Tree rings carbon dioxide and climatic change

Gordon C. Jacoby* and Rosanne D. D'Arrigo

Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, Palisades, NY 10964 © 1997 by The National Academy of Sciences 0027-8424/97/948350-4$2.00/0

PNAS is available online at http://www.pnas.org.

ABSTRACT Tree rings have been used in various applications to reconstruct past climates as well as to assess the effects of recent climatic and environmental change on tree growth. In this paper we briefly review two ways that tree rings provide information about climate change and CO2: (i) in determining whether recent warming during the period of instrumental observations is unusual relative to prior centuries to millennia, and thus might be related to increasing greenhouse gases; and (ii) in evaluating whether enhanced radial growth has taken place in recent decades that appears to be unexplained by climate and might instead be due to increasing atmospheric CO2 or other nutrient fertilization. It is found that a number of tree-ring studies from temperature-sensitive settings indicate unusual recent warming, although there are also exceptions at certain sites. The present tree-ring evidence for a possible CO2 fertilization effect under natural environmental conditions appears to be very limited.

Longer time series than those presently available from instrumental records are needed to evaluate whether recent climatic shifts are unusual and might be evidence of anthropogenic change due to increasing CO2 and other greenhouse gases. Longer records of natural climate variability and forest growth information can also help validate climate and carbon budget models used for prediction of future climate (e.g., see ref. 1).

Large-scale changes in sources and sinks of carbon in the terrestrial biosphere (due to climatic change, direct CO2 fertilization, forest regrowth, increased decay rates, or other factors) can act as either negative or positive feedbacks to the earth's climate system (e.g., see refs. 2 and 3). Recent studies based on isotopic measurements of atmospheric CO2 suggest that there may in fact be a large CO2 sink in the land biosphere of northern temperate latitudes (30-60°N) (4, 5).

Below we outline some of the tree-ring evidence for recent climate and forest growth changes and their relevance for studies of the global carbon cycle. We focus on two issues: (i) whether recent climatic changes during the period of instrumental observations appear to be unusual relative to the past, and (ii) whether enhanced radial growth has taken place that appears to be unexplained by climate and might be due to increasing atmospheric CO2 or other nutrient fertilization.

Do Temperature-Sensitive Tree-Ring Records Indicate that Recent Warming is Unusual?

Tree-ring measurements can help to distinguish anthropogenic from natural environmental change. These data can be used to determine whether recent climatic changes are unusual and possibly due to anthropogenic effects (specifically, increasing CO2 and other trace gases) (e.g., see ref. 6) or are still within the range of natural climate variability. Several recent studies, outlined briefly below, have evaluated tree-ring and other proxy data with this goal in mind.

Cook (7) reviewed high-resolution temperature histories from tree ring and coral proxies to evaluate to what degree the 20th century warming has been anomalous relative to prior centuries to millennia. At northern latitudes, these histories include temperature-sensitive tree-ring series for northern Alaska (8), the north Polar Urals (9), and the Arctic as a whole (10). All three of these series indicate unusual 20th century warming. Recent tree-ring data from Mongolia indicate that there is unusual warming in that region (11), in agreement with the Arctic reconstruction (10).

Jacoby and D'Arrigo (8) describe recent warming in Alaska relative to past tree growth variations (see Fig. 1). This study describes a summer temperature reconstruction based on maximum latewood density which shows evidence of recent warming of 0.5° to 1°C over the past century. By contrast, the ring-width data, which appear to integrate temperature conditions throughout the year (8), indicate more pronounced recent warming of annual temperatures of 2° to 3°C.

Briffa et al. (12) used a 1,000-year long tree-ring temperature record from Siberia to infer that the twentieth century (1901-1990) summer warmth has been unusual relative to the past millennium. Bradley and Jones (13) reconstructed Northern Hemisphere summer temperatures back to A.D. 1400 by using a combination of historical, tree-ring, and ice-core data and found recent conditions to be very warm relative to the past. in contrast, a millennium-long record from Fennoscandia indicates that it was warmer in Fennoscandia during the so-called Medieval Warm Period (14) than it is today, possibly due to cooling of the North Atlantic (12).

In the Southern Hemisphere, a multimillennial summer temperature reconstruction from southern South America shows no evidence of unusual recent warming, in agreement with instrumental records (15, 16). However, tree-ring and other records from Tasmania and New Zealand do indicate anomalous warming in recent decades relative to the past (7, 17, 18). The Tasmanian huon pine record, which is multimillennial in length, indicates that the warming of recent decades is highly unusual, with only one marginally warmer interval over the past several thousand years. This warm-season temperature reconstruction suggests that the recent warming in Tasmania is anomalous although not entirely unprecedented (7).

in summary, a number of temperature-sensitive middle to higher latitude tree-ring records from both hemispheres show evidence that recent warming in these regions may be anomalous. A few of these series are millennial in length. These few very long records allow evaluation of temperature variations prior to the Little ice Age cold interval, which could bias interpretation of recent warm conditions (13). Other sources of proxy data in several areas also support these indications of unusual recent warming relative to the past [e.g., ice cores (19)].

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