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2010 Literature Review Archives - Climate System Studies
Bounoua, L., F.G. Hall, P.J. Sellers, A. Kumar, G.J. Collatz, C.J. Tucker and M.L. Imhoff. 2010. Quantifying the negative feedback of vegetation to greenhouse warming: a modeling approach. Geophysical Research Letters, Vol 37, L23701, doi: 10.1029/2010GL045338.
In a 2x carbon dioxide (CO2) world, enhanced vegetation growth is shown to cause a negative feedback related to increased evapotranspiration that could lessen global warming by 0.3oC globally and 0.6oC over land compared to simulations where the feedback is not included. This particular negative feedback has not been completelyaccounted for in previous studies. Still, despite a marked cooling effect, this feedback is not strong enough to counter the projected global warming in a 2x CO2 world.
Previous studies of vegetation-climate feedbacks in a higher CO2 world have employed interactive vegetation models that incorporated a number of different feedback mechanisms, including changes in leaf area, increased water use efficiency and reduced transpiration under enriched atmospheric CO2 conditions (i.e. CO2 fertilization effects). However, such models have not included the effects of vegetation ‘down-regulation’. Down-regulation occurs when plants reduce their photosynthetic activity in response to long-term exposure to increased CO2. The authors hypothesized that excess photosynthetic capacity and improved water use efficiency might stimulate vegetation growth more than previously simulated. To investigate the impact of this and other carbon and climate-related vegetation interactions, on climate in a 2x CO2 world, Bounoua and colleagues ran a number of simulations with a GCM (the Colorado State University coupled land-ocean-climate model with the Simple Biosphere model). The simulations project an increase in global precipitation in response to increasing atmospheric CO2, which increases vegetation growth in areas previously water-limited. In addition to changes in growth, the researchers found, as hypothesized, an increase in evapotranspiration as well, related to an overall increase in leaf area. This in turn resulted in an additional cooling effect not fully captured in previous 2x CO2 simulations without vegetation down-regulation. The simulations showed that including this feedback (the increased evapotranspiration) reduced warming by about 0.3oC globally and 0.6oC over land compared to simulations where down-regulation and associated changes in leaf area index were not included. Although a negative feedback was identified in this study it is not strong enough to counter the anthropogenic global warming anticipated with 2x CO2 (1.94oC with the low sensitivity model used in this study and projected to be between 2.0-4.5oC in the IPCC AR4). This study provides refined estimates of vegetation-climate feedbacks for use in future climate modeling studies.
Cao,L., G. Bala, K. Caldeira, et al. 2010. Importance of carbon dioxide physiological forcing to future climate change. PNAS 107:9513-9518.
A new study indicates that the physiological response of global vegetation to rising atmospheric CO2 concentrations adds only modestly to the global surface temperature rise caused by the concurrent enhanced greenhouse effect. However, the response of the water cycle to physiological effects is likely to be significantly larger than that for the enhanced greenhouse effect.
Past studies have indicated that rising atmospheric CO2 concentrations have two primary effects on the climate system. First, the enhanced greenhouse effect caused by higher CO2 concentrations increases the net radiation absorbed by the Earth’s climate system, thus heating the Earth. Second, since plant stomata open less widely under elevated atmospheric CO2 this reduces transpiration and suppresses the role of plants in the hydrological cycle. A new study by a team of American and Indian researchers, using an NCAR atmospheric model coupled to a land model and an ocean/sea ice model, provides estimates of the magnitude of this CO2 physiological effect relative to the enhanced greenhouse effect on global average temperature rise and on the hydrological cycle. Simulation results suggest that the physiological effect of a doubling of CO2 would cause a mean global temperature rise of 0.42°C. This adds about 15% to the estimated 2.86°C global warming caused by concurrent enhancement of CO2 forcing. In contrast, the physiological response of plants to rising CO2 concentrations causes a significantly larger impact on the global hydrological cycle than does CO2 radiative forcing. The former, by reducing evapotranspiration, decreases relative humidity over land areas, but increases global river runoff for doubled CO2 conditions by 8.4%. By comparison, the hydrological response to increased radiative forcing has little effect on simulated relative humidity, and increases global runoff by only 5.2%. The results of this study show that temperature and the hydrological cycle respond quite differently to the two forcings and that the effects of the two forcings are roughly additive.
Dessler, A.E. 2010. A determination of the cloud feedback from climate variations over the past decade. Science, Vol 330, pp.1523-15247, doi: 10.1126/science.1192546.
A new observation-based study suggests that the global cloud feedback in response to short-term climate variations over the past decade is positive, The study finds that for each degree Kelvin (or Celsius)of warming, clouds trap an additional 0.54 ± 0.74 Watts per square metre (W/m2/K), enhancing the warming.
Cloud feedbacks are the most complex yet least understood of the feedbacks influencing how much warming will occur in response to the accumulation of greenhouse gases in the atmosphere. In a recent article, Andrew Dessler attempts to better understand cloud feedbacks by quantifying the magnitude of the global cloud-feedback in response to short-term climate variations observed over a 10-year period. Over this time period the El Niño-Southern Oscillation (ENSO) is the primary source of climate variations with global temperatures during the El Niño phase several tenths of a degree Celsius warmer than during the La Niña phase. The top-of-the-atmosphere (TOA) global radiation budget (as measured by instruments onboard NASA’s Terra satellite over the period March 2005-February 2010) was examined to assess the cloud feedback response to variations in global temperature. The part of the TOA global radiation budget caused by changing clouds was extracted by accounting for effects of changing temperatures, water vapour, surface albedo and radiative forcing. The observations showed a positive cloud feedback of 0.54 ± 0.74 W/m2/K over the 10 year study period. Given the range of uncertainty of this observation, Dessler notes that the possibility of a small negative feedback cannot be excluded. Dessler compared the cloud feedback number obtained from the observations with control run output from eight fully coupled climate models processed using the same technique. The cloud feedback estimated by the models ranges from 0.20 to 1.11 ± 0.20 W/m2/K; the observed feedback lies within this range providing some indication that models simulate the response of clouds to variations of climate reasonably well.
Kip, N., J. F. van Winden, Y. Pan et al., 2010. Global prevalence of methane oxidation by symbiotic bacteria in peat-moss ecosystems. Nature Geoscience Vol 3. Sept 2010 pp 617-621.
A study of sphagnum mosses from peat bogs around the world reveals that methane-oxidizing bacteria (methanotrophs) living symbiotically with the mosses enhance carbon uptake by the mosses. As methane oxidation rates increased with increasing temperature, this indicates a potentially important natural feedback mechanism for reducing methane emissions from peatlands under global warming.
Peat bogs contain large amounts of undecomposed plant material. Decomposition of this material is expected to increase with warmer conditions and could result in increased carbon emissions to the atmosphere, either as CO2 under aerobic conditions, or as CH4 under anaerobic conditions, exacerbating global warming. This study by Kip et al. reveals that in peatlands from around the world where sphagnum mosses are dominant, symbiotic methane-oxidizing bacteria could play an important role in reducing those emissions. Mosses were collected from pools, lawns and hummocks from Sphagnum-dominated wetlands in nine regions of the world. Under experimental conditions, methanotrophic activity (conversion of methane to carbon dioxide) was detected in all the samples of living mosses, demonstrating the ubiquitous presence of methanotrophs living on the plants. In addition, no methane oxidation was observed in peat water samples indicating the absence of significant amounts of free-living methanotrophs. Methane oxidation rates were highest in Sphagnum mosses collected from pools and lower in samples from the other environments. Using stable carbon isotopic labelling, the authors showed that significant amounts of methane-derived carbon was being incorporated into plant tissue in submerged mosses (i.e., simulating pool-type environments). Sphagnum plants growing under submerged conditions are hypothesized to benefit from the additional carbon supplied by the methanotrophs, enhancing carbon uptake in conditions that are otherwise often carbon-limited. The methanotrophs in turn are presumed to benefit from the local plant-derived supply of oxygen. When experiments were conducted incubating mosses at different temperatures, methane oxidation rates were shown to be accelerated at higher temperatures. This suggests that under warmer conditions in the future, the presence of methanotrophs could help reduce methane emissions from peatlands even as decomposition rates increase.
Ridley, J., J.M. Gregory, P. Huybrechts and J. Lowe. 2010. Thresholds for irreversible decline of the Greenland ice sheet. Climate Dynamics, Vol 35, pp 1065-1073, DOI: 10.1007/s00382-009- 0646-0.
A new study explores the possibility of regrowth of the Greenland Ice Sheet following different initial amounts of melt if cooler climate conditions are imposed. A threshold of irreversibility is identified between 80-90% of current volume, below which the original ice-sheet volume cannot be regained, indicating an irreversible commitment to sea level rise.
Studies indicate that the volume of the Greenland ice sheet (GIS) will decline with anthropogenic warming. With sufficient warming, the surface mass balance of the GIS will become negative and it has been suggested that this amount of warming, if sustained, could be a threshold triggering irreversible melting of the ice sheet. A question of interest though is the extent to which melting of the ice sheet is reversible if cooler climate conditions were to prevail (e.g., if atmospheric CO2 were reduced). Ridley et al (2010) used model-based experiments to evaluate whether the GIS would recover if the global climate were to cool. A single atmosphere-ocean general circulation model (HadCM3) and an ice sheet model are used to look for a threshold of irreversible melt. The models are initiated under eleven sets of initial GIS conditions followed by an abrupt reduction of atmospheric greenhouse gas concentrations back to pre-industrial concentrations (i.e., an immediate forced cooling), The initial GIS conditions selected range from 0 to 100% of present day volume in 10% increments and were obtained from previously published simulations. When placed in a pre-industrial climate, to test subsequent ice sheet evolution, it was shown that the trajectories of growth are not simply the reverse of the trajectories during decline. Instead, the initial ice sheet volumes converge towards three equilibrium states: 100, 80 and 20% of current volume (over periods of many thousands of years). An initial state of 90% of present day GIS volume grows to 100% volume while the initial state of 80% volume stays at around the same volume. The results, therefore, indicate that the original ice-sheet volume can be regained only if the volume does not drop below a threshold of irreversibility that lies between 80-90% of the current volume. If this 80-90% threshold of irreversibility were passed, the results suggest that the GIS would not recover to more than about 80% of its original volume implying an irreversible global sea level rise of at least 1.3 m. If ice sheet volume were to drop to below half of its current size, an irreversible commitment to sea level rise of 5m was indicated.
Song, X., D. Lubin, and G. Zhang. 2010. Increased greenhouse gases enhance regional climate response to a Maunder Minimum. GRL, Vol. 37, Lo1703, doi:10,1029/2009GL041290, 2010.
A future Maunder-Minimum type drop in solar irradiance is not expected to offset CO 2 -induced warming and may even amplify the warming in certain regions.
During the 11 year solar cycle, total solar irradiance (TSI) fluctuates by about 0.1%. This small but important variation has a detectable influence on the Earth’s climate through changes in atmospheric pressure (AO/NAO) and surface air temperatures, which can be amplified through a sea ice-solar radiation feedback. Imposed on this short term cycle are longer term variations in solar irradiance. A sustained long term drop in solar activity occurred during the Maunder Minimum (1645-1715), when TSI was thought to have been reduced by about 0.2% causing severe winters in the western Baltic regions. This study investigates how a future climate system would respond if the TSI were to decrease in the future, as it did during the Maunder Minimum. Given that future climates will be warmer as a result of elevated greenhouse gases, a question of interest is whether a similar decrease in the TSI sometime later this century would result in similar cooling. The authors examined the climate system responses to a sustained multi-decadal decrease in TSI of 0.2% in a pre-industrial era climate and under a global warming scenario (IPCC B1 scenario) using the NCAR CAM3 model coupled with a mixed-layer slab ocean model. They found that the global average cooling effect induced by the TSI decrease is reduced by about 27% in the B1 scenario relative to that in the pre-industrial era. This difference was due to both the suppression of the sea ice-solar radiation feedback and a stronger greenhouse effect associated with the increase in greenhouse gases. Regionally, however, a more negative AO/NAO affected wind circulation and sea air temperatures which strongly amplified warming in certain regions in the B1 scenario, namely in western Greenland, Central Asia and East Asia. The results showed that a future solar grand minimum is not expected to offset the CO2-induced warming under even a relatively low warming scenario (B1) and may in fact amplify the warming in certain regions.
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