HOLOCENE CENTURY-SCALE CO₂ DYNAMICS RECONSTRUCTED BY STOMATAL FREQUENCY ANALYSIS

Contrary to a longstanding notion, the Holocene epoch is characterized by considerable climatic variability. Climatic fluctuations on millennial, centennial and shorter time scales are forced and amplified by thermohaline circulation adjustments, solar insolation variability and anthropogenic atmospheric greenhouse gas changes. Glaciochemical records of atmospheric carbondioxide concentrations during the Holocene show a relative insensitivity of CO₂ to documented climate changes, although on a larger scale (stadial/interstadials) temperature fluctuations appear concurrent to changes in CO₂. This apparent discrepancy might be partly explained by the relatively low temporal resolution of Holocene ice-core records and problematic dating due to the difference in age between the air bubbles and surrounding ice.

An alternative source of information on atmospheric CO₂ concentrations is provided by the measurements of stomatal frequency on (sub)fossil leaves. A species-specific, inverse relationship between stomatal numbers on leaves and atmospheric CO₂ has been demonstrated for many plant taxa in experiments and analysis of leaves grown under the increasing CO₂ concentration of the last century.

Now, well-dated, high resolution stomatal frequency records for the Holocene are available, using different plant taxa (deciduous trees and conifers) from both the northern and southern hemisphere. These stomatal records clearly reflect the CO₂ rise over the last centuries from low pre-industrial to the high recent CO₂ levels. Moreover, they provide evidence for the occurrence of prominent CO₂ fluctuations on a centennial scale during the Holocene. These global atmospheric CO₂ shifts are contemporaneous with pronounced climatic events in the North Atlantic region, such as the 8.2 kyr event and the Little Ice Age. The linkage between temperature, CO₂ and shifts in North Atlantic terrestrial and oceanic carbon sinks and sources will be discussed.