Vegetation TrendsIn Australia

Peter BriggsMichael Raupach, Edward King, Michael Schmidt, Matt Paget,

Jenny Lovell, Pep CanadellCSIRO Marine and Atmospheric Research

Acknowledgements

Damian Barrett, Susan Campbell, Dean Graetz, Tim McVicar, Udaya Senarath, Stephen Plummer and GlobCarbon (ESA)

Knorr, W., Scholze, M., Gobron, N., Pinty, B., Kaminski, T. (2005). Global scale drought caused atmospheric CO2 increase. EOS 18(18), 178-181.

Trends in fAPAR from SeaWiFS, Oct 1999 to Sep 2003

Decadal Vegetation Greenness TrendsNorthern Hemisphere Change in NDVI

Gains from earlier onset of growing season are almost cancelled out by hotter and drier summers which depress assimilation

Suggests a decreasing net terrestrial C sink

Angert et al. 2005; Dai et al. 2005; Buermann et al. 2005; Courtesy Inez Fung 2005

1980s: d(NDVI)/dt Summer 1982-1991

1990s: d(NDVI)/dt Summer 1994-2002

NDVI AnomalyMonthly 1981-2003

Anomaly as NDVImonth − <NDVImonth> Veg condition relative to

expectations for that time of year "BPAL" AVHRR data series

PAL (Pathfinder AVHRR Land, NASA) dataset 1981-94

CSIRO EOC AVHRR dataset 1992-2003, with BISE filtering and sampling to match PAL

8-11 day max-NDVI composites, aggregated to monthly

No atmos correctionNo BRDF correction“Only a demonstration”

Shows space and time evolution of the drought cycles, e.g.

Droughts 1982-83, 2002-04… Wet La Nina 1988

Less vigour More vigour

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Less vigour More vigourNDVI Anomaly 1992-2004

(BPAL + CATS1)

Test marketed on: Scientific colleagues Senior bureaucrats

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Less vigour More vigourNDVI Anomaly 1992-2004

(BPAL + CATS1)

Test marketed on: Scientific colleagues Senior bureaucrats

Response :1. “Holy sh…”

2. “Is this real?”

3. “What’s going on?”

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NDVI BPAL AVHRR 1982 to 2000

Mean• Rain is main constraint so NDVI follows rain map except:

• Northern tropics: high VPD and strong seasonality of rain limit growth

• Tasmania: light and temp-limited in winter

• Areas of high SD show agricultural cropping zones

• Desert interior: green flush as ephemeral lakes and rivers respond to seasonal runoff from the north

Mean

Std Dev

Confirmation of the 2000-2004 trend in AVHRR NDVI Comparison with

Other AVHRR NDVI treatments NDVI from other sensors Other products from other sensors

How the trend varies with bioclimatic region Comparison of trends within major drainage divisions East Australian drought recovery 2002-2005

Satellite vegetation time series for Australia

AVHRR NDVI BPAL (5 km, 8-11 day max-NDVI composite, BISE-filtered) CATS1 (1 km, ~ 10 day max-NDVI composite, no BRDF, no cloud) CATS2b (= CATS1 with CLAVR cloud clearing) CATS2a (= CATS2b with BRDF correction)

MODIS NDVI SeaWifs NDVI SPOT-Vgt NDVI LAI from GlobCarbon project

SPOT-Vgt, ATSR2/AATSR, MERIS

LAI from MODIS

Converted to

fraction cover fC = 1 ek∙LAI

with k = 0.5

Australia: vegetation greenness trends (1990-2005)

fraction cover fC from GlobCarbon LAI

NDVI: CATS2a(CLAVR, BRDF)

NDVI: MODIS, SeaWiFS, SpotVgt

NDVI: BPAL

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Australian continent averageNDVI (MODIS)NDVI (AVHRR BPAL)NDVI (AVHRR CATS2a; BRDF)NDVI (SeaWiFS)NDVI (SpotVGT)FC (from GlobCarbon LAI)MODIS FC

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NDVI 1992-2004Murray-Darling Basin

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4.3: MDB (semiarid) MODISBPAL CATS1CATS2b CATS2aSeaWiFS SpotVGTGlobCarbFC

Drought recovery (or not) in SE AustraliaSeaWiFS Fraction Cover for 5 Decembers(NDVI scaled by GlobCarbon FC)

Before Drought Drought Max 6 Months Ago

Marginal country slow to recover(if at all yet)

Before Drought Drought Max 6 Months Ago

Annual Rainfall (mm d-1)

Before Drought Drought Max 6 Months Ago

SeaWiFS

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Various NDVIs vs GlobCarbon FCBy Drainage Division, Monthly, 1999-2002

• Up to 36 pts (months) per drainage division

• Periods of known sensor problems removed from AVHRR

• For planned rescaling exercise

Used in previous maps

SeaWiFS

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Various NDVIs vs GlobCarbon FCBy Drainage Division, Monthly, 1999-2002

• Well-defined relationship between all sensors

• Saturation of greenness wrt GlobCarbon in all cases

• Only MODIS showing stratification wrt biogeography

Carbon consequences of vegetation greenness changesModel Let biospheric C obey rate equation dC/dt = FC kC, with mean turnover

rate k. If NPP changes suddenly by FC, then while t << 1/k, the change in C is

Assume NPP ~ green leaf cover fraction: Then biospheric C change associated with a perturbation in green leaf

cover is

Numbers Take t = 1 year; FC = 1 GtC/y; fGL/fGL = 0.2 (a low value)

=> C = 0.2 GtC = 0.2 PgC = 200 MtC = 730 Mt CO2

Compare: Australian GHG emissions (2002 NGGI) were 550 Mt CO2eq

C C GL GLF F f f

C GL GLC t F f f

CC t F

Conclusions (1): Trends in vegetation greenness, are they real?

Broad agreement from multiple sensors on Australian vegetation trends Continent-wide decline 2000-2004 Long AVHRR record suggests decline commenced around 1998

Areas of disagreement MODIS and GlobCarbon give substantially different estimates of LAI

(therefore fraction cover) For Australia, GlobCarbon is closer to the level we expect (from

other RS and flux station work) Trend from AVHRR probably too strong; modern sensors (SeaWiFS,

SpotVGT, MODIS) appear to show some recovery not seen in AVHRR.

Due to continued use of at-launch calibrations by AVHRR? Important to continue to follow trends through the recovery

However Areas of agreement are more significant than areas of disagreement

Conclusions (2): What’s going on?

Strongest relative decline occurs in semi-arid zones (rainfall < 600 mm) Declining trends in heavily forested areas not revealed here, but:

Scale, NDVI saturation may be issues See Helen Cleugh for MODIS/flux station results

Trends are too widespread for land use change to be a significant driver But land use (eg. stocking rates in rangeland) may have an effect

Likely drivers: Rainfall: no major floods in most of the country for 10-15 years (until

Cyclone Larry) Warming: 2002-2004 was a hot drought; 2005 hot and a little wetter

Possible contributing processes: Effects of warming and dryness on fire, heterotrophic respiration Soil evaporation (half of Australian ET) is favoured in the competition

for soil water, causing transpiration to fall

Messages

Loss of terrestrial C in 2002-2004 is similar to total anthropogenic Australian GHG emissions

Terrestrial biospheric C is highly dynamic and vulnerable: to drought and probably also to warming

Long term satellite environmental time series are an important tool for examining trends Modern sensors (with their overlap) will be invaluable for

validating and carrying on the series Too soon to say what will happen to continental vegetation next:

Many parts of the country still waiting for drought recovery Warming, associated with drought, is particularly worrying

Is this a foretaste of the future Australia?