Recent increases in the growing season length at high northern latitudes Nicole Smith-Downey* James T. Randerson Harvard University UC Irvine Sassan S. Saatchi Compton J. Tucker NASA JPL NASA Goddard

Background Hansen et al. [2001] ºC

Our Approach Use satellite-derived estimates of date of soil thaw and freeze to estimate growing season length in northern biomes annually –Use passive microwave brightness temperatures from SSM/I instrument spanning (Armstrong et al. 1994) Calculate changes in the growing season length Compare satellite freeze-thaw record to surface air temperature and NDVI data

Difference between two channels is sensitive to difference in emissivity Plateau shape corresponds to thawed soils Algorithm Smith et al. [2004] 37 GHz 19 GHz GHz

Site Level Validation Smith et al. [2004]  - Satellite + - Field Compared satellite estimate of date of freeze and thaw to soil temperature measurements

Mean Maps Day of Thaw Day of Freeze Growing Season Length (days)

Trend Map - Thaw Trend in Thaw (days/decade)

Trend Map - Freeze Trend in Freeze (days/decade)

Trend Map - Growing Season Length Trend in Growing Season Length (days/decade)

Biome Level Results

1998 Freeze Anomaly Smith et al. [2004]

Surface Air Temperature Comparison Hansen et al. [2001] Smith et al. [2004] Freeze-Thaw Air Temperature Thaw is negatively correlated with SAT (warmer temperatures ~ earlier thaw) Freeze is positively correlated with SAT (warmer temperatures ~ later freeze)

Normalized Difference Vegetation Index (NDVI) A satellite record ( present) of the ‘greenness’ of a pixel NOAA AVHRR instrument 8km spatial resolution - resampled to 1x1 degree, 15 day composite value Tucker et al. [1981, 2004, 2005]

Average NDVI for North American Evergreen Conifer OnsetEnd

NDVI Comparison Freeze-Thaw NDVI Thaw is strongly correlated with the NDVI estimated onset of photosynthesis Freeze is correlated with the NDVI estimated end of the growing season in Conifer and Larch biomes

Implications Satellite record of freeze-thaw is a new metric of global climate change –Global –Extended time series –Fine spatial and temporal resolution Growing season is increasing by 5 days/decade in North American biomes Growing season is shifting forward in Eurasian biomes

Acknowledgements National Snow and Ice Data Center –SSM/I data Global Land Cover Facility –NDVI data NASA GISS –Surface Air Temperature data