| 1. |
- Büntgen, U, et al.
(author)
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Eight centuries of Pyrenees summer temperatures from tree-ring density
- 2008
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 31, s. 615-631
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Journal article (peer-reviewed)abstract
- Two hundred and sixty one newly measured tree-ring width and density series from living and dry-dead conifers from two timberline sites in the Spanish Pyrenees were compiled. Application of the regional curve standardization method for tree-ring detrending allowed the preservation of inter-annual to multi-centennial scale variability. The new density record correlates at 0.53 (0.68 in the higher frequency domain) with May–September maximum temperatures over the 1944–2005 period. Reconstructed warmth in the fourteenth to fifteenth and twentieth century is separated by a prolonged cooling from ∼1450 to 1850. Six of the ten warmest decades fall into the twentieth century, whereas the remaining four are reconstructed for the 1360–1440 interval. Comparison with novel density-based summer temperature reconstructions from the Swiss Alps and northern Sweden indicates decadal to longer-term similarity between the Pyrenees and Alps, but disagreement with northern Sweden. Spatial field correlations with instrumental data support the regional differentiation of the proxy records. While twentieth century warmth is evident in the Alps and Pyrenees, recent temperatures in Scandinavia are relatively cold in comparison to earlier warmth centered around medieval times, ∼1450, and the late eighteenth century. While coldest summers in the Alps and Pyrenees were in-phase with the Maunder and Dalton solar minima, lowest temperatures in Scandinavia occurred later at the onset of the twentieth century. However, fairly cold summers at the end of the fifteenth century, between ∼1600–1700, and ∼1820 were synchronized over Europe, and larger areas of the Northern Hemisphere.
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| 2. |
- Grand Graversen, Rune, et al.
(author)
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Polar amplification in a coupled climate model with locked albedo
- 2009
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In: Climate Dynamics. - : Springer. - 0930-7575 .- 1432-0894. ; 33:5, s. 629-643
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Journal article (peer-reviewed)abstract
- In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling of the atmospheric CO2 content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature amplification.
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| 3. |
- Grudd, H
(author)
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Torneträsk tree-ring width and density AD 500 – 2004 : A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers
- 2008
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 31, s. 843-857
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Journal article (peer-reviewed)abstract
- This paper presents updated tree-ring width(TRW) and maximum density (MXD) from Tornetra¨sk innorthern Sweden, now covering the period AD 500–2004. Byincluding data from relatively young trees for the most recentperiod, a previously noted decline in recent MXD is eliminated.Non-climatological growth trends in the data areremoved using Regional Curve Standardization (RCS), thusproducingTRWandMXDchronologies with preserved lowfrequencyvariability. The chronologies are calibrated usinglocal and regional instrumental climate records. A bootstrappedresponse function analysis using regional climatedata shows that tree growth is forced by April–August temperaturesand that the regression weights for MXD are muchstronger than for TRW. The robustness of the reconstructionequation is verified by independent temperature data andshows that 63–64% of the instrumental inter-annual variationis captured by the tree-ring data. This is a significantimprovement compared to previously published reconstructionsbased on tree-ring data from Tornetra¨sk. Adivergence phenomenon around AD 1800, expressed as anincrease in TRW that is not paralleled by temperature andMXD, is most likely an effect of major changes in the densityof the pine population at this northern tree-line site. The biasintroduced by this TRW phenomenon is assessed by producinga summer temperature reconstruction based onMXDexclusively. The new data show generally higher temperatureestimates than previous reconstructions based onTornetra¨sk tree-ring data. The late-twentieth century, however,is not exceptionally warm in the new record: Ondecadal-to-centennial timescales, periods around AD 750,1000, 1400, and 1750 were equally warm, or warmer. The200-year long warm period centered on AD 1000 was significantlywarmer than the late-twentieth century (p\0.05)and is supported by other local and regional paleoclimatedata. The new tree-ring evidence from Tornetra¨sk suggeststhat this ‘‘Medieval WarmPeriod’’ in northern Fennoscandiawas much warmer than previously recognized.
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| 4. |
- Karlsson, Johannes, et al.
(author)
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Cloud radiative forcing of subtropical low level clouds in global models
- 2008
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 30:7-8, s. 779-788
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Journal article (peer-reviewed)abstract
- Simulations of subtropical marine low clouds and their radiative properties by nine coupled oceanatmosphere climate models participating in the fourth assesment report (AR4) of the intergovernmental panel on climate change (IPCC) are analyzed. Satellite observations of cloudiness and radiative fluxes at the top of the atmosphere (TOA) are utilized for comparison. The analysis is confined to the marine subtropics in an attempt to isolate low cloudiness from tropical convective systems. All analyzed models have a negative bias in the low cloud fraction (model mean bias of –15%). On the other hand, the models show an excess of cloud radiative cooling in the region (model mean excess of 13 W m–2). The latter bias is shown to mainly originate from too much shortwave reflection by the models clouds rather than biases in the clear-sky fluxes. These results confirm earlier studies, thus no major progress in simulating the marine subtropical clouds is noted. As a consequence of the combination of these two biases, this study suggests that all investigated models are likely to overestimate the radiative response to changes in low level subtropical cloudiness.
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| 5. |
- Moberg, Anders, et al.
(author)
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Analysis of the Moberg et al. (2005) hemispheric temperature reconstruction
- 2008
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 31:7-8, s. 957-971
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Journal article (peer-reviewed)abstract
- The Moberg et al. (Nature 433(7026):613–617, 2005. doi:10.1038/nature03265; M05) reconstruction of northern hemisphere temperature variations from proxy data has been criticised; the M05 method may artificially inflate low-frequency variance relative to reality. We test this assertion by undertaking several pseudoproxy experiments in three climate model simulations—one control run and two forced simulations that include several time-varying radiative forcings. The pseudoproxy series are designed to have the same variance spectra as the real M05 proxies, primarily to mimic the low-resolution character of several series. A simple composite-plus-scale (CPS) method is also analysed. In the CPS case all input data behave like annually resolved proxies. The spectral domain performance of both M05 and CPS is found to be dependent on the noise type and noise level in pseudoproxies, on the variance spectrum of the climate model simulation, and on the degree of data smoothing. CPS performs better than M05 in most investigated cases with the control run, but leads to deflated lowfrequency variance in some cases. With M05, low-frequency variance tend to be inflated for the control run but not for one of the forced runs and only very slightly with the other forced simulation. Hence, the M05 approach does not routinelyinflate low-frequency variance. In our experiment, the M05 approach performs better in the spectral domain than CPS when applied to forced climate model simulations. The results underscore the importance of evaluating the variance spectrum of climate reconstructions.
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| 6. |
- Power, M. J., et al.
(author)
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Changes in fire regimes since the Last Glacial Maximum : an assessment based on a global synthesis and analysis of charcoal data
- 2008
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 30:7-8, s. 887-907
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Journal article (peer-reviewed)abstract
- Fire activity has varied globally and continuously since the last glacial maximum (LGM) in response to long-term changes in global climate and shorter-term regional changes in climate, vegetation, and human land use. We have synthesized sedimentary charcoal records of biomass burning since the LGM and present global maps showing changes in fire activity for time slices during the past 21,000 years (as differences in charcoal accumulation values compared to pre-industrial). There is strong broad-scale coherence in fire activity after the LGM, but spatial heterogeneity in the signals increases thereafter. In North America, Europe and southern South America, charcoal records indicate less-than-present fire activity during the deglacial period, from 21,000 to ∼11,000 cal yr BP. In contrast, the tropical latitudes of South America and Africa show greater-than-present fire activity from ∼19,000 to ∼17,000 cal yr BP and most sites from Indochina and Australia show greater-than-present fire activity from 16,000 to ∼13,000 cal yr BP. Many sites indicate greater-than-present or near-present activity during the Holocene with the exception of eastern North America and eastern Asia from 8,000 to ∼3,000 cal yr BP, Indonesia and Australia from 11,000 to 4,000 cal yr BP, and southern South America from 6,000 to 3,000 cal yr BP where fire activity was less than present. Regional coherence in the patterns of change in fire activity was evident throughout the post-glacial period. These complex patterns can largely be explained in terms of large-scale climate controls modulated by local changes in vegetation and fuel load.
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| 7. |
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| 8. |
- Sahlée, Erik, et al.
(author)
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Influence of the boundary layer height on the global air-sea surface fluxes
- 2009
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 33:1, s. 33-44
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Journal article (peer-reviewed)abstract
- Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by the boundary layer height, z (i) , during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind profile, phi (m) , has been found to be a function of both z/L and z (i) /L. At constant z/L, phi (m) decreases with decreasing boundary layer height. Since phi (m) is directly related to the parameterizations of the air-sea surface fluxes, these results will have an influence when calculating the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979 to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase by 0.77 W m(-2) or about 1% and the mean surface stress increase by 1.4 mN m(-2) or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger response. The greatest impact is found over the tropical oceans between 30A degrees S and 30A degrees N. In this region the boundary layer height influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase of the mean annual heat fluxes by 2.0 W m(-2) and an increase of the mean annual surface stress by 2.6 mN m(-2).
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| 9. |
- Seppa, H, et al.
(author)
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Low-frequency and high-frequency changes in temperature and effective humidity during the Holocene in south-central Sweden: implications for atmospheric and oceanic forcings of climate
- 2005
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In: Climate Dynamics. - : Springer Science and Business Media LLC. - 1432-0894 .- 0930-7575. ; 25:2-3, s. 285-297
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Research review (peer-reviewed)abstract
- An integrated use of independent palaeoclimatological proxy techniques that reflect different components of the climate system provides a potential key for functional analysis of past climate changes. Here we report a 10,000 year quantitative record of annual mean temperature (T-ann), based on pollen-climate transfer functions and pollen-stratigraphical data from Lake Flarken, south-central Sweden. The pollen-based temperature reconstruction is compared with a reconstruction of effective humidity, as reflected by a delta(18)O record obtained on stratigraphy of lacustrine carbonates from Lake Igelsjon, c. 10 km from Lake Flarken, which gives evidence of pronounced changes in effective humidity. The relatively low T-ann, and high effective humidity as reflected by a low evaporation/inflow ratio suggest a maritime early Holocene climate (10,000-8,300 cat year BP), seemingly incompatible with the highly seasonal solar insolation configuration. We argue that the maritime climate was due to the stronger-than-present zonal flow, enhanced by the high early Holocene sea-surface temperatures in the North Atlantic. The maritime climate mode was disrupted by the abrupt cold event at 8,200 cat year BP, followed at 8,000 cat year BP by a stable Holocene Thermal Maximum. The latter was characterized by T-ann values about 2.5 degrees C higher than at present and markedly dry conditions, indicative of stable summertime anti-cyclonic circulation, possibly corresponding with modern blocking anticyclonic conditions. The last 4,300 year period is characterized by an increasingly cold, moist, and unstable climate. The results demonstrate the value of combining two independent palaeoclimatic proxies in enhancing the reliability, generality, and interpretability of the palaeoclimatic results. Further methodological refinements especially in resolving past seasonal climatic contrasts are needed to better understand the role of different forcing factors in driving millennial-scale climate dynamics.
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| 10. |
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