The changing carbon cycle at Mauna Loa Observatory

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   Fig. 1.

   Time series of the relative amplitude of the seasonal cycle of atmospheric CO₂ at the MLO (black) and anomalies in observed annual land temperatures (red) for the latitudinal band from 30°N to 80°N (except Greenland). Plotted are both annual means (triangles connected by dashed lines) and a smoothed time series based on a five-point binomial filter (thick solid curves). The relative amplitudes are in respect to the mean amplitude of the first 5 yr of CO₂ record (1959–1963). Temperature anomalies are relative to the 1959–2004 study period. For both time series, annual values correspond to the annual tick marks on the time axis.

   
   
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   Fig. 2.

   Correlations of the MLO amplitude time series with mean growing-season (May to October) climate and NDVI gridded fields for two 23-yr study periods: 1959–1981 (a and b) and 1982–2004 (c–e). The maps show correlation patterns for land surface temperature (a and c), SPI6 (b and d), and NDVI (e). Contoured are only correlations that are statistically significant at the 90% level (r ≥ 0.28; Student's t test, one-tailed).

   
   
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   Fig. 3.

   Standardized anomalies in the MLO amplitude (black) and spatial averages of mean growing-season (May to October) NDVI (green), SPI6 (blue), PDSI (orange), and temperature (red), and moving-window correlations between these indices and the MLO amplitude. (Upper) For the NDVI, SPI6, and PDSI, the spatial averaging was performed from 20°N to 50°N and 90°W to 120°W for North America (a) and 20°N to 50°N and 100°E to the eastern coast for Eurasia (b). For temperature, the spatial averaging domain spans 30°N to 80°N for North America (c) and from 30°N to 80°N and 60°E to the eastern coast for Eurasia (d). Nonvegetated areas were masked out in the spatial averaging. Positive anomalies in SPI6 and PDSI indicate wetter conditions, whereas those in the NDVI and temperature correspond to greener and warmer conditions. For a and b, all standardized anomalies are relative to the 1982–2004 period of the NDVI satellite record, whereas for c and d, the standardized anomalies are relative to 1959–2004. Plotted are both annual values (triangles connected by dashed lines) and a smoothed curve based on a five-point binomial filter (thick solid curves). One tick mark on the y scale corresponds to 1 SD. (Lower) The window length for the moving correlations between amplitude and the corresponding climate and the NDVI indices is 11 yr, and the corresponding correlation is plotted in the middle (year 6) of each interval (diamonds). For a and b, the moving correlations between the amplitude and the indices are plotted by using the same color assignments as in Upper. For c and d, moving correlations between amplitude and temperature are shown for zero (green) and 1-yr (blue) and 2-yr (red) lags, with the amplitude always lagging. Nonshaded correlations are statistically significant at the 95% level (r ≥ 0.52; Student's t test, one-tailed).

   
   
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   Fig. 4.

   Standardized anomalies in the MLO amplitude (black) and spatial averages of mean cold-season (November to April) temperature (red) and corresponding moving-window correlations. (Upper) The temperature spatial means encompass 30°N to 80°N for North America (a) and 30°N to 80°N and 60°E to the eastern coast for Eurasia (b). Nonvegetated areas were masked out in the spatial averaging. All standardized anomalies are relative to the whole study period, 1959–2004. Plotted are both annual values (triangles connected by dashed lines) and a smoothed curve based on a five-point binomial filter (thick solid curves). One tick mark on the y scale corresponds to 1 SD. (Lower) The window length for the moving correlations between amplitude and the temperature time series is 11 yr, and the respective correlation is plotted in the middle (year 6) of each interval (diamonds). Moving correlations between amplitude and temperature are shown for zero (green) and 1-yr (blue) and 2-yr (red) lags, with the amplitude always lagging. Nonshaded correlations are statistically significant at the 95% level (r ≥ 0.52; Student's t test, one-tailed).

   
   
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   Fig. 5.

   Time series of observed (black) and simulated (blue) relative amplitude of the seasonal cycle of atmospheric CO₂ at the MLO, and standardized anomalies in mean springtime (April to June) simulated fossil fuel (FF) only (dark red) and observed radon-222 (green) concentrations. Plotted are both annual (dashed) and 5-yr running (thick line) means. Following ref. 4, simulated monthly CO₂ concentrations from the transport-model (MATCH) output correspond to three vertical and nine horizontal grid-point averages centered at the MLO station (155°W, 19°N). For consistency, the seasonal cycle from the monthly MATCH record was extracted with the same curve-fitting algorithm that was used for the observed record (see Methods). The observed and simulated relative amplitudes are in respect to the corresponding mean amplitude of the first 5 yr of simulations (1972–1976). Simulated FF and radon anomalies are relative to the 1992–2002 radon record.