AGW Observer

Observations of anthropogenic global warming

New research from last week 25/2011

Posted by Ari Jokimäki on June 27, 2011

Here is the new research published last week. I’m not including everything that was published but just some papers that got my attention. Those who follow my Facebook page (and/or Twitter) have already seen most of these, as I post these there as soon as they are published. Here, I’ll just put them out in one batch. Sometimes I might also point out to some other news as well, but the new research will be the focus here. Here’s the archive for the news of previous weeks. By the way, if this sort of thing interests you, be sure to check out A Few Things Illconsidered, they have a weekly posting containing lots of links to new research and other climate related news. Planet 3.0 also reports new research.

Published last week:

Positive cloud feedback possibly observed

Variations in cloud cover and cloud types over the ocean from surface observations, 1954–2008 – Eastman et al. (2011) “Synoptic weather observations from ships throughout the world ocean have been analyzed to produce a climatology of total cloud cover and the amounts of nine cloud types. About 54 million observations contributed to the climatology, which now covers 55 years, from 1954 to 2008. In this work interannual variation of seasonal cloud amounts are analyzed in 10-degree grid boxes. Long-term variations on the order of 5 – 10 years, coherent across multiple latitude bands, remain present in the updated cloud data. A comparison to coincident data on islands indicates that the coherent variations are probably spurious. An exact cause for this behavior remains elusive. The globally-coherent variations are removed from the grid-box time series using a Butterworth filter before further analysis. Before removing the spurious variation, the global average time series of total cloud cover over the ocean shows low-amplitude, long-term variations on the order of 2 percent over the 55-year span. High-frequency, year-to-year variation is seen on the order of 1-2%. Among the cloud types, the most widespread and consistent relationship is found for the extensive marine stratus and stratocumulus clouds (MSC) over the eastern parts of the subtropical oceans. Substantiating and expanding upon previous work, strong negative correlation is found between MSC and sea surface temperature (SST) in the eastern north Pacific, eastern south Pacific, eastern south Atlantic, eastern north Atlantic, and the Indian Ocean west of Australia. By contrast, a positive correlation between cloud cover and SST is seen in the central Pacific. High clouds show a consistent low-magnitude positive correlation with SST over the equatorial ocean. In regions of persistent MSC, time series show decreasing MSC amount. This decrease could be due to further spurious variation within the data. However, the decrease combined with observed increases in SST and the negative correlation between marine stratus and sea surface temperature suggests a positive cloud feedback to the warming sea surface. The observed decrease of MSC has been partly but not completely offset by increasing cumuliform clouds in these regions; a similar decrease in stratiform and increase in cumuliform clouds had previously been seen over land. Interannual variations of cloud cover in the tropics show strong correlation with an ENSO index.” Ryan Eastman, Stephen G. Warren, Carole J. Hahn, Journal of Climate 2011, doi: 10.1175/2011JCLI3972.1. [Full text]

Nature of forcing and climate state affect climate sensitivity

Dependency of Feedbacks on Forcing and Climate State in Physics Parameter Ensembles – Yoshimori et al. (2011) “Climate sensitivity is one of the most important metrics for future climate projections. In previous studies the climate of the last glacial maximum has been used to constrain the range of climate sensitivity, and similarities and differences of temperature response to the forcing of the last glacial maximum and to idealized future forcing have been investigated. The feedback processes behind the response have not, however, been fully explored in a large model parameter space. In this study, we first examine the performance of various feedback analysis methods that identify important feedbacks for a physics parameter ensemble in experiments simulating both past and future climates. The selected methods are then used to reveal the relationship between the different ensemble experiments in terms of individual feedback processes. We, for the first time, evaluate all the major feedback processes for an ensemble of paleoclimate simulations. It is shown that the feedback and climate sensitivity parameters depend on the nature of the forcing and background climate state. The forcing-dependency arises through the shortwave cloud feedback while the state-dependency arises through the combined water vapor and lapse-rate feedback. The forcing-dependency is, however, weakened when the feedback is estimated from the forcing that includes the tropospheric adjustments. Despite these dependencies, past climate can still be used to provide a useful constraint on climate sensitivity as long as the limitation is properly taken into account, because the strength of each feedback correlates reasonably well between the ensembles. It is, however, shown that our physics parameter ensemble does not cover the range of results simulated by structurally different models, which suggests the need for further study exploring both structural and parameter uncertainties.” Masakazu Yoshimori, Julia C. Hargreaves, James D. Annan, Tokuta Yokohata, Ayako Abe-Ouchi, Journal of Climate 2011, doi: 10.1175/2011JCLI3954.1.

Mankind controls the sediment flux to western Pacific Ocean

Recent changes of sediment flux to the western Pacific Ocean from major rivers in East and Southeast Asia – Wang et al. (2011) “The five largest rivers in East and Southeast Asia (Yellow, Yangtze, Pearl, Red and Mekong) are important contributors of terrigenous sediment to the western Pacific Ocean. Although they have annually delivered ~ 2000 × 109 kg of sediment to the ocean since 1000 yr BP, they presently contribute only ~ 600 × 109 kg/yr, which is reverting to a level typical of the relatively undisturbed watersheds before the rise in human activities in East and Southeast Asia at 2000 yr BP. During the most recent decades flow regulation by dams and sediment entrapment by reservoirs, as well as human-influenced soil erosion in the river basins, have sharply reduced the sediment delivered from the large river basins to the ocean. We constructed a time series of data on annual water discharges and sediment fluxes from these large rivers to the western Pacific Ocean covering the period 1950–2008. These data indicate that the short–term (interannual scale) variation of sediment flux is dominated by natural climatic oscillations such as the El Niño/La Niña cycle and that anthropogenic causes involving dams and land use control the long–term (decadal scale) decrease in sediment flux to the ocean. In contrast to the relatively slow historical increase in sediment flux during the period 2000–1000 yr BP, the recent sediment flux has been decreased at an accelerating rate over centennial scales. The alterations of these large river systems by both natural and anthropogenic forcing present severe environmental challenges in the coastal ocean, including the sinking of deltas and declines in coastal wetland areas due to the decreasing sediment supply. Our work thus provides a regional perspective on the large river–derived sediment flux to the ocean over millennial and decadal scales, which will be important for understanding and managing the present and future trends of delivery of terrigenous sediment to the ocean in the context of global change.” Houjie Wang, Yoshiki Saito, Yong Zhang, Naishuang Bi, Xiaoxiao Sun and Zuosheng Yang, Earth-Science Reviews, doi:10.1016/j.earscirev.2011.06.003.

Contrary to land-based wind farms, offshore farms cool their surroundings

Potential climatic impacts and reliability of large-scale offshore wind farms – Wang & Prinn (2011) “The vast availability of wind power has fueled substantial interest in this renewable energy source as a potential near-zero greenhouse gas emission technology for meeting future world energy needs while addressing the climate change issue. However, in order to provide even a fraction of the estimated future energy needs, a large-scale deployment of wind turbines (several million) is required. The consequent environmental impacts, and the inherent reliability of such a large-scale usage of intermittent wind power would have to be carefully assessed, in addition to the need to lower the high current unit wind power costs. Our previous study (Wang and Prinn 2010 Atmos. Chem. Phys. 10 2053) using a three-dimensional climate model suggested that a large deployment of wind turbines over land to meet about 10% of predicted world energy needs in 2100 could lead to a significant temperature increase in the lower atmosphere over the installed regions. A global-scale perturbation to the general circulation patterns as well as to the cloud and precipitation distribution was also predicted. In the later study reported here, we conducted a set of six additional model simulations using an improved climate model to further address the potential environmental and intermittency issues of large-scale deployment of offshore wind turbines for differing installation areas and spatial densities. In contrast to the previous land installation results, the offshore wind turbine installations are found to cause a surface cooling over the installed offshore regions. This cooling is due principally to the enhanced latent heat flux from the sea surface to lower atmosphere, driven by an increase in turbulent mixing caused by the wind turbines which was not entirely offset by the concurrent reduction of mean wind kinetic energy. We found that the perturbation of the large-scale deployment of offshore wind turbines to the global climate is relatively small compared to the case of land-based installations. However, the intermittency caused by the significant seasonal wind variations over several major offshore sites is substantial, and demands further options to ensure the reliability of large-scale offshore wind power. The method that we used to simulate the offshore wind turbine effect on the lower atmosphere involved simply increasing the ocean surface drag coefficient. While this method is consistent with several detailed fine-scale simulations of wind turbines, it still needs further study to ensure its validity. New field observations of actual wind turbine arrays are definitely required to provide ultimate validation of the model predictions presented here.” Chien Wang and Ronald G Prinn, 2011, Environ. Res. Lett. 6 025101 doi: 10.1088/1748-9326/6/2/025101. [Full text]

The ecosystems of clouds

Atmospheric cloud water contains a diverse bacterial community – Kourtev et al. (2011) “Atmospheric cloud water contains an active microbial community which can impact climate, human health and ecosystem processes in terrestrial and aquatic systems. Most studies on the composition of microbial communities in clouds have been performed with orographic clouds that are typically in direct contact with the ground. We collected water samples from cumulus clouds above the upper U.S. Midwest. The cloud water was analyzed for the diversity of bacterial phylotypes by denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene amplicons. DGGE analyses of bacterial communities detected 17-21 bands per sample. Sequencing confirmed the presence of a diverse bacterial community; sequences from seven bacterial phyla were retrieved. Cloud water bacterial communities appeared to be dominated by members of the cyanobacteria, proteobacteria, actinobacteria and firmicutes.” Peter S. Kourtev, Kimberly A. Hill, Paul B. Shepson and Allan Konopka, Atmospheric Environment, doi:10.1016/j.atmosenv.2011.06.041.

Carbon sink of European forests has increased

Reconstruction and attribution of the carbon sink of European forests between 1950 and 2000 – Bellassen et al. (2011) “European forests are an important carbon sink, yet the relative contributions to this sink of climate, atmospheric CO2 concentration ([CO2]), nitrogen deposition and forest management are under debate. We attributed the European carbon sink in forests using ORCHIDEE-FM, a process-based vegetation model that differs from earlier versions of ORCHIDEE by its explicit representation of stand growth and idealized forest management. The model was applied on a grid across Europe to simulate changes in the net ecosystem productivity (NEP) of forests with and without changes in climate, [CO2] and age structure, the three drivers represented in ORCHIDEE-FM. The model simulates carbon stocks and volume increment that are comparable – RMSE of 2 m3 ha-1 yr-1 and 1.7 kgC m-2 respectively – with inventory-derived estimates at country level for 20 European countries. Our simulations estimate a mean European forest NEP of 175 ± 52 gC m-2 yr-1 in the 1990s. The model simulation that is most consistent with inventory records provides an upwards trend of forest NEP of 1 ± 0.5 gC m-2 yr-2 between 1950 and 2000 across the EU 25. Further, the method used for reconstructing past age structure was found to dominate its contribution to temporal trends in NEP. The potentially large fertilizing effect of nitrogen deposition cannot be told apart as the model does not explicitly simulate the nitrogen cycle. Among the three drivers that were considered in this study, the fertilizing effect of increasing [CO2] explains about 61% of the simulated trend, against 26% to changes in climate, and 13% only to changes in forest age structure. The major role of [CO2] at the continental scale is due to its homogeneous impact on NPP. At the local scale, however, changes in climate and forest age structure often dominate trends in NEP by affecting NPP and heterotrophic respiration.” Valentin Bellassen, Nicolas Viovy, Sebastiaan Luyssaert, Guerric Le Maire, Mart-Jan Schelhaas, Philippe Ciais, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02476.x.

Snow cover has generally decreased in Arctic

The Changing Cryosphere: Pan-Arctic Snow Trends (1979–2009) – Liston & Hiemstra (2011) “Arctic snow presence, absence, properties, and water amount are key components of Earth’s changing climate system that incur far-reaching physical and biological ramifications. Recent dataset and modeling developments permit relatively high-resolution (10 km horizontal grid; 3 hour time-step) pan-Arctic snow estimates for 1979–2009. Using MicroMet and SnowModel in conjunction with land cover, topography, and 30 years of NASA-MERRA atmospheric reanalysis data, we created a distributed snow-related dataset including air temperature, snow precipitation, snow-season timing and length, maximum snow water equivalent depth (SWE), average snow density, snow sublimation, and rain-on-snow events. Regional variability is a dominant feature of the modeled snow-property trends. Both positive and negative regional trends are distributed throughout the pan-Arctic domain, featuring, for example, spatially distinct areas of increasing and decreasing SWE or snow season length. In spite of strong regional variability, the data clearly show a general snow decrease throughout the Arctic: maximum winter SWE has decreased, snow cover onset is later, the snow-free date in spring is earlier, and snow cover duration has decreased. The domain-averaged air temperature trend when snow was on the ground was 0.17 °C decade−1 with minimum and maximum regional trends of −0.55 and 0.78 °C decade−1, respectively. The trends for total number of snow days in a year averaged −2.49 days decade−1 with minimum and maximum regional trends of −17.21 and 7.19 days decade−1, respectively. The average trend for peak SWE in a snow season was −0.17 cm decade−1 with minimum and maximum regional trends of −2.50 and 5.70 cm decade−1, respectively.” Glen E. Liston, Christopher A. Hiemstra, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00081.1.

New analysis of climate in medieval times

Spatial and Temporal Characteristics of Climate in Medieval Times Revisited – Diaz et al. (2011) “Development of accurate reconstructions of past climate regimes, and enhancing our understanding of the causal factors that may have contributed to their occurrence is important for a number of reasons; these include improvements in the attribution of climate change to natural and anthropogenic forcing, gaining a better appreciation for the range and magnitude of low frequency variability and previous climatic regimes in comparison with the modern instrumental period, and developing greater insights into the relationship between human society and climatic changes. Here, we examine up-to-date evidence regarding the characteristics of the climate in medieval times (~ 950 to 1400 A.D.). Long and high-resolution climate proxy records reported in the scientific literature, which form the basis for the climate reconstructions, have greatly expanded in the last few decades, with greater numbers of sites that now cover more areas of the globe. Some comparisons with the modern climate record and discussion on potential mechanisms associated with the patterns of medieval climate are presented here, but our main goal is to provide the reader with some appreciation of the richness of past natural climate variability in terms of its spatial and temporal characteristics.” Henry F. Diaz, Ricardo Trigo, Malcolm K. Hughes, Michael E. Mann, Elena Xoplaki, and David Barriopedro, Bulletin of the American Meteorological Society 2011, doi: 10.1175/BAMS-D-10-05003.1. [Full text]

Potential climate tipping elements in Europe

Potential climatic transitions with profound impact on Europe Review of the current state of six ‘tipping elements of the climate system’ – Levermann et al. (2011) “We discuss potential transitions of six climatic subsystems with large-scale impact on Europe, sometimes denoted as tipping elements. These are the ice sheets on Greenland and West Antarctica, the Atlantic thermohaline circulation, Arctic sea ice, Alpine glaciers and northern hemisphere stratospheric ozone. Each system is represented by co-authors actively publishing in the corresponding field. For each subsystem we summarize the mechanism of a potential transition in a warmer climate along with its impact on Europe and assess the likelihood for such a transition based on published scientific literature. As a summary, the ‘tipping’ potential for each system is provided as a function of global mean temperature increase which required some subjective interpretation of scientific facts by the authors and should be considered as a snapshot of our current understanding.” Anders Levermann, Jonathan L. Bamber, Sybren Drijfhout, Andrey Ganopolski, Winfried Haeberli, Neil R. P. Harris, Matthias Huss, Kirstin Krüger, Timothy M. Lenton and Ronald W. Lindsay, et al., Climatic Change, DOI: 10.1007/s10584-011-0126-5. [Full text]

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