134 W e a t h e r and Climate (1991) 11: 134-138

IMPACTS OF CLIMATE ON AGRICULTURE : A SOUTHERNPERSPECTIVE

M J SalingerNational Cl imate Centre, N Z Meteorological Service

Global c l i m a t e change w i l l pa r t i cu l a r l y a f f e c t ag r i cu l tu ra l pa t te rns i n temperateand sub t rop ica l l a t i t udes o f t h e sou the rn hemisphere. M a n y c o u n t r i e s i n t h e s elatitudes h a v e a m a r i t i m e , o r semi -mar i t ime c l i m a t e i n compar i son w i t h t h e i rnorthern c o u n t e r p a r t s . B e s t e s t i m a t e s o f f u t u r e c l i m a t e p l a c e t e m p e r a t u r eincreases a t 1.5°C, w i t h a range f r o m 0.5 t o 2.5°C b y 2030 A D . Here the impacts o fthese a n d associated changes i n o t h e r c l i m a t e f a c t o r s o n sou the rn ag r i cu l tu ra lsystems a r e considered.

IMPACTS O N CROP DEVELOPMENT

Crop development i s t h e progress a p l an t makes f r o m germinat ion t o ma tu r i t y f o ran a n n u a l c r o p ; a n d t h e a n n u a l c y c l e f r o m b u d i n i t i a t i o n t o m a t u r i t y f o r aperennial c r o p . T h e r a t e o f development i n t h e f i e l d i s genera l l y determined b ytemperature, a n d t o s o m e ex ten t b y daylength.

T e m p e r a t u r e

Temperature increases change t h e l e n g t h o f t h e po ten t ia l g r o w i n g season i n t h esouthern hemisphere, r e s u l t i n g i n substant ia l s h i f t s o f agr icu l tu ra l po ten t ia l . F o ra 1 ° C increase i n temperature, present c rops a n d pasture c o u l d b e g r o w n a t 2 0 0metres h i g h e r i n a l t i tude a n d abou t 2 0 0 k m fu r the r polewards.

An i n c r e a s e i n m e a n t e m p e r a t u r e i n c r e a s e s t h e r m a l t i m e ( d e g r e e - d a y )accumulat ion a b o v e t h r e s h o l d b a s e t e m p e r a t u r e s . A s t h e t h e r m a l t i m erequirement f o r a c r o p v a r i e t y t o ma tu re i s constant , c r o p s w i l l m a t u r e ear l ie r.About 2 0 0 - 3 0 0 degree-days a r e a d d e d t o t h e g r o w i n g season, decreas ing c r o pmaturation t ime b y t w o t o f o u r weeks.

Similarly, a n increase i n mean temperature decreases t h e numbe r o f days o f f ros tand lengthens t h e f r o s t f r e e season, t h e in te rva l between t h e l a s t f r o s t i n sp r ingand t h e f i r s t f r o s t i n au tumn . I n con t inenta l no r t he rn hemisphere env i ronmentsthe season lengthens b y 1 0 d a y s p e r 1 ° C inc rease i n m e a n temperature. F o rmaritime sou thern hemisphere env i ronments t h e increase i n f r o s t f r e e season i sbetween 2 5 and 3 0 days.

Many temperate c r o p s b r e d i n t h e m o r e cont inenta l n o r t h h a v e w i n t e r c h i l l i n grequirements ( a per iod o f l o w temperatures i n w in te r ) t o i n i t i a te o r accelerate t h eflowering process. A s temperature increases reduce t h e amoun t o f c h i l l i n g i n t h esouthern c l imates , t hese c r o p s h a v e t h e i r potent ia l r a n g e cont racted polewards.

U l t imate ly, y e a r - t o - y e a r t e m p e r a t u r e v a r i a b i l i t y w i l l s t i l l b e c r u c i a l i ndetermining t h e success o f arab le a n d hor t icu l tu ra l c rops , espec ia l l y w h e r e c ropsare g r o w n a t t h e ext reme marg ins o f t h e i r range.

D a y l e n g t h

The greatest daylength effects w o u l d b e f e l t w i t h t h e la t i tud inal sh i f t s i n c r o p andpasture var ie t ies a s a consequence o f warmer temperatures. C r o p s w o u l d need t oadapt t o a n e w l i g h t environment.

Climate Change and Agriculture

IMPACTS O N CROP GROWTH

Growth i s r e g a r d e d a s t h e i nc rease i n d r y w e i g h t o f a p l a n t because o fphotosynthesis. I t i s m a i n l y a consequence o f amb ien t C O 2 concent ra t ion , l i g h tinterception a n d m o i s t u r e .

Carbon D i o x i d e

In t h e m i d s t o f a l l t h e c l ima te uncertainties, atmospher ic C O 2 l e v e l i s present lyincreasing b y 1 . 5 ppm/year and i t w i l l cont inue t o r i se f o r some t ime. H ighe r CO2levels w i l l increase p l a n t g rowth , especia l ly t hose adapted t o a temperate c l ima te(C3 plants). H o w much w i l l b e expressed as y i e l d i s uncertain. T h e effects w i l l b emost pronounced u n d e r h i g h w a t e r st ress a n d h i g h temperatures. T h e w a t e r u s eefficiencies o f a l l c r o p s w o u l d i m p r o v e s ign i f i can t ly.

C3 p lan ts use t h e C 3 biochemical pathway f o r photosynthesis, w h i c h i s l i m i t e d b yC 0 2 concen t ra t ion . T h e C 4 p a t h w a y, u s e d b y C 4 p l a n t s , i s n o t l i m i t e d b yatmospheric C O 2 concent ra t ion. M a n y temperate c e r e a l a n d ho r t i cu l t u ra l c r o p suse t h e C 3 pathway. Subtropical pasture a n d ma ize a r e C 4 crops a n d t h e i r g rowthrates w o u l d b e o n l y s l i gh t l y affected.

M o i s t u r e

This i s a v e r y impor tan t reg ional var iab le f o r agr icu l tura l product ion. T h e impac tof changes i n r a i n f a l l w i l l ove r r i de temperature e f f ec t s i f hea t requirements a r eadequate f o r c rop matu r i t y. L a c k o f s o i l mois ture i s t h e m a i n constraint t o y i e l dsin m a n y temperate a n d subt rop ica l a reas o f t h e sou thern hemisphere, espec ia l l yin t h e s u m m e r season. R e g i o n s w h e r e g r o w i n g season r a i n f a l l increases w o u l dhave h i g h e r y i e l d s , a n d reg ions w i t h r a i n f a l l decreases w o u l d exper ience l o w e ryields w i t h o u t i r r i g a t i o n .

Climate change poses v e r y s ign i f i can t i m p a c t s o n agr i cu l tu ra l p roduc t ion b y i t seffects o n ext reme events: i n some regions droughts w o u l d become m o r e frequent.The changes i n va r iab i l i t y o f extreme events w i l l cause impor tan t changes i n t h ecomparative advantages o r competit iveness o f d i f f e ren t c rops .

Soil m o i s t u r e i s t h e r e s u l t o f a n u m b e r o f w a t e r ba lance processes: r a i n f a l lduration a n d i n t e n s i t y , evapo t ransp i ra t ion a n d r u n o f f . U l t i m a t e l y i t i s t h edecrease o r increase i n s o i l moisture a n d resul t ing changes i n t h e number o f daysat w i l t i ng po in t , ( a d a y w h e n a l l t h e avai lable wa te r capacity i n t h e s o i l f o r p lan tgrowth i s deple ted) t h a t impacts o n c r o p g r o w t h a n d pasture y i e l ds . T h e h i g h e rthe number o f days a t w i l t i ng point , t h e more t i m e l o s t f o r p lan t growth.

Models s h o w t h a t areas t h a t rece ive 1 0 % m o r e prec ip i ta t ion g a i n f r o m 4 t o 1 5extra d a y s ava i l ab le f o r p l a n t g r o w t h pe rm i t t ed b y t h e w a t e r s u p p l y. I n t heseareas, y i e l ds w o u l d increase. Conversely areas w h i c h receive 1 0 % less have 1 2 t o17 ex t ra days o f w i l t i ng po i n t (drought days). Un less ex t ra wa te r can b e added b yirrigation, t h e a r e a s t h a t b e c o m e d r i e r w i l l h a v e l o w e r y i e l d s , a s t h e a c t u a lduration o f p lan t g rowth i s less o v e r t h e g row ing season.

W i n d

Arable a n d hor t icu l tura l c rops a r e a t m o s t r i s k f r o m increases i n w i n d . G a l e a n dstorm f o r c e w i n d s c a u s e mechanica l damage t o p l a n t s w h i c h m a y damage t h eplant t o such a n extent t ha t t h e p lan t needs replacing, o r t ha t t h e present season'scrop i s uneconomic .

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136 C l i m a t e Change and Agriculture

Many areas i n t h e southern hemisphere a re i n i s land locations, a n d a r e exposed t ostrong w i n d s . A n y changes i n t r o p i c a l c y c l o n e (hu r r i cane ) f r e q u e n c y o r p a t h swi l l h a v e s ign i f i can t e f f e c t s o n c r o p product ion.

IMPACTS O N AGRICULTURAL PESTS A N D DISEASES

Climate change w i l l a l t e r t h e inc idence o f agr icul tural pes ts a n d diseases, o w i n gto c h a n g e s i n t h e w e a t h e r c o n d i t i o n s t h a t f a v o u r o r d i s c o u r a g e t h e m .Introduction o f n e w c r o p s a n d var ie t ies w o u l d b e accompanied b y n e w diseasesand p e s t s . T h e g r e a t e s t t h r e a t c o m e s f r o m t h e m i g r a t i o n p o l e w a r d s o r t h einadvertent in t roduct ion o f n e w pests a n d diseases, especial ly o n is lands.

Fungal a n d B a c t e r i a l D i s e a s e s

Rainfall a n d temperature a r e t h e t w o c l ima t i c fac to rs t h a t s ign i f i can t a f f e c t thesediseases. T h e y r e q u i r e f r e e w a t e r t o i n f e c t t h e h o s t c r o p . R a i n f a l l a n d t h efrequency a n d d u r a t i o n o f su r face l e a f wetness w i t h h i g h h u m i d i t y a r e m a j o rrequirements f o r d i s e a s e e p i d e m i c s . W a r m e r t empera tu res a l s o s h o r t e n t h egeneration t i m e , i nc reas ing t h e deve lopment r a t e o f ep idemics . I n t h o s e a reaswhich b e c o m e w e t t e r , t h e d i sease t h r e a t f r o m f u n g a l a n d b a c t e r i a l ep idem icsincreases.

Insect a n d M i t e P e s t s

The gene ra l e f f e c t s o f a w a r m e r c l i m a t e o n t hese pes t s a r e t o increase t h e i rdevelopment ra tes , resu l t i ng i n m o r e generat ions i n a g r o w i n g season. W h e r e anatural pes t enemy i s present, b io log ica l con t ro l m a y b e m o r e o r less e ffec t ive i nwarmer temperatures, depend ing o n t h e o p t i m u m temperature f o r t h e p e s t a n dnatural e n e m y conce rned .

Animal D i s e a s e s a n d P a r a s i t e s

The i n c i d e n c e o f m a n y e x i s t i n g l i v e s t o c k d iseases a n d paras i tes i s l i k e l y t oincrease w i t h temperature increases, because o f fas te r l i f e c y c l e comp le t ion a n denhanced s u r v i v a l . I n s o u t h e r n l i v e s t o c k a reas d iseases a n d paras i tes s u c h a snematodes, sa lmonel la a n d f a c i a l eczema w o u l d increase. A t t h e s a m e t i m e pestssuch a s midges, mosquitos a n d t i c k s w o u l d a l l spread polewards. These a r e vectorsfor a range o f new diseases w h i c h cou ld become established.

FIELD CROPS

Annual c r o p s a r e sens i t i ve t o c l i m a t e change, b u t because t h e i r r o ta t i on cyc lesare shor t , t h e y c a n b e replaced q u i c k l y, s o adjustment c a n b e rap id . T h e cur rentgenetic base o f crop types and varieties i s w ide. I t wou ld b e possible t o change thecrop t y p e s a n d var ie t ies g r o w n i n a n a r e a f as te r t h a n t h e c l i m a t e i s l i k e l y t ochange, p rov i ded p l a n t b reed ing a n d v a r i e t y t es t i ng a r e mainta ined.

The potent ia l f o r a m a j o r s h i f t i n la t i tude range poleward a n d t o h i ghe r elevationwould o c c u r w i t h temperature increases f o r a l l f i e l d c r o p s (whea t , b a r l e y, oa t s ,maize, s o r g h u m , p e a s , p o t a t o e s , soybeans , s u n f l o w e r , c o t t o n a n d r i c e ) . T h eincreases o f t e m p e r a t u r e a n d C O 2 w i l l i n c r e a s e t h e r a t e s o f b i o l o g i c a lproductivity, s o c r o p p roduc t i on r a t e s w i l l increase. O n t h e o t h e r hand , h i g h e rtemperatures w i l l i n c r e a s e c r o p d e v e l o p m e n t r a t e s a n d h e n c e t h e d u r a t i o n .Therefore t h e t i m e avai lable f o r accumulation o f y i e ld w i l l b e shorter.

The m a i n f i e l d c r o p s a r e annual species. There i s n o need t o encourage prematurerelocation o f c rop product ion t o n e w regions. T h e f a c t t h a t t h i s t y p e o f cropping

Climate Change and Agriculture 1 3 7

operates o n a n annual cyc le means t h e ac t i v i t y c a n s h i f t i n a v e r y shor t t ime.Plant b r e e d i n g p rogrammes, w h i c h ope ra te i n a l o n g t i m e f r a m e , s h o u l d b eencouraged t o m a i n t a i n s t r o n g s t ra teg ic bases s o t h e y h a v e t h e f l e x i b i l i t y t oproduce n e w var ie t ies adapted t o changed condi t ions.

HORTICULTURAL CROPS

Horticultural c rops a r e especial ly sensi t ive t o c l ima te change because t h e y h a v e along r o t a t i o n c y c l e a n d s o m e c r o p t y p e s h a v e a s m a l l g e n e t i c base . Va r i e t ydiversity a l l o w s s u b s t i t u t i o n s o f n e w v a r i e t i e s t o m a t c h n e w p a t t e r n s o fagr icul tural p o t e n t i a l .

Temperature increases m o v e t h e po ten t ia l l a t i t u d e r a n g e po leward a n d t o h i g h e relevations. H o w e v e r , t h i s c h a n g e i n r a n g e w i l l b e m o d i f i e d b y c h a n g e s i nrainfall. R e g i o n s t h a t b e c o m e w e t t e r f o r m a n y tempera te c r o p s w o u l d b e c o m emore unsui table because o f increased disease pressure.

Other s t ra teg ies t o a d a p t , a p a r t f r o m m o v i n g c r o p r a n g e s sou thward , i n c l u d ebreeding programmes f o r n e w var ie t ies w i t h d i f f e r e n t responses t o c l ima te . T h edegree o f adaptation strategies w o u l d v a r y. Perhaps t h e bes t strategies w o u l d b e amixture o f m o v e m e n t o f c r o p ranges po lewards a n d u p w a r d s i n t r o d u c i n g n e wvarieties o f t h e same c r o p w h i c h a r e be t te r su i ted t o t h e d i s t r i c t c l imate. F i n a l l y,the s e a r c h i n g f o r c r o p s t h a t n a t u r a l l y m a t c h t h e n e w c l i m a t e i s a n o t h e rresponse.

PASTURE

Pastures a r e sensi t ive t o c l imate change, b u t because t h e i r l i f e cyc le c a n b e shor tor t h e var ie ty c a n b e replaced qu ick ly, adjustment w o u l d b e rap id .

Temperate pastures a r e a t p e a k p roduc t ion i n s p r i n g . I n compar ison, subt rop ica lpastures p e a k i n summer. A l t h o u g h temperate grasses w o u l d spread po leward a n dupwards, s o t o o w o u l d subt rop ica l grasses. C l i m a t e w a r m i n g i n m a n y sou the rnlatitudes w o u l d increase t h e a rea covered b y subtropica l pasture a n d decrease t h earea i n t e m p e r a t e p a s t u r e . I f m o i s t u r e r e q u i r e m e n t s a r e m a i n t a i n e d , h i g h e ryields o f b o t h pas tu re t y p e s w o u l d o c c u r. T h e r e w o u l d b e m a r k e d e f f e c t s o nseasonality o f pasture product ion depending o n t h e ba lance o f temperate grasses,legumes a n d sub t rop ica l grasses.

With a s h i f t towards m o r e subtropical pastures, pasture product ion w o u l d increaseor decrease depend ing o n w h e t h e r t h e sub t rop ica l pas tu re w a s b e t t e r o r w o r s ethan t h e tempera te p a s t u r e i t rep laced . A m a j o r l e g u m e c o m p o n e n t o f b o t htemperate a n d subt rop ica l pastures i s w h i t e c l ove r.

For e x i s t i n g pas tures t h e e f f e c t s o f c l i m a t e c h a n g e o n p r o d u c t i v i t y w o u l d b eimmediate. H o w e v e r , t h e expans ion o f sub t rop ica l g rasses p o l e w a r d w o u l d l a gbehind c l ima te change. Fa rm ing systems w i l l need t o adapt t o t h e change i n f e e dquality a n d q u a n t i t y. Subt rop ica l grasses a r e character ised b y a s u m m e r g r o w t hpeak a n d a w i n t e r t r o u g h . Te m p e r a t e pastures. o n t h e o t h e r h a n d , w i l l t e n dtowards a l ess marked seasonal g r o w t h pat tern, excep t whe re mois ture i s l i m i t i n gin s u m m e r.

More impo r tan t l y, t h e th resho ld a l t i tudes f o r temperate pasture p roduc t ion w o u l dbe h igher. T h e u p p e r l i m i t o f g raz ing l a n d w o u l d r i s e b y 2 0 0 met res p e r 1 ° Ctemperature i n c r e a s e . I n h i g h e r e l e v a t e d a r e a s o f S o u t h A m e r i c a , s o u t h e r nAfrica, Aus t ra l i a a n d N e w Zealand such a w a r m i n g w i l l s ign i f i cant ly increase t h earea tha t c a n b e used f o r grazing l ivestock.

138 C l i m a t e Change and Agriculture

L IVESTOCK

Direct e f f e c t s o f c l i m a t e c h a n g e o n metabo l i sm, g r o w t h a n d pe r fo rmance o fanimals w o u l d n o t b e important, except tha t c o l d stress w o u l d b e reduced. Howeverheat stress w o u l d have s ign i f icant e f fec ts o n l o w e r concept ion ra tes i n l ivestock.

Indirect e f f e c t s w o u l d h a v e v e r y s i g n i f i c a n t i m p a c t s o n a n i m a l p r o d u c t i o n .Changes i n t h e amoun t a n d seasonal d is t r ibu t ion o f f eed ava i l ab i l i t y a n d q u a l i t y,and i n t h e t y p e a n d i n f e c t i v i t y o f pests, diseases a n d parasi tes w o u l d h a v e t h emain e f f e c t o n product ion.

Climate w a r m i n g w i l l r e d u c e t h e c h i l l i n g t h a t o c c u r s o f n e w b o r n l i v e s t o c k i nwinter a n d s p r i n g , r e d u c i n g m o r t a l i t y r a t e s o f s h e e p a n d c a t t l e . I n w a r m e rclimates, h e a t stress w i l l a f f e c t f eed in take , metabol ism, b o d y we igh t , m i l k y i e lds(dairy c a t t l e ) a n d rep roduc t i ve per formance.

Increased q u a n t i t y o f f e e d u s u a l l y r e s u l t s i n i m p r o v e d a n i m a l p e r f o r m a n c eprovided f e e d i n t a k e a n d d iges t i b i l i t y a r e n o t l i m i t i n g . I n temperate zones , t h eincrease i n CO2 a n d temperature s h o u l d m a r k e d l y inc rease s p r i n g , s u m m e r a n dautumn product ion o f pasture, p rov ided r a i n f a l l d o e s n o t decrease. H i g h e r w i n t e rtemperatures r e s u l t i n i m p r o v e d w i n t e r g r o w t h a n d q u a l i t y pas tu res dom ina tedby temperate C 3 pasture species. I n m a n y w a n n e r temperate a n d subtropical areastemperature increases w i l l a l l o w t h e spread o f subtropical C 4 grasses, w h i c h havea l o w e r d igest ib i l i ty a n d h ighe r f i b r e con ten t than temperate C 3 grasses. T h i s w i l lresult i n r e d u c e d a n i m a l i n t a k e s , g r o w t h r a t e s a n d m i l k y i e l d s f o r sou the rnlivestock product ion i n these areas. T h e h i g h e r CO2 l e v e l s s h o u l d i m p r o v e w a t e ruse e f f i c i e n c i e s , w h i c h p a r t i a l l y c o m p e n s a t e f o r r e d u c t i o n s i n r a i n f a l l o rincreases i n evapo t ransp i ra t i on .

Increases i n tempera ture a n d a l s o h u m i d i t y t e n d t o p r o d u c e m o r e f avou rab leconditions f o r an ima l pests, diseases a n d parasites.

A l l t hese changes r a i s e a n u m b e r o f adaptat ion strategies f o r l i ves tock fa rm ing .The change i n seasonal i ty o f f e e d product ion w i l l m e a n a change i n t i m i n g o freproduction a n d g r o w t h cyc les . H o w e v e r, a dec l i ne i n f e e d q u a l i t y i n w a r m e rtemperate r e g i o n s m e a n s r e l o c a t i o n o f l i v e s t o c k i n d u s t r i e s s u c h a s d a i r y i n gpoleward. Sheep f ann ing act iv i t ies shou ld b e ab le t o spread t o h i ghe r elevations.

Apart f r o m re loca t ion o f l i ves tock r e a r i n g a t t h e p o l e w a r d a n d h i g h e r a l t i t udemargins, w a r m i n g w o u l d inc rease t h e inc idence o f a n d po ten t i a l l osses d u e t opests, diseases a n d parasi tes, a n d hence increase t h e n e e d f o r e f f ec t i ve c o n t r o lmeasures. Changes i n f a r m management i n response t o chang ing pas tu re g r o w t hpatterns, together w i t h changes i n d is t r ibu t ion a n d a c t i v i t y o f pests, diseases a n dparasites w i l l r e q u i r e changes t o f a r m i n g c o n t r o l programmes.