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USGS2005

National Acid Precipitation Assessment Program Report to Congress: An integrated assessment

Acid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere (with water, oxygen, and oxidants) to form various acidic compounds. These acidic compounds then fall to earth in either a wet form (rain, snow, and fog) or a dry form (gases, aerosols, and particles). Prevailing winds transport the acidic compounds hundreds of miles, often across state and national borders. At certain levels the acidic compounds, including small particles such as sulfates and nitrates can cause many negative human health and environmental effects. While ecosystems are subject to many stresses, including land-use changes, climate change, and variations in hydrologic and meteorologic cycles, the scientific literature has clearly demonstrated that these pollutants can: • Degrade air quality, • Impair visibility, • Damage public health, • Acidify lakes and streams, • Harm sensitive forests, • Harm sensitive coastal ecosystems, and • Accelerate the decay of building materials, paints, and cultural artifacts such as buildings, statues, and sculptures. Title IV was passed by Congress as part of the 1990 Clean Air Act Amendments to reduce emissions of SO 2 and NO x from fossil fuel-burning power plants in order to protect ecosystems suffering damage from acid deposition and to improve air quality. At the same time, the National Acid Precipitation Assessment Program (NAPAP) was asked to periodically assess and report to Congress on the implementation of the Acid Rain Program, recent scientific knowledge surrounding acid deposition and its effects, and the reduction in acid deposition necessary to prevent adverse ecological effects. This NAPAP Report focuses primarily on emission reductions from power plants, both in terms of assessing past reductions under the Acid Rain Program and in projecting the ecological effects of additional reductions of SO 2 and NO x . It should be noted that power generation currently contributes approximately 69% of the SO 2 emissions and 22% of the NO x emissions nationwide. This contribution is decreasing as emissions from power generation continue to decrease, making the other sources of these pollutants more prominent. Modeling suggests that even if SO 2 emissions from power generation were reduced to zero, some lakes and streams would remain acidic due to acid deposition. However, there are several other regulations that reduce emissions of SO 2 and NO x from these non-power generation sources, such as the Tier II mobile source standards, the Heavy Duty Diesel standards, and the Clean Air Non-Road Diesel Rule, that have also been promulgated since 1990. These regulations, primarily designed to bring counties into attainment with fine particle and ozone air quality standards, also incidentally reduce emissions that contribute to acid deposition. Implementation of Title IV has successfully and substantially reduced emissions of SO 2 and NO x from power generation at a significantly lower cost than expected: • In 2002, SO 2 emissions were 10.2 million tons, 35% lower than 1990 emissions and 40% lower than 1980 emissions.* • In 2002, NO x emissions were 4.5 million tons, 33% lower than 1990 emissions. In addition, SO 2 emissions from all sources have decreased by 32% since 1990 and emissions of NO x from all sources have decreased by 12% since 1990. Power generating sources continue to close in on the goal of reducing power plant SO 2 emissions from 1980 levels by 50% (to 8.95 million tons) as required by the 1990 Clean Air Act. Power generating sources have also exceeded the goal of a two million ton reduction in NO x emissions from projected 2000 levels as required by the 1990 Clean Air Act. These emission reductions have contributed to measurable improvements in air quality, reductions in acid deposition, and the beginnings of recovery of acid-sensitive waters in some areas: • SO2 concentrations in the atmosphere (a precursor to fine particles and acid deposition) have decreased since 1990. Average annual SO2 concentrations in the Northeast in 2000–2002 were 40% lower than they were in 1989–1991, concentrations in the mid-Atlantic were 30% lower, concentrations in the Southeast were 35% lower, and concentrations in the Midwest were 45% lower. • Sulfate concentrations in the atmosphere (a major component of fine particles, especially in the East) have decreased since 1990 as well. Average annual sulfate concentrations in the Northeast and Midwest in 2000–2002 were approximately 30% lower than they were in 1989–1991, and concentrations in the mid-Atlantic and Southeast were 25% lower. • Wet sulfate deposition, a major component of acid rain, has also decreased since 1990. Average annual sulfate deposition in the Northeast in 2000–2002 was 40% lower than it was in 1989–1991, deposition in the mid-Atlantic and Midwest was 35% lower, and deposition in the Southeast was 25% lower. • Wet nitrate deposition has not decreased regionally from historical levels because of the relatively moderate NO x reduction from power plants and the continuing large contribution (over 50% of total NOx emissions) from other sources of NO x such as vehicles and nonroad vehicles. • Although visibility has begun to improve in some parts of the U.S., there is still significant impairment of visibility in many national parks and other Class I areas throughout the U.S. • Acid neutralizing capacity is beginning to rise in some surface waters in the Northeast, including lakes in the Adirondack Mountains (see graphic below). This is an indication that recovery from acidification is occurring in those areas.

Report to Congress↗

USGS2010

Global change and water resources in the next 100 years

We are in the midst of a continental-scale, multi-year experiment in the United States, in which we have not defined our testable hypotheses or set the duration and scope of the experiment, which poses major water-resources challenges for the 21st century. What are we doing? We are expanding population at three times the national growth rate in our most water-scarce region, the southwestern United States, where water stress is already great and modeling predicts decreased streamflow by the middle of this century. We are expanding irrigated agriculture from the west into the east, particularly to the southeastern states, where increased competition for ground and surface water has urban, agricultural, and environmental interests at odds, and increasingly, in court. We are expanding our consumption of pharmaceutical and personal care products to historic high levels and disposing them in surface and groundwater, through sewage treatment plants and individual septic systems. These substances are now detectable at very low concentrations and we have documented significant effects on aquatic species, particularly on fish reproduction function. We don’t yet know what effects on human health may emerge, nor do we know if we need to make large investments in water treatment systems, which were not designed to remove these substances. These are a few examples of our national-scale experiment. In addition to these water resources challenges, over which we have some control, climate change models indicate that precipitation and streamflow patterns will change in coming decades, with western mid-latitude North America generally drier. We have already documented trends in more rain and less snow in western mountains. This has large implications for water supply and storage, and groundwater recharge. We have documented earlier snowmelt peak spring runoff in northeastern and northwestern States, and western montane regions. Peak runoff is now about two weeks earlier than it was in the first half of the 20th century. Decreased summer runoff affects water supply for agriculture, domestic water supply, cooling needs for thermoelectric power generation, and ecosystem needs. In addition to the reduced volume of streamflow during warm summer months, less water results in elevated stream temperature, which also has significant effects on cooling of power generating facilities and on aquatic ecosystem needs. We are now required to include fish and other aquatic species in negotiation over how much water to leave in the river, rather than, as in the past, how much water we could remove from a river. Additionally, we must pay attention to the quality of that water, including its temperature. This is driven in the US by the Endangered Species Act and the Clean Water Act. Furthermore, we must now better understand and manage the whole hydrograph and the influence of hydrologic variability on aquatic ecosystems. Man has trimmed the tails off the probability distribution of flows. We need to understand how to put the tails back on but can’t do that without improved understanding of aquatic ecosystems. Sea level rise presents challenges for fresh water extraction from coastal aquifers as they are compromised by increased saline intrusion. A related problem faces users of ‘run-of-the-river’ water-supply intakes that are threatened by a salt front that migrates further upstream because of higher sea level. We face significant challenges with water infrastructure. The U.S. has among the highest quality drinking water in the world piped to our homes. However, our water and sewage treatment plants and water and sewer pipelines have not had adequate maintenance or investment for decades. The US Environmental Protection Agency estimates that there are up to 3.5M illnesses per year from recreational contact with sewage from sanitary sewage overflows. Infrastructure investment needs have been put at 5 trillion nationally. Global change and water resources c

Conference Paper↗

USGS2013

Statewide summary for Florida

Throughout the past century, emergent wetlands have been declining across the Gulf of Mexico. Emergent wetland ecosystems provide a multitude of resources, including plant and wildlife habitat, commercial and recreational economic activity, and natural barriers against storms. As emergent wetland losses increase, so does the need for information on the causes and effects of this loss; emergent wetland mapping, monitoring, and restoration efforts; and education. This report provides scientists, managers, and citizens with valuable baseline information on the status and trends of emergent wetlands along the coast of the Gulf of Mexico. The Statewide Summary for Florida provides status and trends information for Florida using what data is available during the 1950-2010 time period. The State of Florida (Figure 1) is approximately 151,670 km2 (58,560 mi2 ) large with an average elevation of 30.5 m (100 ft) (Dahl, 2005). The Florida gulf coast stretches approximately 1,000 km (621 miles) from the Alabama State line to the Dry Tortugas in the Florida Keys (Handley et al., 2007). The climate varies along the coast, ranging from temperate continental in the panhandle to oceanic subtropical in the Keys. Due to this climatic gradient, the Gulf coast of Florida is divisible into two ecoregions, the Louisianian in the north along the panhandle, and the West Indian in the south along the length of the peninsula (Bailey 1978). The Lousianian ecoregion extends from Cedar Key north and west along the panhandle to the Alabama state line. It is characterized by extensive emergent coastal wetlands, temperate fauna, small tidal ranges (

Florida↗

USGS2020

Ancient Egyptian mummified shrews (Mammalia: Eulipotyphla: Soricidae) and mice (Rodentia: Muridae) from the Spanish Mission to Dra Abu el-Naga, and their implications for environmental change in the Nile valley during the past two millennia

Excavation of Ptolemaic Period (ca. 309–30 BC) strata within Theban Tombs 11, 12, -399-, and UE194A by the Spanish Mission to Dra Abu el-Naga (also known as the Djehuty Project), on the west bank of the Nile River opposite Luxor, Egypt, yielded remains of at least 175 individual small mammals that include four species of shrews (Eulipotypha: Soricidae) and two species of rodents (Rodentia: Muridae). Two of the shrews ( Crocidura fulvastra and Crocidura pasha ) no longer occur in Egypt, and one species ( Crocidura olivieri ) is known in the country only from a disjunct population inhabiting the Nile delta and the Fayum. Although deposited in the tombs by humans as part of religious ceremonies, these animals probably derived originally from local wild populations. The coexistence of this diverse array of shrew species as part of the mammal community near Luxor indicates greater availability of moist floodplain habitats than occur there at present. These were probably made possible by a greater flow of the Nile, as indicated by geomorphological and palynological evidence. The mammal fauna recovered by the Spanish Mission provides a unique snapshot of the native Ptolemaic community during this time period, and it permits us to gauge community turnover in the Nile valley of Upper Egypt during the last 2000 years. It also serves as a relevant example for understanding the extinction and extirpation of mammal species as effects of future environmental changes predicted by current climatic models.

northern Egypt↗

USGS2016

An international borderland of concern: Conservation of biodiversity in the Lower Rio Grande Valley

The Lower Rio Grande Valley (LRGV) of southern Texas is located on the United States-Mexico borderland and represents a 240-kilometer (150-mile) linear stretch that ends at the Gulf of Mexico. The LRGV represents a unique transition between temperate and tropical conditions and, as such, sustains an exceptionally high diversity of plants and animals—some of them found in few, or no other, places in the United States. Examples include Leopardus pardalis albescens (northern ocelot) and Falco femoralis septentrionalis (northern aplomado falcon)—both endangered in the United States and emblematic of the LRGV. The U.S. Fish and Wildlife Service (USFWS) manages three national wildlife refuges (Santa Ana, Lower Rio Grande Valley, and Laguna Atascosa) that together make up the South Texas Refuge Complex, which actively conserves biodiversity in about 76,006 hectares (187,815.5 acres) of native riparian and upland habitats in the LRGV. These diminished habitats harbor many rare, threatened, and endangered species. This report updates the widely used 1988 USFWS biological report titled “Tamaulipan Brushland of the Lower Rio Grande Valley of South Texas: Description, Human Impacts, and Management Options” by synthesizing nearly 400 peer-reviewed scientific publications that have resulted from biological and sociological research conducted specifically in the four Texas counties of the LRGV in the past nearly 30 years. This report has three goals: (1) synthesize scientific insights gained since 1988 related to the biology and management of the LRGV and its unique biota, focusing on flora and fauna of greatest conservation concern; (2) update ongoing challenges facing Federal and State agencies and organizations that focus on conservation or key natural resources in the LRGV; and (3) redefine conservation opportunities and land-acquisition strategies that are feasible and appropriate today, given the many new and expanding constraints that challenge conservation activities in the LRGV. The LRGV faces every contemporary conservation challenge of the 21st century, but ongoing human population growth and its associated demands, international border issues, and oil, gas, and alternative energy development dominate impacts that affect conservation in the LRGV. Continued careful syntheses of existing and future information collected in the LRGV are needed on many biological and sociological topics to guide conservation activities. Quick response will no doubt be necessary to face contemporary and difficult-to-predict challenges such as climate change, diminished water availability and quality, spread of invasive species, and habitat loss and fragmentation. Complexities of a guarded international borderland add pressure to small patches of native habitat that remain in many places of the LRGV, particularly along the Rio Grande. Large connected corridors of restored native habitat could be the best option to maintain, and even enhance, the exceptional biodiversity of the LRGV in the face of exceptional human demand.

Texas↗

USGS2026

Erosion potential and flood vulnerability of streams and stream crossings at Acadia National Park, Maine

Acadia National Park has had increases in the frequency and magnitude of precipitation in recent years, leading to increased flood flows, stream erosion, and costly infrastructure damage. To improve infrastructure management in a changing climate, the U.S. Geological Survey, in cooperation with the National Park Service, has developed multiple datasets that can help natural resource managers identify stream reaches and stream crossings that have the highest potential for erosion and flood damage within Acadia National Park. To develop these datasets, we first created a lidar-derived hydrography based on a 1-meter digital elevation model and then estimated peak flows at stream crossings and along the stream network using regional regression equations for Maine. We assessed the erosion potential of stream reaches by computing channel morphologic and hydrologic metrics associated with erosive power, such as stream steepness, topographic openness, and percent storage in the contributing watershed. Stream crossing flood vulnerability was assessed by comparing estimated peak flows to stream crossing conveyance capacities. Our results indicate that stream reaches in the headwaters of the Acadia National Park highlands such as Sargent, Penobscot, and Cadillac Mountain, have the highest erosion potential and generally coincide with reaches that have had erosion and infrastructure damage in the past. Stream crossings with the highest flood vulnerability are distributed throughout Mount Desert Island and Acadia National Park, especially south of Jordan Pond, north of Sargent Mountain, and surrounding Eagle Lake. Over a quarter of the total stream crossings have insufficient information to compute flood vulnerability and are often on the parts of the stream with the highest potential for erosion. The datasets allow users to identify stream reaches with the highest erosion potential, stream crossings that are most vulnerable to flood damage, and to highlight areas where supplemental field assessments could most effectively be completed.

Maine↗

USGS2012

State of the Earth’s cryosphere at the beginning of the 21st century: Glaciers, global snow cover, floating ice, and permafrost and periglacial environments

This chapter is the tenth in a series of 11 book-length chapters, collectively referred to as “this volume,” in the series U.S. Geological Survey Professional Paper 1386, Satellite Image Atlas of Glaciers of the World. In the other 10 chapters, each of which concerns a specific glacierized region of Earth, the authors used remotely sensed images, primarily from the Landsat 1, 2, and 3 series of spacecraft, in order to analyze that glacierized region and to monitor changes in its glaciers. Landsat images, acquired primarily during the period 1972 through 1981, were used by an international team of glaciologists and other scientists to study the various glacierized regions and (or) to discuss related glaciological topics. In each glacierized region, the present distribution of glaciers within its geographic area is compared, wherever possible, with historical information about their past areal extent. The atlas provides an accurate regional inventory of the areal extent of glacier ice on our planet during the 1970s as part of an expanding international scientific effort to measure global environmental change on the Earth’s surface. However, this chapter differs from the other 10 in its discussion of observed changes in all four elements of the Earth’s cryosphere (glaciers, snow cover, floating ice, and permafrost) in the context of documented changes in all components of the Earth System. Human impact on the planet at the beginning of the 21st century is pervasive. The focus of Chapter A is on changes in the cryosphere and the importance of long-term monitoring by a variety of sensors carried on Earth-orbiting satellites or by a ground-based network of observatories in the case of permafrost. The chapter consists of five parts. The first part provides an introduction to the Earth System, including the interrelationships of the geosphere (cryosphere, hydrosphere, lithosphere, and atmosphere), the biosphere, climate processes, biogeochemical cycles, and the critically important hydrologic cycle, in which glacier ice is the second largest reservoir of water after the oceans. The second part assesses the state of glaciers in all of the glacierized regions of the planet, primarily as drawn in the other 10 chapters. It includes sections on ice cores and the climate record they contain, volumetric changes in glaciers, harnessing spaceborne sensors to measure changes in glaciers, and related topics. The third part summarizes trends in global snow cover. The fourth part summarizes long-term changes in area and thickness of floating ice, including polar sea ice and freshwater (lake and river) ice. The fifth part assesses the loss of permafrost and changes in periglacial environments at high latitudes and high altitudes.

Professional Paper↗

USGS2007

Spatial and temporal variability in oceanographic and meteorologic forcing along Central California and its implications on nearshore processes

In the past two decades, the understanding of the important large-scale phenomena (El Niño, upwelling, California current, etc) that drive physical, chemical, and biological processes along the US West Coast has greatly improved. However, the ability to predict the influence of annual and inter-annual events on a regional scale still remains limited. High-resolution hourly data from 6 National Oceanographic and Atmospheric Administration (NOAA) buoys deployed since the early 1980's off Central California were analyzed to improve our understanding of spatial and temporal variability of oceanographic and meteorologic forcing along the coastline. Seasonal to inter-annual trends in wave height, wave period, sea level barometric pressure, sea-surface temperature, and wind direction were identified, as were significant departures in these trends during El Niño and La Niña periods. The results suggest there are increasing wave heights and wave periods, decreasing sea level barometric pressures and variability in sea-surface temperatures, and increasingly variable winds off Central California between 1980 and 2002. The impact of these climatic trends on coastal physical, geological and biologic processes will also be addressed.

California↗

USGS2017

Reconstructing Common Era relative sea-level change on the Gulf Coast of Florida

To address a paucity of Common Era data in the Gulf of Mexico, we reconstructed ~ 1.1 m of relative sea-level (RSL) rise over the past ~ 2000 years at Little Manatee River (Gulf Coast of Florida, USA). We applied a regional-scale foraminiferal transfer function to fossil assemblages preserved in a core of salt-marsh peat and organic silt that was dated using radiocarbon and recognition of pollution, 137 Cs and pollen chronohorizons. Our proxy reconstruction was combined with tide-gauge data from four nearby sites spanning 1913–2014 CE. Application of an Errors-in-Variables Integrated Gaussian Process (EIV-IGP) model to the combined proxy and instrumental dataset demonstrates that RSL fell from ~ 350 to 100 BCE, before rising continuously to present. This initial RSL fall was likely the result of local-scale processes (e.g., silting up of a tidal flat or shallow sub-tidal shoal) as salt-marsh development at the site began. Since ~ 0 CE, we consider the reconstruction to be representative of regional-scale RSL trends. We removed a linear rate of 0.3 mm/yr from the RSL record using the EIV-IGP model to estimate climate-driven sea-level trends and to facilitate comparison among sites. This analysis demonstrates that since ~ 0 CE sea level did not deviate significantly from zero until accelerating continuously from ~ 1500 CE to present. Sea level was rising at 1.33 mm/yr in 1900 CE and accelerated until 2014 CE when a rate of 2.02 mm/yr was attained, which is the fastest, century-scale trend in the ~ 2000-year record. Comparison to existing reconstructions from the Gulf coast of Louisiana and the Atlantic coast of northern Florida reveal similar sea-level histories at all three sites. We explored the influence of compaction and fluvial processes on our reconstruction and concluded that compaction was likely insignificant. Fluvial processes were also likely insignificant, but further proxy evidence is needed to fully test this hypothesis. Our results indicate that no significant Common Era sea-level changes took place on the Gulf and southeastern Atlantic U.S. coasts until the onset of modern sea-level rise in the late 19th century.

Florida↗

USGS2024

Drought prediction and water availability: A report on the 2022 USGS-NIDIS National Listening Session Series

The U.S. Geological Survey (USGS) and NOAA’s National Integrated Drought Information System (NIDIS) conducted a series of four Listening Sessions in 2022 – each with a different application or topical focus – to seek input on priorities and needs related to predicting water availability changes under drought conditions at national and regional scales. This input was gathered to help inform the USGS Drought Program, regional and national drought efforts at NIDIS, and other national drought efforts. The series started with a February 2022 kick-off that introduced the series of Listening Sessions being held from March through September 2022. This kickoff also provided an overview of the USGS Drought Program’s work to characterize hydrological (e.g., streamflow and groundwater) drought, drought variability, drivers, and trends over the past century. Participants in these Listening Sessions included diverse stakeholder representation and perspectives. The first of the four Listening Sessions focused on streamflow (March 3, 2022), and included a short introduction to the USGS national streamflow drought research, the properties of a national drought prediction system, as well as presentations by other agencies on different drought prediction and forecasting efforts. The second session focused on groundwater (May 5, 2022), and included presentations on groundwater drought, sustainable groundwater management, and improving our understanding of soil moisture, groundwater, and surface water drought. The third session focused on water use (July 14, 2022), and included a discussion of the different drought types, as well as an introduction to several key projects, including the USGS Upper Colorado River Basin Study, the Ogallala Data Directory project, and a multi-agency drought prediction partnership in Oklahoma. The fourth and final Listening Session focused on water availability prediction for ecosystems (September 8, 2022), and included presentations on the development of a national capacity for eco-hydrological and drought science, building climate resilience, and actionable ecodrought resources.

Report↗

USGS2017

Snow and ice

Temperature and precipitation are key determinants of snowpack levels. Therefore, climate change is likely to affect the role of snow and ice in the landscapes and hydrology of the Chugach National Forest region. Downscaled climate projections developed by Scenarios Network for Alaska and Arctic Planning (SNAP) are useful for examining projected changes in snow at relatively fine resolution using a variable called “snowday fraction (SDF),” the percentage of days with precipitation falling as snow. We summarized SNAP monthly SDF from five different global climate models for the Chugach region by 500 m elevation bands, and compared historical (1971–2000) and future (2030–2059) SDF. We found that: Snow-day fraction and snow-water equivalent (SWE) are projected to decline most in late autumn (October to November) and at lower elevations. Snow-day fraction is projected to decrease 23 percent (averaged across five climate models) from October to March, between sea level and 500 m. Between sea level and 1000 m, SDF is projected to decrease by 17 percent between October and March. Snow-water equivalent is projected to decrease most in autumn (October and November) and at lower elevations (below 1500 m), an average of -26 percent for the 2030–2059 period compared to 1971– 2000. Averaged across the cool season and the entire domain, SWE is projected to decrease at elevations below 1000 m because of increased temperature, but increase at higher elevations because of increased precipitation. Compared to 1971–2000, the percentage of the landscape that is snowdominant in 2030–2059 is projected to decrease, and the percentage in which rain and snow are co-dominant (transient hydrology) is projected to increase from 27 to 37 percent. Most of this change is at lower elevations. Glaciers on the Chugach National Forest are currently losing about 6 km3 of ice per year; half of this loss comes from Columbia Glacier (Berthier et al. 2010). Over the past decade, almost all glaciers surveyed within the Chugach have lost mass (with one exception), including glaciers that have advancing termini (Larsen et al. 2015). Glaciers that are not calving into the ocean are typically thinning by 3 m/year at their termini (Larsen et al. 2015). In the future, glaciers not calving into the ocean will retreat and shrink at rates equivalent to or higher than current rates of ice loss (Larsen et al. 2015). Columbia Glacier will likely retreat another 15 km and break into multiple tributaries over the next 20 years before stabilizing. Other tidewater glaciers have uncertain futures, but likely will not advance significantly in coming decades. These impacts will likely affect recreation and tourism through changes in reliable snowpack and access to recreation and viewsheds.

Alaska↗

USGS2003

Evolution of CAM and C 4 carbon-concentrating mechanisms

Mechanisms for concentrating carbon around the Rubisco enzyme, which drives the carbon-reducing steps in photosynthesis, are widespread in plants; in vascular plants they are known as crassulacean acid metabolism (CAM) and C 4 photosynthesis. CAM is common in desert succulents, tropical epiphytes, and aquatic plants and is characterized by nighttime fixation of CO2. The proximal selective factor driving the evolution of this CO2-concentrating pathway is low daytime CO2, which results from the unusual reverse stomatal behavior of terrestrial CAM species or from patterns of ambient CO2 availability for aquatic CAM species. In terrestrials the ultimate selective factor is water stress that has selected for increased water use efficiency. In aquatics the ultimate selective factor is diel fluctuations in CO2 availability for palustrine species and extreme oligotrophic conditions for lacustrine species. C4 photosynthesis is based on similar biochemistry but carboxylation steps are spatially separated in the leaf rather than temporally as in CAM. This biochemical pathway is most commonly associated with a specialized leaf anatomy known as Kranz anatomy; however, there are exceptions. The ultimate selective factor driving the evolution of this pathway is excessively high photorespiration that inhibits normal C3 photosynthesis under high light and high temperature in both terrestrial and aquatic habitats. CAM is an ancient pathway that likely has been present since the Paleozoic era in aquatic species from shallow-water palustrine habitats. While atmospheric CO2 levels have undoubtedly affected the evolution of terrestrial plant carbon-concentrating mechanisms, there is reason to believe that past atmospheric changes have not played as important a selective role in the aquatic milieu since palustrine habitats today are not generally carbon sinks, and the selective factors driving aquatic CAM are autogenic. Terrestrial CAM, in contrast, is of increasing selective value under extreme water deficits, and undoubtedly, high Mesozoic CO2 levels reduced the amount of landscape perceived by plants as water limited. Late Tertiary and Quaternary reductions in atmospheric CO2, coupled with increasing seasonality, were probably times of substantial species radiation and ecological expansion for CAM plants. C4 photosynthesis occurs in only about half as many families as CAM, and three-fourths of C4 species are either grasses or sedges. Molecular phylogenies indicate C4 is a more recent innovation than CAM and that it originated in the mid-Tertiary, 20–30 Ma, although some data support an earlier origin. While the timing of the origin of C4 remains controversial, the nearly explosive increase in C4 species is clearly documented in the late Miocene, 4–7 Ma. Increasing seasonality has been widely suggested as an important climatic stimulus for this C4 expansion. Alternatively, based on models of photosynthetic quantum yield at different temperatures and CO2 concentration, it has been hypothesized that the late Miocene C4 expansion resulted from declining atmospheric CO2 levels. This model is most appropriate for explaining the transition from C3 grasslands to C4 grasslands but by itself may not be sufficient to explain the more likely scenario of a late Miocene transition from C3 woodland/ savanna to C4 grasslands. A largely unexplored hypothesis is that climatic changes in late Miocene altered disturbance regimes, in particular the incidence of fires, which today are often associated with maintenance of C4 grasslands. Oceanic charcoal sediments that appear to represent Aeolian deposits from continental wildfires follow a strikingly similar pattern of explosive increase in late Miocene. Climate, CO2, and disturbance are not mutually exclusive explanations and probably all acted in concert to promote the expansion of C4 grasslands. More recently, late Quaternary changes in CO2 may have been responsible for driving major changes in the landscape distribution of C4 species. The theory is sound; however, many of the studies cited in support of this model are open to alternative interpretations, and none has eliminated climatic factors as important selective agents. CAM and C4 evolution required coupling of biochemical pathways with structural changes in photosynthetic tissues, succulence in CAM and Kranz in C4. This was apparently accomplished by piecemeal evolution beginning with mechanisms for recapturing respiratory CO2, although this need not have been so in aquatic CAM species. It has been proposed that the extreme rarity of both pathways in the same plant results from biochemical and structural incompatibilities (Sage 2002). Equally important is the fact that the selective environments are quite different, with CAM evolution thriving on stressful sites inhospitable to C3 species whereas C4 evolution has selected for rapid growth capable of outcompeting associated C3 plants.

International Journal of Plant Sciences↗

USGS2014

Shifts in plant functional types have time-dependent and regionally variable impacts on dryland ecosystem water balance

Summary 1. Terrestrial vegetation influences hydrologic cycling. In water-limited, dryland ecosystems, altered ecohydrology as a consequence of vegetation change can impact vegetation structure, ecological functioning and ecosystem services. Shrub steppe ecosystems dominated by big sagebrush (Artemisia tridentata) are widespread across western North America, and provide a range of ecosystem services. While sagebrush abundance in these ecosystems has been altered over the past century, and changes are likely to continue, the ecohydrological consequences of sagebrush removal and reestablishment remain unclear. 2. To characterize the immediate and medium-term patterns of water cycling and availability following sagebrush plant community alteration, we applied the SOILWAT ecosystem water balance model to 898 sites across the distribution of sagebrush ecosystems, representing the three primary sagebrush ecosystem types: sagebrush shrublands, sagebrush steppe and montane sagebrush. At each site, we examined three vegetation conditions representing intact sagebrush, recently disturbed sagebrush and recovered but grass-dominated vegetation. 3. Transition from shrub to grass dominance decreased precipitation interception and transpiration and increased soil evaporation and deep drainage. Relative to intact sagebrush vegetation, simulated soils in the herbaceous vegetation phases typically had drier surface layers and wetter deep layers. 4. Our simulations suggested that alterations in ecosystem water balance may be most pronounced in vegetation representing recently disturbed conditions (herbaceous vegetation with low biomass) and only modest in conditions representing recovered, but still grass-dominated vegetation. Furthermore, the ecohydrological impact of simulated sagebrush removal depended on climate; while short-term changes in water balance were greatest in wet areas represented by the montane sagebrush ecosystem type, medium-term impacts were greatest in dry areas of sagebrush shrublands and sagebrush steppe. 5. Synthesis. This study provides a novel, regional-scale assessment of how plant functional type transitions may impact ecosystem water balance in sagebrush-dominated ecosystems of North America. Results illustrate that the ecohydrological consequences of changing vegetation depend strongly on climate and suggest that decreasing woody plant abundance may have only limited impact on evapotranspiration and water yield.

Journal of Ecology↗

USGS2000

Holocene and recent sediment accumulation rates in southern Lake Michigan

Rates of sediment accumulation in Lake Michigan are a key component of its geologic history and provide important data related to societal concerns such as shoreline erosion and the fate of anthropogenic pollutants. Previous attempts to reconstruct Holocene rates of sediment accumulation in Lake Michigan, as well as in the other Laurentian Great Lakes, have been bedeviled by the effect of refractory terrestrial material on radiocarbon ages from total organic carbon samples of lake sediments. AMS radiocarbon ages on small samples of biogenic carbonate (ostracodes and mollusks) in Lake Michigan provide accurate Holocene ages. The present bicarbonate reservoir effect is estimated from shells of mollusks collected live before atmospheric nuclear testing to be 250 yr. From paired samples of biogenic carbonate and terrestrial macrofossils, the past reservoir effect is thought to be less than 500 yr. The radiocarbon ages indicate a distinct decrease in sediment accumulation rates throughout the southern basin of Lake Michigan at about 5 ka, about the time when lake level stabilized at the Nipissing level after rising rapidly for several thousand years. Average rates of sediment accumulation for the historic period (the last 150 yr) can be estimated from radioisotopes ( 210 Pb and 137 Cs), pollen stratigraphy, and changes in sediment properties associated with human activity. Multiple methods are necessary because at any given site, problems arise in the assumptions or applicability of one or more methods. In general, the mass accumulation calculations suggest that sediments were deposited 4 to 11 times faster in the historic period than before human settlement. The character of the sediment did not change in a dramatic way, but sediment magnetic properties suggest shifts in the sources of sediment. The data suggest that some of the changes in sources and (or) character of the sediment occurred just before human settlement and were probably related to climatic changes associated with the Little Ice Age.

Michigan↗

USGS2021

Current activity on the Martian surface: A key subject for future exploration

One of the fundamental discoveries in Mars science in the last decade has been the extent and importance of current surface activity. Recent results have shifted our view of Mars from a world where the most interesting geologic events were in the distant past (similar to the Moon) to a world that undergoes active evolution and one where understanding the present is key to deciphering the planet’s history. When input was requested for the last Planetary Science Decadal Survey, some observations of surface changes had been published, but the number of detections was small and their significance not fully appreciated. Since that time, detections have proliferated, driven primarily by the long-term operation of the Mars Reconnaissance Orbiter (MRO) and the High Resolution Imaging Science Experiment (HiRISE) as well as landed observations of aeolian activity. In addition to observed changes, theory suggests that additional important surface processes are likely active but not yet observed because orbital data are limited in space and time and landed studies are rare. Understanding current Martian surface processes is a fundamental science question in itself, as it provides a test for physical and terrestrial analog-based models of specific geological processes acting under non-Earth planetary and environmental conditions. It is also an essential step for reading Mars’ geologic history and providing input to climate models: without understanding current dynamic processes, we cannot understand how they have varied during recent climate cycles, nor how they are reflected in ancient rock or modern ice records. Understanding the rates and types of current surface activity is also highly relevant to selecting geological samples, setting Planetary Protection rules, and understanding the hazards and environment that would be experienced by future human explorers.

Bulletin of the AAS↗

USGS2007

Sediment geochemical records of productivity and oxygen depletion along the margin of western North America during the past 60,000 years: Teleconnections with Greenland Ice and the Cariaco Basin

Many sediment records from the margins of the Californias (Alta and Baja) collected in water depths between 60 and 1200 m contain anoxic intervals (laminated sediments) that can be correlated with interstadial intervals as defined by the oxygen-isotope composition of Greenland ice (Dansgaard–Oeschger, D–O, cycles). These intervals include all or parts of Oxygen Isotope Stage 3 (OIS3; 60–24 cal ka), the Bölling/Alleröd warm interval (B/A; 15–13 cal ka), and the Holocene. This study uses organic carbon (Corg) and trace-element proxies for anoxia and productivity, namely elevated concentrations and accumulation rates of molybdenum and cadmium, in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom-water oxygen (BWO) conditions on open, highly productive continental margins. The main conclusion from these proxies is that during the last glacial interval (LGI; 24–15 cal ka) and the Younger Dryas cold interval (YD; 13–11.6 cal ka) productivity was lower and BWO levels were higher than during OIS3, the B/A, and the Holocene on all margins of the Californias. The Corg and trace-element profiles in the LGI–B/A–Holocene transition in the Cariaco Basin on the margin of northern Venezuela are remarkably similar to those in the transition on the northern California margin. Correlation between D–O cycles in Greenland ice with gray-scale measurements in varved sediments in the Cariaco Basin also is well established. Synchronous climate-driven changes as recorded in the sediments on the margins of the Californias, sediments from the Cariaco Basin, and in the GISP-2 Greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka. Millennial-scale cycles in productivity and oxygen depletion on the margins of the Californias demonstrate that the California Current System was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern North Pacific Ocean. It is likely that the Pacific and Atlantic Oceans were linked through the atmosphere. Warmer air temperatures during interstadials would have strengthened Hadley and Walker circulations, which, in turn, would have strengthened the subtropical high pressure systems in both the North Pacific and the North Atlantic, producing increased rainfall over the Cariaco Basin and increased upwelling along the margins of the Californias.

California↗

USGS2020

Opportunities and challenges for restoration of the Merced River through Yosemite Valley, Yosemite National Park, USA

Successful river restoration requires understanding and integration of multiple disciplinary perspectives, including evaluations of past and ongoing watershed processes, local geomorphic response, and impacts unique to human activity. Nowhere is this more apparent than along the Merced River in Yosemite National Park, USA, where both an outstanding natural landscape and the consequences of over a century of human disturbances continue to interact. An intact upstream watershed highlights the importance here of local impacts on geomorphic response. Incision and the resulting decoupling of the channel from its adjacent late-Holocene floodplain are consequences of reduced channel roughness, likely from de-snagging the river, and instream gravel mining in the 19th and early 20th century. Riparian-zone disturbance by visitor use has damaged riparian vegetation and soils, inducing channel widening. Revetments and channel-spanning bridges, the latter being visible and oft-cited impacts to fluvial processes, have distorted the natural evolution of meanders and induced local channel narrowing. The historical rate of sediment export from Yosemite Valley has greatly exceeded replenishment from upstream and lateral sources, creating a deficit that now inhibits recovery via passive restoration of more natural channel form and function. Climate change may amplify now-diminished fluvial processes but also exacerbate the rate of sediment export. These conditions, reflecting a complex intersection of geologic history, modern geomorphic processes, and human interactions, demonstrate how a limited influx of sediment coupled with intensive human use can have long-term consequences for riverine conditions, restoration opportunities, and social engagement with the riverine landscape.

California↗

USGS2019

A landscape model of variable social-ecological fire regimes

Fire regimes are now recognized as the product of social processes whereby fire on any landscape is the product of human-generated drivers: climate change, historical patterns of vegetation manipulation, invasive species, active fire suppression, ongoing fuel management efforts, prescribed burning, and accidental ignitions. We developed a new fire model (Social-Climate Related Pyrogenic Processes and their Landscape Effects: SCRPPLE) that emphasizes the social dimensions of fire and enables simulation of fuel-treatment effects, fire suppression, and prescribed fires. Fire behavior was parameterized with daily fire weather, ignition, and fire-boundary data. SCRPPLE was initially parameterized and developed for the Lake Tahoe Basin (LTB) in California and Nevada, USA although its behavior is general and could be applied worldwide. We demonstrate the behavior and utility of our model via four simple scenarios that emphasize the social dimensions of fire regimes: a) Recent Historical: simulated recent historical patterns of lightning and accidental fires and current patterns of fire suppression, b) Natural-Fire-Regime: simulated wildfire without suppression, accidental fires, or prescribed fires, holding all other factors the same as Recent Historical, c) Enhanced Suppression: simulated a doubling of the effectiveness of suppression, holding all other factors the same as Recent Historical, and d) Reduced Accidental Ignitions: within which the number of accidental fires was reduced by half, holding all other factors the same as Recent Historical. Results indicate that SCRPPLE can recreate past fire regimes, including size, intensity, and locations. Furthermore, our results indicate that the ‘Enhanced Suppression’ and ‘Reduced Accidental Ignitions’ scenarios had similar capacity to reduce fire and related tree mortality over time, suggesting that within the broad outlines of the scenarios, reducing accidental fires can be as effective as substantially increasing resources for suppression.

California, Nevada↗