Forum Article / Obituary
The Holocene
Corresponding author: Timothy Osborn, Climatic Research Unit, School of
Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.
t.osborn@uea.ac.uk
In Memoriam:
Keith R. Briffa, 1952-2017
Timothy J. Osborn
1
, Phil D. Jones
1
and Ed R. Cook
2
1
Climatic Research Unit, School of Environmental Sciences, University of East Anglia,
Norwich, UK
2
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA
Submitted version: 15 May 2018
Abstract
Keith R. Briffa was one of the most influential palaeoclimatologists of the last thirty
years. His primary research interests lay in late Holocene climate change with a
geographical emphasis on northern Eurasia. His greatest impact was in the field of
dendroclimatology, a field that he helped to shape. His contributions have been seminal
to the development of sound methods for tree-ring analysis and in their proper application
to allow the interpretation of climate variability from tree rings. This led to the
development of many important records that allow us to understand natural climate
variability on timescales from years to millennia and to set recent climatic trends in their
historical context.
Keywords
Dendrochronology, Tree-rings, Holocene, Climate Change, Palaeoclimate
Keith R. Briffa (Plate 1), who helped to shape the field of dendroclimatology, died
peacefully on 29 October 2017 at the age of 64. Keith studied Biological Sciences at the
University of East Anglia, graduating in 1974. His scientific career began when he joined
the Climatic Research Unit (CRU) at the University of East Anglia (UEA) in 1977 and
ended with his death as Emeritus Professor 40 years later. His work ranged across such
topics as European droughts, glacier fluctuations, cooling by volcanic eruptions, beetle
fossils, and temperatures in ice age Britain. His greatest and most sustained contribution
was to unravel the complex climatic signals encrypted in annual tree rings, and to set
their interpretation on a sound and rigorously tested footing.
Keith made outstanding achievements in the reconstruction, interpretation and
understanding of climate variability and change. His greatest impact has been in the field
of dendroclimatology, where he led many methodological developments that have
become widely used in this field (and in other palaeoclimate disciplines). He applied
these methods to develop many highly-cited tree-ring chronologies, interpreting their
climatic information on timescales from centuries to millennia.
His first major research area was methodological advances in dendroclimatology.
A key methodological advance was the introduction of widely-used measures of
chronology confidence (such as the Expressed Population Signal, EPS and the sub-
sample signal strength, SSS), based on the average inter-core correlation value (r-bar).
Keith identified these relationships empirically and Tom Wigley undertook a more
formal mathematical derivation. Their article (Wigley et al., 1984) is among the most
widely cited in dendroclimatology, with over 2,100 citations in Google Scholar in May
2018 (and related methodological work – Briffa and Jones, 1990 – has almost 500
citations). It is notable that, more than three decades after it was first published, this work
is being cited almost 200 times each year. The approach was recently extended to
measure the reliability of chronologies on longer timescales, such as their centennial tree-
growth variability (Briffa et al., 2013).
Another major methodological focus, with improvements and advances
continuing every few years, was on approaches to objectively remove non-climatic
variability from tree-ring chronologies while retaining long-timescale climatic
information. This challenge was later referred to as the “segment length curse” (Cook et
al., 1995). Keith first experimented with Gaussian filters (Briffa et al., 1983, 1988a;
Briffa, 1984) and splines, but realized that more fundamental approaches were needed,
particularly if long tree-ring chronologies were to be developed combining samples from
living trees, archaeological and sub-fossil material. His first approach was “Regional
Curve Standardization” (RCS), realigning all the samples to their tree age instead of their
calendrical age (Briffa et al., 1992a). Although this worked well there were issues with
samples coming from different populations, such as modern samples growing on
relatively dry ground whereas those preserved as sub-fossil trees had grown close to, and
fell into, small lakes (Plate 1). The most recent approaches use “Signal-Free” methods to
minimise modern sample bias (Melvin and Briffa, 2008; Briffa and Melvin, 2011) and
multiple RCS curves to represent different populations (Briffa et al., 2013). These
innovations in tree-ring standardisation methods will continue to influence the future
development of the field for many years and are implemented in freely available software
(CRUST – Melvin and Briffa, 2014). Keith’s methodological advances pervade the
dendroclimatic literature such that most studies cite one or more of his papers.
His second major research area was the development of long (multi-century to
multi-millennial) tree-ring chronologies from Fennoscandia, North America, northern
Russia and most recently the Tibetan Plateau (Briffa et al., 1992ab, 1995, 2008 and 2013;
Yang et al., 2014). They provide annually-resolved information about growth-rate
changes on interannual to millennial timescales in these regions, from which summer
temperatures or annual precipitation can be inferred. None of these would have been
possible without the methodological advances that address the segment length curse.
His third major research area was the combination of chronologies to infer the
spatial patterns of past climate variability (Cook et al., 1994; Briffa et al., 2002b) or to
reconstruct climate variability of large area averages, such as the northern extratropics
(Briffa et al., 2001). Using extensive tree-ring networks compiled by Fritz Schweingruber
(Plate 2), Keith demonstrated that maximum latewood density (MXD) measurements
have a more sensitive response than ring widths to summer temperatures in high northern
latitudes (Briffa et al., 1992b, 2002a). MXD reconstructions led to better isolation of the
effects of explosive volcanic eruptions (Briffa et al., 1998a) and with exact dating the
realization that these events could be used to improve ice core dating (Vinther et al.,
2010). Keith was also the first to demonstrate that a widespread divergence between
some MXD data and instrumental summer temperatures in northern high latitude had
apparently occurred since about 1960 (Briffa et al., 1998bc). This provided a challenge to
the interpretation of long tree-ring reconstructions and a new urgency to more fully
understand the impacts of standardization and modern sampling bias, and that many more
samples are needed to evaluate the low-frequency climate signal in the presence of
multiple climatic and non-climatic influences.
Looked at individually (e.g. for Fennoscandia and locations in northern Russia)
the timing of cooler and warmer periods differed (Briffa, 2000). In terms of past
temperature variability, local response functions indicated that the trees were responding
to variability during high summer (for ring widths) and an extended summer season (for
MXD) from May to September. One issue was whether reconstructions for different
seasons could be combined and how they would relate to our understanding of the
centennial variability over the Northern Hemisphere, where there was then believed to
have been a Medieval Warm Period (MWP from ~900 to 1250) and a Little Ice Age (LIA
from ~1550 to 1850). Concepts of both the MWP and LIA have evolved since their initial
use in their modern form in the 1970s (see discussions in Matthews and Briffa, 2005;
Jones et al., 2009), especially as the amount of available information from an expanding
array of proxies had multiplied from a handful of reconstructions in the early 1970s to the
several hundred available today. Keith was involved in a number of studies that looked at
the sensitivity of using only certain proxies (e.g. trees only) or the effects of only using a
limited number of series that all extended back for the whole millennium (Jones et al.,
1998; Rutherford et al., 2005; Osborn and Briffa, 2006; Juckes et al., 2007; Kaufman et
al., 2009), as well as reconstructions of precipitation and drought patterns in mid-latitude
regions (Cooper et al., 2013; Yang et al., 2014; Cook et al., 2015).
Keith’s scientific contributions extend far beyond dendroclimatology. Examples
include other palaeoclimatic areas, such as the use of beetle remains for reconstructions
since the last glacial maximum (Atkinson et al., 1987) and combining tree-ring
reconstructions with glacier snout reconstructions from Scandinavia and Canada (Raper
et al., 1996; Luckman et al., 1997). He investigated the evidence for recent climate
change based on instrumental records, encompassing surface air temperature (Jones and
Briffa, 1992, 1995; Briffa and Jones, 1993), drought (Briffa et al., 1994, 2009; Trenberth
et al., 2014) and atmospheric circulation variability (Briffa et al., 1990a; Cornes et al.,
2013).
In all, Keith published more than 140 articles in journals and chapters in books.
These have amassed more than 16,000 citations, illustrating his influence on the field
(with an average of more than 100 citations per article). Beyond his personal research, he
was an associate editor of The Holocene (of which he was a founding member) for more
than two decades, and held similar editorial roles for the journals Dendrochronologia (for
21 years) and Boreas (for 12 years). Keith saw at an early stage the importance of
bringing together communities working with palaeoclimate data and those studying
climate variability from a dynamical and numerical modelling perspective. He used his
international roles within the International Geosphere-Biosphere Programme Past Global
Changes (IGBP PAGES) programme (as a member of its scientific steering committee,
1994–2000 and its executive committee, 1998–2000) and within the World Climate
Research Programme (WRCP) CLIVAR / PAGES Intersection Working Group (2005–
2011) to foster closer ties between these communities.
Keith’s amenable, friendly personality, combined with enthusiasm for his science
and a searching and constructively critical mind led to building contacts with many
people in the field of palaeoclimatology (e.g., Plates 1 to 3). The fruits of these long-
lasting and successful collaborations have been felt across climate science, with Keith
leading the development of many important records allowing us to understand natural
climate variability on timescales from years to millennia and to set recent climatic trends
in their historical context. This spirit of collaboration is apparent in the Assessment
Reports of the Intergovernmental Panel on Climate Change (IPCC), to which Keith made
important contributions over four of its cycles. In particular, he was a lead author of the
Palaeoclimate chapter (Jansen et al., 2007) for the IPCC’s Fourth Assessment Report,
with a focus on the, at times controversial, topic of climate variations during the last 2000
years. With his characteristic objectivity and openness about the strengths and limitations
of palaeoclimate data, he led the careful assessment of scientific understanding about this
topic.
Keith Briffa was a great friend to many colleagues throughout his 40 years in the
Climatic Research Unit and the School of Environmental Sciences at UEA, and was an
admired lecturer among students, especially for his never-ending enthusiasm for his
subject. Despite his great achievements, Keith remained remarkably modest and self-
effacing. He helped many colleagues and particularly early stage researchers to achieve
their potential, and he provided enduring support for the development of
dendroclimatology communities in many regions of the world.
Acknowledgements
This article benefitted from discussions with Tim Atkinson about Keith’s career.
Funding
The authors received no financial support for preparation of this article.
Selected publications by Keith R. Briffa (in chronological
order)
Briffa KR, Jones PD, Wigley TML, Pilcher JR and Baillie MGL (1983) Climate reconstruction
from tree rings: part 1, basic methodology and preliminary results for England. Journal of
Climatology 3, 233-242.
Briffa KR (1984) Tree-climate Relationships and Dendroclimatological Reconstruction in the
British Isles. Unpublished Ph.D. dissertation. University of East Anglia, U.K.
Wigley TML, Briffa KR and Jones PD (1984) On the average value of correlated time series,
with applications in dendroclimatology and hydrometeorology. Journal of Climate and
Applied Meteorology 23, 201-213.
Baillie MGL, Hillam J, Briffa KR and Brown DM (1985) Re-dating the English art-historical
tree-ring chronologies. Nature 315, 317-319.
Briffa KR, Jones PD, Wigley TML, Pilcher JR and Baillie MGL (1986) Climate reconstruction
from tree rings: part 2, spatial reconstruction of summer mean sea level pressure patterns
over Great Britain. Journal of Climatology 6, 1-15.
Atkinson TC, Briffa KR and Coope GR (1987) Seasonal temperatures in Britain during the last
22,000 years, reconstructed using beetle remains. Nature 325, 587-592.
Briffa KR, Jones PD, Pilcher JR and Hughes MK (1988a) Reconstructing summer temperatures
in northern Fennoscandinavia back to A.D. 1700 using tree-ring data from Scots Pine.
Arctic and Alpine Research 20, 385-394.
Briffa KR, Jones PD and Schweingruber FH (1988b) Summer temperature patterns in Europe: a
reconstruction to 1750 based on maximum latewood density indices of conifers.
Quaternary Research 30, 36-52.
Briffa KR and Jones PD (1990) Basic chronology statistics and assessment. (In) Methods of
Dendrochronology: Applications in the Environmental Sciences (ER Cook and LA
Kairiukstis, Eds.) 137-152. Kluwer, Dordrecht.
Briffa KR, Jones PD and Kelly PM (1990a) Principal component analysis of the Lamb catalogue
of daily weather types: Part 2, Seasonal frequencies and update to 1987. International
Journal of Climatology 10, 549-563.
Briffa KR
, Bartholin T, Eckstein D, Jones PD, Karlén W, Schweingruber FH and Zetterberg P
(1990b) A 1,400-year tree-ring record of summer temperatures in Fennoscandia. Nature
346, 434-439.
Schweingruber FH, Briffa KR and Jones PD (1991) Yearly maps of summer temperature in
western Europe from A.D. 1750 to 1975 and western North America from 1600 to 1982:
Results of a radiodensitometrical study on tree rings. Vegetatio 92, 5-71.