recent increases in the growing season length at high northern latitudes nicole smith-downey* james...
TRANSCRIPT
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 1988-2005 (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
GH
z1
9 G
Hz
37-
19
GH
z
€
Tb (υ ) = Ts ⋅ε(υ )
Tb (37 −19) = Ts ⋅ε(37 −19)
Site LevelValidation
Smith et al. [2004] - Satellite + - Field
Compared satellite estimate of date of freeze and thaw to soil temperature measurements
Mean Maps 1988-2005
0 50 100 150 200 0 100 200 300 0 100 200 300
Day of Thaw Day of Freeze Growing Season Length (days)
Trend Map - Thaw
-10 0 10
Trend in Thaw(days/decade)
Trend Map - Freeze
-10 0 10
Trend in Freeze(days/decade)
Trend Map - Growing Season Length
-10 0 10
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 (1981 - present) of the ‘greenness’ of a pixel
• NOAA AVHRR instrument• 8km spatial resolution - resampled to 1x1
degree, 15 day composite value
€
NDVI =NIR −VIS
NIR +VIS
Tucker et al. [1981, 2004, 2005]
Average NDVI for North American Evergreen Conifer
Onset End
NDVI Comparison
Freeze-Thaw
NDVI
Thaw is strongly correlated with the NDVI estimated onset of photosynthesisFreeze 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