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2009 Literature Review Archives - Climate System Studies

Clement, A.C., R. Burgman and J.R. Norris, 2009. Observational and Model Evidence for positive Low-Level Cloud Feedback, Science, vol. 325, 460, DOI: 10.1126/science.1171255.
In the NE Pacific, decreasing low-cloud cover has amplified warming over the past decades. The challenge now is to improve the ability of global climate models to properly represent this feedback so that there is greater confidence in the sign of the low-cloud feedback under future changes in greenhouse gas concentrations.
Low clouds have a cooling effect on global climate since they enhance Earth's albedo and reflect incoming solar radiation. Whether changes in low-cloud cover in the future will have a negative or positive feedback on global warming is not yet known and low-cloud feedbacks remain a primary cause of uncertainty in global climate model (GCM) projections. This study helps address this question by examining the decadal variability of low-clouds over a large section of the NE Pacific ocean over the period 1952 to 2006. Two independent cloud data sets are used: one based on observations and one based on satellite measurements. The results of the study show that during regional warming periods, cloud cover, especially low-cloud layers, decreased, ocean temperature rose, sea level pressure fell and atmospheric circulation weakened. The decrease in low-cloud cover provided a positive climate feedback, by allowing more sunlight to warm the surface. During a cooling episode, the trends were reversed. The authors also analysed 20th century climate simulations in 18 GCMs to see if the low-cloud feedback in this region is properly represented in these models. Only two models were able to reproduce the appropriate changes in low cloud cover. Under a 2XCO2 simulation, only one of these (HadGEM1) could also produce temperature and circulation changes that were consistent with the 18 model ensemble mean. This model produces, as observed, a reduction in cloud throughout much of the Pacific in response to greenhouse gas forcing, that is, a positive low-level cloud feedback.

McGuire, D.A., L.G. Anderson, T.R. Christensen et al., 2009, Sensitivity of the carbon cycle in the Arctic to climate change, Ecological Monographs, 79 (4): 523-555.
A comprehensive review of the carbon cycle in the Arctic and its response to ongoing climate changes shows that carbon cycle feedbacks and responses over the next 50-100 years are highly uncertain.
The response of the carbon cycle to climate change in the Arctic is an issue of global concern because of the large stores of carbon in this region that could be released with ongoing warming. A comprehensive review of this issue is provided by McGuire and colleagues in the journal Ecological Monographs that aimed to clarify key uncertainties and vulnerabilities relevant over the next 50-100 years. They estimate carbon stocks in the Arctic at between 1400 and 1850 PgC soil organic matter in both the surface (0-3 m) and deeper, with much of this stored in peatlands and deep permafrost soils in Siberia. The Arctic was a sink for atmospheric CO2 of between 0 and 0.8 PgC per year during the 1990s, which is between 0 and 25% of the global net land/ocean flux of 3.2 PgC per year reported in the IPCC-AR4 for the same time period. Most of the sink activity is due to the growth of trees in the boreal forest. The Arctic was a source of methane (CH4) to the atmosphere of between 15 and 50 Tg CH4 per year over 1984-2004, which is between 3% and 9% of the net land/ocean source of 552 Tg CH4 per year estimated in the IPCC-AR4. Most of this source is from wetland ecosystems. Over the next 50 to 100 years, the sensitivity of the carbon cycle in the Arctic to projected climate change is highly uncertain in terms of magnitude, and direction (positive or negative feedbacks) depending on the response of vegetation photosynthesis to increases in atmospheric CO2. The Arctic carbon cycle involves many complex processes and a key uncertainty is what the balance of competing feedbacks will be. Future research must thus focus on regional studies to better understand the processes involved and integration of this knowledge into improved models.

Phillips, O.L., Aragão, L.E.O.C., Lewis, S.L. et al. 2009. Drought sensitivity of the Amazon rainforest. Science 323:1344-1347.; Malhi,Y., Aragão, L.E.O.C., Galbraith, D. et al. 2009. Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest. PNAS 106, doi/10.1073/pnas.0804619106.
Warmer climates are likely to result in large losses of biomass in the Amazon forests due to dieback and fire - but the extent of loss may not be as large as previously thought.
Two recent papers once again highlight the concern about the potential future die-back of the Amazonian forests under warmer climates. A large dieback would cause a large release of CO2 from the region into the atmosphere which would be a major positive feedback within the climate system. In one of the studies, published in Science, a large team of 67 international experts analyzed data from long term monitoring plots in the region to determine the potential impacts of intense droughts like that of 2005 on Amazon forest biomass. During the 2005 drought year, these forests changed from their traditional role as a net sink for atmospheric carbon dioxide to that of a large net source equivalent to between 1.2 and 1.6 billion metric tonnes of carbon. This is equal to about 20 to 25% of annual global carbon dioxide emissions from fossil fuel combustion. Since many climate models project increased drought conditions for the region under warmer climates, the authors warn that the large net emissions of CO2 from the Amazon forests in 2005 could be a harbinger of things to come. However, the second study (published in the Proceedings of the Natural Academy of Sciences) cautions that the future fate of the Amazon may be less dire than many of the model projections might suggest. In it, a team of British scientists compare simulations of the current climate of the eastern Amazon region generated by 19 different state-of-the-art global climate models with observations. They note that the models, in general, tend to underestimate current rainfall amounts for the region. The model simulations for the next century suggest a regional trend towards intensified water stress. This increases the risks of fire and hence episodes of large loss of biomass. However, when allowing for the general model bias in total precipitation amounts implied in their simulations of current climates, the researchers suggest that the region may revert to a lower biomass seasonal forest rather than experience a complete dieback. They also note that deliberate limitation of deforestation and enhanced fire suppression may help the region's forests to retain their resilience.

Piao, S., Ciais, P., Friedlingstein, P. et al. 2009. Spatiotemporal patterns of terrestrial carbon cycle during the 20th century. Global Biogeochemical Cycles 23, GB4026, doi:10.1029/2008GB0033339.
Simulations of global land ecosystem response to land use change, climate change and rising CO2 concentrations suggest that the growth in land carbon sinks due to enhanced net primary productivity has exceeded emissions from land use change since 1980. However, ecosystem respiration due to warmer climates is also rising, and may return global land ecosystems to net sources of atmospheric CO2 in coming decades.
The response of global land ecosystems to changes in human land use and climate change is challenging to project, in part because the fertilization effects of rising CO2 concentrations and nitrogen deposition is complex. However, a number of dynamic global vegetation models have now been successful in capturing many of the processes involved and approximating the behaviour of ecosystems in response to these causal factors, or drivers. In a recent paper published in Global Biogeochemical Cycles, a team of French and Chinese researchers report results achieved with the ORCHIDEE model, which considers land use change, climate and CO2 concentration change as drivers. Nitrogen deposition effects were not included. Results suggest that, over the last century, about three-quarters of the net emissions caused by land use change were offset by the enhanced net primary production (NPP) due to the fertilization effects of rising CO2 concentrations and changes in climate. Since 1980, NPP has been increasing at the rate of 0.4%/year. This has resulted in a net carbon sink during the past two decades that has exceeded concurrent emissions from land use change. During the 1990s, the net global sink that results when considering both natural and anthropogenic contributions reached 1 billion tonnes of carbon per year. This is in close agreement with results from some inversion model assessments of atmospheric CO2 distributions and profiles. However, the model also suggests that natural release of CO2 from ecosystems through respiration is rising, and that global ecosystems may once again become a source of carbon dioxide in coming decades.

Schuur, E.A.G., J.G. Vogel, K.G. Crummer, H. Lee, J.O. Sickman and T.E. Osterkamp. 2009. The effect of permafrost thaw on old carbon release and net carbon exchange from tundra. Nature Vol 459. May 28, 2009. pp 556-559.
A study of carbon fluxes in areas of permafrost experiencing different amounts of thaw reveals that over decadal timescales, the decomposition of older, stored carbon overwhelms carbon uptake through increased plant growth. The results indicate that in a warmer world, thawing of permafrost could become a large source of carbon to the atmosphere, providing a positive feedback on the climate system.
As Arctic warming progresses, one of the key uncertainties is how much of the vast store of buried carbon within the permafrost will be released to the atmosphere as the permafrost thaws, providing a positive feedback on the climate system. In this study, a team of scientists from the United States undertake measurements at several Alaskan permafrost thaw sites to provide some insights into how carbon fluxes from areas of permafrost thaw may change over time and what the potential is for release of old carbon. They find that, although losses of old carbon are greater for sites with more extensive permafrost thaw, moderate thaw sites (sites that thawed over the past 15 years) had overall net ecosystem carbon uptake, since increased plant growth more than offset losses from respiration. In contrast, older sites with more extensive thaw were net carbon sources, with decomposition of older carbon overwhelming plant carbon uptake. The authors estimate that about 9-13% of the carbon stored in the 80 cm active layer of soil in this watershed could be lost by the end of the century. Applying similar rates of carbon loss to estimates of the global surface permafrost carbon pool suggests that 0.8-1.1PgC/yr could be lost if surface permafrost thaws. Hence, the release of carbon dioxide into the atmosphere from this positive feedback alone could potentially approach the magnitude of the current biospheric flux from land use change (1.5±0.5 PgC/yr.)

Wang, X., D. Wang and W. Zhou, 2009. Decadal variability of twentieth-century El Niño and la Niña occurrence from observations and IPCC AR4 coupled models. Geophys. Res. Lett., 36, L11701, doi:10.1029/2009GL037929).
A recent study indicates that the multi-decadal variability in ENSO behaviour may be linked to long term changes in other ocean circulation patterns in complex ways. In particular, it suggests that la Nina behaviour is influenced by changes in the Pacific Decadal Oscillation index, while El Ninos are affected by the Atlantic Mulltidecadal Oscillation. Models do poorly on capturing these linkages.
Chinese scientists analysed the frequency of El Niño and la Niña events and their associated sea surface temperature (SST) variations, to investigate the linkages between these events and both the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO). The study was done for the period 1900 to 2003, using observational datasets and simulations by 15 coupled global climate models (CGCMs) used in the IPCC-AR4. Analysis of the observations shows that the El Niño activity index has a decadal oscillation with a period of about 12 years and that these events are related to the AMO index. The La Niña activity index, on the other hand, shows decadal variations with a period of 16 years, influenced by the PDO index. Therefore, on a decadal timescale, the planetary teleconnections related to El- Niño and La Niña differ from each other. In a second step, the authors evaluated the ability of 15 CGCMs to simulate the decadal variability of ENSO activity and the relationships between ENSO events and the Pacific and Atlantic SSTs. They found that very few of the CGCMs could simulate well one or both types of events and furthermore, that none of the CGCMs could simulate completely the relationship between El Niño or La Niña activity and the Pacific and Atlantic SSTs. The results of this study are encouraging as they help to further understand how multidecadal Pacific and Atlantic oscillations impact ENSO variability, which may improve our ability to forecast ENSO behaviour.

Wild, M. (2009), Global dimming and brightening: A review, J. Geophys. Res., 114, D00D16, doi:10.1029/2008JD011470, 31 p.
Throughout the 20th century, anthropogenic aerosol emissions have played an important role in the multi-decadal variations of solar radiation reaching the Earth's surface. Current GCM simulations do not capture these variations well. Future changes in these emissions will continue to have implications for climate change, especially at the regional level.
A recent review of the 'twinned' issues of global dimming and brightening examined changes during the 20th century in the amount of solar radiation reaching the surface of the planet. It also considered possible reasons for these changes, and their potential to influence climate change. The evidence indicates that surface solar radiation (SSR) has experienced significant decadal variations over time, with an early brightening (increase in SSR) in the 1930s and 1940s, followed by a global dimming (decrease in SSR) between the 1950s and 1980s and, more recently, brightening again at many locations. These variations are important as they influence surface temperatures. Variations in solar radiation at the surface of the Earth are not linked with changes in solar output, such as those associated with the 11-year sunspot cycle, but are mainly the result of changes in clouds and in atmospheric aerosols from anthropogenic pollution. Current GCMs, in general, generate weaker dimming and brightening variations than observed. The author notes that whether dimming or brightening prevails in the future will depend on how anthropogenic emissions of aerosols and aerosols precursors will change in response to economic developments and air pollution regulations. The author suggests that future regional dimming is likely in heavy polluted areas like Southeast Asia, while weak additional brightening might be expected in western industrialized countries, where aerosol levels have stabilized at low values since about 2000.

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