No description available.

No description available.

No description available.

We develop and use a new version of the Terrestrial Ecosystem Model (TEM) to study how rates of methane (CH4) emissions and consumption in high-latitude soils of the Northern Hemisphere have changed over the past century in response to observed changes in the region’s climate. We estimate that the net emissions of CH4 (emissions minus consumption) from these soils have increased by an average 0.08 Tg CH4 per year during the 20th century. Our estimate of the annual net emission rate at the end of the century for the region is 51 Tg CH4 per year. Russia, Canada, and Alaska are the major CH4 regional sources to the atmosphere; responsible for 64%, 11%, and 7% of these net emissions, respectively. Our simulations indicate that large inter-annual variability in net CH4 emissions occurred over the last century. If CH4 emissions from the soils of the pan-Arctic region respond to future climate changes as our simulations suggest they have responded to observed climate changes over the 20th century, a large increase in high latitude CH4 emissions is likely and could lead to a major positive feedback to the climate system. / Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/). / This study was supported by a NSF biocomplexity grant (ATM-0120468), the NASA Land Cover and Land Use Change Program (NAG5-6257), and by funding from MIT Joint Program on the Science and Policy of Global Change, which is supported by a consortium of government, industry and foundation sponsors.

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A flux measurement setup was established at a communications tower north of downtown Houston, Texas, to measure energy and trace gas fluxes from a variety of emission sources in the urban surface layer. The first part of this study details the development of a correction for latent heat and carbon dioxide (CO2) fluxes due to a low-pass filtering of the true water vapor and CO2 atmospheric signals. A method of spectral analysis was used to develop a correction scheme for this flux underestimation through the observation that the low-pass filtering is a strong function of atmospheric relative humidity and wind speeds. The determined flux corrections for latent heat fluxes (average correction of 34%) were significantly larger than CO2 fluxes (3-4%), suggesting the low-pass filtering had a more dominant effect on the water vapor signal. For the second part of this study, we describe a quadrant analysis technique for separating measured net fluxes into their biogenic (c’ < 0 and q’ > 0 signals) and anthropogenic (c’ > 0 and T’ > 0 signals) components, and quantify these fluxes through the use of a refined relaxed eddy accumulation model. A method of minimizing a defined net – (biogenic + anthropogenic) residual was used to determine an accurate “hole” around the origin to exclude points in the desired quadrants that appear there randomly. The magnitude of the biogenic flux contribution showed expected relationships with temperature and irradiance. The anthropogenic flux contribution showed a strong correlation with measured traffic counts collected on local thoroughfares. Due to a presumed small bias in the flux calculation methodology, neither flux contribution truly measured zero, so anthropogenic and biogenic “background” fluxes were calculated (0.01 µmol mo^-1 m s^-1 and 0.01 – 0.02 µmol mol^-1 m s^-1 respectively). Future work involves perfecting the model, with the goal being widespread use of the theory at urban flux sites. The accurate separation of the net flux gives a more complete picture of the carbon cycle and allows for more accurate analysis of the effects of increased temperature and CO2 concentration on urban vegetation as compared to non-urban vegetation.

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No description available.

No description available.

The establishment and intensive management of forests for the production of timber can have significant effects on the soil carbon dynamics. The establishment of forest on organic soils under grasslands may lead to substantial losses in soil carbon, due to the site preparation for the planting of trees and other disturbances. This is gradually compensated by carbon accumulation in tree biomass as the forest grows until clearfelling at maturity may initiate another substantial carbon loss. This study had two main aims. The first was to investigate the long-term effects of forest establishment on natural grassland as well as clearfelling and re-growth of the forest during second rotation, by looking at the changes in soil carbon stocks and soil carbon balance in a Sitka spruce (Picea sitchsensis) in Harwood (N. E. England). Secondly, to investigate the short-term effects of forest clearfelling on the fluxes of soil CO2, N2O and CH4 and on the environmental factors (soil temperature, water content and water table depth) affecting them. The fluxes were initially measured in two mature stands (40-years old) during one growth season. One of the two stands was subsequently clearfelled while the other was kept intact and fluxes were measured for a further ten months after clearfelling. The relationships between these fluxes and the environmental factors were also examined. The study also investigated the spatial variability of soil CO2 emissions using geostatistical approaches. The soil CO2 fluxes were measured with two methods, a closed dynamic chamber and a closed static chamber, giving the opportunity to compare their relative performance. A performance further investigation on this discrepancy between the two methods took place in lab experiments and on a soil monolith, excavated from the 40-year old stand and kept under controlled conditions in the greenhouse.