On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes Rosanne D'Arrigo a, ⁎ , Rob Wilson a,b , Beate Liepert a , Paolo Cherubini a,c a Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, New York 10964, USA b University of Edinburgh, Grants Institute, West Mains Road, Edinburgh, EH9 3JW, UK c Forest Ecosystems and Ecological Risks, WSL Swiss Federal Institute for Forest, Snow and Landscape Research Zuercherstrasse 111, CH-8903, Birmensdorf, Switzerland Received 28 November 2006; received in revised form 23 February 2007; accepted 9 March 2007 Available online 24 March 2007 Abstract An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. These possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt and related changes in seasonality, and differential growth/climate relationships inferred for maximum, minimum and mean temperatures. Another possible cause of the divergence described briefly herein is ‘global dimming’, a phenomenon that has appeared, in recent decades, to decrease the amount of solar radiation available for photosynthesis and plant growth on a large scale. It is theorized that the dimming phenomenon should have a relatively greater impact on tree growth at higher northern latitudes, consistent with what has been observed from the tree-ring record. Additional potential causes include “end effects” and other methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations. © 2007 Elsevier B.V. All rights reserved. Keywords: tree rings; dendrochronology; divergence; paleoclimate; reconstructions 1. Introduction Tree rings are a critically important proxy for reconstructing the high resolution climate of the past millennium and are the dominant data type in most large Available online at www.sciencedirect.com Global and Planetary Change 60 (2008) 289 – 305 www.elsevier.com/locate/gloplacha ⁎ Corresponding author. Tel.: +1 845 365 8617; fax: +1 845 365 8152. E-mail address: rdd@ldeo.columbia.edu (R. D'Arrigo). 0921-8181/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2007.03.004