Tg C for 2014 using this improved model, which is twice the value calculated using the original model. Nonetheless, the sea-to-air fluxes have to be at least 1 order of magnitude higher to account for measured atmospheric isoprene mixing ratios. These findings suggest that there is at least one missing oceanic source of isoprene and, possibly, other unknown factors in the ocean or atmosphere influencing the atmospheric values. The discrepancy between calculated fluxes and atmospheric observations must be reconciled in order to fully understand the importance of marine-derived isoprene as a precursor to remote marine boundary layer particle formation.
Biomass burning drives changes in greenhouse gases, climate-forcing aerosols, and global atmospheric chemistry. There is controversy about the magnitude and timing of changes in biomass burning ...emissions on millennial time scales from preindustrial to present and about the relative importance of climate change and human activities as the underlying cause. Biomass burning is one of two notable sources of ethane in the preindustrial atmosphere. Here, we present ice core ethane measurements from Antarctica and Greenland that contain information about changes in biomass burning emissions since 1000 CE (Common Era). The biomass burning emissions of ethane during the Medieval Period (1000–1500 CE) were higher than present day and declined sharply to a minimum during the cooler Little Ice Age (1600–1800 CE). Assuming that preindustrial atmospheric reactivity and transport were the same as in the modern atmosphere, we estimate that biomass burning emissions decreased by 30 to 45% from the Medieval Period to the Little Ice Age. The timing and magnitude of this decline in biomass burning emissions is consistent with that inferred from ice core methane stable carbon isotope ratios but inconsistent with histories based on sedimentary charcoal and ice core carbon monoxide measurements. This study demonstrates that biomass burning emissions have exceeded modern levels in the past and may be highly sensitive to changes in climate.
Marine phytoplankton play a critical role in modulating marine-based food webs, fishery yields, and the global drawdown of atmospheric CO2. Due to sparse measurements prior to 21st century satellite ...monitoring, however, little is known of the long-term response of planktonic stocks to climate forcing. Here we produce the first continuous, multi-century record of subarctic Atlantic marine productivity, showing a marked 10 ± 7% decline has occurred across this highly-productive ocean basin over the last two centuries. We support this conclusion through the application of a novel marine-productivity proxy, established using a unique signal of planktonic-derived aerosol commonly identified across an array of Greenlandic ice cores. Utilizing contemporaneous satellite-era observations, we demonstrate this signal’s use as a robust and high-resolution proxy for spatially-integrated marine productivity variations. We show that the initiation of declining subarctic Atlantic productivity broadly coincides with the onset of Arctic surface warming, and that productivity strongly covaries with regional sea-surface temperatures and basin-wide gyre circulation strength over recent decades. Taken together, our results suggest the industrial era productivity decline may be evidence of the predicted5 collapse of northern Atlantic planktonic stocks in response to a weakened Atlantic Meridional Overturning Circulation (AMOC). Continued AMOC weakening, as projected for the 21st century, may therefore result in further productivity declines across this globally-relevant region.
Acetylene is a short‐lived trace gas produced during combustion of fossil fuels, biomass, and biofuels. Biomass burning is likely the only major source of acetylene in the preindustrial atmosphere, ...making ice core acetylene a powerful tool for reconstructing paleofire emissions. Here we present a 2,000‐year atmospheric record of acetylene reconstructed from analysis of air bubbles trapped in Greenland and Antarctic ice cores and infer pyrogenic acetylene emissions using a chemistry transport model. From 0 to 1500 CE, Antarctic acetylene averages 36 ± 1 pmol mol−1 (mean ± 1 SE), roughly double the annual mean over Antarctica today. Antarctic acetylene declines during the Little Ice Age by over 50% to 17 ± 2 pmol mol−1 from 1650 to 1750 CE. Acetylene over Greenland declines less dramatically over the same period. Modeling results suggest that pyrogenic acetylene emissions during 1000–1500 CE were sustained at rates significantly greater than modern day and declined by over 50% during the 1650–1750 CE period.
Plain Language Summary
Acetylene is an atmospheric trace gas produced by combustion of fossil fuels, agricultural and domestic burning, and wildfires. In the preindustrial atmosphere, the major source of acetylene is from wildfires. We measured the abundance of acetylene in the air bubbles trapped inside polar ice cores from Greenland and Antarctica over the last 2,000 years for the first time. Variations in the atmospheric abundance of acetylene over Antarctica indicate large changes in preindustrial wildfire emissions. Using a model, we find that preindustrial wildfire emissions of acetylene during the Medieval Period (1000–1500 CE) could have been several fold greater than what is observed today. Acetylene emissions declined by about 50% at the onset of the Little Ice Age (1650–1750 CE).
Key Points
First ever ice core record of acetylene covering the last 2,000 years is used to reconstruct paleofire emissions
Preindustrial acetylene levels over Antarctica were nearly double the modern‐day mean level
Inferred biomass burning emissions of acetylene from 1000 to 1500 CE are several times greater than modern‐day rates and decline sharply around 1650 CE
During 2013–15, prolonged near-surface warming in the northeastern Pacific was observed and has been referred to as the Pacific warm blob. Here, statistical analyses are conducted to show that the ...generation of the Pacific blob is closely related to the tropical Northern Hemisphere (TNH) pattern in the atmosphere. When the TNH pattern stays in its positive phase for extended periods of time, it generates prolonged blob events primarily through anomalies in surface heat fluxes and secondarily through anomalies in wind-induced ocean advection. Five prolonged (≥24 months) blob events are identified during the past six decades (1948–2015), and the TNH–blob relationship can be recognized in all of them. Although the Pacific decadal oscillation and El Niño can also induce an arc-shaped warming pattern near the Pacific blob region, they are not responsible for the generation of Pacific blob events. The essential feature of Pacific blob generation is the TNH-forced Gulf of Alaska warming pattern. This study also finds that the atmospheric circulation anomalies associated with the TNH pattern in the North Atlantic can induce SST variability akin to the so-called Atlantic cold blob, also through anomalies in surface heat fluxes and wind-induced ocean advection. As a result, the TNH pattern serves as an atmospheric conducting pattern that connects some of the Pacific warm blob and Atlantic cold blob events. This conducting mechanism has not previously been explored.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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