This study investigates modes of atmospheric variability in response to changes in Earth's orbit and changes in 20th century sea surface temperatures (SST). The orbital forcing is manifested by a change in obliquity and precession, and changes the distribution of the top-of-atmosphere insolation. A smaller obliquity reduces the the annual insolation that the poles receive and increases the annual insolation in the tropics. As the meridional insolation gradient increases, the zonal mean atmospheric-ocean circulation increases. The resulting climate also has a reduced global mean temperature due to the effect of climate feedbacks. This cooling can be attributed to a reduced lapse rate, increased cloud fraction. reduced water vapor in the atmosphere, and an increase in the surface albedo. A change in the precession, as the perihelion shifts from the winter to the summer solstice, causes a strengthening as well as an expansion of the N. Pacific summer subtropical anticyclone. This anticyclonic anomaly can be attributed to the weakening of the baroclinic activity, but also represents the circulation response to remote and local diabatic heating. The remote diabatic heating is associated with monsoonal activity in the SE Asia and North Africa. Regarding the 20th century SST forcing, it is represented by a multidecadal variability in the inter-hemispheric SST difference. This change in the SST causes a latitudinal shift in the ascending branch of the Hadley cell and precipitation in the tropics, as well as an increase in the atmospheric meridional heat transport from the warmer to the colder hemisphere.

Radiative feedbacks associated with changes in water vapor, temperature, surface albedo and clouds remain a major source of uncertainty in our understanding of climate's response to anthropogenic forcing. In this dissertation climate model data is used to investigate variations in feedbacks that result from changing CO��� forcing and the time scales on which feedbacks operate, focusing on the applicability of one method in particular, the radiative kernel technique, to these problems. This computationally efficient technique uses a uniform, incremental change in feedback variables to infer top-of-atmosphere (TOA) radiative flux changes. The first chapters explore the suitability of the linear radiative kernel technique for large forcing scenarios. We show that kernels based on the present-day climate misestimate TOA flux changes for large perturbations, translating into biased feedback estimates. We address this issue by calculating additional kernels based on a large forcing climate state with eight times present day CO��� concentrations. Differences between these and the present-day kernels result from added absorption of radiation by CO��� and water vapor, and increased longwave emission due to higher temperatures. Combining present-day and 8xCO��� kernels leads to significant improvement in the approximation of TOA flux changes and accuracy of feedback estimates. While climate sensitivity remains constant with increasing CO��� forcing when the inaccurate present-day kernels are used, sensitivity increases significantly when new kernels are used. Comparison of feedbacks in climate models with observations is one way towards understanding the disagreement among models. However, climate change feedbacks operate on time scales that are too long to be evaluated from the observational record. Rather, short-term proxies for greenhouse-gas-driven warming are often used to compute feedbacks from observations. The third chapter of this dissertation examines links between the seasonal cycle and global warming using pattern correlations of spatial distribution of feedback variables and radiative flux changes. We find strong correlations between time scales for changes in surface temperature and climate variables, but not for TOA flux anomalies, reaffirming conclusions drawn in previous work. Finally, we investigate the fitness of the radiative kernel technique for evaluation of short-term feedbacks in a comparison with the more accurate, but more computationally expensive, partial radiative perturbations. / Graduation date: 2013

Land-use changes impact the energy balance of the Earth system, and feedbacks in the Earth system can dampen or amplify this perturbation. We analyze here from satellite data the response of clouds and subsequently radiation to a change of land use for the example of deforestation in the Amazon Basin. In this region, the characteristics of different cloud types over two vegetation types (forest and crop-/grasslands) were calculated for a time period of five years by using satellite data from the instruments MODIS and CERES. The cloud types are defined according to height, optical thickness, and fraction of cloud cover. For calculating the radiative forcing caused by deforestation, the dependency of spatial and temporal averages for the reflected shortwave and outgoing longwave radiation of the top of the atmosphere on vegetation types were determined as well. The results show distinct differences in cloud cover and radiative forcing over crop-/grasslands and forests for the two vegetation regimes, implying a potentially significant positive cloud feedback to deforestation.

Land-use changes impact the energy balance of the Earth system, and feedbacks in the Earth system can dampen or amplify this perturbation. We analyze here from satellite data the response of clouds and subsequently radiation to a change of land use for the example of deforestation in the Amazon Basin. In this region, the characteristics of different cloud types over two vegetation types (forest and crop-/grasslands) were calculated for a time period of five years by using satellite data from the instruments MODIS and CERES. The cloud types are defined according to height, optical thickness, and fraction of cloud cover. For calculating the radiative forcing caused by deforestation, the dependency of spatial and temporal averages for the reflected shortwave and outgoing longwave radiation of the top of the atmosphere on vegetation types were determined as well. The results show distinct differences in cloud cover and radiative forcing over crop-/grasslands and forests for the two vegetation regimes, implying a potentially significant positive cloud feedback to deforestation.

Sammanfattning: Wood decay fungi are the main decomposer of lignocellulose material stored in wood. Thus, all factors that affect them could affect their ecological function. This in return, may affect ecosystem functioning in terms of altered carbon emissions from dead wood. Increased temperature is one of the main factors influencing fungal decay. The aim of the current study is to explore the effects of temperature and assembly history (order of species arrival), two important regulators of fungal communities, on wood decomposition. I conducted a microcosm experiment with two temperature treatments and eight assembly histories where each species was allowed to colonize the wood two weeks ahead of the rest of the species. The temperature treatments were set to mimic the effect of climate induced warming. Therefore, I had one treatment with relatively high temperature, representing the expected temperatures year 2100 given the current emission trends of the northern inland of Sweden, and another treatment representing the current normal temperature (1961-1990). The temperature treatments had an average difference of 5°C. In order to see how climate induced warming and fungal assembly history influenced decomposition, I measured and analyzed initial fungal growth, fungal respiration and wood weight loss. Both temperature and assembly history had a significant influence on fungal growth, fungal respiration and wood decomposition. There was also strong interaction between the two factors. The average increase in mass loss under elevated temperature was 19% compared to 14% under normal temperature. The highest mass loss (25%) was when Phlebia centrifuga was the initial species under elevated temperature and the lowest (12%) was when Climacocystis borealis was initial species under normal temperature. All assembly histories had higher mass loss under elevated temperature, but the magnitude varied. For example, when C. borealis was the initial species, mass loss increased by 60% compared to only 7% when Antrodia sinuosa was the initial species. Six out of eight assembly histories had higher CO2 under elevated temperature, with the highest increase (88%) in P. centrifuga histories and the lowest (7%) in C. borealis histories. Even if the results need to be confirmed by field studies, my data illustrates that climate induced warming probably results in higher fungal respiration and deadwood decomposition and that the magnitude of this effect depends on fungal assembly history.