B31C-01 08:00h
Considerations of Socio-Economic and Global Change Effects on Eurasian Steppes Ecosystem and Land-Atmosphere Interactions
Dramatic changes occurred in pastoral systems of Eurasia ranging from Mongolia, China and Central Asia for the past decades. Recently, evaluation of the pastoral systems has been conducted in the region. Pastoral systems, where humans depend on livestock, exist largely in arid or semi-arid ecosystems where climate is highly variable. Interaction between ecosystems and nomadic land use systems co-shaped them in mutual adaptive ways for hundreds of years, thus making both the Mongolian rangeland ecosystem and nomadic pastoral system resilient and sustainable. Current changes in environmental conditions are affecting land-atmosphere interactions. Regional dust events, changes in hydrological cycle, and land use changes contribute to changing interactions between ecosystem and landscape processes which affect regional climate. The general trend involves greater intensification of resource exploitation at the expense of traditional patterns of extensive range utilization. This set of drivers is orthogonal to the above described climate drivers. Thus we expect climate-land use-land cover relationships to be crucially modified by the socio-economic forces.
B31C-02 08:15h
Feedbacks Between Agriculture and Climate: An Illustration of the Potential Unintended Consequences of Human Land-use Activities
Agriculture has significantly transformed the face of the planet -- in particular, croplands have replaced natural vegetation over large areas of the global land surface. To cultivate the land, humans take advantage of the resource provided by climate -- optimum temperature and precipitation. However, land clearing for cultivation might have resulted in an inadvertent change in the climate. This feedback might, in turn, have altered the suitability of land for growing crops. In this study, we used a combination of land cover data sets, numerical models, and cropland suitability analysis, to estimate the degree to which the replacement of natural vegetation by croplands might have altered the land suitability for cultivation. We found that the global changes in cropland suitability are likely to have been fairly small, however large regional changes in land suitability might have occurred. Our study showed that the major changes in suitability occurred in Canada, Eastern Europe, the Former Soviet Union, and Mexico and Central America. Our study also showed that local land cover changes may very likely cause changes in climate elsewhere through changing the general circulation of the atmosphere. Therefore, similar to climate problems related to fossil-fuel emissions, local land use changes have the potential to become a global climate problem. Although the magnitude, sign, and spatial patterns of change indicated by this study may be an artifact of our particular model and experimental design, our study is illustrative of the potential inadvertent consequences of human activities on the land. Moreover, it offers a methodology for evaluating how climate changes due to human activities on the land, may alter the multiple services offered by ecosystems to human beings.
B31C-03 08:30h
Carbon Budget in High-Arctic Greenland Analyzed by a Cascade System Approach
The High Arctic is considered one of the most vulnerable climatic zones with respect to the potential effect of a global warming. To be able to monitor this development, the carbon budget has for the last decade been observed at Zackenberg research station of NE Greenland (74.5 oN, 20.5 oW). The CO2 flux measurements has comprised terrestrial (wetlands and dwarf shrub heaths), fluvial and coastal ecosystems and this is supplemented by CO2 exchange measurements over the Greenlandic sea. Rather than a detailed study of the individual components a cascade system approach is used starting with the fixation of CO2 by the vegetation and the sea, followed by carbon release through dissolution and erosion, and lateral transport by rivers and currents until a final sedimentation takes place in the fjord system. The results show that the High Arctic is an important carbon sink also when the methane emission is taken into account. It is found that there is large diversity in the CO2 exchange rates among the terrestrial ecosystem which may be scaled by use of the leaf area index and the snow cover. This is verified by comparison with fluxes derived by planetarian boundary layer budgets. The largest CO2 uptake rates are measured in the wetlands during July and August (2 g C m-2 d-1) but due to the net CO2 emission simulated for the rest of the year the annual NEE is only around 18 g C m-2 yr-1 This is only one third of the annual carbon uptake in the open Greenlandic Sea because the carbon uptake is nearly constant throughout the year, only in case of sea ice the CO2 uptake is reduced to near zero. Finally, climatic feedback couplings are discussed. For the land surface increasing summertime temperatures leads to an increase in carbon uptake which more than compensates for an increased CO2 emission during wintertime. A negative feedback coupling with temperature is also found for the sea, because the observed decrease in sea ice will also imply an increase in the CO2 uptake rate.
B31C-04 08:45h
Planet-wide volcanics correlated with Last Glacial abrupt climate changes
We recently reported a correlation in excess of 99.5% between volcanic ash layers recorded in the deep ice core site at Siple Dome, West Antarctica and millennium-timescale abrupt cold periods (Dansgaard-Oeschger events) recorded at Summit, Greenland (GISP2) during the last glacial period. These data, obtained with our deep borehole optical dust logger, are the best evidence yet for a causal connection between volcanism and millennial climate change on the planetary scale, and lead to possibilities of a direct causal relationship. We now present a comparison with other volcanic proxies which demonstrates that the heaviest ash layers we detected at Siple Dome, those sufficiently concentrated for detailed chemical analysis in the core, appear to have come from local sources in West Antarctica, whereas the majority correspond to volcanic events detected throughout the Antarctic continent that correlate strongly with millennial climate changes in the Northern Hemisphere. Excluding the several heaviest ash signals in the Siple Dome data set increases the correlation with climate above the 3-sigma level, more than 800-to-one rejection of the null hypothesis. In June 2004 we deployed a high-resolution logger in the GRIP borehole at Summit, Greenland. We detected of order $\sim$100 volcanic ash layers which correlate weakly if at all with millennial climate change, consistent with studies of other Greenlandic records of volcanism. This contrast may provide an important clue to understanding global volcano-climate interaction as well as the role of the Southern Hemisphere. Of interest is a scenario in which volcanic ash and sulfate abruptly increase the soluble iron in large surface areas of nutrient-limited oceans, particularly the Southern Ocean, and stimulate growth of phytoplankton which enhance cooling by altering ocean albedo and atmospheric chemistry through mechanisms not fully understood. Viewed from another perspective, crustal stresses from ice-sheet loading/unloading and changes in sea-level driven by climate fluctuations are believed to trigger volcanic eruptions, and these data could be evidence supporting a bipolar seesaw model. Large global temperature swings may even be limited by feedback within the volcano-climate system.
B31C-05 09:00h
Clouds Versus Carbon: Predicting Vegetation Roughness by Maximizing Productivity
Surface roughness is one of the dominant vegetation properties that affects land surface exchange of energy, water, carbon, and momentum with the overlying atmosphere. We hypothesize that the canopy structure of terrestrial vegetation adapts optimally to climate by maximizing productivity, leading to an optimum surface roughness. An optimum should exist because increasing values of surface roughness cause increased surface exchange, leading to an increased supply of carbon dioxide for photosynthesis. At the same time, increased roughness enhances evapotranspiration and cloud cover, thereby reducing the supply of photosynthetically active radiation. We demonstrate the optimum through sensitivity simulations using a coupled dynamic vegetation-climate model for present day conditions, in which we vary the value of surface roughness for vegetated surfaces. We find that the maximum in productivity occurs at a roughness length of 2 meters, a value commonly used to describe the roughness of today's forested surfaces. The sensitivity simulations also illustrate the strong climatic impacts of vegetation roughness on the energy and water balances over land: with increasing vegetation roughness, solar radiation is reduced by up to 20 W/m2 in the global land mean, causing shifts in the energy partitioning and leading to general cooling of the surface by 1.5 K. We conclude that the roughness of vegetated surfaces can be understood as a reflection of optimum adaptation, and it is associated with substantial changes in the surface energy and water balances over land. The role of the cloud feedback in shaping the optimum underlines the importance of an integrated perspective that views vegetation and its adaptive nature as an integrated component of the Earth system.
B31C-06 09:15h
Understanding urban atmospheric CO$_{2}$: Challenges and Opportunities
Many studies have shown that atmospheric CO$_{2}$ concentrations are elevated far above ambient levels in cities due to strong local sources. Measurements of urban atmospheric CO2 mixing ratio, its isotopic composition, and its sources and sinks provide opportunities to understand the local carbon cycle and biogeochemistry of cities, which is increasingly important in studies of regional and global change as well as urban sustainability and planning. In an ongoing project in the Salt Lake Valley, Utah, measurements of CO$_{2}$ mixing ratio and the isotopic composition of CO$_{2}$ have shown that vehicle exhaust significantly elevates CO$_{2}$ mixing ratios above ambient, particularly in the wintertime when temperature inversions create stable conditions. Natural gas combustion also makes a large contribution to CO$_{2}$ mixing ratio in the winter, but becomes negligible in the summer. However, the urban "forest" in the Salt Lake Valley plays an active role in influencing CO$_{2}$ mixing ratio during the spring, summer, and fall through photosynthesis and respiration. Atmospheric CO$_{2}$ measurements in the Salt Lake Valley are also useful in that they correlate with air pollutants such as aerosols, particularly in the wintertime when CO$_{2}$ sources are dominated by combustion. The relationship between CO$_{2}$ mixing ratio and other pollutants varies as a function of fuel source (natural gas versus gasoline) and meteorological variables that affect atmospheric chemistry of reactive compounds; therefore, these relationships provide additional information about sources and sinks for atmospheric constituents. Finally, CO$_{2}$ is a stable atmospheric tracer in that it does not undergo chemical transformations in the atmosphere. Measurements in the Salt Lake Valley showed that the temporal and spatial distribution of CO$_{2}$ in the wintertime may provide information about atmospheric transport during complex cold pools events if mixing ratios are monitored at multiple locations. These results suggest that studies of the processes and controls on land-atmosphere exchange of CO$_{2}$ are warranted in human-dominated, urban systems as well as more "pristine" natural ecosystems.
B31C-07 09:30h
Quantifying Urban-Ecosystem Atmosphere Fluxes of N$_2$O, CO$_2$, CH$_4$ over Denver, Colorado.
Urban ecosystems are complex, rapidly expanding, and the location of strong emissions of greenhouse gases. Ecosystem-atmosphere exchanges of carbon dioxide (CO$_2$), methane (CH$_4$), nitrous oxide (N$_2$O) and water vapor were measured over a ~30 km$^2$ portion of Denver, Colorado. Equipment used in these measurements included eddy covariance sensors at two levels on a tall 120m tower for CO$_2$ flux and evapotranspiration (ET), soil surface chambers for the measurement of CO$_2$, CH$_4$, and N$_2$O fluxes at 33 locations, and direct flux measurements of CO$_2$ and N$_2$O from the water treatment system. Measurement of CO$_2$ flux and ET has been continuous for more than two years (2002-2004). Chamber measurements were conducted on selected dates during two growing seasons. Direct flux measurements from the water treatment system were conducted on a monthly basis in the 2004 summer. Measurements indicate both strong temporal and spatial heterogeneity of fluxes owing to characteristics of natural and anthropogenic ecosystem components. Tower-based eddy covariance measurements indicated that the urban ecosystem was a net CO$_2$ source when measurements were averaged over a day or longer. However, during the majority of mid-day summer hours it was a weak sink, sequestering atmospheric CO$_2$. In this case, the weak sink may indicate that the vegetative sink for CO$_2$ was stronger than anthropogenic emissions of CO$_2$ from heavy traffic on nearby highways in the flux footprint. Chamber-based measurements of CH$_4$ flux over most dry land surfaces with vegetation varied from -0.3 to 2.6mmol/m$^2$/d while the range in N$_2$O flux was ~0 to 0.2\mu mol/m$^2$/d. For both fluxes, the higher values were observed over wet soils ($<$20%, volumetric). In contrast, at certain locations over a landfill (closed more than 30years ago) and converted to other uses (light industry, city park and golf course), CO$_2$, CH$_4$ and N$_2$O fluxes were a few orders of magnitude higher than that found anywhere else in the city. However, for the majority of the landfill surface, fluxes were near that found elsewhere in the city. N$_2$O flux from a waste water treatment facility within the study area was 3.2X10$^5$ mol/yr or about 1 mol/yr/capita. This does not include emissions from waste water in transit to the waste water treatment facility or departing it. There will also be discussion of other significant sources of N$_2$O within the urban ecosystem.
B31C-08 09:45h
Stable Carbon Isotope Characterization of Non-Methane Hydrocarbons in Vancouver and Toronto Airsheds
We have developed an approach using stable carbon isotopes as tool to understanding distribution and free radical chemistry of Volatile Organic Compounds (VOCs) in urban airsheds. Carbon isotope ratios are indirect tracers for the reactions of non-methane hydrocarbons (NMHC) with OH radicals. The carbon isotope signatures are diagnostic of the source inputs and trajectories and potentially fingerprint emissions. Compound Specific Isotope Analyses (CSIA) by Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS) of NMHC were made on ambient air samples from urban, suburban, rural and source sites in the airsheds of Toronto and Vancouver, Canada. The extent of chemical processing due to OH radical reactions that the individual NMHC has experienced since emission is quantitatively determined. In combination with concentration measurements it is shown that isotope ratio measurements are an extremely valuable new tool for studying spatial and temporal differences in chemical removal mechanisms, mixing and dilution processes.
http://www.biogeochemistry.uvic.ca