Increases in early season ecosystem uptake explain recent changes in the seasonal cycle of atmospheric CO₂ at high northern latitudes

Abstract

Abstract. We report changes in the seasonal cycle of atmospheric CO2 at high northern latitudes from 1980 to 1997 based on NOAA/CMDL observation stations. Using a combination of biogeochemical and atmospheric modeling approaches, we show that increases in early season net ecosystem uptake explain the recent trends in the seasonal cycle. A strong year-to-year correlation between spring temperatures and early season uptake further suggests that increased photosynthetic activity is the primary mechanism.' At the end of the growing season, a strong correlation between fall temperatures and late season releases provides evidence for a large active pool of decomposing soil carbon. Taken together, our results suggest that the seasonal timing of temperature anomalies may have important consequences for the interannual carbon balance of northern ecosystems.

The changing carbon cycle at Mauna Loa Observatory

“…Reports of a greening trend in the satellite-derived normalized difference vegetation index (NDVI) throughout the 1980s at northern high latitudes consistent with springtime warming provided additional support for this hypothesis (5). The warming trend at northern high latitudes stimulated wintertime respiration as well as summertime photosynthesis, with both contributing to the observed CO 2 amplitude increases (6). In addition to these temperature-based analyses, positive trends in precipitation from 1950 to 1993 over the continental United States have also contributed to higher rates of carbon sequestration (7), consistent with the CO 2 record.…”

Species and Genotype Effects of Bioenergy Crops on Root Production, Carbon and Nitrogen in Temperate Agricultural Soil

“…On occasions, we observed other plants to be present on the plot, despite weed control, though we could not quantify the areal extent. Other factors include trends in atmospheric CO 2 [53,54], and variations in plant input signatures relating to physiological stress [55,56], which all affect the δ 13 C determinations used to estimate Miscanthus-derived inputs. The former would be unlikely to have been significant given the short period between measurements, but the latter may have been important.…”

Attribution of divergent northern vegetation growth responses to lengthening non-frozen seasons using satellite optical-NIR and microwave remote sensing

“…Since the GLOBALVIEW-CO 2 record is derived from the integration of surface, tower, and aircraft measurements by an atmospheric transport model (Masarie and Tans 1995), this record is considered suitable for representing aggregated HNL atmospheric CO 2 variability (Higuchi et al 2003;Murayama, Taguchi, and Higuchi 2004). The timing of atmospheric CO 2 drawdown by vegetation net photosynthesis in spring (C spr ) was computed from the weekly GLOBALVIEW-CO 2 record (>50°N) on an annual basis as the timing (day of year) of the downward 0-ppm crossing of the normalized (weekly -annual mean) atmospheric CO 2 seasonal record Randerson et al 1999), and coinciding with the HNL growing season initiation Zhang et al 2008). The annual minimum CO 2 concentration of the normalized seasonal record (C min ) was used as a relative measure of net photosynthetic carbon uptake Randerson et al 1997).…”