John Topham

Bio: John Topham is an academic researcher. The author has contributed to research in topics: Spatial ecology. The author has an hindex of 1, co-authored 1 publications receiving 20 citations.

Spatial reconstruction of summer temperatures in Central Europe for the last 500 years using annually resolved proxy records: problems and opportunities

Abstract: Most palaeoclimate studies in Central Europe, utilizing annually resolved proxies such as tree-ring and documentary sources to reconstruct past temperatures, have focused mainly upon single sites or regional studies. The combined information of published summer temperature reconstructions from the Alpine region show a generally coherent picture of cool conditions for the periods c. 1450–1475, 1575–1610, 1660–1710, 1800–1850 and 1875–1925. These reconstructed cool periods can be partly explained by external forcing (e.g. low solar activity and volcanic events). However, these reconstructions, in their present form, cannot be used to comparatively assess spatial summer temperature variability through the region due to methodological differences in their development and the fact that many of them were not originally developed to emphasize spatial patterns. We propose that a network of tree-ring chronologies which have been processed in a consistent way would allow the robust reconstruction of spatial summer temperature variability for high elevations in Central Europe. Unfortunately, most living tree-ring chronologies only go back into the 18th century — so restricting the length of reconstruction. As a possible solution, we introduce a historical database of ring-width series, measured from string instruments, that could be used to extend high elevation spruce chronologies in Central Europe back for at least 500 years.

Summer Temperature Variations in the European Alps, a.d. 755–2004

Abstract: Annually resolved summer temperatures for the European Alps are described. The reconstruction covers the a.d. 755–2004 period and is based on 180 recent and historic larch [Larix decidua Mill.] density series. The regional curve standardization method was applied to preserve interannual to multicentennial variations in this high-elevation proxy dataset. Instrumental measurements from high- (low-) elevation grid boxes back to 1818 (1760) reveal strongest growth response to current-year June–September mean temperatures. The reconstruction correlates at 0.7 with high-elevation temperatures back to 1818, with a greater signal in the higher-frequency domain (r = 0.8). Low-elevation instrumental data back to 1760 agree with the reconstruction’s interannual variation, although a decoupling between (warmer) instrumental and (cooler) proxy data before ∼1840 is noted. This offset is larger than during any period of overlap with more recent high-elevation instrumental data, even though the proxy time series...

Testing for tree-ring divergence in the European Alps

Abstract: Evidence for reduced sensitivity of tree growth to temperature has been reported from multiple forests along the high northern latitudes. This alleged circumpolar phenomenon described the apparent inability of temperature-sensitive tree-ring width and density chronologies to parallel increasing instrumental temperature measurements since the mid-20th century. In addition to such low-frequency trend offset, the inability of formerly temperature-sensitive tree growth to reflect high-frequency temperature signals in a warming world is indicated at some boreal sites, mainly in Alaska, the Yukon and Siberia. Here, we refer to both of these findings as the ‘divergence problem’ (DP), with their causes and scale being debated. If DP is widespread and the result of climatic forcing, the overall reliability of tree-ring-based temperature reconstructions should be questioned. Testing for DP benefits from well-replicated tree-ring and instrumental data spanning from the 19th to the 21st century. Here, we present a network of 124 larch and spruce sites across the European Alpine arc. Tree-ring width chronologies from 40 larch and 24 spruce sites were selected based on their correlation with early (1864‐1933) instrumental temperatures to assess their ability of tracking recent (1934‐2003) temperature variations. After the tree-ring series of both species were detrended in a manner that allows low-frequency variations to be preserved and scaled against summer temperatures, no unusual late 20th century DP is found. Independent tree-ring width and density evidence for unprecedented late 20th century temperatures with respect to the past millennium further reinforces our results.

Interannual growth response of Norway spruce to climate along an altitudinal gradient in the Tatra Mountains, Poland

Abstract: Ring-width chronologies of Picea abies (L.) Karst. from ten sites in the Tatra Mountains, Poland, were developed to explore growth/climate responses in stands along an altitudinal gradient ranging from 839 to 1468 m a.s.l. There were positive relationships between current-year radial growth and mean monthly temperatures in March, April, June and July, but with increasing elevation, the strength of this correlation declined for March–April and increased for June–July temperatures. The mean monthly temperature in October of the previous year positively influenced radial growth of trees at all sites. Lower mean temperatures in January negatively affected growth of trees at the high-elevation sites. Trees at the low-elevation sites responded positively to a warm early spring, whereas trees at the high-elevation sites showed positive growth responses to higher summer temperatures. Growth of trees at the high-elevation sites was better explained by the temperature regime than was growth at the lower-elevation sites. Therefore, it is likely that these sites may be particularly sensitive to potential effects of temperature change.

Europe's southernmost glaciers: response and adaptation to climate change

Abstract: The southernmost glaciers in Europe are located on the Iberian, Apennine and Balkan Peninsulas in mid-latitudes between 41 � � N and 44 � N at altitudes ranging from 2000 to 3000 m a.s.l. All these glaciers are a legacy of the Little Ice Age (LIA). They survive in a relatively warm environment (mean annual temperature 0 � Ct o +1 � C) due to local topographic controls and high levels of accumulation as a result of avalanche and wind-blown snow. In the Pirin Mountains, Bulgaria, Snezhnika glacieret has been cored, providing an archive of recent climate change. Small glaciers such as this respond quickly to climatic extremes. Since the LIA maximum during the 19th century, all southern European glaciers have retreated, losing 30-100% of their volume. However, despite the trend towards warmer years since the late 1970s, some glaciers still survive, even after some of the hottest summers on record. Predicted future warming, especially in summer, and drier conditions in the Mediterranean basin may result in the disappearance of all glacier features at these latitudes in Europe within the next few decades.

Trends and uncertainties in Siberian indicators of 20th century warming

Abstract: Estimates of past climate and future forest biomass dynamics are constrained by uncertainties in the relationships between growth and climatic variability and uncertainties in the instrumental data themselves. Of particular interest in this regard is the borealforest zone, where radial growth has historically been closely connected with temperature variability, but various lines of evidence have indicated a decoupling since about the 1960s. We here address this growth-vs.-temperature divergence by analyzing tree-ring width and density data from across Siberia, and comparing 20th century proxy trends with those derived from instrumental stations. We test the influence of approaches considered in the recent literature on the divergence phenomenon (DP), including effects of tree-ring standardization and calibration period, and explore instrumental uncertainties by employing both adjusted and nonadjusted temperature data to assess growth-climate agreement. Results indicate that common methodological and data usage decisions alter 20th century growth and temperature trends in a way that can easily explain the post-1960 DP. We show that (i) Siberian station temperature adjustments were up to 1.31C for decadal means before 1940, (ii) tree-ring detrending effects in the order of 0.6‐0.81C, and (iii) calibration uncertainties up to about 0.41C over the past 110 years. Despite these large uncertainties, instrumental and tree growth estimates for the entire 20th century warming interval match each other, to a degree previously not recognized, when care is taken to preserve long-term trends in the tree-ring data. We further show that careful examination of early temperature data and calibration of proxy timeseries over the full period of overlap with instrumental data are both necessary to properly estimate 20th century longterm changes and to avoid erroneous detection of post-1960 divergence.