“Torneträsk tree-ring width and density AD 500–2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers.” Håkan Grudd. Climate Dynamics. 10.1007/s00382-007-0358-2, published online January 2008
Abstract: This paper presents updated tree-ring width (TRW) and maximum density (MXD) from Torneträsk in northern Sweden, now covering the period AD 500–2004. By including data from relatively young trees for the most recent period, a previously noted decline in recent MXD is eliminated. Non-climatological growth trends in the data are removed using Regional Curve Standardization (RCS), thus producing TRW and MXD chronologies with preserved low-frequency variability. The chronologies are calibrated using local and regional instrumental climate records. A boot-strapped response function analysis using regional climate data shows that tree growth is forced by April–August temperatures and that the regression weights for MXD are much stronger than for TRW. The robustness of the reconstruction equation is verified by independent temperature data and shows that 63–64% of the instrumental inter-annual variation is captured by the tree-ring data. This is a significant improvement compared to previously published reconstructions based on tree-ring data from Torneträsk. A divergence phenomenon around AD 1800, expressed as an increase in TRW that is not paralleled by temperature and MXD, is most likely an effect of major changes in the density of the pine population at this northern tree-line site. The bias introduced by this TRW phenomenon is assessed by producing a summer temperature reconstruction based on MXD exclusively. The new data show generally higher temperature estimates than previous reconstructions based on Torneträsk tree-ring data. The late-twentieth century, however, is not exceptionally warm in the new record: On decadal-to-centennial timescales, periods around AD 750, 1000, 1400, and 1750 were equally warm, or warmer. The 200-year long warm period centered on AD 1000 was significantly warmer than the late-twentieth century (p \ 0.05) and is supported by other local and regional paleoclimate data. The new tree-ring evidence from Torneträsk suggests that this ‘‘Medieval Warm Period’’ in northern Fennoscandia was much warmer than previously recognized.
http://www.springerlink.com/content/8j71453650116753/
"Global warming and United States landfalling hurricanes." Wang, Chunzai; Lee, Sang-Ki. Geophys. Res. Lett., Vol. 35, No. 2, 23 January 2008
Abstract: A secular warming of sea surface temperature occurs almost everywhere over the global ocean. Here we use observational data to show that global warming of the sea surface is associated with a secular increase of tropospheric vertical wind shear in the main development region (MDR) for Atlantic hurricanes. The increased wind shear coincides with a weak but robust downward trend in U.S. landfalling hurricanes, a reliable measure of hurricanes over the long term. Warmings over the tropical oceans compete with one another, with the tropical Pacific and Indian Oceans increasing wind shear and the tropical North Atlantic decreasing wind shear. Warmings in the tropical Pacific and Indian Oceans win the competition and produce increased wind shear which reduces U.S. landfalling hurricanes. Whether future global warming increases the vertical wind shear in the MDR for Atlantic hurricanes will depend on the relative role induced by secular warmings over the tropical oceans.
http://www.agu.org/pubs/crossref/2008/2007GL032396.shtml
“Vertical structure of recent Arctic warming.” Graversen, R.G., et al., 2008. Nature, 541, 53-57.
Abstract: Near-surface warming in the Arctic has been almost twice as large as the global average over recent decades-a phenomenon that is known as the 'Arctic amplification'. The underlying causes of this temperature amplification remain uncertain. The reduction in snow and ice cover that has occurred over recent decades may have played a role. Climate model experiments indicate that when global temperature rises, Arctic snow and ice cover retreats, causing excessive polar warming. Reduction of the snow and ice cover causes albedo changes, and increased refreezing of sea ice during the cold season and decreases in sea-ice thickness both increase heat flux from the ocean to the atmosphere. Changes in oceanic and atmospheric circulation, as well as cloud cover, have also been proposed to cause Arctic temperature amplification. Here we examine the vertical structure of temperature change in the Arctic during the late twentieth century using reanalysis data. We find evidence for temperature amplification well above the surface. Snow and ice feedbacks cannot be the main cause of the warming aloft during the greater part of the year, because these feedbacks are expected to primarily affect temperatures in the lowermost part of the atmosphere, resulting in a pattern of warming that we only observe in spring. A significant proportion of the observed temperature amplification must therefore be explained by mechanisms that induce warming above the lowermost part of the atmosphere. We regress the Arctic temperature field on the atmospheric energy transport into the Arctic and find that, in the summer half-year, a significant proportion of the vertical structure of warming can be explained by changes in this variable. We conclude that changes in atmospheric heat transport may be an important cause of the recent Arctic temperature amplification.
http://www.nature.com/nature/journal/v451/n7174/abs/nature06502.html
“Difficulties in tracking the long-term global trend in tropical forest area.” Grainger, Alan. Proceedings of the National Academy of Sciences USA 105: 818-823 (2008).
Abstract: The long-term trend in tropical forest area receives less scrutiny than the tropical deforestation rate. We show that constructing a reliable trend is difficult and evidence for decline is unclear, within the limits of errors involved in making global estimates. A time series for all tropical forest area, using data from Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization, is dominated by three successively corrected declining trends. Inconsistencies between these trends raise questions about their reliability, especially because differences seem to result as much from errors as from changes in statistical design and use of new data. A second time series for tropical moist forest area shows no apparent decline. The latter may be masked by the errors involved, but a "forest return" effect may also be operating, in which forest regeneration in some areas offsets deforestation (but not biodiversity loss) elsewhere. A better monitoring program is needed to give a more reliable trend. Scientists who use FRA data should check how the accuracy of their findings depends on errors in the data.
www.pnas.org/cgi/content/abstract/105/2/818
“Meteorologically driven trends in sea level rise.” Kolker, A. S., and S. Hameed (2007), Geophys. Res. Lett.
Abstract: Determining the rate of global sea level rise (GSLR) during the past century is critical to understanding recent changes to the global climate system. However, this is complicated by non-tidal, short-term, local sea-level variability that is orders of magnitude greater than the trend. While the non-dimensional North Atlantic Oscillation (NAO) index can explain some of this variability in the Atlantic, significant results have been largely restricted to Europe. We show that dimensional indices of the position and intensity of the atmospheric centers of action (COAs) comprising the NAO are correlated with a major fraction of the variability and trend at 5 Atlantic Ocean tide gauges since 1900. COA fluctuations are shown to influence winds, pressure and sea-surface temperatures, thereby influencing sea level via a suite of coastal oceanographic processes. These findings reduce variability in regional sea level rise estimates and indicate a meteorological driver of sea-level trends.
www.agu.org/pubs/crossref/2007/2007GL031814.shtml
"Remote sensing of Greenland ice sheet using multispectral near-infrared and visible radiances." Chylek, P., M. McCabe, M. K. Dubey, and J. Dozier (2007), J. Geophys. Res. 14 December 2007.
Abstract: We present the physical basis of and validate a new remote-sensing algorithm that utilizes reflected visible and near-infrared radiation to discriminate between dry and wet snow. When applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, our discrimination algorithm has the potential to retrieve melting regions of the ice sheet at a spatial resolution of 0.25 km2, over three orders of magnitude higher than the resolution of current microwave methods. The method should be useful for long-term monitoring of the melt area of the Greenland ice sheet, especially regions close to ice sheet margins and of the outflow glaciers. Our analysis of MODIS retrievals of the western portion of the Greenland ice sheet over the period 2000 to 2006 indicates significant interannual variability with a maximum melt extent in 2005. Collocated in situ meteorological data reveal a high correlation (0.80) between the MODIS melt-day area and the average summer temperature. Our analysis suggests that it is the magnitude of the summer temperature that dominates the melting (not the variability of the length of the melting season). Furthermore, we find that the melt-day area increases by about 3.8% for each 0.1 K increase in the average surface air summer temperature. We combine this empirical relationship with historic temperature data to infer that the melt-day area of the western part of the ice sheet doubled between the mid-1990s and mid-2000s and that the largest ice sheet surface melting probably occurred between 1920s and 1930s, concurrent with the warming in that period.
www.agu.org/pubs/crossref/2007/2007JD008742.shtml
"Phenomenological reconstructions of the solar signature in the Northern Hemisphere surface temperature records since 1600." Scafetta, N., and B. J. West (2007), J. Geophys. Res., 112, D24S03. November 2007.
Abstract: A phenomenological thermodynamic model is adopted to estimate the relative contribution of the solar-induced versus anthropogenic-added climate forcing during the industrial era. We compare different preindustrial temperature and solar data reconstruction scenarios since 1610. We argue that a realistic climate scenario is the one described by a large preindustrial secular variability (as the one shown by the paleoclimate temperature reconstruction by Moberg et al. (2005)) with the total solar irradiance experiencing low secular variability (as the one shown by Wang et al. (2005)). Under this scenario the Sun might have contributed up to approximately 50% (or more if ACRIM total solar irradiance satellite composite (Willson and Mordvinov, 2003) is implemented) of the observed global warming since 1900
www.agu.org/pubs/crossref/2007/2007JD008437.shtml
"Climate change cannot be entirely responsible for soil carbon loss observed in England and Wales 1978–2003." By: Smith, Pete et. al. Global Change Biology, Dec2007, Vol. 13 Issue 12, p2605-2609.
Abstract: We present results from modelling studies, which suggest that, at most, only about 10–20% of recently observed soil carbon losses in England and Wales could possibly be attributable to climate warming. Further, we present reasons why the actual losses of SOC from organic soils in England and Wales might be lower than those reported.
http://www.ingentaconnect.com/content/bsc/gcb/2007/00000013/00000012/art00010
"Cloud and radiation budget changes associated with tropical intraseasonal oscillations." Spencer, R.W., Braswell, W.D., Christy, J.R., Hnilo, J., 2007. Geophysical Research Letters, 34.
Abstract: We explore the daily evolution of tropical intraseasonal oscillations in satellite-observed tropospheric temperature, precipitation, radiative fluxes, and cloud properties. The warm/rainy phase of a composited average of fifteen oscillations is accompanied by a net reduction in radiative input into the ocean-atmosphere system, with longwave heating anomalies transitioning to longwave cooling during the rainy phase. The increase in longwave cooling is traced to decreasing coverage by ice clouds, potentially supporting Lindzen's “infrared iris” hypothesis of climate stabilization. These observations should be considered in the testing of cloud parameterizations in climate models, which remain sources of substantial uncertainty in global warming prediction.
http://www.agu.org/pubs/crossref/2007/2007GL029698.shtml
"Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide." Wöppelmann, G., B. Martin Miguez, M.-N. Bouin, and Z. Altamimi. 2007. Global and Planetary Change, 57, 396–406.
Abstract: The problem of correcting the tide gauge records for the vertical land motion upon which the gauges are settled has only been partially solved. At best, the analyses so far have included model corrections for one of the many processes that can affect the land stability, namely the Glacial-Isostatic Adjustment (GIA). An alternative approach is to measure (rather than to model) the rates of vertical land motion at the tide gauges by means of space geodesy. A dedicated GPS processing strategy is implemented to correct the tide gauges records, and thus to obtain a GPS-corrected set of ‘absolute’ or geocentric sea-level trends. The results show a reduced dispersion of the estimated sea-level trends after application of the GPS corrections. They reveal that the reference frame implementation is now achieved within the millimetre accuracy on a weekly basis. Regardless of the application, whether local or global, we have shown that GPS data analysis has reached the maturity to provide useful information to separate land motion from oceanic processes recorded by the tide gauges or to correct these latter. For comparison purposes, we computed the global average of sea-level change according to Douglas [Douglas, B.C., 2001. Sea level change in the era of the recording tide gauge. Int. Geophys. Ser., 75, pp. 37–64.] rules, whose estimate is 1.84 ± 0.35 mm/yr after correction for the GIA effect [Peltier, W.R., 2001. Global glacial isostatic adjustment and modern instrumental records of relative sea level history. Int. Geophys. Ser., 75, pp. 65–95.]. We obtain a value of 1.31 ± 0.30 mm/yr, a value which appears to resolve the ‘sea level enigma’ [Munk, W., 2002. Twentieth century sea level: an enigma. Proc. Natl. Acad. Sci. U.S.A., 99(10), pp. 6550–6555].
linkinghub.elsevier.com/retrieve/pii/S0921818107000239
"A comparison of tropical temperature trends with model predictions." Douglass, D.H., J.R. Christy, B.D. Pearson, and S.F. Singer. 2007. International Journal of Climatology.
Abstract: We examine tropospheric temperature trends of 67 runs from 22 ‘Climate of the 20th Century’ model simulations and try to reconcile them with the best available updated observations (in the tropics during the satellite era). Model results and observed temperature trends are in disagreement in most of the tropical troposphere, being separated by more than twice the uncertainty of the model mean. In layers near 5 km, the modelled trend is 100 to 300% higher than observed, and, above 8 km, modelled and observed trends have opposite signs. These conclusions contrast strongly with those of recent publications based on essentially the same data.
http://www.isepp.org/Comparison.pdf
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