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Kim, Y; Kimball, JS; Zhang, K; McDonald, KC (2012). Satellite detection of increasing Northern Hemisphere non-frozen seasons from 1979 to 2008: Implications for regional vegetation growth. REMOTE SENSING OF ENVIRONMENT, 121, 472-487.

The landscape freeze-thaw (FT) signal from satellite microwave remote sensing is closely linked to vegetation phenology and land-atmosphere trace gas exchange where seasonal frozen temperatures are a major constraint to plant growth. We applied a temporal change classification of 37 GHz brightness temperature (T-b) series from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I) to classify daily FT status over global land areas where seasonal frozen temperatures influence ecosystem processes. A temporally consistent, long-term (30 year) FT record was created, ensuring cross-sensor consistency through pixel-wise adjustment of the SMMR T-b record based on empirical analyses of overlapping SMMR and SSM/I measurements. The resulting FT record showed mean annual spatial classification accuracies of 91 (+/-8.6) and 84 ( +/-9.3) percent for PM and AM overpass retrievals relative to in situ air temperature measurements from the global weather station network. The FT results were compared against other measures of biosphere activity including CO2 eddy flux tower measurements and satellite (MODIS) vegetation greenness (NDVI). The FT defined non-frozen season largely bounds the period of active vegetation growth and net ecosystem CO2 uptake for tower sites representing major biomes. Earlier spring thawing and longer non-frozen seasons generally benefit vegetation growth inferred from NDVI spring and summer growth anomalies where the non-frozen season is less than approximately 6 months, with greater benefits at higher (>45 degrees N) latitudes. A strong (P<0.001) increasing (0.189 days yr(-1)) trend in the Northern Hemisphere mean annual non-frozen season is largely driven by an earlier (-0.149 days yr(-1)) spring thaw trend and coincides with a 0.033 degrees C yr(-1) regional warming trend. The FT record also shows a positive (0.199 days yr(-1)) trend in the number of transitional (AM frozen and PM non-frozen) frost days, which coincide with reduced vegetation productivity inferred from tower CO2 and MODIS NDVI measurements. The relative benefits of earlier and longer non-frozen seasons for vegetation growth under global warming may be declining due to opposing increases in disturbance, drought and frost damage related impacts. (c) 2012 Elsevier Inc. All rights reserved.



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