30-year Northern Hemisphere Freeze/Thaw seasonal trends and associated impacts to vegetation growing seasons and Carbon Exchange

Youngwook Kim, University of Montana-NTSG/FLBS, youngwook.kim@ntsg.umt.edu (Presenter)
John S Kimball, University of Montana, johnk@ntsg.umt.edu
Ke Zhang, Harvard University, kezhang@oeb.harvard.edu
Kyle C McDonald, The City College of New York, kmcdonald2@ccny.cuny.edu

Landscape freeze-thaw (FT) state 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 (Tb) 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 Tb 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 also compared against other measures of biosphere activity including satellite (MODIS) vegetation greenness (NDVI) and tower CO2 flux measurements. A strong (P<0.001) increasing (0.189 days yr-1) trend in Northern Hemisphere mean annual non-frozen period is largely driven by an earlier (-0.149 days yr-1) spring thaw trend and coincides with a 0.033 oC yr-1 regional warming trend. The FT defined non-frozen period largely bounds the season of active vegetation growth and net ecosystem CO2 uptake for tower sites representing major ecoregion types. Earlier spring thawing and a longer non-frozen season 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°N) latitudes and upper elevations. 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 photosynthetic activity inferred from tower and 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.

Portions of this work were conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract to the National Aeronautics and Space Administration.

Key Words: Freeze thaw, SMMR, SSM/I, climate change, global warming, MODIS, NDVI, vegetation growing season, phenology, ESDR, CDR, NASA MEaSUREs.

Presentation: 2011_Poster_Kim_220_17.pdf (1870k)

Presentation Type:  Poster

Session:  Global Change Impact & Vulnerability   (Tue 11:30 AM)

Associated Project(s): 

• McDonald, Kyle: Satellite Monitoring of Landscape Freeze-Thaw State and Associated Constraints to the North American Carbon Budget ...details
• McDonald, Kyle: Vegetation Phenology Assessment Using Satellite Radar Remote Sensing: Global Monitoring of Daily and Seasonal Changes in Canopy Structure and Water Status ...details

Poster Location ID: 220