Warming Climate, Greening Earth, Carbon Sink

Growing Season Leaf Area Changes: Further analysis of the data and model results (figure below) reveals an increasing trend in the annual maximum leaf area index (green leaf area per unit ground area, LAI) during 1982-1991 (+0.23, observed), an abrupt decline from 1991-1992 and a resumed increase during 1992-1998 (+0.19). The model reproduces 65% of the amplitude of the increasing trend before 1992, 85% of the trend after 1992, and 63% of the overall trend (Table S1). According to the AVHRR satellite data, the overall greening trend is associated with an advance of spring budburst by several days and a delay of autumn leaf-fall by a similar amount. The model agrees closely with the trend in spring onset, and its interannual variability (r = 0.91). A delay in autumn onset is also shown by the model. The amplitude of this delay is underestimated, but this quantitative comparison is less appropriate than for spring onset because the model estimates the timing of cessation of carbon assimilation rather than the timing of leaf abscission, which is what is visible from space.

In order to determine the climatic factors leading to the observed trends and variability we ran the vegetation model while keeping selected climatic inputs constant at 1966-1995 average values. We find that variations in temperature alone account for nearly all of the modeled behavior. Changes in precipitation, and physiological effects of atmospheric CO2 concentration, contribute only marginally (figure below). Stratospheric aerosol generated by the Pinatubo eruption in 1991 depressed incoming shortwave radiation. The global effect was still -1 W/m 2 even a year afterwards. As a result, an anomalous cooling of -0.5 K was observed in the northern high-latitude growing seasons of 1992-1993. The AVHRR satellite data indicate a large negative anomaly of LAI in the boreal zone during these years. The reliability of this finding taken in isolation is uncertain because volcanic aerosols increase atmospheric scattering, and could have spuriously lowered the apparent LAI. However, the LAI values estimated from the satellite data agree closely with those we simulate from climate data during this period. Both data and model results indicate that the northern biosphere suffered an abrupt setback after 1991, followed by a resumption of the greening trend in 1993. The decline in LAI from 1991 to 1992 was 0.27 in the satellite data and 0.22 in the model results. Spring onset was delayed by 4 days according to the satellite data and 3 days in the model. Autumn onset was brought forward by 9 days according to the satellite data and 4 days in the model. These reactions to the Pinatubo eruption occurred more or less uniformly on all three northern continents.

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    May-23-2002
    rmyneni@bu.edu