Skip to main content
eScholarship
Open Access Publications from the University of California

Faculty Publications

The Department of Earth System Science (ESS) focuses on how the atmosphere, land, and oceans interact as a system, and how the Earth will change over a human lifetime.

Cover page of Phylogenetic conservation of bacterial responses to soil nitrogen addition across continents.

Phylogenetic conservation of bacterial responses to soil nitrogen addition across continents.

(2019)

Soil microbial communities are intricately linked to ecosystem functioning such as nutrient cycling; therefore, a predictive understanding of how these communities respond to environmental changes is of great interest. Here, we test whether phylogenetic information can predict the response of bacterial taxa to nitrogen (N) addition. We analyze the composition of soil bacterial communities in 13 field experiments across 5 continents and find that the N response of bacteria is phylogenetically conserved at each location. Remarkably, the phylogenetic pattern of N responses is similar when merging data across locations. Thus, we can identify bacterial clades - the size of which are highly variable across the bacterial tree - that respond consistently to N addition across locations. Our findings suggest that a phylogenetic approach may be useful in predicting shifts in microbial community composition in the face of other environmental changes.

Cover page of Building bottom-up aggregate-based models (ABMs) in soil systems with a view of aggregates as biogeochemical reactors.

Building bottom-up aggregate-based models (ABMs) in soil systems with a view of aggregates as biogeochemical reactors.

(2019)

In our recent article in Global Change Biology (Wang et al., 2019), we proposed to develop aggregate-based models (ABMs) based on a view of soil aggregates as biogeochemical reactors in the context of soil heterogeneity. Using a bottom-up philosophy, we argued for developing ABMs based on a systematic and dynamic view of soils as a constellation of aggregate reactors of different sizes. We envision that these ABMs offer the potential to bring new mechanistic perspectives into soil system modelling. This article is protected by copyright. All rights reserved.

Cover page of Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018.

Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018.

(2019)

We reconstruct the mass balance of the Greenland Ice Sheet using a comprehensive survey of thickness, surface elevation, velocity, and surface mass balance (SMB) of 260 glaciers from 1972 to 2018. We calculate mass discharge, D, into the ocean directly for 107 glaciers (85% of D) and indirectly for 110 glaciers (15%) using velocity-scaled reference fluxes. The decadal mass balance switched from a mass gain of +47 ± 21 Gt/y in 1972-1980 to a loss of 51 ± 17 Gt/y in 1980-1990. The mass loss increased from 41 ± 17 Gt/y in 1990-2000, to 187 ± 17 Gt/y in 2000-2010, to 286 ± 20 Gt/y in 2010-2018, or sixfold since the 1980s, or 80 ± 6 Gt/y per decade, on average. The acceleration in mass loss switched from positive in 2000-2010 to negative in 2010-2018 due to a series of cold summers, which illustrates the difficulty of extrapolating short records into longer-term trends. Cumulated since 1972, the largest contributions to global sea level rise are from northwest (4.4 ± 0.2 mm), southeast (3.0 ± 0.3 mm), and central west (2.0 ± 0.2 mm) Greenland, with a total 13.7 ± 1.1 mm for the ice sheet. The mass loss is controlled at 66 ± 8% by glacier dynamics (9.1 mm) and 34 ± 8% by SMB (4.6 mm). Even in years of high SMB, enhanced glacier discharge has remained sufficiently high above equilibrium to maintain an annual mass loss every year since 1998.

Cover page of 14C-Free Carbon Is a Major Contributor to Cellular Biomass in Geochemically Distinct Groundwater of Shallow Sedimentary Bedrock Aquifers.

14C-Free Carbon Is a Major Contributor to Cellular Biomass in Geochemically Distinct Groundwater of Shallow Sedimentary Bedrock Aquifers.

(2019)

Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for subsurface microorganisms in biogeochemical distinct shallow aquifers (Critical Zone Exploratory, Thuringia Germany). Despite the presence of younger OC, the microbes assimilated 14C-free OC to varying degrees; ~31% in groundwater within the oxic zone, ~47% in an iron reduction zone, and ~70% in a sulfate reduction/anammox zone. The persistence of trace amounts of mature and partially biodegraded hydrocarbons suggested that autochthonous petroleum-derived hydrocarbons were a potential 14C-free C source for heterotrophs in the oxic zone. In this zone, Δ14C values of dissolved inorganic carbon (-366 ± 18‰) and 11MeC16:0 (-283 ± 32‰), an important component in autotrophic nitrite oxidizers, were similar enough to indicate that autotrophy is an important additional C fixation pathway. In anoxic zones, methane as an important C source was unlikely since the 13C-fractionations between the PLFAs and CH4 were inconsistent with kinetic isotope effects associated with methanotrophy. In the sulfate reduction/anammox zone, the strong 14C-depletion of 10MeC16:0 (-942 ± 22‰), a PLFA common in sulfate reducers, indicated that those bacteria were likely to play a critical part in 14C-free sedimentary OC cycling. Results indicated that the 14C-content of microbial biomass in shallow sedimentary aquifers results from complex interactions between abundance and bioavailability of naturally occurring OC, hydrogeology, and specific microbial metabolisms.

Cover page of Urban pollution greatly enhances formation of natural aerosols over the Amazon rainforest.

Urban pollution greatly enhances formation of natural aerosols over the Amazon rainforest.

(2019)

One of the least understood aspects in atmospheric chemistry is how urban emissions influence the formation of natural organic aerosols, which affect Earth's energy budget. The Amazon rainforest, during its wet season, is one of the few remaining places on Earth where atmospheric chemistry transitions between preindustrial and urban-influenced conditions. Here, we integrate insights from several laboratory measurements and simulate the formation of secondary organic aerosols (SOA) in the Amazon using a high-resolution chemical transport model. Simulations show that emissions of nitrogen-oxides from Manaus, a city of ~2 million people, greatly enhance production of biogenic SOA by 60-200% on average with peak enhancements of 400%, through the increased oxidation of gas-phase organic carbon emitted by the forests. Simulated enhancements agree with aircraft measurements, and are much larger than those reported over other locations. The implication is that increasing anthropogenic emissions in the future might substantially enhance biogenic SOA in pristine locations like the Amazon.

Cover page of Four decades of Antarctic Ice Sheet mass balance from 1979-2017

Four decades of Antarctic Ice Sheet mass balance from 1979-2017

(2019)

We use updated drainage inventory, ice thickness, and ice velocity data to calculate the grounding line ice discharge of 176 basins draining the Antarctic Ice Sheet from 1979 to 2017. We compare the results with a surface mass balance model to deduce the ice sheet mass balance. The total mass loss increased from 40 ± 9 Gt/y in 1979-1990 to 50 ± 14 Gt/y in 1989-2000, 166 ± 18 Gt/y in 1999-2009, and 252 ± 26 Gt/y in 2009-2017. In 2009-2017, the mass loss was dominated by the Amundsen/Bellingshausen Sea sectors, in West Antarctica (159 ± 8 Gt/y), Wilkes Land, in East Antarctica (51 ± 13 Gt/y), and West and Northeast Peninsula (42 ± 5 Gt/y). The contribution to sea-level rise from Antarctica averaged 3.6 ± 0.5 mm per decade with a cumulative 14.0 ± 2.0 mm since 1979, including 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Peninsula (i.e., East Antarctica is a major participant in the mass loss). During the entire period, the mass loss concentrated in areas closest to warm, salty, subsurface, circumpolar deep water (CDW), that is, consistent with enhanced polar westerlies pushing CDW toward Antarctica to melt its floating ice shelves, destabilize the glaciers, and raise sea level.

Cover page of A cloud-free MODIS snow cover dataset for the contiguous United States from 2000 to 2017

A cloud-free MODIS snow cover dataset for the contiguous United States from 2000 to 2017

(2019)

This article presents a cloud-free snow cover dataset with a daily temporal resolution and 0.05° spatial resolution from March 2000 to February 2017 over the contiguous United States (CONUS). The dataset was developed by completely removing clouds from the original NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) Snow Cover Area product (MOD10C1) through a series of spatiotemporal filters followed by the Variational Interpolation (VI) algorithm; the filters and VI algorithm were evaluated using bootstrapping test. The dataset was validated over the period with the Landsat 7 ETM+ snow cover maps in the Seattle, Minneapolis, Rocky Mountains, and Sierra Nevada regions. The resulting cloud-free snow cover captured accurately dynamic changes of snow throughout the period in terms of Probability of Detection (POD) and False Alarm Ratio (FAR) with average values of 0.955 and 0.179 for POD and FAR, respectively. The dataset provides continuous inputs of snow cover area for hydrologic studies for almost two decades. The VI algorithm can be applied in other regions given that a proper validation can be performed.