Cloud Cover Feedback Moderates Fennoscandian Summer Temperature Changes Over the Past 1,000 Years

Northern Fennoscandia has experienced little summer warming over recent decades, in 24 contrast to the hemispheric trend, which is strongly linked to greenhouse gas emissions. A likely25 explanation is the feedback between cloud cover and temperature. We establish the long- and26 short-term relationship between summer cloud cover and temperature over Northern27 Fennoscandia, by analysing meteorological and proxy climate data. We identify opposing28 feedbacks operating at different timescales. At short timescales, dominated by internal29 variability, the cloud cover-temperature feedback is negative; summers with increased cloud30 cover are cooler and sunny summers are warmer. However, over longer timescales, at which31 forced climate changes operate, this feedback is positive, rising temperatures causing increased32 regional cloud cover and vice versa. This has occurred both during warm (Medieval Climate33 Anomaly and at present) and cool (Little Ice Age) periods. This two-way feedback relationship34 therefore moderates Northern Fennoscandian temperatures during both warm and cool35 hemispheric periods

Non-stationary Response of Tree Growth to Climate Trends Along the Arctic Margin

Climate change modulates cold-marginal forest ecosystems through changing growth constraints. Understanding spatiotemporal variations in climate– growth relationships is essential to project forest ecosystem dynamics, and climate–environmental feedbacks. We explored variations in growth and climate–growth relationships, along the Arctic margin in north-western Europe, using Scots pine radial growth chronologies, climate data and links between the geographical origin of dominant air masses and growth-controlling variables. Analyses covered nineteenth century to early twenty-first century, with emphasis on two separate warming periods (early twentieth century, and late twentieth to early twenty-first century) and the intervening cooling period. The analyses revealed spatiotemporally unstable growth responses to climate along the Arctic margin. Spatial growth patterns were most similar during the cooling period. However, climate trends (warming, cooling) were weak drivers of growth-limiting climate variables. Instead, a transition in growth-limiting variables occurred throughout the analysed period. A wide range of growing season and non-growing season climate variables limited growth during the early twentieth century. Thereafter the number of growth-limiting variables progressively decreased. This change was accompanied by a contraction in the spatial correspondence between growth and climate, and by a shift in the geographical origin of dominant air masses. This was particularly emphasized close to the Atlantic during recent warming period. The weak association between growth-limiting variables and climate trends question projections of future ecosystem dynamics based on climate variables identified during specific periods (for example, recent warming period). Such projections may be misleading as the diversity of climate conditions constraining cold-marginal forests will be underestimated. climate trends; cold-marginal forests; dendroclimatology; growth-controlling climate; tree-climate interactions; Pinus sylvestris; spatiotemporal growth responses.publishedVersio

Growth variability of Scots pine (Pinus sylvestris) along a West-East gradient across northern Fennoscandia: a dendroclimatic approach

We performed a spatiotemporal analysis of a network of 21 Scots pine (Pinus sylvestris) ring-width chronologies in northern Fennoscandia by means of chronology statistics and multivariate analyses. Chronologies are located on both sides (western and eastern) of the Scandes Mountains (67°N-70°N, 15°E-29°E). Growth relationships with temperature, precipitation, and North Atlantic Oscillation (NAO) indices were calculated for the period 1880-1991. We also assessed their temporal stability. Current July temperature and, to a lesser degree, May precipitation are the main growth limiting factors in the whole area of study. However, Principal Component Analysis (PCA) and mean interseries correlation revealed differences in radial growth between both sides of the Scandes Mountains, attributed to the Oceanic-Continental climatic gradient in the area. The gradient signal is temporally variable and has strengthened during the second half of the 20th century. Northern Fennoscandia Scots pine growth is positively related to early winter NAO indices previous to the growth season and to late spring NAO. NAO/growth relationships are unstable and have dropped in the second half of the 20th century. Moreover, they are noncontinuous through the range of NAO values: for early winter, only positive NAO indices enhance tree growth in the next growing season, while negative NAO does not. For spring, only negative NAO is correlated with radial growth

Middelalderfiske, fellefangst og fraflytting : en dendrokronologisk undersøkelse av et 1300-talls stasjonært fiskeanlegg i Nord-Mesna, Sørøst-Norge

Fishing in rivers and lakes has traditionally been a longstanding and dependable means of sustenance for the people of inland Scandinavia. However, our understanding of pre-modern fishing traditions has been hampered by a lack of written sources, as well as a scarcity of archaeological data. However, a recent excavation and comprehensive dendrochronological analysis of a fish trapping enclosure system in Lake Nord-Mesna (520 masl.), in the boreal forests of inland Norway, has provided unique insight into freshwater fishing traditions, techniques and organization. The excavated structure, believed to be a fish weir with lath screen traps set into shallow water, was established in the late 1200s. In the following years it was regularly maintained in the spring/early summer. The last documented repair was in 1343. Its abandonment is interpreted as resulting from a recession likely induced by factors such as plague and climatic deterioration, that affected inland areas of Scandinavia in the 1300s. These results provide archaeological evidence of medieval utilization of effective enclosure traps in the region. This contradicts the prevailing notion that this form of fishing was later introduced by Forest Finns who migrated to the area in the 17th century. In addition, the findings give new and significant information about the organization and practice of medieval fishing in inland Scandinavia

Growth variability of Scots pine (Pinus sylvestris) along a West-East gradient across northern Fennoscandia: a dendroclimatic approach

We performed a spatiotemporal analysis of a network of 21 Scots pine (Pinus sylvestris) ring-width chronologies in northern Fennoscandia by means of chronology statistics and multivariate analyses. Chronologies are located on both sides (western and eastern) of the Scandes Mountains (67°N-70°N, 15°E-29°E). Growth relationships with temperature, precipitation, and North Atlantic Oscillation (NAO) indices were calculated for the period 1880-1991. We also assessed their temporal stability. Current July temperature and, to a lesser degree, May precipitation are the main growth limiting factors in the whole area of study. However, Principal Component Analysis (PCA) and mean interseries correlation revealed differences in radial growth between both sides of the Scandes Mountains, attributed to the Oceanic-Continental climatic gradient in the area. The gradient signal is temporally variable and has strengthened during the second half of the 20th century. Northern Fennoscandia Scots pine growth is positively related to early winter NAO indices previous to the growth season and to late spring NAO. NAO/growth relationships are unstable and have dropped in the second half of the 20th century. Moreover, they are noncontinuous through the range of NAO values: for early winter, only positive NAO indices enhance tree growth in the next growing season, while negative NAO does not. For spring, only negative NAO is correlated with radial growth

Micro-site conditions affect Fennoscandian forest growth

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Compound response of marine and terrestrial ecosystems to varying climate : pre-anthropogenic perspective from bivalve shell growth increments and tree-rings

Annual growth increments were examined from shells of the ocean quahog (Arctica islandica L.) from northwest Norway and from tree-ring samples of the Scots pine (Pinus sylvestris L.) from nearby coastal areas. The reconstructed annual growth increments were used to compare growth variability in marine and terrestrial ecosystems. Spatiotemporal comparison of the growth records showed statistically significant correlation during the 19th century A.D., indicative of ecosystemindependent response to pre-anthropogenic climate variations. Geographical correlation between marine and terrestrial records was only observed at the local scale. Years with particularly low winter or high summer North Atlantic Oscillation (NAO) indices showed the best synchronization of marine and terrestrial growth. Despite strong correlation during historical time, our palaeoecological evidence suggests that marine and terrestrial ecosystems may show dissimilar growth reaction to recently observed positive winter-NAO phases

A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes

Tree-ring stable carbon isotope ratios (d(13)C) in environments of low moisture stress are likely to be controlled primarily by photosynthetic rate. Therefore, sunshine, rather than temperature, represents the more direct controlling factor. Temperature reconstructions based on tree-ring d(13)C results thus rest on the assumption that temperature and sunshine are strongly coupled. This assumption is tested using a d(13)C series from pine trees in NW Norway, where there are long (>100 yr) records of both summer temperature and cloud cover. It is demonstrated that when summer temperature and d(13)C diverge, summer temperature and cloud cover also diverge, and that cloud cover/sunshine may provide a stronger and more consistent parameter with which to calibrate tree-ring d(13)C series in this area. When a 500-year reconstruction of summer cloudiness is compared with a published reconstruction of summer temperatures in northern Sweden based on tree-ring maximum densities, the two time-series are largely parallel, with high levels of annual-decadal coherence. We identify, however, three distinct periods of lower frequency divergence: two (AD 1600-1650 and ad 1900-1927) when we propose summers were cool but sunny and one during the first half of the sixteenth century when summers were warm but cloudy. These episodes where temperature and sunshine decouple may represent large-scale changes in circulation as recorded in the Arctic Oscillation (AO) index. Strongly negative values of the summer AO index, as occurred during the early twentieth century, are associated with persistent high pressure over northern Norway and Fennoscandia, bringing cool summers with clear skies. Long reconstructions of cloudiness (near-ground radiation), based on tree-ring d(13)C series from suitable sites, would be extremely valuable for testing General Circulation Models (GCMs), because the generation of cloud is a strong control on temperature evolution, but remains a major source of uncertainty