Posted on 19 June 2012 by Ari Jokimäki

This week we have temperature studies from Saudi-Arabia, European Alps, and the whole globe. Clouds disturb satellite measurements. Tree rings measure wind too. Some future related issues are methane under glaciers, snow albedo feedback, and AMOC slowdown. Lightning flashes take weekends off. What are the causes for El Niño intensification and for UV radiation increase?

Evidence from Last Interglacial suggests that Greenland melt induced AMOC slowdown might not cool Europe

European climate optimum and enhanced Greenland melt during the Last Interglacial - Goñi et al. (2012) [FULL TEXT]

Abstract: "The Last Interglacial climatic optimum, ca. 128 ka, is the most recent climate interval significantly warmer than present, providing an analogue (albeit imperfect) for ongoing global warming and the effects of Greenland Ice Sheet (GIS) melting on climate over the coming millennium. While some climate models predict an Atlantic meridional overturning circulation (AMOC) strengthening in response to GIS melting, others simulate weakening, leading to cooling in Europe. Here, we present evidence from new proxy-based paleoclimate and ocean circulation reconstructions that show that the strongest warming in western Europe coincided with maximum GIS meltwater runoff and a weaker AMOC early in the Last Interglacial. By performing a series of climate model sensitivity experiments, including enhanced GIS melting, we were able to simulate this configuration of the Last Interglacial climate system and infer information on AMOC slowdown and related climate effects. These experiments suggest that GIS melt inhibited deep convection off the southern coast of Greenland, cooling local climate and reducing AMOC by ∼24% of its present strength. However, GIS melt did not perturb overturning in the Nordic Seas, leaving heat transport to, and thereby temperatures in, Europe unaffected."

Citation: Maria Fernanda Sánchez Goñi, Pepijn Bakker, Stéphanie Desprat, Anders E. Carlson, Cédric J. Van Meerbeeck, Odile Peyron, Filipa Naughton, William J. Fletcher, Frédérique Eynaud, Linda Rossignol and Hans Renssen, Geology, v. 40 no. 7 p. 627-630, doi: 10.1130/G32908.1.

Less lightning activity during weekends in Atlanta region

Weekend–weekday aerosols and geographic variability in cloud-to-ground lightning for the urban region of Atlanta, Georgia, USA - Stallins et al. (2012)

Abstract: "We characterized the differences in warm-season weekday and weekend aerosol conditions and cloud-to-ground (CG) flashes (1995–2008) for an 80,000 square kilometer region around Atlanta, Georgia, a city of 5.5 million in the humid subtropics of the southeastern United States. An integration of distance-based multivariate techniques (hierarchical agglomerative clustering, multiresponse permutation procedures, fuzzy kappa statistics, and Mantel tests) indicated a greater concentration of CG flash activity within a 100 km radius around Atlanta under weekday aerosol concentrations. On weekends, these effects contracted toward the city. This minimized any weekly anthropogenic cycle over the more densely populated urban center even though this location had a higher flash density, a higher percentage of days with flashes, and stronger peak currents over the course of a week compared to the surrounding region. The sharper contrasts in weekday and weekend lightning regime developed outside the perimeter of the city over nonurban land uses. Here, lightning on weekend and weekdays differed more in its density, frequency, polarity, and peak current. Across the full extent of the study region, weekday peak currents were stronger and flash days more frequent, suggesting that weekly CG lightning signals have a regional component not tied to a single city source. We integrate these findings in a conceptual model that illustrates the dependency of weekly anthropogenic weather signals on spatial and temporal extent."

Citation: J. Anthony Stallins, James Carpenter, Mace L. Bentley, Walker S. Ashley and James A. Mulholland, Regional Environmental Change, 2012, DOI: 10.1007/s10113-012-0327-0.

Ozone changes cannot explain increase of UV radiation in Austria

Trends in spectral UV radiation from long-term measurements at Hoher Sonnblick, Austria - Fitzka et al. (2012)

Abstract: "High-quality long-term records of spectral UV irradiance from the Network for the Detection of Atmospheric Composition Change-affiliated Bentham spectroradiometer at the high-mountain site Hoher Sonnblick (47.05° N, 12.95° E, 3,106 m above sea level) from the period 1997–2011 have been investigated for the existence of trends. Throughout the year, significant upward trends are found at wavelengths of 315 nm and longer. The magnitudes at 315 nm range from +9.3 ± 4.5 %/dec at 45° solar zenith angle (SZA) to +14.2 ± 3.7 %/dec at SZA 65° for all-sky conditions. The trend estimates at 305 nm are considerably smaller and less significant, yielding between +5.1 ± 6.5 and +7.9 ± 7.3 %/dec, depending on SZA. Seasonally, the largest trends are found during winter and spring. Total ozone has significantly increased by year-round +1.9 ± 1.3 %/dec since 1997 and therefore cannot explain these significant increases. They are rather attributed to decreases in total cloud cover and aerosol optical depth."

Citation: M. Fitzka, S. Simic and J. Hadzimustafic, Theoretical and Applied Climatology, 2012, DOI: 10.1007/s00704-012-0684-0.

Evaluation of the strength of snow albedo feedback

Using models and satellite observations to evaluate the strength of snow albedo feedback - Fletcher et al. (2012) [FULL TEXT]

Abstract: "Snow albedo feedback (SAF) is important for global climate change, with strong regional impacts over northern continental areas. SAF calculated from the seasonal cycle is a good predictor of SAF in climate change among a suite of coupled climate models. A previous linear decomposition of the simulated total SAF (NET) found 80% was related to the albedo contrast of snow covered and snow-free land (SNC), and 20% was related to the temperature dependence of snow albedo (TEM). By contrast, recent work using snow cover and surface albedo derived from APP-x satellite observations found that TEM and SNC contributed almost equally to NET. In the present study, revised estimates of TEM and SNC for northern land areas are calculated for the period 1982–99 using a simplified and reproducible method for comparing SAF in models and observations. The observed NET is −1.11% K⁻¹, of which 69% comes from SNC and 31% from TEM; the approximate additivity of SNC and TEM indicates that these two terms fully explain the total SAF. Regionally, the SNC term dominates equatorward of 65°N, while TEM dominates over the Arctic. The mean of 17 CMIP3 climate models shows NET is 7% larger than observed, caused primarily by a bias in TEM equatorward of 65°N. A newer model (NCAR CCSM4) with improved land surface and snow schemes reproduces observed values of NET and SNC closely. However, TEM in all models examined is 50–100% weaker than observed over the Arctic. There is a strong correlation between SAF in the seasonal cycle and SAF in climate change for all components, but the correlation is weakest for TEM. The TEM term also exhibits a much larger spread in the seasonal cycle than in climate change, which partially explains a discrepancy between previous published studies examining TEM."

Citation: Fletcher, C. G., H. Zhao, P. J. Kushner, and R. Fernandes (2012), Using models and satellite observations to evaluate the strength of snow albedo feedback, J. Geophys. Res., 117, D11117, doi:10.1029/2012JD017724.

Recent intensification of central Pacific El Niño could be due to natural variability

Statistical evidence for the natural variation of the central Pacific El Niño - Kim et al. (2012)

Abstract: "Extensive studies claimed that the central equatorial Pacific (CP) El Niño has occurred more frequently and strongly than the eastern equatorial Pacific El Niño in recent years. To explain this phenomenon, spatial patterns and principal component time series from several sea surface temperature (SST) data sets in the tropical Pacific are analyzed for the period of 1951–2010. Cyclostationary empirical orthogonal function analysis separates two modes of SST variability, which explain about 50% and 10% of the total SST variability, respectively. Their spatial and temporal patterns are similar among the different SST data sets. The first mode captures the typical El Niño pattern, while the second mode is a dipole pattern in the tropical Pacific. The two modes are, by definition, uncorrelated over the analysis period but are in phase since the late 1990s; superposition of the two modes results in a significant warming in the CP region, which is a potential explanation for a more frequent occurrence of the CP El Niño in the recent decades. Similar analysis is conducted based on the 500 year data from the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 under the preindustrial condition. The result is generally consistent with the observations yielding occasional in-phase relationship between the two modes. Thus, it cannot be ruled out that a more frequent occurrence of the CP El Niño in recent years is a natural feature of the equatorial climate system."

Citation: Kim, J.-S., K.-Y. Kim, and S.-W. Yeh (2012), Statistical evidence for the natural variation of the central Pacific El Niño, J. Geophys. Res., 117, C06014, doi:10.1029/2012JC008003.

Methane under glaciers and ice sheets - what happens when glaciers retreat?

Methanogenic potential of Arctic and Antarctic subglacial environments with contrasting organic carbon sources - Stibal et al. (2012)

Abstract: "Subglacial environments are largely anoxic, contain organic carbon (OC) overridden by glacier ice during periods of advance, and harbour active microbial communities. This creates favourable conditions for OC degradation via methanogenesis. It has been hypothesised that OC beneath ice sheets is converted to methane (CH₄) and may be released to the atmosphere during retreat. However, there are limited data available to support this contention. Here, we present new data on the abundance, diversity and activity of methanogenic archaea and the amount and character of OC in subglacial sediments from Arctic and Antarctic glacial systems based on different substrate types. We employed long-term laboratory incubations to quantify the CH₄ production potential in different subglacial settings. Significant numbers of methanogens (up to 7×10⁴ cells g⁻¹) were detected in the samples and clones of Methanomicrobiales and Methanosarcinales were identified in clone libraries. Long lag periods (up to >200 days) were observed before significant CH₄ concentrations were measured. We report order of magnitude differences in rates of CH₄ production (10¹-10⁵ fmol g⁻¹ d⁻¹) in different subglacial sediments, reflecting contrasts in the origin of the sediment and the OC character. Hence, we predict that contrasting rates of CH₄ production are likely to occur beneath glaciers and ice sheets that overran different types of substrate. We subsequently estimated the potential for CH₄ production beneath the Laurentide/Inuitian/Cordilleran and Fennoscandian Ice Sheets during a typical 85 ka Quaternary glacial/interglacial cycle. CH₄ production from lacustrine-derived OC is likely to be an order of magnitude higher (~6.3-27 Pg C) than that from overridden soils (~0.55-0.68 Pg C), possibly due to a difference in lability between lacustrine and soil OC. While representing a fraction of the entire OC pool (~418-610 Pg C), this finding highlights the importance of considering the character of different OC pools when calculating subglacial CH₄ production."

Citation: Marek Stibal, Jemma L. Wadham, Grzegorz P. Lis, Jon Telling, Richard D. Pancost, Ashley Dubnick, Martin J. Sharp, Emily C. Lawson, Catriona E. H. Butler, Fariha Hasan, Martyn Tranter, Alexandre M. Anesio, Global Change Biology, DOI: 10.1111/j.1365-2486.2012.02763.x.

Tree rings as windstorm indicators

A 300-Year History of Pacific Northwest Windstorms Inferred from Tree Rings - Knapp & Hadley (2012)

Abstract: "Hurricane-force winds are frequently allied with mid-latitude cyclones yet little is known about their historical timing and geographic extent over multiple centuries. This research addresses these issues by extending the historical record of major mid-latitude windstorms along North America's Pacific Northwest (PNW) coast using tree-ring data collected from old-growth (> 350 yrs.), wind-snapped trees sampled at seven coastal sites in Oregon, USA. Our objectives were to: 1) characterize historical windstorm regimes; 2) determine the relationship between high-wind events (HWEs) and phases of the PDO, ENSO and NPI; and 3) test the hypothesis that PNW HWEs have migrated northward. We based our study on the identification of tree-growth anomalies resulting from windstorm-induced canopy changes corresponding to documented (1880–2003) and projected HWEs (1701–1880). Our methods identified all major windstorm events recorded since the late 1800 s and confirmed that variations in coastal tree-growth are weakly related to temperature, precipitation, and drought, but are significantly related to peak wind speeds. These results suggest wind-induced changes in canopy conditions control tree growth at all sites. Comparisons between the tree-ring record and the PDO, NPI, and ENSO revealed a significant positive correlation between HWEs and neutral to warm PDO conditions and a slightly weaker correlation with the NPI. ENSO events were not significantly related to the occurrence of HWEs. Latitudinal groupings of our sites revealed a gradual and non-significant northerly shift of HWEs until the late 19th century followed by a significant northward shift during the past 120 years. These results mark the application of dendroanemology as a method for characterizing windstorm regimes for multiple centuries."

Citation: Paul A. Knapp, Keith S. Hadley, Global and Planetary Change, http://dx.doi.org/10.1016/j.gloplacha.2012.06.002.

20th century surface temperature evolution as a test for climate models

Reproduction of twentieth century intradecadal to multidecadal surface temperature variability in radiatively forced coupled climate models - Brown et al. (2012)

Abstract: "Coupled Model Intercomparison Project 3 simulations that included time-varying radiative forcings were ranked according to their ability to consistently reproduce twentieth century intradecadal to multidecadal (IMD) surface temperature variability at the 5° by 5° spatial scale. IMD variability was identified using the running Mann-Whitney Z method. Model rankings were given context by comparing the IMD variability in preindustrial control runs to observations and by contrasting the IMD variability among the ensemble members within each model. These experiments confirmed that the inclusion of time-varying external forcings brought simulations into closer agreement with observations. Additionally, they illustrated that the magnitude of unforced variability differed between models. This led to a supplementary metric that assessed model ability to reproduce observations while accounting for each model's own degree of unforced variability. These two metrics revealed that discernable differences in skill exist between models and that none of the models reproduced observations at their theoretical optimum level. Overall, these results demonstrate a methodology for assessing coupled models relative to each other within a multimodel framework."

Citation: Brown, P. T., E. C. Cordero, and S. A. Mauget (2012), Reproduction of twentieth century intradecadal to multidecadal surface temperature variability in radiatively forced coupled climate models, J. Geophys. Res., 117, D11116, doi:10.1029/2011JD016864.

Reasons for different amount of warming at low and high elevations in European Alps region

Greenhouse warming and solar brightening in and around the Alps - Philipona (2012)

Abstract: "At low elevations (500 m a.s.l.) Central Europe's surface temperature increased about 1.3 °C since 1981. Interestingly, at high elevations (2200 m a.s.l.) in the Alps, temperature rose less than 1 °C over the same period. Detailed investigations of temperature, humidity and the radiation budget at lowland and alpine climate stations now show that the difference in temperature rise is likely related to unequal solar- and greenhouse warming. The analysis shows that the important decline of anthropogenic aerosols in Europe since the mid-1980s led to solar brightening at low elevations, whereas inherent low aerosol concentrations at high elevations led to only minor changes of solar radiation in the Alps. In the Lowland, absolute humidity and also total net radiation show an about 6% K⁻¹ Clausius–Clapeyron conform increase with temperature since the 1980s. In the Alps, however, the percentage increase rate of humidity and total net radiation is more than twice as large. This large water vapour increase in the Alps is likely related to strong warming and thermal advection in the Lowlands, and may also have increased due to atmospheric circulation changes. Hence, while in the Alps temperature increased primarily due to strong water vapour enhanced greenhouse warming, solar brightening combined with anthropogenic greenhouse gas and water vapour feedback greenhouse warming led to a higher temperature increase at low elevations in Central Europe."

Citation: Rolf Philipona, International Journal of Climatology, DOI: 10.1002/joc.3531.

Clouds make trace gas measurements from satellites more difficult

Decadal Biases in long-term NO₂ averages inferred from satellite observations due to cloud selection criteria - Geddes et al. (2012)

Abstract: "Retrievals of atmospheric trace gas column densities from space are compromised by the presence of clouds, requiring most studies to exclude observations with significant cloud fractions in the instrument's field of view. Using NO₂ observations at three ground stations representing urban, suburban, and rural environments, and tropospheric vertical column densities measured by the Ozone Monitoring Instrument (OMI) over each site, we show that the observations from space represent monthly averaged ground-level pollutant conditions well (R = 0.86) under relatively cloud-free conditions. However, by analyzing the ground-level data and applying the OMI cloud fraction as a filter, we show there is a significant bias in long-term averaged NO₂ as a result of removing the data during cloudy conditions. For the ground-based sites considered in this study, excluding observations on days when OMI-derived cloud fractions were greater than 0.2 causes 12:00–14:00 mean summer mixing ratios to be underestimated by 12% ± 6%, 20% ± 7%, and 40% ± 10% on average (± 1 standard deviation) at the urban, suburban, and rural sites respectively. This bias was investigated in particular at the rural site, a region where pollutant transport is the main source of NO₂, and where long-term observations of NO_y were also available. Evidence of changing photochemical conditions and a correlation between clear skies and the transport of cleaner air masses play key roles in explaining the bias. The magnitude of a bias is expected to vary from site to site depending on meteorology and proximity to NO_x sources, and decreases when longer averaging times of ground station data (e.g. 24-h) are used for the comparison."

Citation: Jeffrey A. Geddes, Jennifer G. Murphy, Jason M. O'Brien, Edward A. Celarier, Remote Sensing of Environment, Volume 124, September 2012, Pages 210–216, http://dx.doi.org/10.1016/j.rse.2012.05.008.

Saudi Arabia has gotten even warmer

Decadal variability of the observed daily temperature in Saudi Arabia during 1979–2008 - Athar (2012)

Abstract: "Variability in the observed daily temperature for the 30-year period (1979–2008) is studied from a total of 19 stations in Saudi Arabia (SA) by calculating the empirical anomaly probability distribution functions (PDFs) on annual basis. The 30-year period is divided into three decades. As compared with the first decade, the PDFs for the remaining decades display a relative frequency rise in warmer temperatures. The mean values of the PDFs depict an average decadal positive shift of 0.83, 0.66, and 0.49 °C, for the maximum, the mean, and the minimum temperature, respectively, relative to the 30-year base value."

Citation: H. Athar, Atmospheric Science Letters, DOI: 10.1002/asl.390.

CLASSIC OF THE WEEK: Ohring & Mariano (1963)

The effect of cloudiness on a greenhouse model of the venus atmosphere - Ohring & Mariano (1963) [FULL TEXT]

Abstract: "In previous models of the greenhouse effect in the Venus atmosphere, it has been assumed that infrared-absorbing atmospheric gases provide the sole contribution to the infrared opacity of the Venus atmosphere. In the present study, the influence of an extensive cloud-cover, opaque to infrared radiation, is also included in the greenhouse model. The magnitude of the greenhouse effect, which is defined here as the ratio of the surface temperature produced by the greenhouse to the surface temperature of an atmosphere-less Venus, is computed as a function of infrared opacity of the atmosphere, and amount and height (actually ratio of cloud-top pressure to surface pressure) of clouds. It is assumed that the Venus atmosphere is grey, the absorbing gas has a constant mixing ratio, and the temperature variation with altitude is linear. Calculations are made for two temperature lapse rates: the adiabatic lapse rate, and nine-tenths of the adiabatic lapse rate. The adiabatic lapse rate maximizes the greenhouse effect; for this case estimates of the minimum infrared opacity required to maintain the observed surface temperature can be determined. For a surface temperature of 700K, 99% cloudiness, and cloud-top temperature of 240K, the minimum required infrared opacity is six. Uncertainties and questionable side effects of the model are discussed."

Citation: Ohring, G., Mariano, J. F., NASA report NASA-CR-51176.

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