Abstract Round Up: 11/07-11/15


Image courtesy of Flickr user "marsi", used under Creative Commons

A contribution to attribution of recent global warming by out-of-sample Granger causality analysis

ABSTRACT:  The topic of attribution of recent global warming is usually faced by studies performed through global climate models (GCMs). Even simpler econometric models have been applied to this problem, but they led to contrasting results. In this article, we show that a genuine predictive approach of Granger analysis leads to overcome problems shown by these models and to obtain a clear signal of linear Granger causality from greenhouse gases (GHGs) to the global temperature of the second half of the 20th century. In contrast, Granger causality is not evident using time series of natural forcing.


Growth and Development Rates Have Different Thermal Responses

ABSTRACT:  Growth and development rates are fundamental to all living organisms. In a warming world, it is important to determine how these rates will respond to increasing temperatures. It is often assumed that the thermal responses of physiological rates are coupled to metabolic rate and thus have the same temperature dependence. However, the existence of the temperature-size rule suggests that intraspecific growth and development are decoupled. Decoupling of these rates would have important consequences for individual species and ecosystems, yet this has not been tested systematically across a range of species. We conducted an analysis on growth and development rate data compiled from the literature for a well-studied group, marine pelagic copepods, and use an information-theoretic approach to test which equations best describe these rates. Growth and development rates were best characterized by models with significantly different parameters: development has stronger temperature dependence than does growth across all life stages. As such, it is incorrect to assume that these rates have the same temperature dependence. We used the best-fit models for these rates to predict changes in organism mass in response to temperature. These predictions follow a concave relationship, which complicates attempts to model the impacts of increasing global temperatures on species body size.


Estimating the effects of ENSO upon the observed freshening trends of the western tropical Pacific Ocean

ABSTRACT:  A significant surface freshening trend and an eastward expansion of fresh surface waters have been documented in the western tropical Pacific, consistent with the expected effects of climate change. The highest El Niño Southern Oscillation (ENSO) variability in Sea Surface Salinity (SSS) has been also documented in that region, with different quantitative signatures for the Eastern and Central Pacific ENSO events (EP and CP ENSO, respectively). This study hence analyses to what extent have the EP and CP ENSO events contributed to the long-term freshening trends, relying on 1955–2008 in situ SSS data and on EP and CP ENSO main features. We show the influence of EP ENSO events to be negligible, while CP El Niño events contribute to enhance the long-term freshening trend (up to 30%) in the far western equatorial Pacific and moderately reduce that freshening (up to 10%) in the South Pacific Convergence Zone (SPCZ). Our results thus suggest that the observed eastward expansion of the surface covered by low-salinity waters in the western half of the tropical Pacific is mostly due to climate change rather than to the documented possible increased occurrence and intensity of CP El Niño events. The sensitivity of the trend estimates to the different periodicity of the SSS data records is also discussed.


Simulating the impacts of disturbances on forest carbon cycling in North America: Processes, data, models, and challenges

ABSTRACT:  Forest disturbances greatly alter the carbon cycle at various spatial and temporal scales. It is critical to understand disturbance regimes and their impacts to better quantify regional and global carbon dynamics. This review of the status and major challenges in representing the impacts of disturbances in modeling the carbon dynamics across North America revealed some major advances and challenges. First, significant advances have been made in representation, scaling, and characterization of disturbances that should be included in regional modeling efforts. Second, there is a need to develop effective and comprehensive process-based procedures and algorithms to quantify the immediate and long-term impacts of disturbances on ecosystem succession, soils, microclimate, and cycles of carbon, water, and nutrients. Third, our capability to simulate the occurrences and severity of disturbances is very limited. Fourth, scaling issues have rarely been addressed in continental scale model applications. It is not fully understood which finer scale processes and properties need to be scaled to coarser spatial and temporal scales. Fifth, there are inadequate databases on disturbances at the continental scale to support the quantification of their effects on the carbon balance in North America. Finally, procedures are needed to quantify the uncertainty of model inputs, model parameters, and model structures, and thus to estimate their impacts on overall model uncertainty. Working together, the scientific community interested in disturbance and its impacts can identify the most uncertain issues surrounding the role of disturbance in the North American carbon budget and develop working hypotheses to reduce the uncertainty.


Methane emissions from the surface of the Three Gorges Reservoir

ABSTRACT:  After our previous study about methane (CH4) emissions from littoral marshes of the Three Gorges Reservoir (TGR), Chinese dams have raised a world-wide concern. Through measurements from the surface of the TGR, a CH4 emission rate was recorded as 0.26 ± 0.38 mg CH4 m−2 h−1 (Mean ± SD), relatively low compared with those from other hydropower reservoirs. We also recorded CH4 emission rate from the surface of downstream water, which was also relatively low (0.24 ± 0.37 mg CH4 m−2 h−1). Such result may indicate that TGR is not a great CH4 emitter (not “CH4 menace”). One possible reason for such a low emission rate is that measures to maintain water quality and protect environment and ecosystem decrease the input of organic materials (for methanogenesis), which in turn limits the CH4 production in the sediment of the TGR. We also found that CH4 emission from the flooding drawdown area (0.29 ± 0.37 mg CH4 m−2 h−1) was higher than other permanently flooded sites (0.23 ± 0.38 mg CH4 m−2 h−1). Because of annual vegetation re-growth, the drawdown zone is the especially important carbon source for methanogenesis in flooding season. Interestingly, we also observed that mean CH4 emission was significantly higher in winter than in spring and summer. This was partly due to seasonal dynamics of hydrology. In order to estimate the net CH4 emissions caused by the reservoir and reservoir operation, the best approach would be Life Cycle Analysis.


Protracted storage of biospheric carbon in the Ganges–Brahmaputra basin

ABSTRACT:  The amount of carbon stored in continental reservoirs such as soils, sediments and the biosphere greatly exceeds the amount of carbon in the atmosphere1. As such, small variations in the residence time of organic carbon in these reservoirs can produce large variations in the atmospheric inventory of carbon dioxide. One such reservoir is the Ganges–Brahmaputra system draining the Himalayas, which represents one of the largest sources of terrestrial biospheric carbon to the ocean2. Here, we examine the radiocarbon content of river sediments collected from the Ganges–Brahmaputra drainage basin to determine the residence time of organic carbon in this reservoir. We show that the average age of biospheric organic carbon in the drainage basin ranges from 0.5 to 17 thousand years. The radiocarbon age of plant-derived fatty acids—a proxy for labile terrestrial vegetation—ranges from just 0.05 to 1.3 thousand years. We propose that the bulk ages can be explained by the existence of a refractory, slowly cycling component of the organic carbon pool that is mixed with a younger labile pool. We estimate that this refractory component has an average age of over 15,000 years, and represents up to 20% of total biospheric carbon exported by the Ganges–Brahmaputra system. We suggest that global warming might destabilize this ancient pool of carbon, if warming stimulates microbial decomposition of organic carbon reserves.


Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures

ABSTRACT:  Abrupt periods of global warming between 57 and 50 million years ago—known as the Early Palaeogene hyperthermal events—were associated with the repeated injection of massive amounts of carbon into the atmosphere1, 2, 3, 4. The release of methane from the sea floor following the dissociation of gas hydrates is often invoked as a source5. However, seafloor temperatures before the events were at least 4–7 °C higher than today1, which would have limited the area of sea floor suitable for hosting gas hydrates6, 7. Palaeogene gas hydrate reservoirs may therefore not have been sufficient to provide a significant fraction of the carbon released. Here we use numerical simulations of gas hydrate accumulation8 at Palaeogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted in the Palaeogene. Our simulations show that warmer temperatures during the Palaeogene would have enhanced the amount of organic carbon reaching the sea floor as well as the rate of methanogenesis. We find that under plausible temperature and pressure conditions, the abundance of gas hydrates would be similar or higher in the Palaeogene than at present. We conclude that methane hydrates could have been an important source of carbon during the Palaeogene hyperthermal events.


Sensitivity of the attribution of near surface temperature warming to the choice of observational dataset

ABSTRACT:  A number of studies have demonstrated that much of the recent warming in global near surface temperatures can be attributed to increases in anthropogenic greenhouse gases. While this conclusion has been shown to be robust in analyses using a variety of climate models there have not been equivalent studies using different available observational datasets. Here we repeat the analyses as reported previously using an updated observational dataset and other independently processed datasets of near surface temperatures. We conclude that the choice of observational dataset has little impact on the attribution of greenhouse gas warming and other anthropogenic cooling contributions to observed warming on a global scale over the 20th century, however this robust conclusion may not hold for other periods or for smaller sub-regions. Our results show that the dominant contributor to global warming over the last 50 years of the 20th century is that due to greenhouse gases.


High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core

ABSTRACT:  Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise. Yet, it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability. To address this need, we reconstruct Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilises argon and nitrogen isotopic ratios from occluded air bubbles. The estimated average Greenland snow temperature over the past 4000 years was −30.7°C with a standard deviation of 1.0°C and exhibited a long-term decrease of roughly 1.5°C, which is consistent with earlier studies. The current decadal average surface temperature (2001–2010) at the GISP2 site is −29.9°C. The record indicates that warmer temperatures were the norm in the earlier part of the past 4000 years, including century-long intervals nearly 1°C warmer than the present decade (2001–2010). Therefore, we conclude that the current decadal mean temperature in Greenland has not exceeded the envelope of natural variability over the past 4000 years, a period that seems to include part of the Holocene Thermal Maximum. Notwithstanding this conclusion, climate models project that if anthropogenic greenhouse gas emissions continue, the Greenland temperature would exceed the natural variability of the past 4000 years sometime before the year 2100.


Varying boreal forest response to Arctic environmental change at the Firth River, Alaska

ABSTRACT:  The response of boreal forests to anthropogenic climate change remains uncertain, with potentially significant impacts for the global carbon cycle, albedo, canopy evapotranspiration and feedbacks into further climate change. Here, we focus on tree-ring data from the Firth River site at treeline in northeastern Alaska, in a tundra–forest transition region where pronounced warming has already occurred. Both tree-ring width (TRW) and maximum latewood density (MXD) chronologies were developed to identify the nature of tree growth and density responses to climatic and environmental changes in white spruce (Picea glauca), a dominant Arctic treeline species. Good agreement was found between the interannual fluctuations in the TRW chronology and summer temperatures from 1901 to 1950, whereas no significant relationships were found from 1951 to 2001, supporting evidence of significant divergence between TRW and summer temperature in the second half of the 20th century. In contrast to this unstable climatic response in the TRW record, the high frequency July–August temperature signal in the MXD series seems reasonably stable through the 20th century. Wider and denser rings were more frequent during the 20th century, particularly after 1950, than in previous centuries. Finally, comparison between the tree-ring proxies and a satellite-derived vegetation index suggests that TRW and MXD correlate with vegetation productivity at the landscape level at different times of the growing season.


Are secular correlations between sunspots, geomagnetic activity, and global temperature significant?

ABSTRACT:  Recent studies have led to speculation that solar-terrestrial interaction, measured by sunspot number and geomagnetic activity, has played an important role in global temperature change over the past century or so. We treat this possibility as an hypothesis for testing. We examine the statistical significance of cross-correlations between sunspot number, geomagnetic activity, and global surface temperature for the years 1868–2008, solar cycles 11–23. The data contain substantial autocorrelation and nonstationarity, properties that are incompatible with standard measures of cross-correlational significance, but which can be largely removed by averaging over solar cycles and first-difference detrending. Treated data show an expected statistically-significant correlation between sunspot number and geomagnetic activity, Pearson p < 10−4, but correlations between global temperature and sunspot number (geomagnetic activity) are not significant, p = 0.9954, (p = 0.8171). In other words, straightforward analysis does not support widely-cited suggestions that these data record a prominent role for solar-terrestrial interaction in global climate change. With respect to the sunspot-number, geomagnetic-activity, and global-temperature data, three alternative hypotheses remain difficult to reject: (1) the role of solar-terrestrial interaction in recent climate change is contained wholly in long-term trends and not in any shorter-term secular variation, or, (2) an anthropogenic signal is hiding correlation between solar-terrestrial variables and global temperature, or, (3) the null hypothesis, recent climate change has not been influenced by solar-terrestrial interaction.


Recent global trends in atmospheric fronts

ABSTRACT:  An automated, objective method is used to identify atmospheric fronts in four independent reanalysis data sets for the period 1989–2009 and to calculate changes in their frequency. The analysis highlights several coherent regions of statistically significant change in the frequency of fronts. The front frequency in the North Atlantic storm track has decreased by about 10–20%, whereas changes observed over the Southern Ocean are relatively small. In the subtropical Pacific the front frequency has increased significantly, which is consistent with an expansion of the dry subtropics. The sensitivity of these trends to the detection method is tested and the results are found to be robust. The results provide a concise summary of the recent changes in a major component of synoptic weather conditions, providing a benchmark for climate models as well as an additional tool for interpreting climate change predictions.


Testing the robustness of semi-empirical sea level projections

ABSTRACT:  We determine the parameters of the semi-empirical link between global temperature and global sea level in a wide variety of ways, using different equations, different data sets for temperature and sea level as well as different statistical techniques. We then compare projections of all these different model versions (over 30) for a moderate global warming scenario for the period 2000–2100. We find the projections are robust and are mostly within ±20% of that obtained with the method of Vermeer and Rahmstorf (Proc Natl Acad Sci USA 106:21527–21532, 2009), namely ~1 m for the given warming of 1.8°C. Lower projections are obtained only if the correction for reservoir storage is ignored and/or the sea level data set of Church and White (Surv Geophys, 2011) is used. However, the latter provides an estimate of the base temperature T 0 that conflicts with the constraints from three other data sets, in particular with proxy data showing stable sea level over the period 1400–1800. Our new best-estimate model, accounting also for groundwater pumping, is very close to the model of Vermeer and Rahmstorf (Proc Natl Acad Sci USA 106:21527–21532, 2009).


The role of the intra-daily SST variability in the Indian monsoon variability and monsoon-ENSO–IOD relationships in a global coupled model

ABSTRACT:  The impact of diurnal SST coupling and vertical oceanic resolution on the simulation of the Indian Summer Monsoon (ISM) and its relationships with El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events are studied through the analysis of four integrations of a high resolution Coupled General Circulation Model (CGCM), but with different configurations. The only differences between the four integrations are the frequency of coupling between the ocean and atmosphere for the Sea Surface Temperature (SST) parameter (2 vs. 24 h coupling) and/or the vertical oceanic resolution (31 vs. 301 levels) in the CGCM. Although the summer mean tropical climate is reasonably well captured with all the configurations of the CGCM and is not significantly modified by changing the frequency of SST coupling from once to twelve per day, the ISM–ENSO teleconnections are rather poorly simulated in the two simulations in which SST is exchanged only once per day, independently of the vertical oceanic resolution used in the CGCM. Surprisingly, when 2 h SST coupling is implemented in the CGCM, the ISM–ENSO teleconnection is better simulated, particularly, the complex lead-lag relationships between the two phenomena, in which a weak ISM occurs during the developing phase of an El Niño event in the Pacific, are closely resembling the observed ones. Evidence is presented to show that these improvements are related to changes in the characteristics of the model’s El Niño which has a more realistic evolution in its developing and decaying phases, a stronger amplitude and a shift to lower frequencies when a 2-hourly SST coupling strategy is implemented without any significant changes in the basic state of the CGCM. As a consequence of these improvements in ENSO variability, the lead relationships between Indo-Pacific SSTs and ISM rainfall resemble the observed patterns more closely, the ISM–ENSO teleconnection is strengthened during boreal summer and ISM rainfall power spectrum is in better agreement with observations. On the other hand, the ISM–IOD teleconnection is sensitive to both SST coupling frequency and the vertical oceanic resolution, but increasing the vertical oceanic resolution is degrading the ISM–IOD teleconnection in the CGCM. These results highlight the need of a proper assessment of both temporal scale interactions and coupling strategies in order to improve current CGCMs. These results, which must be confirmed with other CGCMs, have also important implications for dynamical seasonal prediction systems or climate change projections of the monsoon.


Atmospheric CO2 decline during the Pliocene intensification of Northern Hemisphere glaciations

ABSTRACT:  Several hypotheses have been put forward to explain the onset of intensive glaciations on Greenland, Scandinavia, and North America during the Pliocene epoch between 3.6 and 2.7 million years ago (Ma). A decrease in atmospheric CO2 may have played a role during the onset of glaciations, but other tectonic and oceanic events occurring at the same time may have played a part as well. Here we present detailed atmospheric CO2 estimates from boron isotopes in planktic foraminifer shells spanning 4.6–2.0 Ma. Maximal Pliocene atmospheric CO2 estimates gradually declined from values around 410 μatm to early Pleistocene values of 300 μatm at 2.0 Ma. After the onset of large-scale ice sheets in the Northern Hemisphere, maximal pCO2 estimates were still at 2.5 Ma +90 μatm higher than values characteristic of the early Pleistocene interglacials. By contrast, Pliocene minimal atmospheric CO2 gradually decreased from 310 to 245 μatm at 3.2 Ma, coinciding with the start of transient glaciations on Greenland. Values characteristic of early Pleistocene glacial atmospheric CO2 of 200 μatm were abruptly reached after 2.7 Ma during the late Pliocene transition. This trend is consistent with the suggestion that ocean stratification and iron fertilization increased after 2.7 Ma in the North Pacific and Southern Ocean and may have led to increased glacial CO2 storage in the oceanic abyss after 2.7 Ma onward.


Warming and drying of the eastern Mediterranean: Additional evidence from trend analysis

ABSTRACT:  The climate of the eastern Mediterranean (EM), at the transition zone between the Mediterranean climate and the semi-arid/arid climate, has been studied for a 39-year period to determine whether climate changes have taken place. A thorough trend analysis using the nonparametric Mann-Kendall test with Sen’s slope estimator has been applied to ground station measurements, atmospheric reanalysis data, synoptic classification data and global data sets for the years 1964–2003. In addition, changes in atmospheric regional patterns between the first and last twenty years were determined by visual comparisons of their composite mean. The main findings of the analysis are: 1) changes of atmospheric conditions during summer and the transitional seasons (mainly autumn) support a warmer climate over the EM and this change is already statistically evident in surface temperatures having exhibited positive trends of 0.2–1°C/decade; 2) changes of atmospheric conditions during winter and the transitional seasons support drier conditions due to reduction in cyclogenesis and specific humidity over the EM, but this change is not yet statistically evident in surface station rain data, presumably because of the high natural precipitation variance masking such a change. The overall conclusion of this study is that the EM region is under climate change leading to warmer and drier conditions.


Renewed methane increase for five years (2007–2011) observed by solar FTIR spectrometry

ABSTRACT:  Trends of column-averaged methane for the time period [1996, September 2011] are derived from the mid-infrared (mid-IR) solar FTIR time series at the Zugspitze (47.42° N, 10.98° E, 2964 m a.s.l.) and Garmisch (47.48° N, 11.06° E, 743 m a.s.l.). Trend analysis comprises a fit to the de-seasonalized time series along with bootstrap resampling for quantifying trend uncertainties. We find a positive trend during [1996, 1998] (9.0 [3.2, 14.7] ppb yr−1, Zugspitze, 95 % confidence interval), a non-significant growth during [1999, mid 2006] (0.8 [−0.1, 1.7] ppb yr−1, Zugspitze), and a significant renewed increase during [mid 2006, September 2011] of 5.1 [4.2, 6.0] ppb yr−1 for Garmisch, which is in agreement with 4.8 [3.8, 5.9] ppb yr−1 for Zugspitze.

The agreement of methane trends at the two closely neighboring FTIR sites with strongly differing levels of integrated water vapor (min/max = 0.2 mm/12.7 mm for Zugspitze, 1.9 mm/34.9 mm for Garmisch) proves that potentially significant water-vapor-methane interference errors do not affect the trend results, if the updated mid-IR retrieval strategy MIR-GBM v1.0 is used. Furthermore, agreement of the trend of 6.6 ppb yr−1 derived from SCIAMACHY (WFMD v2.0) data for the time period [mid 2006, mid 2009] is found within the 95 % confidence interval of the ground-based FTIR result.

While earlier studies using surface network data revealed changes of 8.0±0.6 ppb in 2007 and 6.4±0.6 ppb in 2008 (update from Dlugokencky et al., 2009), our updated result proves that meanwhile, the renewed methane increase has been persisting for >5 yr [mid 2006, September 2011]. This is either the longest and largest positive trend anomaly since >25 yr when systematic observations began or the onset of a new period of strongly increasing CH4 levels in the atmosphere.

The 2007–2008 part of the anomaly was previously attributed to increased natural wetland emissions. For the full period from 2007 to 2011, our analysis of ECMWF ERA-INTERIM precipitations and 2-m temperatures shows that precipitations above tropical wetland areas increased in 2007–2008, decreased in 2009, and have been increasing again since 2010, while tropical land temperatures increased only slightly. As recent estimates of anthropogenic emissions are not yet available, it is not possible to finally conclude that the 2009–2011 period of methane increase was dominated by natural wetland emissions, although they probably play a significant role.

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