Tree rings, carbon dioxide, and climatic change

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.

Figure 1

Reconstructions of central Alaska temperatures. (Upper) Five-year averaged annual (October–September) temperature reconstructed by using ring widths. (Lower) Summer (May–August) temperatures reconstructed by using maximum latewood density. Note the increase in reconstructed summer temperature over the past 100 years is only about 0.5° to 1.0°C, whereas the reconstructed annual temperature has increased about 1.5° to 2°C. The cooler period in annual temperatures prior to 1900 was broken by several warm intervals. Dashed line in Upper is 5-year recorded temperatures for central Alaska. Note that the reconstruction underestimates temperatures since about 1970. This is attributed to the effects of moisture stress (8).