14 research outputs found
Internal and external forcing of multidecadal Atlantic climate variability over the past 1,200 years
The North Atlantic experiences climate variability on multidecadal scales, which is sometimes referred to as Atlantic multidecadal variability. However, the relative contributions of external forcing such as changes in solar irradiance or volcanic activity and internal dynamics to these variations are unclear. Here we provide evidence for persistent summer Atlantic multidecadal variability from AD 800 to 2010 using a network of annually resolved terrestrial proxy records from the circum-North Atlantic region. We find that large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic multidecadal variability and collectively explain about 30% of the variance in the reconstruction on timescales greater than 30 years. We are then able to isolate the internally generated component of Atlantic multidecadal variability, which we define as the Atlantic multidecadal oscillation. We find that the Atlantic multidecadal oscillation is the largest contributor to Atlantic multidecadal variability over the past 1,200 years. We also identify coherence between the Atlantic multidecadal oscillation and Northern Hemisphere temperature variations, leading us to conclude that the apparent link between Atlantic multidecadal variability and regional to hemispheric climate does not arise solely from a common response to external drivers, and may instead reflect dynamic processes
A multi-proxy reconstruction of spatial and temporal variations in Asian summer temperatures over the last millennium
To investigate climate variability in Asia during the last millennium, the spatial and temporal evolution of summer (June–July–August; JJA) temperature in eastern and south-central Asia is reconstructed using multi-proxy records and the regularized expectation maximization (RegEM) algorithm with truncated total least squares (TTLS), under a point-by-point regression (PPR) framework. The temperature index reconstructions show that the late 20th century was the warmest period in Asia over the past millennium. The temperature field reconstructions illustrate that temperatures in central, eastern, and southern China during the 11th and 13th centuries, and in western Asia during the 12th century, were significantly higher than those in other regions, and comparable to levels in the 20th century. Except for the most recent warming, all identified warm events showed distinct regional expressions and none were uniform over the entire reconstruction area. The main finding of the study is that spatial temperature patterns have, on centennial time-scales, varied greatly over the last millennium. Moreover, seven climate model simulations, from the Coupled Model Intercomparison Project Phase 5 (CMIP5), over the same region of Asia, are all consistent with the temperature index reconstruction at the 99 % confidence level. Only spatial temperature patterns extracted as the first empirical orthogonal function (EOF) from the GISS-E2-R and MPI-ESM-P model simulations are significant and consistent with the temperature field reconstruction over the past millennium in Asia at the 90 % confidence level. This indicates that both the reconstruction and the simulations depict the temporal climate variability well over the past millennium. However, the spatial simulation or reconstruction capability of climate variability over the past millennium could be still limited. For reconstruction, some grid points do not pass validation tests and reveal the need for more proxies with high temporal resolution, accurate dating, and sensitive temperature signals, especially in central Asia and before AD 1400
Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE
This study was funded by the WSL-internal COSMIC project (5233.00148.001.01), the ETHZ (Laboratory of Ion Beam Physics), the Swiss National Science Foundation (SNF Grant 200021L_157187/1), and as the Czech Republic Grant Agency project no. 17-22102s.Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.Publisher PDFPeer reviewe
Spatio-temporal variability of Arctic summer temperatures over the past 2 millennia
In this article, the first spatially resolved and millennium-length summer
(June–August) temperature reconstruction over the Arctic and sub-Arctic
domain (north of 60° N) is presented. It is based on a set of 44
annually dated temperature-sensitive proxy archives of various types from the
revised PAGES2k database supplemented with six new recently updated proxy
records. As a major advance, an extension of the Bayesian BARCAST climate
field (CF) reconstruction technique provides a means to treat climate
archives with dating uncertainties. This results not only in a more precise
reconstruction but additionally enables joint probabilistic constraints to be
imposed on the chronologies of the used archives. The new seasonal CF
reconstruction for the Arctic region can be shown to be skilful for the
majority of the terrestrial nodes. The decrease in the proxy data density
back in time, however, limits the analyses in the spatial domain to the period
after 750 CE, while the spatially averaged reconstruction covers the entire
time interval of 1–2002 CE.The centennial to millennial evolution of the reconstructed temperature is in
good agreement with a general pattern that was inferred in recent studies for
the Arctic and its subregions. In particular, the reconstruction shows a
pronounced Medieval Climate Anomaly (MCA; here ca. 920–1060 CE), which was
characterised by a sequence of extremely warm decades over the whole domain.
The medieval warming was followed by a gradual cooling into the Little Ice
Age (LIA), with 1766–1865 CE as the longest centennial-scale cold period,
culminating around 1811–1820 CE for most of the target region.In total over 600 independent realisations of the temperature CF were
generated. As showcased for local and regional trends and temperature
anomalies, operating in a probabilistic framework directly results in
comprehensive uncertainty estimates, even for complex analyses. For the
presented multi-scale trend analysis, for example, the spread in different
paths across the reconstruction ensemble prevents a robust analysis of
features at timescales shorter than ca. 30 years. For the spatial
reconstruction, the benefit of using the spatially resolved reconstruction
ensemble is demonstrated by focusing on the regional expression of the recent
warming and the MCA. While our analysis shows that the peak MCA summer
temperatures were as high as in the late 20th and early
21st centuries, the spatial coherence of extreme years over
the last decades of the reconstruction (1980s onwards) seems unprecedented at
least back until 750 CE. However, statistical testing could not provide
conclusive support of the contemporary warming to exceed the peak of the MCA
in terms of the pan-Arctic mean summer temperatures: the reconstruction
cannot be extended reliably past 2002 CE due to lack of proxy data and thus
the most recent warming is not captured
The 10Be deglaciation chronology of the Göschenertal, central Swiss Alps, and new insights into the Göschenen Cold Phases
The Göschenertal (Göschenen valley) is the type locality of the so‐called Göschenen Cold Phases I (~3–2.3 ka) and II (~1.8–1.1 ka). According to earlier studies, these Late Holocene climatic cooling periods were characterized by changes in vegetation and pronounced glacier advances. As a peculiarity, the Göschenen Cold Phase I was thought to be connected to a local surge‐type advance of the Chelengletscher (Chelen glacier) – an exceptional event of unparalleled dimension in the European Alps. Based on cosmogenic 10Be exposure ages from moraine boulders, we investigated the local glacier chronology. In contrast to former research, moraines at different positions within the Göschenen valley (central Swiss Alps) have been dated to the Younger Dryas and the Early Holocene. This questions the applicability of palaeo‐Equilibrium Line Altitude (ELA) calculations for stadial attributions without additional numerical age constraints. Furthermore, we have found compelling evidence that the proposed non‐climatic glacier advance attributed to the Göschenen Cold Phase I did not occur. The present results, along with a reappraisal of the original study, question the scientific reliability and the glaciological definition of the Göschenen Cold Phases as glacier advances that clearly exceeded the Little Ice Age positions. While our data do not exclude potential changes in climate and vegetation, we nonetheless show that the Göschenen Cold Phases are not suitable as reference stadials in the system of Alpine Holocene glacier fluctuations