Landscape Partnership Resources Library
The Perfect Ocean for Drought
The 1998 –2002 droughts spanning the United States, southern Europe, and South- west Asia were linked through a common oceanic influence. Cold sea surface temperatures (SSTs) in the eastern tropical Pacific and warm SSTs in the western tropical Pacific and Indian oceans were remarkably persistent during this period. Climate models show that the climate signals forced separately by these regions acted synergistically, each contributing to widespread mid-latitude drying: an ideal scenario for spatially expansive, synchronized drought. The warmth of the Indian and west Pacific oceans was unprecedented and consistent with greenhouse gas forcing. Some implications are drawn for future drought.
Moisture transport across Central America as a positive feedback on abrupt climatic changes
Moisture transport from the Atlantic to the Pacific ocean across Central America leads to relatively high salinities in the North Atlantic Ocean1 and contributes to the formation of North Atlantic Deep Water2. This deep water formation varied strongly between Dansgaard/Oeschger interstadials and Heinrich events— millennial-scale abrupt warm and cold events, respectively, during the last glacial period3. Increases in the moisture transport across Central America have been proposed to coincide with northerly shifts of the Intertropical Convergence Zone and with Dansgaard/ Oeschger interstadials, with opposite changes for Heinrich events4. Here we reconstruct sea surface salinities in the eastern equatorial Pacific Ocean over the past 90,000 years by comparing palaeotemperature estimates from alkenones and Mg/Ca ratios with foraminiferal oxygen isotope ratios that vary with both tem- perature and salinity. We detect millennial-scale fluctuations of sea surface salinities in the eastern equatorial Pacific Ocean of up to two to four practical salinity units. High salinities are associated with the southward migration of the tropical Atlantic Intertropical Convergence Zone, coinciding with Heinrich events and with Greenland stadials5. The amplitudes of these salinity variations are significantly larger on the Pacific side of the Panama isthmus, as inferred from a comparison of our data with a palaeoclimate record from the Caribbean basin6. We conclude that millennial- scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes, modulating the North Atlantic freshwater budget and hence North Atlantic Deep Water formation.
FIXING THE SKY
When nations made plans to save the ozone layer, they didn’t factor in global warming. Quirin Schiermeier reports on how two environmental problems complicate each other.
The challenge of hot drought
1st paragraph: rought is heating up around the warm- ing world. Particularly hot drought has cost more than US$40 billion and claimed 218 human lives since 2010 in the United States alone1. These hot and dry conditions have also contributed to unusually widespread and devastating wildfires1, fuelled by wide expanses of weakened and dead trees that were unable to deal with heat stress and subsequent insect attack2. Yet, to get a real sense of how this recent change in drought severity might shape the future, one has to look to the past. An analysis of regional and pan- continental North American drought over the past 1,000 years, reported by Cook et al.3 in the Journal of Climate, makes it clear that recent droughts, as costly as they have been, are only a taste of what might lie ahead, independently of any big climate change.
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.
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
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.
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.
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.
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.
Southward movement of the Pacific intertropical convergence zone AD 1400–1850
Tropical rainfall patterns control the subsistence lifestyle of more than one billion people. Seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone, which is characterized by deep convection causing heavy rainfall near 10◦ N in boreal summer and 3◦ N in boreal winter. Dynamic controls on the position of the intertropical convergence zone are debated, but palaeoclimatic evidence from continental Asia, Africa and the Americas suggests that it has shifted substantially during the past millennium, reaching its southernmost position some time during the Little Ice Age (AD 1400–1850). However, without records from the meteorological core of the intertropical convergence zone in the Pacific Ocean, quantitative constraints on its position are lacking. Here we report microbiological, molecular and hydrogen isotopic evidence from lake sediments in the Northern Line Islands, Galápagos and Palau indicating that the Pacific intertropical convergence zone was south of its modern position for most of the past millennium, by as much as 500 km during the Little Ice Age. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the intertropical convergence zone south. We conclude that small changes in Earth’s radiation budget may profoundly affect tropical rainfall.
Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation
Anthropogenic greenhouse-gas emissions continue to increase rapidly despite efforts aimed at curbing the release of such gases. One potentially long-term solution for offsetting these emissions is the capture and storage of carbon dioxide. In principle, fluid or gaseous carbon dioxide can be injected into the Earth’s crust and locked up as carbonate minerals through chemical reactions with calcium and magnesium ions supplied by silicate minerals. This process can lead to near-permanent and secure sequestration, but its feasibility depends on the ease and vigour of the reactions. Laboratory studies as well as natu- ral analogues indicate that the rate of carbonate mineral formation is much higher in host rocks that are rich in magnesium- and calcium-bearing minerals. Such rocks include, for example, basalts and magnesium-rich mantle rocks that have been emplaced on the continents. Carbonate mineral precipitation could quickly clog up existing voids, presenting a challenge to this approach. However, field and laboratory observations suggest that the stress induced by rapid precipitation may lead to fracturing and subsequent increase in pore space. Future work should rigorously test the feasibility of this approach by addressing reaction kinetics, the evolution of permeability and field-scale injection methods.
