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Impacts of reforestation upon sediment load and water outflow in the Lower Yazoo River Watershed, Mississippi
Among the world’s largest coastal and river basins, the Lower Mississippi River Alluvial Valley (LMRAV) is one of the most disturbed by human activities. This study ascertained the impacts of reforestation on water outflow attenuation (i.e., water flow out of the watershed outlet) and sediment load reduction in the Lower Yazoo River Watershed (LYRW) within the LMRAV using the US-EPA’s BASINS-HSPF model. The model was calibrated and validated with available experimental data prior to its application. Two simulation scenarios were then performed: one was chosen to predict the water outflow and sediment load without reforestation and the other was selected to project the potential impacts of reforestation upon water outflow attenuation and sediment load reduction following the conversion of 25, 50, 75, and 100% of the agricultural lands with most lands near or in the batture of the streams. Comparison of the two simulation scenarios (i.e., with and without reforestation) showed that a conversion of agricultural land into forests attenuated water outflow and reduced sediment load. In general, a two-fold increase in forest land area resulted in approximately a two-fold reduction in annual water outflow volume and sediment load mass, which occurred because forests absorb water and reduce surface water runoff and prevent soil erosion. On average, over a 10-year simulation, the specific water outflow attenuation and sediment load reduction were, respectively, 250 m3 /ha/y and 4.02 metric ton/ha/y. Seasonal variations of water outflow attenuation and sediment load reduction occurred with the maximum attenuation/reduction in winter and the minimum attenuation/reduction in summer. Our load duration curve analysis further confirmed that an increase in forest land area reduced the likelihood of a given sediment load out of the watershed outlet. This study suggests that reforestation in or around the batture of streams is a useful practice for water outflow attenuation and sediment load reduction.
Oligocene CO2 Decline Promoted C4 Photosynthesis in Grasses
C4 photosynthesis is an adaptation derived from the more common C3 photosynthetic pathway that con- fers a higher productivity under warm temperature and low atmospheric CO2 concentration [1, 2]. C4 evolution has been seen as a consequence of past atmospheric CO2 decline, such as the abrupt CO2 fall 32–25 million years ago (Mya) [3–6]. This relationship has never been tested rigorously, mainly because of a lack of accurate estimates of divergence times for the different C4 lineages [3]. In this study, we inferred a large phylogenetic tree for the grass family and es- timated, through Bayesian molecular dating, the ages of the 17 to 18 independent grass C4 lineages. The first transition from C3 to C4 photosynthesis occurred in the Chloridoideae subfamily, 32.0–25.0 Mya. The link between CO2 decrease and transition to C4 pho- tosynthesis was tested by a novel maximum likeli- hood approach. We showed that the model incorpo- rating the atmospheric CO2 levels was significantly better than the null model, supporting the importance of CO2 decline on C4 photosynthesis evolvability. This finding is relevant for understanding the origin of C4 photosynthesis in grasses, which is one of the most successful ecological and evolutionary innovations in plant history.
Old-Growth Forests Can Accumulate Carbon in Soils
1st paragraph: ld-growth forests have traditionally been considered negligible as carbon sinks because carbon uptake has been thought to be balanced by respiration (1). We show that soils in the top 20-cm soil layer in preserved old-growth forests in southern China accumulated atmospheric carbon at an unexpectedly high rate from 1979 to 2003. This phenomenon indicates the need for future research on the complex responses and adaptation of belowground processes to global environmental change.
Comment:Nuclear winter is a real and present danger
Models show that even a ‘small’ nuclear war would cause catastrophic climate change. Such findings must inform policy, says Alan Robock.
Growth, carbon-isotope discrimination, and drought-associated mortality across a Pinus ponderosa elevational transect
Drought- and insect-associated tree mortality at low-elevation ecotones is a widespread phenomenon but the underlying mechanisms are uncertain. Enhanced growth sensitivity to climate is widely observed among trees that die, indicating that a predisposing physiological mechanism(s) underlies tree mortality. We tested three, linked hypotheses regarding mortality using a ponderosa pine (Pinus ponderosa) elevation transect that experienced low-elevation mortality following prolonged drought. The hypotheses were: (1) mortality was associated with greater growth sensitivity to climate, (2) mortality was associated with greater sensitivity of gas exchange to climate, and (3) growth and gas exchange were correlated. Support for all three hypotheses would indicate that mortality results at least in part from gas exchange constraints. We assessed growth using basal area increment normalized by tree basal area [basal area increment (BAI)/basal area (BA)] to account for differences in tree size. Whole-crown gas exchange was indexed via estimates of the CO2 partial pressure difference between leaf and atmosphere (pa-pc) derived from tree ring carbon isotope ratios (d13C), corrected for temporal trends in atmospheric CO2 and d13C and elevation trends in pressure. Trees that survived the drought exhibited strong correlations among and between BAI, BAI/BA, pa-pc, and climate. In contrast, trees that died exhibited greater growth sensitivity to climate than trees that survived, no sensitivity of pa-pc to climate, and a steep relationship between pa-pc and BAI/BA. The pa-pc results are consistent with predictions from a theoretical hydraulic model, suggesting trees that died had a limited buffer between mean water availability during their lifespan and water availability during drought – i.e., chronic water stress. It appears that chronic water stress predisposed low-elevation trees to mortality during drought via constrained gas exchange. Continued intensification of drought in mid-latitude regions may drive increased mortality and ecotone shifts in temperate forests and woodlands. Keywords: altitude, climate change, die-off, photosynthesis, stomatal conductance, water availability
Elevation-dependent influence of snow accumulation on forest greening
Rising temperatures and declining water availability have influenced the ecological function of mountain forests over the past half-century. For instance, warming in spring and summer and shifts towards earlier snowmelt are associated with an increase in wildfire activity and tree mortality in mountain forests in the western United States (1,2). Temperature increases are expected to continue during the twenty-first century in mountain ecosystems across the globe (3,4), with uncertain consequences. Here, we examine the influence of interannual variations in snowpack accumulation on forest greenness in the Sierra Nevada Mountains, California, between 1982 and 2006. Using observational records of snow accumulation and satellite data on vegetation greenness we show that vegetation greenness increases with snow accumulation. Indeed, we show that variations in maximum snow accumulation explain over 50% of the interannual variability in peak forest greenness across the Sierra Nevada region. The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited mid- elevations, between 2,000 and 2,600 m. In situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.
winds of change
On average, terrestrial near-surface winds have slowed down in recent decades. This change will affect both wind energy and hydrology.
Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands
Climate change has increased the area affected by forest fires each year in boreal North America1,2. Increases in burned area and fire frequency are expected to stimulate boreal carbon losses3–5. However, the impact of wildfires on carbon emissions is also affected by the severity of burning. How climate change influences the severity of biomass burning has proved difficult to assess. Here, we examined the depth of ground-layer combustion in 178 sites dominated by black spruce in Alaska, using data collected from 31 fire events between 1983 and 2005. We show that the depth of burning increased as the fire season progressed when the annual area burned was small. However, deep burning occurred throughout the fire season when the annual area burned was large. Depth of burning increased late in the fire season in upland forests, but not in peatland and permafrost sites. Simulations of wildfire-induced carbon losses from Alaskan black spruce stands over the past 60 years suggest that ground-layer combustion has accelerated regional carbon losses over the past decade, owing to increases in burn area and late-season burning. As a result, soils in these black spruce stands have become a net source of carbon to the atmosphere, with carbon emissions far exceeding decadal uptake.
Trends in the sources and sinks of carbon dioxide
Efforts to control climate change require the stabilization of atmospheric CO2 concentrations. This can only be achieved through a drastic reduction of global CO2 emissions. Yet fossil fuel emissions increased by 29% between 2000 and 2008, in conjunction with increased contributions from emerging economies, from the production and international trade of goods and services, and from the use of coal as a fuel source. In contrast, emissions from land-use changes were nearly constant. Between 1959 and 2008, 43% of each year’s CO2 emissions remained in the atmosphere on average; the rest was absorbed by carbon sinks on land and in the oceans. In the past 50 years, the fraction of CO2 emissions that remains in the atmosphere each year has likely increased, from about 40% to 45%, and models suggest that this trend was caused by a decrease in the uptake of CO2 by the carbon sinks in response to climate change and variability. Changes in the CO2 sinks are highly uncertain, but they could have a significant influence on future atmospheric CO2 levels. It is therefore crucial to reduce the uncertainties.
Observational evidence for soil-moisture impact on hot extremes in southeastern Europe
Climate change is expected to affect not only the means of climatic variables, but also their variabilities1,2 and extremes such as heat waves2–6. In particular, modelling studies have postulated a possible impact of soil-moisture deficit and drought on hot extremes7–11. Such effects could be responsible for impending changes in the occurrence of heat waves in Europe7. Here we analyse observational indices based on measurements at 275 meteorological stations in central and southeastern Europe, and on publicly available gridded observations12. We find a relationship between soil-moisture deficit, as expressed by the standardized precipitation index13, and summer hot extremes in southeastern Europe. This relationship is stronger for the high end of the distribution of temperature extremes. We compare our results with simulations of current climate models and find that the models correctly represent the soil-moisture impacts on temperature extremes in southeastern Europe, but overestimate them in central Europe. Given the memory associated with soil moisture storage, our findings may help with climate-change- adaptation measures, such as early-warning and prediction tools for extreme heat waves.
Brownness of organics in aerosols from biomass burning linked to their black carbon content
Atmospheric particulate matter plays an important role in the Earth’s radiative balance. Over the past two decades, it has been established that a portion of particulate matter, black carbon, absorbs significant amounts of light and exerts a warming effect rivalling that of anthropogenic carbon dioxide1,2. Most climate models treat black carbon as the sole light-absorbing carbonaceous particulate. However, some organic aerosols, dubbed brown carbon and mainly associated with biomass burning emissions3–6 , also absorbs light7 . Unlike black carbon, whose light absorption properties are well understood8, brown carbon comprises a wide range of poorly characterized compounds that exhibit highly variable absorptivities, with reported values spanning two orders of magnitude3–6,9,10. Here we present smog chamber experiments to characterize the effective absorptivity of organic aerosol from biomass burning under a range of conditions. We show that brown carbon in emissions from biomass burning is associated mostly with organic compounds of extremely low volatility11. In addition, we find that the effective absorptivity of organic aerosol in biomass burning emissions can be parameterized as a function of the ratio of black carbon to organic aerosol, indicating that aerosol absorptivity depends largely on burn conditions, not fuel type. We conclude that brown carbon from biomass burning can be an important factor in aerosol radiative forcing.
Autopsy of two mega-heatwaves
Record-breaking heatwaves in 2003 and 2010 surprised both the public and experts. Observations provide new insights into how temperatures escalated to unprecedented values through the interaction of boundary-layer dynamics and land surface drying.
Impacts in the third dimension
Despite reports of no trends in snow- and rainfall, rivers in the northwest USA have run lower and lower in recent decades. A closer look at high- and low-altitude precipitation suggests that observational networks have missed a decline in mountain rain and snow that can explain the discrepancy.
Strong increase in convective precipitation in response to higher temperatures
Precipitation changes can affect society more directly than variations in most other meteorological observables1–3, but precipitation is difficult to characterize because of fluctuations on nearly all temporal and spatial scales. In addition, the intensity of extreme precipitation rises markedly at higher temperature4–9, faster than the rate of increase in the atmosphere’s water-holding capacity1,4 , termed the Clausius– Clapeyron rate. Invigoration of convective precipitation (such as thunderstorms) has been favoured over a rise in stratiform precipitation (such as large-scale frontal precipitation) as a cause for this increase4,10, but the relative contributions of these two types of precipitation have been difficult to disentan- gle. Here we combine large data sets from radar measurements and rain gauges over Germany with corresponding synoptic ob- servations and temperature records, and separate convective and stratiform precipitation events by cloud observations. We find that for stratiform precipitation, extremes increase with temperature at approximately the Clausius–Clapeyron rate, without characteristic scales. In contrast, convective precipi- tation exhibits characteristic spatial and temporal scales, and its intensity in response to warming exceeds the Clausius– Clapeyron rate. We conclude that convective precipitation responds much more sensitively to temperature increases than stratiform precipitation, and increasingly dominates events of extreme precipitation.
Continuous flux of dissolved black carbon from a vanished tropical forest biome
Humans have used fire extensively as a tool to shape Earth’s vegetation. The slash-and-burn destruction of Brazil’s Atlantic forest, which once covered over 1.3 million km2 of present-day Brazil and was one of the largest tropical forest biomes on Earth1, is a prime example. Here, we estimate the amount of black carbon generated by the burning of the Atlantic forest, using historical records of land cover, satellite data and black carbon conversion ratios. We estimate that before 1973, destruction of the Atlantic forest generated 200–500 million tons of black carbon. We then estimate the amount of black carbon exported from this relict forest between 1997 and 2008, using measurements of polycyclic aromatic black carbon collected from a large river draining the region, and a continuous record of river discharge. We show that dissolved black carbon (DBC) continues to be mobilized from the watershed each year in the rainy season, despite the fact that widespread forest burning ceased in 1973. We estimate that the river exports 2,700 tons of DBC to the ocean each year. Scaling our findings up, we estimate that 50,000–70,000 tons of DBC are exported from the former forest each year. We suggest that an increase in black carbon production on land could increase the size of the refractory pool of dissolved organic carbon in the deep ocean.
Reduction in carbon uptake during turn of the century drought in western North America
Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present1–3. Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity3,4. The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming5–8. Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30–298Tg C yr−1 during the 2000–2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions7 and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177–623 Tg C yr−1 in western North America could disappear by the end of the century.
Editorial : Beyond forest carbon
The preservation of forests, both on land and in mangrove swamps, has received much attention in the move to protect biological carbon stores. Less conspicuous communities of organisms deserve some scrutiny, too.
Response of the North Atlantic storm track to climate change shaped by ocean– atmosphere coupling
A poleward shift of the mid-latitude storm tracks in response to anthropogenic greenhouse-gas forcing has been diagnosed in climate model simulations1,2. Explanations of this effect have focused on atmospheric dynamics3–7 . However, in contrast to storm tracks in other regions, the North Atlantic storm track responds by strengthening and extending farther east, in particular on its southern flank8. These adjustments are associated with an intensification and extension of the eddy- driven jet towards western Europe9 and are expected to have considerable societal impacts related to a rise in storminess in Europe10–12. Here, we apply a regression analysis to an ensemble of coupled climate model simulations to show that the coupling between ocean and atmosphere shapes the distinct storm-track response to greenhouse-gas forcing in the North Atlantic region. In the ensemble of simulations we analyse, at least half of the differences between the storm-track responses of different models are associated with uncertainties in ocean circulation changes. We compare the fully coupled simulations with both the associated slab model simulations and an ocean-forced experiment with one climate model to establish causality. We conclude that uncertainties in the response of the North Atlantic storm track to anthropogenic emissions could be reduced through tighter constraints on the future ocean circulation.
Wood and river landscapes
The influence of trees and dead wood on river dynamics has long been overlooked. Recent work suggests that large wood pieces can stabilize the land surface, contributing to a large-wood cycle that profoundly affects floodplain morphology and ecology.
Riverine carbon dioxide release
Inland waters are increasingly recognized as important to the global carbon cycle. Detailed measurements in the United States suggest that significant amounts of carbon dioxide are released from streams and rivers, particularly the smaller ones.
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