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Trends in Global Agricultural Land Use : Implications for Environmental Health and Food Security Ramankutty, Navin; Mehrabi, Zia; Waha, Katharina; Kremen, Claire; Herrero, Mario; Rieseberg, Loren H. 2018

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	 1	Trends	in	Global	Agricultural	Land	Use:	Implications	for	Environmental	Health	and	Food	Security	Navin	Ramankutty1,	Zia	Mehrabi1,	Katharina	Waha2,	Claire	Kremen3,	Mario	Herrero2,	Loren	Rieseberg4		1UBC	School	of	Public	Policy	and	Global	Affairs	and	Institute	for	Resources,	Environment	and	Sustainability,	University	of	British	Columbia,	Vancouver,	BC,	Canada.	Emails:	navin.ramankutty@ubc.ca;	zia.mehrabi@ubc.ca.	2Commonwealth	Scientific	and	Industrial	Research	Organization,	St	Lucia,	QLD,	Australia.	Emails:	wah006@csiro.au;	Mario.Herrero@csiro.au.	3Department	of	Environmental	Science,	Policy	and	Management,	University	of	California	Berkeley,	Berkeley	California,	USA.	Email:	ckremen@gmail.com.	4Department	of	Botany	and	Biodiversity	Research	Centre,	University	of	British	Columbia,	Vancouver,	BC,	Canada.	Email:	loren.rieseberg@botany.ubc.ca.		Running	title:	Agriculture,	environment	and	food	security		Corresponding	author:	Navin	Ramankutty,	Liu	Institute	for	Global	Issues,	6476	NW	Marine	Drive,	Vancouver,	BC,	V6T	1Z2.	Ph:	(604)	827-1745.	Email:	navin.ramankutty@ubc.ca.			 2	Abstract	The	18th	century	Malthusian	prediction	of	population	growth	outstripping	food	production	have	not	yet	come	to	bear.	Unprecedented	agricultural	land	expansion	since	1700,	and	technological	innovations	from	the	Green	Revolution	of	the	1950s,	have	enabled	more	calorie	production	per	capita	than	was	ever	available	before	in	history.	This	remarkable	success,	however,	has	come	at	a	great	cost.	Agriculture	is	a	major	cause	of	global	environmental	degradation.	Undernourishment	and	micronutrient	deficiencies	persist	among	large	parts	of	the	population,	and	a	new	epidemic	of	obesity	is	on	the	rise.	We	review	both	the	successes	and	failures	of	the	global	food	system,	addressing	ongoing	debates	on	pathways	to	environmental	health	and	food	security.	To	deal	with	these	challenges,	a	new	coordinated	research	program	blending	modern	breeding	with	agro-ecological	methods	is	needed.	We	call	on	plant	biologists	to	lead	this	effort,	and	help	steer	humanity	toward	a	“safe	operating	space”	for	agriculture.				Keywords:	agriculture,	food	production,	food	security,	environment,	land	use			 		 3	1. INTRODUCTION	Agricultural	lands	constitute	the	largest	biome	on	this	planet	(42),	occupying	a	third	of	the	global	ice-free	land	area	(118).	Agriculture	is	still	a	major	livelihood	for	40%	of	the	world’s	population	and	contributes	to	~30%	of	GDP	in	low	income	countries	(179).	It	also	provides	food,	fiber,	biofuels,	and	other	products	for	the	current	human	population	of	7	billion.		Agriculture	provides	more	than	enough	calories	for	all	people	on	the	planet,	yet	800	million	people	remain	undernourished	(50),	and	approximately	two	billion	suffer	from	micronutrient	deficiencies	(164).	Furthermore,	human	populations	are	projected	to	grow	to	nearly	10	billion	by	2050	and	more	than	11	billion	by	2100	(167).	At	the	same	time,	with	increasing	wealth,	there	is	greater	per-capita	consumption	of	meat,	refined	fats,	refined	sugars,	alcohols,	and	oils,	which	are	more	resource-consuming	to	produce	than	crops	directly	consumed	by	humans	(157).	Thus,	there	is	increasing	pressure	on	agriculture	to	meet	the	needs	of	current	and	future	human	populations.		Agriculture,	however,	is	already	one	of	the	greatest	environmental	threats	(158).	Clearing	forests	and	other	natural	vegetation	results	in	climate	change	and	biodiversity	loss.	Agriculture	is	the	biggest	user	of	freshwater	on	this	planet,	and	is	the	major	cause	of	freshwater	eutrophication.	Balancing	the	environmental	costs	of	agriculture	with	the	need	to	feed	current	and	future	populations	is	a	major	challenge.	This	is	doubly	so,	as	global	environmental	changes	can	feed	back,	and	hamper	future	production.	Climate	change	is	already	a	major	threat	to		 4	production,	estimated	to	have	caused	~4-5%	declines	in	maize	and	wheat	production	over	the	last	30	years	(96).		Many	solutions	have	been	proposed	for	navigating	the	pathway	to	a	sustainable	food	system	(55).	Some	scholars	advocate	for	new	technological	systems,	such	as	genetic	modification	(51)	or	vertical	farming	(37),	while	others	argue	for	organic	agriculture	(11)	or	local	food	systems	(70).	Still	others	argue	that	agriculture	does	not	need	a	revolution	and	that	we	simply	need	to	improve	current	farming	practices	(31).	Other	arguments	shift	the	focus	from	farm-level	solutions	to	the	entire	food	supply	chain	from	production	to	processing	to	consumption	(79),	and	consider	issues	such	as	food	waste	and	diets	(88,	157).	Some	authors	question	the	entire	framing	of	the	sustainable	food	challenge,	suggesting	food	sovereignty	as	an	alternate	paradigm	(90,	178).		In	this	paper,	we	will	start	by	reviewing	the	major	trends	in	the	evolution	of	agriculture	from	the	Industrial	Revolution	to	the	emerging	trends	and	projections	for	the	21st	Century.	As	alluded	to	above,	these	trends	generally	depict	success	in	terms	of	increasing	production,	but	problems	of	hunger,	malnutrition	and	environmental	impacts	remain.	Accordingly,	the	remaining	sections	of	this	paper	address	the	implications	of	these	land	use	trends	for	environmental	health	and	food	security,	touching	on	current	debates	and	controversies.	We	conclude	by	drawing	implications	for	plant	biology.	The	scope	of	our	review	is	limited	to	crops	and	livestock,	and	does	not	consider	fisheries	or	forestry.			 5	2. THREE	CENTURIES	OF	EXPANSION	–	CHANGES	SINCE	1700	Humans	have	modified	the	Earth’s	landscapes	since	time	immemorial	(125,	138).	First	through	the	control	of	fire,	then	the	domestication	of	plants	and	animals,	and	finally	through	harnessing	the	energy	from	fossil-fuels,	humans	have	greatly	expanded	their	footprint	on	this	planet	(166).			But	the	extent	and	pace	of	human	land	use	activities	accelerated	over	the	last	300	years	(165),	with	the	emergence	of	the	Industrial	Revolution	and	associated	rapid	growth	and	transformation	of	human	societies.	Between	1700	and	2007,	croplands	and	pasturelands	each	expanded	5-fold	(	~3	to	~15		and	~5		to	~27	million	km2	respectively,	Figure	1).	Most	cropland	expansion	replaced	forests,	while	most	pastureland	expansion	replaced	grasslands,	savannas,	and	shrublands	(Figure	1),	with	some	notable	exceptions	(e.g,	the	North	American	Prairies	were	replaced	by	croplands,	while	a	large	amount	of	Latin	American	deforestation	today	is	still	for	grazing).			The	global	expansion	of	agriculture	followed	the	development	of	human	settlements	and	the	world	economy	(67,	100,	126).	In	1700,	large-scale	agriculture	was	mainly	confined	to	the	Old	World	(Figure	2),	to	Europe,	India,	China,	and	Africa	(119).	European	colonization	created	new	settlement	frontiers	in	the	Americas,	Australasia,	and	South	Africa,	while	Russians	moved	east	in	the	Former	Soviet	Union	(67).	Between	1850	and	1950,	agriculture	expanded	rapidly	in	North	America,	starting	on	the	eastern	seaboard	and	migrating	westward	over	time,	and	also	pushed	eastward	in	the	Former	Soviet	Union	(119).	However,	in	the	last	50	years,	the	agricultural	frontiers	have	shifted	to	the	tropics,	towards	Latin	America,	Southeast	Asia,	and	Africa	(119).		 6	Meanwhile,	many	temperate	regions	of	the	world	witnessed	stabilization	of	agricultural	lands	and	even	abandonment.	In	North	America,	as	the	agricultural	frontier	shifted	west,	croplands	were	abandoned	along	the	eastern	seaboard	around	the	turn	of	the	20th	century,	followed	by	regeneration	of	the	eastern	forests	during	the	20th	century	(71,	121,	177).	Similarly,	croplands	areas	have	decreased	in	China	and	Western	Europe	(100).	More	recently,	post-Soviet	abandonment	of	agriculture	occurred	in	Russia,	Ukraine	and	Belarus	(133).	Some	abandonment	of	agriculture	followed	by	regrowth	of	forests	has	also	occurred	in	parts	of	Latin	America,	although	rapid	deforestation	continues	elsewhere	in	that	continent	(2).		3. THE	GREEN	REVOLUTION	–	CHANGES	SINCE	1960	Despite	inexorable	agricultural	expansion	over	the	past	300	years,	clearing	has	slowed	since	the	1950s.	Thus,	while	rapid	clearing	of	tropical	forests	and	savannas	for	agriculture	continues	(92),	these	rates	are	small	compared	to	those	affecting	temperate	latitudes	between	1850	and	1950	(Figure	2).			Despite	reduced	clearing	rates,	and	reduced	agricultural	land	area	per-capita	globally,	our	agricultural	lands	have	continued	to	provide	food	and	other	agricultural	products	for	the	rapidly	rising	human	population.	Indeed,	cereal	production	per	capita	increased	from	0.29	to	0.39	tonnes	per	person	between	1961	and	2014	(49)	as	a	result	of	increasing	productivity	of	land	over	time.	The	“Green	Revolution”	is	the	term	commonly	used	to	denote	the	suite	of	technologies	that	enabled	crop	yields	(i.e.,	crop	production	per	unit	area)	to	increase	rapidly	since	the	1950s.			 7		3.1.	The	package	deal:	seeds,	water,	nutrients,	machinery	The	increased	productivity	of	land	was	enabled	by	a	suite	of	technological	advances	that	can	broadly	be	divided	into	three	categories.	First,	advances	in	plant	biology	improved	our	understanding	of	genetics,	development,	and	physiology,	and	their	relationship	to	crop	performance.	Plant	breeders	were	able	to	develop	new	varieties	of	crops	with	desirable	traits	such	as	dwarfing,	high	yields,	and	increased	resistance	to	pests	and	diseases	(45).	These	new	“high	yield	varieties”	of	maize,	wheat,	and	rice	were	rapidly	developed	and	deployed	around	the	world	in	the	1950s	and	1960s	(45)	(Figure	3),	albeit	with	bias	toward	certain	world	regions	(Latin	America	and	Asia,	but	not	the	Middle	East	or	Africa).			Evenson	and	Gollin	(46)	conducted	an	exhaustive	study	of	the	impact	of	international	agricultural	research	on	the	development,	diffusion,	and	influence	of	modern	crop	varieties	over	1960	to	2000.	They	found	that	more	than	8000	modern	varieties	had	been	released	for	11	major	crops	by	2000.		With	the	exception	of	wheat,	farmer	adoption	of	new	cultivars	occurred	soon	after	their	release	(with	the	notable	exception	of	Sub-Saharan	Africa).	Remarkably,	the	use	of	modern	varieties	accounted	for	21%	of	the	growth	in	yields	in	the	early	phase	of	the	Green	Revolution	in	all	developing	countries	between	1961	and	1980,	and	nearly	50%	of	the	growth	in	yield	in	the	late	phase	from	1981	to	2000.			The	second	major	advance	was	the	development	of	the	Haber-Bosch	process,	which	permitted	synthesis	of	nitrogen	fertilizer	from	the	plentiful	nitrogen	available	in	the	atmosphere.	This		 8	discovery	was	a	major	breakthrough	for	agriculture	as	nitrogen	is	a	major	limiting	nutrient	in	soils.	The	application	of	additional	nutrients,	in	combination	with	irrigation,	pesticides,	and	new	crop	varieties,	led	to	a	major	boost	in	crop	productivity	(45).	Total	fertilizer	use	quadrupled	during	1961-2014,	with	the	biggest	increases	in	Asia	(and	also	through	much	of	the	rest	of	the	world),	but	with	little	increases	in	Africa	(Figure	4).	It	has	been	estimated	that	40-60%	of	yields	in	the	USA	and	England	(and	much	higher	proportions	in	the	tropics)	are	attributable	to	commercial	fertilizers	(148).	More	than	a	quarter	of	the	world	population	over	the	past	century	is	estimated	to	have	been	fed	by	synthetic	nitrogen	fertilizers	(44).			The	third	major	advance	was	the	harnessing	of	energy	from	fossil	fuels,	which	enabled	other	technological	advances,	including	vast	improvements	in	the	mechanization	of	agriculture,	as	well	as	the	production	of	synthetic	fertilizers	and	pesticides.	These	developments,	coupled	with	low	(subsidized)	energy	costs,	allowed	farmers	to	efficiently	exploit	(and	over-exploit)	groundwater	resources.	Over	the	1961-2014	period,	the	global	area	equipped	for	irrigation	doubled,	from	0.16	billion	ha	(12%	of	cropland)	to	0.33	million	ha	(21%	of	cropland)	(Figure	4).	Asia	contributed	predominantly	(75%)	to	this	growth.	Irrigated	yields	were	1.6	times	higher	than	rainfed	yields	during	1988-2002,	and	24%	of	the	total	harvested	area	that	was	irrigated	contributed	to	33%	of	the	total	production	(136).		Irrigation	also	enabled	farmers	to	extend	the	growing	season	into	the	dry	season.	Coupled	with	new	shorter-season	varieties	of	crops,	farmers	were	able	to	increase	production	through	multiple	cropping	of	existing	cropland	(34).	Total	harvested	land	area	(i.e.,	area	counted	twice		 9	when	two	crops	are	grown	in	a	season)	increased	faster	than	standing	cropland	area	during	the	1961-2011	period	(34).	For	example,	double-cropped	area	in	Brazil’s	Mato	Grosso	increased	six-fold	from	roughly	0.5	to	2.9	million	hectares	between	2001	and	2011	(141).	On	the	global	level,	these	increases	in	harvest	intensity	contributed	to	9%	of	production	growth	during	1961-2007	(5).			In	summary,	the	Green	Revolution	was	a	package	deal	of	new	seeds	and	new	inputs,	made	possible	by	the	availability	of	cheap	energy.		3.2.	Changes	in	crop	types	and	crop	yields	While	the	Green	Revolution	led	to	general	increases	in	crop	yields,	there	is	massive	variation	in	how	yields,	harvested	area,	and	production	changed	across	different	crop	types	(Figure	5).	For	some	crops	we	saw	marked	yield	increases.	Maize	yield	in	the	USA	remained	around	1.7	tonnes/ha	from	1866	to	1935,	but	has	since	increased	to	~10	tonnes/ha	(168);	similarly	wheat	yield	in	the	UK	remained	around	2	tonnes/ha	until	the	1930s,	but	has	increased	since	to	~8	tonnes/ha	today	(6).			Since	1961,	the	biggest	production	increases	have	occurred	in	oil	crops	(8-fold	increase)	–	especially	oil	palm,	rapeseed	and	soy	–	due	to	increases	in	both	harvested	area	and	yields	(Figure	5).	In	contrast,	production	increases	in	major	cereals	–	rice,	wheat,	and	maize—occured	through	yield	increases,	and	saw	little	change	in	harvested	area.	The	minor	cereal	crops	(e.g.	sorghum,	millet)	decreased	in	harvested	area	by	31%	between	1961	and	2014.	Yet	their	total		 10	production	increased	by	33%,	reflecting	a	93%	increase	in	yields	per	hectare.	While	most	crops	increased	production	between	1961	and	2014,	a	few	did	not,	such	as	oats	whose	production	declined	by	54%.			Green	Revolution	yield	increases	have	not	continued	apace	everywhere.	Overall,	for	24-39%	of	maize,	rice,	wheat,	and	soy	growing	regions	of	the	world	(for	example,	maize	in	Kansas,	wheat	in	France,	and	rice	in	Nigeria),	yields	either	never	improved,	stagnated	or	collapsed	(124),	with	the	situation	being	worse	for	food	crops	(rice,	wheat)	versus	fodder	crops	(maize	and	soy).	Based	on	this	information,	current	yield	trends	were	estimated	to	be	insufficient	to	meet	the	needs	of	the	future	(123),	although	this	is	debated	(77)	as	discussed	further	in	Section	7.1.	One	important	potential	reason	for	yield	stagnation	is	climate	change,	which	is	estimated	to	have	decreased	maize	and	wheat	production	by	3.8%	and	5.5%,	respectively	over	the	1980-2008	period	(96).	However,	stagnating	yields	could	also	be	attributed	to	a	multitude	of	other	reasons	including	loss	of	soil	fertility	and	salinization,	cultivars	approaching	yield	potentials,	pest	and	disease	buildup,	water	scarcity,	and	policies	supporting	environmental	outcomes	over	yields	(124).		Another	important	trend	to	consider	is	changes	in	crop	yield	variability	from	year	to	year,	as	more	volatile	crop	yields	can	lead	to	unstable	farmer	incomes	and	price	hikes	affecting	consumers.	Recent	estimates	suggest	that	year-to-year	variability	in	climate	accounted	for	roughly	a	third	of	the	observed	year-to-year	variability	in	yields	between	1979	and	2008	(122).	But,	at	the	global	scale,	there	is	no	strong	and	clear	pattern	that	crop	yields	have	become	more		 11	volatile	over	time	(78,	109),	although	statistically	significant	increases	in	yield	variability	were	detected	in	9-22%	of	maize,	rice,	wheat,	and	soy	harvested	areas	over	the	1981-2010	period	(78).	There	is	some	evidence	that	climate	trends	are	partly	responsible	for	these	increases	in	yield	variability	(78,	109).		3.3.	Trends	in	crop	diversity	Large	swaths	of	agricultural	land	currently	operate	under	monocultures	or	monoculture	rotations,	with	double	or	triple	crops	per	year	(29).	Increases	in	farm	size	in	upper	income	countries	(97),	and	preponderance	of	monoculture	suggests	that	spatial	diversity	of	cropping	has	also	declined	at	the	landscape	level	(e.g.	1).	Further,	a	number	of	studies	suggest	that	the	industrial	agricultural	transition	led	to	a	reduction	in	area	cropped	with	traditional	varieties	(111).	Nevertheless,	farmers	in	traditional	agro-ecosystems	often	maintain	high	varietal	and	species	diversity	on	their	farms	and	across	communities	and	regions,	although	this	is	higher	for	staple	than	non-staple	crops	(82).			The	current	hotspots	of	crop	diversity	are	concentrated	in	Europe,	Africa,	Asia,	and	West	South	America,	with	low	diversity	in	Australia,	North	America,	and	South	America	(74).	A	global	map	of	the	major	crop	belts	highlights	specialized	locations	for	particular	crop	groups,	such	as	major	cereals,	and	luxury	crops	such	as	cocoa	and	coffee	(Figure	6).	Historically	genetic	diversity	has	been	eroded	by	domestication	of	wild	crop	precursors	(17)	and	major	concern	exists	for	the	erosion	of	wild	types	and	crop	genetic	resources	of	the	world	today	(30).	However,	meta-	 12	analysis	suggests	that	in	recent	decades	genetic	diversity	of	breeder	varieties	does	not	show	clear	downward	trends	(169).			3.4.	Trends	in	livestock	intensification	Alongside	the	Green	Revolution,	there	was	also	a	‘livestock	revolution’,	which	largely	occurred	as	a	result	of	people	consuming	more	animal	products	as	they	get	richer.	Since	incomes	are	increasing	faster	in	low-income	countries,	which	also	often	have	higher	rates	of	human	population	growth,	accelerated	growth	in	animal	numbers	has	taken	place	(35).	In	2014,	the	world	had	23.4	billion	poultry	birds,	1.7	billion	cattle	and	buffaloes,	2.2	billion	sheep	and	goats,	and	0.9	billion	pigs	(49).	The	global	stocks	of	chickens	and	pigs	increased	at	a	faster	pace	than	human	population	between	1960	and	2000,	by	a	factor	of	5	and	2.5,	respectively	(63).	The	number	of	cattle	and	buffaloes	and	sheep	and	goats	have	increased	by	62%	and	64%	respectively	between	1961	and	2014	(Figure	7).	The	largest	increases	were	witnessed	in	Asia	and	Africa	(Figure	7).			In	addition	to	increases	in	animal	numbers,	significant	livestock	intensification	has	also	taken	place.	The	has	largely	been	achieved	by	increasing	animal	densities,	production	units,	the	use	of	concentrated	feeds,	pharmaceuticals	and	vaccinations,	and	improved	efficiencies	(63).	Globally,	62%	less	land	and	46%	less	GHG	emissions	are	used	now	to	produce	one	kilocalorie	from	livestock	than	was	used	in	1961.	This	intensification	of	production	has	occurred	at	the	expense	of	an	188%	increase	in	nitrogen	use	for	increasing	feed	production	(89).	A	shift	from	ruminants	to	more	intensive	pig	and	poultry	production	has	been	partly	responsible	for	this	trade-off;		 13	intensive	systems	need	less	land	and	fewer	livestock	implies	less	methane	emissions,	but	increased	feed	requirements	imply	more	nitrogen	fertilizer	use.	Collectively,	livestock	intensification	has	resulted	in	about	36%	of	the	calories	produced	on	global	croplands	being	diverted	to	animal	feed	(28),	and	the	rise	in	livestock	numbers	have	generated	large	concentrations	of	animal	wastes	(see	Section	3.5).			Intensification	of	livestock	has	nevertheless	occurred	at	different	rates	in	different	parts	of	the	world	and	in	some	cases	has	led	to	reductions	in	animal	numbers.	For	example,	the	US	produces	60%	more	milk	with	80%	fewer	cows	now	than	in	the	1940s	(25).	Significant	intensification	and	also	growth	of	the	livestock	sector	has	occurred	primarily	in	Latin	America	and	Asia.	This	is	in	stark	contrast	with	Sub-Saharan	Africa,	where	productivity	per	animal	has	remained	stagnant	for	decades,	and	all	the	growth	in	the	sector	has	occurred	due	to	increases	in	animal	numbers.		3.5.	Separation	of	crops	and	livestock	Mixed	crop-livestock	systems	are	a	traditional	form	of	agriculture	that	remains	predominant	in	most	smallholder	and	subsistence	farming	systems	in	developing	countries	(73).	The	integration	of	crops	and	livestock	offers	many	management	benefits.	Animals	can	deliver	nutrient-rich	manures	for	the	crops	and	draft	power	for	bed	preparation,	while	crops	and	their	residues	can	be	used	for	forage	(73).	Mixed-crop	livestock	systems	are	nevertheless	on	the	decline	in	many	parts	of	the	world	(114,	131).	This	separation	of	cropping	and	livestock	systems	has	increased	the	problem	of	manure	waste	management,	increased	the	need	to	import	feed	in	livestock		 14	systems,	and	for	chemical	fertilizers	in	cropping	systems.	Recoupling	livestock	and	cropping	systems	offers	a	major	path	to	sustainable	management	in	agriculture	(73).	In	sub-Saharan	Africa,	such	recoupling	has	the	potential	to	nearly	close	the	nutrient	cycle,	returning	up	to	80%	of	the	nutrients	extracted	by	crops	back	into	the	soil	system	(142).	However,	while	mixed	crop-livestock	systems	offer	many	benefits,	they	also	require	higher	capital	to	establish	and	can	be	extremely	difficult	to	manage	(153).		4. EMERGING	TRENDS	AND	FUTURE	PROJECTIONS	It	is	clear	that	the	Green	Revolution	was	a	major	success	in	terms	of	increasing	crop	production.	Crop	production	has	more	than	kept	pace	with	population	growth	over	the	last	50	years,	with	cereal	production	per	capita	increasing	from	0.29	to	0.39	tonnes	per	person	between	1961	and	2014,	even	while	human	populations	more	than	doubled	from	3	billion	in	1961	to	7	billion	in	2014.	However,	many	questions	for	the	future	remain	unanswered.	Will	agriculture	be	able	to	keep	pace	with	future	population	demand?	Will	we	reach	peak	cropland?	And	where	might	we	expect	future	productivity	gains	to	come	from?		4.1.	Projections	of	future	production	and	demand	With	rising	human	populations	and	increasing	per-capita	wealth,	the	demand	for	food,	feed,	and	other	agricultural	products	is	expected	to	increase	in	the	future.	There	are	two	major	studies	that	have	projected	future	demand	to	2050.	Alexandratos	and	Bruinsma	from	the	Food	and	Agriculture	Organization	(FAO)	projected	that	aggregate	agricultural	production	(of	all	crop	and	livestock	products)	will	increase	60%	by	2050,	compared	to	a	2005/2007	baseline	(5).	But		 15	this	aggregate	is	difficult	to	interpret	since	it	includes	multiple	dissimilar	products	that	are	weighted	by	international	prices	(5,	161).	Alexandratos	and	Bruinsma	also	estimated	demand	for	different	commodity	groups	on	a	tonnage	basis,	and	found	that	between	2005/2007	and	2050,	global	demand	for	meat	production	and	sugarcane	&	sugarbeet	production	will	increase	by	76%,	oilcrop	production	by	90%,	and	cereal	production	by	50%.			David	Tilman	and	colleagues	also	projected	future	food	demand	to	2050	using	future	projections	of	population	growth	and	GDP	coupled	with	income-dependent	estimates	of	per-capita	crop	demand	(155,	157).	Their	analysis	projected	a	100%	increase	in	global	demand	for	calories	and	a	110%	increase	in	protein	by	2050	(155).	It	is	difficult	to	compare	the	aggregate	figures	from	Tilman	et	al.	with	those	from	Alexandratos	and	Bruinsma,	because	of	the	different	units	(calories	versus	value-weighted	production)	used	by	these	studies.			However,	as	we	will	discuss	in	section	7.1,	several	recent	studies	challenge	these	estimates	of	future	food	demand.	Current	trends	should	not	necessarily	be	a	guide	to	the	future.	Diets	heavy	in	meat,	oils,	and	sugars	are	a	major	contributor	to	the	global	health	burdens	of	diabetes,	cancer,	and	heart	disease	(157),	and	future	realization	of	these	negative	impacts	may	cause	demand	to	be	much	lower	than	projected.	Policies	that	promote	greater	dietary	reliance	on	grains,	fruits,	vegetables	and	dairy	would	in	turn	also	alter	future	demand	on	global	agriculture.		4.2.	Will	we	reach	peak	cropland?		 16	Peak	cropland	is	a	term	used	to	describe	a	time	when	humanity	might	reach	its	most	extensive	use	of	the	earth’s	land	surface	area	for	agriculture.	A	recent	study	(8)	suggested	this	might	occur	soon.	Analyzing	historical	trends,	they	showed	a	reduction	in	rates	of	cropland	expansion	over	1961-2010,	with	expansions	of	0.24%	per	year	over	the	whole	of	1961-2010,	but	only	0.04%	per	year	during	1995-2010.	They	showed	that	this	was	a	result	of	rising	yields	and	relatively	slower	growth	in	consumption	(than	expected	based	on	changing	affluence)	countering	increased	pressure	on	croplands	from	growth	in	population	and	affluence.	Projecting	forward,	they	showed	possible	scenarios	whereby	cropland	areas	would	peak	and	then	decline.	This	projection	of	peak	cropland	requires	slower	diet	shifts	toward	meat	consumption	and	the	abandonment	of	biofuels	or	other	non-food	uses	of	crops	(8).	It	is	debatable	whether	these	projections	are	realistic.		But	whether	cropland	actually	peaks	or	not,	the	FAO	study	of	Alexandratos	and	Bruinsma	(5)	supports	the	slowdown	of	cropland	expansion.	Recent	historical	trends	suggests	that	77%	of	increased	production	over	the	1961-2005	period	came	from	increased	yields,	14%	from	expansion	of	croplands,	and	9%	from	increases	in	cropping	intensity	(5).	Looking	forward	into	2050,	they	projected	that	80%	of	future	production	growth	will	come	from	yield	growth,	and	10%	each	from	cropland	expansion	and	increases	in	cropping	intensity	(5).	This	suggests	that	the	contribution	of	cropland	expansion	to	production	growth	is	expected	to	reduce	by	~4%	in	the	future.			 17	Notwithstanding	projections	of	future	cropland,	for	environmental	reasons	it	is	imperative	that	we	slow	cropland	expansion,	as	most	of	the	new	lands	available	for	clearing	are	in	the	tropics	and	of	high	carbon	and	biodiversity	value	(120).	The	threat	of	climate	change	is	an	especially	important	reason	to	avoid	deforestation	for	agriculture	(52).			4.3.	Future	growth	through	improvements	in	efficiency	Increases	in	crop	productivity	since	the	Green	Revolution	have	been	driven	in	part	by	increases	in	external	inputs	(e.g.,	water	and	nutrients).	However,	increasingly,	some	of	the	improvements	in	productivity	are	being	driven	by	improvements	in	the	efficiency	of	input	use.	Agricultural	economists	use	the	concept	of	‘total	factor	productivity’	(TFP)	to	examine	the	efficiency	of	input	use.	A	recent	study	(57)	estimated	that	since	1990	overall	contributions	to	global	agricultural	output	has	switched	from	input	intensification	(growth	due	to	addition	of	new	land,	irrigation,	labour,	machinery,	etc)	to	improvements	in	TFP	(more	output	per	input).		Future	increases	in	TFP	can	be	sustained	by	further	increases	in	input	efficiency	-	getting	‘more	crop	per	drop’	of	fertilizers	or	water.	Economists	argue	that	past	successes	have	resulted	from	high	investments	in	research	and	development	in	agricultural	technology,	such	as	witnessed	in	Brazil	and	China	in	recent	decades	(57).	Precision	agriculture	(PA)	and	variable	rate	applications	of	inputs	can	increase	efficiencies	by	applying	nutrients	and	inputs	where	they	are	required	to	achieve	the	best	productivity	gains	on	a	given	piece	of	land	(99).	However,	the	cost	of	obtaining	information	to	enable	TFP	increases	through	PA	are	high	(23),	and	has	slowed	adoption	(19).	Other	potential	opportunities	to	increase	TFP	exist	through	ecological	intensification	(e.g.	68),		 18	conventional	breeding,	or	genetic	engineering	approaches,	although	economic	gains	from	these	advances	are	not	always	clear	(e.g.	128).			5. IMPLICATIONS	FOR	ENVIRONMENTAL	HEALTH		5.1.	Forest	loss	and	fragmentation	Agriculture	is	responsible	for	converting	~30%	of	forests	worldwide	(118).	From	1980-2000,	more	than	half	of	new	agricultural	land	in	the	tropics	came	from	deforestation	of	intact	forests,	and	just	under	a	third	from	disturbed	forest	(61).	Globally	between	2000-2010,	it	is	thought	that	80%	of	deforestation	resulted	from	conversion	to	agriculture	and	grazing	lands	(84).	Just	two	countries,	Indonesia	and	Brazil,	were	responsible	for	over	50%	of	this	tropical	forest	loss	(10).	Agriculture	has	also	massively	fragmented	forests,	with	large	stretches	of	natural	habitat,	such	as	the	Brazilian	Atlantic	Forest,	now	existing	in	degraded	fragments	of	<1000	ha	in	size,	all	within	1km	of	the	forest	edge	(69).			5.2.	Greenhouse	gas	emissions	Agriculture,	including	deforestation	and	land	use	change,	currently	contributes	~22%	of	global	greenhouse	gas	emissions	(GHGe)	(140).	Around	9%	of	GHGe	(4.3	–	5.5	GtCO2	eq/yr)	comes	from	ongoing	deforestation	and	land	conversion	(140).	The	conversion	of	tropical	forests	to	cropland	releases	about	three	times	more	carbon	into	the	atmosphere	compared	to	temperate	forests	(176).	GHGe	from	agriculture	have	changed	over	time	and	space	as	a	result	of	land	use	regime	shifts.	For	example,	in	the	great	plains	of	North	America,	conversion	of	prairie	habitat		 19	and	plowing	were	the	greatest	contributors	of	GHGe	in	earlier	times,	but	livestock	are	now	the	largest	emitters	(110).	Globally	today,	agricultural	management	on	already	converted	lands	are	thought	to	make	up	~13%	of	GHGe	(5.0	–	5.8	GtCO2	eq	/	yr).	Over	a	third	of	this	results	from	CH4	from	enteric	fermentation,	~15%	from	N20	emissions	from	manure	and	synthetic	fertilizer	application,	and	~12%	from	CH4	in	rice	paddies	(140).	Like	carbon	losses	from	deforestation,	management-based	emissions	are	concentrated	geographically	in	particular	hotspots:	CH4	enteric	fermentation	largely	occurs	in	India,	sub-Saharan	Africa,	Brazil	and	W.	Europe	(72),	while	more	than	50%	of	all	N20	emissions	from	nutrient	application	is	from	China,	India	and	the	USA	(175),	and	~60%	of	CH4	from	rice	is	emitted	by	India,	China	and	Vietnam	(26).		5.3.	Biodiversity	loss	Agriculture	affects	biodiversity	through	habitat	replacement	and	management	choices	on	converted	lands.	Across	biomes	and	taxonomic	groups,	conversion	to	pasture	and	cropland	results	in	losses	of	~20-30%	of	local	species	richness	(106).	Biodiversity	loss	is	non-random,	with	marked	declines	in	functionally	important	species	in	ecosystems,	such	as	large	bodied	pollinators	(91).	In	the	tropics,	species	losses	have	shown	to	be	persistent	after	abandonment	of	agricultural	lands	(62).	Fragmentation	breaks	down	essential	plant-animal	interactions	required	for	regeneration	and	persistence	of	native	vegetation	(105),	and	causes	species	diversity	to	erode	over	time	beyond	initial	disturbance	events	(69).		In	addition	to	habitat	loss	and	fragmentation,	agricultural	management	impacts	biodiversity	through	management	choices	such	as	use	of	pesticides,	fertilizers,	and	crop	choice.		 20	Fertilization,	from	nitrogen-fixing	legumes	and	application	of	manure	and	synthetic	fertilizers,	has	contributed	to	a	global	increase	in	nitrogen	(N)	flow	(172).	This	results	in	species	loss	in	terrestrial	(146)	and	freshwater	environments	(103).	The	longest	running	experiment	of	N	addition	at	Rothamsted	Research	station	in	the	UK	shows	that	plant	diversity	rebounds	after	reductions	in	N	application,	although	it	is	unclear	if	recovery	is	possible	in	other	systems	(149).	Intense	management,	which	includes	tillage	and	short	rotations,	also	negatively	affects	soil	biodiversity	and	food	web	structure	(162).		Pesticide	application	has	also	been	linked	to	declines	in	populations	of	non-target	plants	and	insects	(20),	and	the	development,	foraging	patterns,	and	effectiveness	of	bees	and	natural	enemies	of	crop	pests	(36,	143).	Conversely,	management	options	to	increase	the	diversity	of	cropping	systems	have	been	shown	to	improve	both	the	abundance	of	natural	enemies	(94),	and	species	diversity	and	yield	contributions	of	pollinators	(58).	Organic	agriculture	typically	has	higher	species	richness	than	conventional	systems	across	a	range	of	taxonomic	groups	(163),	but	lower	yields	make	it	less	efficient	for	local	species	richness	on	a	per-unit	product	basis	than	conventional	systems	(135).	However,	crop	diversification	closes	the	yield	gap	between	organic	and	conventional	systems	(115),	suggesting	that	this	trade-off	is	dependent	on	management	of	crop	choice	and	scheduling	of	rotations.		5.4.	Soil	health	The	impact	of	agriculture	on	soils	is	tightly	linked	to	land	use	change	and	agricultural	management.	The	alteration	of	vegetative	cover,	through	replacement	of	forests	or	grasslands		 21	with	annual	crops,	influences	infiltration,	erosion,	and	organic	matter	inputs.	Three	major	soil	erosion	linked	agricultural	transitions	have	occurred	–	the	expansion	of	river-based	populations	up	forested	slopes	around	2000	BE,	the	invention	of	sharp	plough	and	deep	tillage	in	16th-19th	Century,	and	crop	expansion	into	tropical	biomes	after	World	War	II	(101).	It	is	estimated	that	by	1990	~15%	of	the	world’s	soils	were	in	some	way	degraded	(108).	Current	rates	of	erosion	on	agricultural	land	are	estimated	to	be	~35	Pg	yr-1	(28	Pg	yr-1	from	water,	~5Pg	yr-1	from	tillage	and	~2	Pg	yr-1	from	wind)	(117)	–	rates	that	are	an	order	of	magnitude	higher	than	that	of	natural	erosion	or	soil	formation	processes	(151).	Land	clearing	for	agriculture	has	also	led	to	soil	degradation	through	other	means,	with	vegetation	removal	in	semi-arid	Western	Australia	resulting	in	recharging	of	ground	water	at	two	orders	of	magnitude	above	the	background	rate,	causing	water	tables	to	rise,	and	salinization	of	~10%	of	agricultural	lands	in	the	region	(60).			The	management	of	soils,	through	fertilization,	tillage,	grazing,	crop	type	and	rotation	planning,	also	has	had	marked	influence	on	soil	health.	The	loss	of	soil	organic	matter,	which	results	from	replenishing	soil	nutrients	with	synthetic	mineral	fertilizers	(N-P-K)	without	replenishing	organic	material,	has	pushed	agricultural	systems	into	a	state	of	rapid	nutrient	cycling	with	high	rates	of	nutrient	loss	(98).	This,	in	combination	with	shorter	rotations	and	loss	of	cover	crops,	has	led	to	increases	in	soil	borne	pathogens	(171),	increases	in	crop	susceptibility	to	droughts	(33),	and	crop	yield	declines	(15).			5.5.	Water	use	&	quality		 22	Agricultural	production	accounts	for	92%	of	the	human	water	footprint,	~77%	of	which	can	be	attributed	to	rainfed	agricultural	systems	(76).	Twelve	percent	of	agricultural	water	footprint	is	in	freshwater,	with	irrigation	accounting	for	~64%	of	withdrawals	worldwide	(41).	Agricultural	water	use	has	had	catastrophic	impacts	on	freshwater	resources,	for	example,	the	complete	loss	of	the	68,000	km2	of	the	Aral	sea	at	the	end	of	the	last	century	(102),	and	groundwater	depletion	crises	in	North	West	India	(127).			Importantly,	water	use	in	production	systems	is	concentrated	in	space	and	by	crop	type.	China,	India,	Pakistan	and	the	USA	account	for	~68%	of	irrigated	water	used,	half	by	India	alone,	with	rice	and	wheat	covering	~69%	of	irrigated	area	and	consuming	~54%	of	irrigated	water	globally	(175).			In	addition	to	effects	on	quantity	used,	loading	of	nutrients	(27),	pesticides	(4),	and	livestock	antibiotics	(83)	from	agriculture	all	have	negative	effects	on	water	quality,	and	pose	public	health	problems	for	humans.	Phosphorous	and	nitrogen	fertilizer	pollution	in	particular	is	notorious	for	forcing	algal	blooms	and	anoxic	dead	zones	in	both	freshwater	(27)	and	coastal	marine	systems	(39),	which	kill	fish	and	reduce	the	palatability	of	drinking	water	for	human	consumption.		5.6.	Summary	of	environmental	impacts	Greenhouse	gas	emissions,	biodiversity	loss,	soil	degradation,	and	water	impacts	of	agriculture	all	negatively	feedback	and	reduce	the	benefits	that	can	be	received	from	the	food	system.	For		 23	example,	agricultural	greenhouse	gas	emissions	contribute	to	an	increase	in	extreme	events	(40)	and	global	crop	production	losses	(93),	soil	erosion	is	leading	to	declines	in	crop	productivity	(151)	and	pollinator	declines	threaten	yields	of	increasingly	pollination-dependent	cropping	choices	(116).	These	negative	feedbacks	within	agriculture	represent	significant	long-term	financial	and	business	risks.	While	humans	have	become	more	environmentally	efficient	on	a	per	capita	basis	at	producing	food	(e.g.	16),	in	aggregate	these	negative	effects	of	agriculture	are	a	major	concern	for	both	the	future	of	agriculture	and	for	the	safe	operating	space	for	humanity	on	our	planet	(144).		6. IMPLICATIONS	FOR	FOOD	SECURITY		6.1.	More	production,	more	calories,	but	less	nutrition	The	Green	Revolution	was	a	massive	success	in	terms	of	producing	calories	for	humanity.	Average	available	calories	per	person	from	crop	and	livestock	production	increased	from	2196	kcal	day-1	per	person	in	1961	to	2884	kcal	day-1	per	person	in	2013.	There	was	more	than	enough	energy	available	in	2013	to	supply	every	person	on	the	planet	(49).		However,	this	energy	is	not	distributed	evenly	and	is	not	as	nutritious	as	it	could	be.	While	the	U.S.A.	had	~3680	kcal	day-1	per	person	available	in	2013,	the	Central	African	Republic	had	only	half,	~1880	kcal	day-1	per	person.	The	number	of	undernourished	in	the	world	remains	unacceptably	high,	with	~795	million	people	still	lacking	sufficient	calories	in	the	world	today	(50).	Furthermore,	2	billion	suffer	from	iron	deficiencies	(164).	Deficiencies	in	iron	and	other		 24	micronutrients	such	as	iodine,	folate,	Vitamin	A	and	zinc,	are	particularly	important	for	human	growth	and	are	together	associated	with	a	range	of	pathologies,	including	cognitive	impairment,	anemia,	blindness,	and	pregnancy	complications	(164).			A	new	problem	exists	today.	From	1975-2014,	the	world	transitioned	from	a	state	in	which	the	prevalence	of	underweight	was	double	that	of	obesity,	to	one	in	which	more	people	are	obese	than	underweight	(38).	Currently	around	37%	of	the	world’s	population	is	overweight	or	obese	(107),	carrying	a	heavy	burden	of	non-communicable	diseases	such	as	diabetes,	heart	disease,	and	morbidity	(150).			The	global	reliance	on	very	few	crops	for	energy,	with	some	84%	of	calories	globally	coming	from	just	17	crops	(175)	is	a	primary	reason	for	the	human	nutrition	gap.	This	is	most	clearly	demonstrated	by	the	South/South-East	Asian	regions	which	have	micronutrient	deficiency	prevalences	of	~30%,	due	to	dominance	of	white	rice	in	the	diet	(13).	Moreover,	in	some	regions,	marked	declines	in	micronutrient	density	in	diets	have	taken	place	in	recent	decades	with	shifts	away	from	fruits,	nuts,	and	pulses	toward	calorie-dense	but	nutrient-poor	foods	(e.g.	maize,	rice,	wheat,	vegetable	oils),	such	as	in	Sub-Saharan	Africa	during	1979	-1993	(13).	Worryingly	there	have	also	been	downward	trends	in	the	nutritional	quality	of	crops,	with	declines	for	some	items	observed	in	U.S.A.	between	1950	and	1999,	due	to	optimization	for	increased	yield	(32).	The	world	produces	22%	less	fruits	and	vegetables	than	required	to	meet	the	World	Health	Organization	recommendation	to	consume	five	portions	of	fruits	and	vegetables	per	day	to	achieve	a	healthy	diet	(137).		 25		6.2.	More	production,	more	calories,	but	access	remains	the	bottleneck	A	recent	study	found	that	improvements	in	caloric	supply	was	not	the	main	cause	of	improvements	in	child	nutritional	status	over	1970-2012	(139);	instead,	dietary	diversity,	sanitation,	clean	water,	and	women’s	education	were	equally	or	more	important	drivers.	The	prevalence	of	malnutrition	despite	sufficient	caloric	availability	at	the	national	and	global	levels	led	the	Food	and	Agricultural	Organization	of	the	UN	to	revise	their	definition	of	food	insecurity	in	1996	to	include	availability,	access,	utilization,	and	stability	(48).	Purchasing	power	is	a	central	component	of	access,	and	reliable	cash	transfer	programs	have	been	shown	to	increase	the	quality	and	quantity	of	food	in	diets	of	the	poor	(159),	as	has	off-farm	income	for	smallholders	(56).	Globally	there	is	an	inverse	relationship	between	GDP	and	the	proportion	of	labor	force	in	agriculture	(152)	—	and	this	lack	of	purchasing	power	means	that	predominantly	farming	countries	are	typically	food	insecure.	Currently	69%	of	the	world’s	farms	exist	in	Southeast	Asia,	South	Asia	and	Sub-saharan	Africa	(97),	with	30%	of	their	produce	coming	from	holdings	<2ha	in	size	(74).	Economists	have	suggested	that	the	solution	to	economic	development	is	agricultural	development,	but	the	directionality	of	this	relationship	on	the	national	level	is	widely	context	dependent	(9).	The	urban	environment	brings	new	access	opportunities	(134),	but	malnutrition	remains	prevalent	in	populations	of	the	urban	poor	(132).	It	has	been	widely	documented	that	populations	that	exist	in	perpetual	states	of	caloric	and	nutritional	food	insecurity	from	poverty	limited	access	are	also	often	those	that	are	most	at	risk	for	acute	food	insecurity	from	extreme	climate	events	or	political	disasters	(12).				 26	6.3.	More	production,	but	less	stability	It	has	been	suggested	that	intensive	crop	production	systems	might	be	more	fragile	than	less	intensive	systems	(93).	Some	evidence	exists	at	the	regional	scale	that	maize	yields	(but	not	wheat	or	rice)	follow	Taylors	power	law,	with	the	variance	in	crop	yields	increasing	non-linearly	with	increases	in	yield	(14).	However	maize	cultivar	trials	do	not	support	the	existence	of	a	trade-off	between	yield	and	stability	(160).	High	input	systems,	with	nutrient	and	water	additions,	also	provide	a	fundamental	means	to	decouple	growth	from	external	stressors,	and	protect,	at	least	in	the	short	term,	against	environmental	water	stress	(24)	or	nutrient	exhaustion	(104).	Pollinator	dependent	plants	typically	display	higher	production	instability	than	non-pollination	dependent	crops	(59),	which	suggests	stability	benefits	for	agricultural	systems	due	to	decoupling	from	nature.	The	increase	in	pollination	dependency	(3),	could	therefore	be	destabilizing	production.	However,	the	benefits	of	decoupling	from	nature	may	fail	when	systems	are	pushed	to	their	limits,	such	as	under	extreme	weather	events	(95),	or,	over	the	long-term,	when	intensified	practices	lead	to	ecosystem	degradation	(156)	which	negatively	feedback	onto	crops.				An	alternative	perspective	is	to	develop	diversified	farming	systems	that	rely	on	a	diversity	of	ecosystem	service	providers	for	production	and	stability	(85,	87).	Ecological	theory	and	experiments	suggest	that	it	is	possible	to	obtain	high	yields	and	reduce	production	variability	simultaneously	(80).	While	diversification	practices	have	remained	at	the	sidelines	of	agricultural	development,	the	few	local	scale	tests	of	diversified	agro-ecological	systems	suggest	stability	benefits:	with	evidence	that	polycultures	increase	the	temporal	stability	of		 27	yields	(64),	increase	pollination	(58),	and	decrease	losses	to	pests	(81).	Facilitation	between	plants	is	maximized	under	environmental	stress	(22),	suggesting	that	diversified	systems	might	actually	increase	their	adaptive	capacity	under	extreme	shocks	(87,	95).	Nevertheless,	while	there	is	some	evidence	that	polycultures	may	increase	supply	stability	by	providing	portfolio	effects	and	increasing	nutrient	and	water	use	efficiencies	(21,	80),	widespread	adoption	on	large	scale	farms	has	not	yet	taken	place.	Links	between	crop	biodiversity	and	stability	at	higher	levels	of	organization	(e.g.	national,	regional,	or	global	levels)	have	not	yet	been	made	empirically.			7. CURRENT	DEBATES	In	this	section	we	review	some	of	the	major	ongoing	debates	in	the	agriculture,	food	security,	and	environmental	literature.			7.1.	Challenging	the	doubling	narrative	In	section	4.1,	we	reviewed	two	future	projections	of	crop	production	to	2050:	a	60%	growth	in	aggregate	production	in	dollar-weighted	terms	from	a	2005/2007	baseline,	and	a	100-110%	increase	in	calories/protein	demand	from	a	2005	baseline.	These	studies	have	resulted	in	a	general	tendency	in	the	literature	to	suggest	that	a	“doubling”	of	food	production	is	needed	by	2050	(54,	123,	154).				 28	Several	recent	papers	have	challenged	this	narrative.	Tomlinson	(161),	referring	to	an	older	FAO	estimate	of	70%	increase	by	2050,	pointed	out	that	it	was	not	a	normative	estimate	(desirable	production	in	2050),	but	rather	a	projection	of	the	most	likely	future	according	to	the	authors.	Moreover,	she	pointed	out	that	the	FAO	estimate	is	not	of	production	or	calories,	but	of	dollar-weighted	aggregate	production	(also	excluding	fruit	and	vegetables).	Alexandratos	and	Bruinsma	take	pains	to	make	the	same	point	in	their	updated	2012	report	(5).	Another	recent	study	also	critiqued	the	doubling	narrative	for	ignoring	baselines	(77),	pointing	that	the	baseline	for	both	the	FAO	and	Tilman	studies	was	~2005,	and	that	production	growth	experienced	since	then	actually	suggests	only	a	25%-70%	increase	is	needed	between	2014	and	2050.			7.2.	Land	sparing	versus	land	sharing	Land	sparing	is	the	idea	that	intensifying	agricultural	production,	and	thereby	growing	the	same	amount	of	food	on	less	agricultural	land,	can	spare	land	for	nature.	The	idea	goes	back	to	Norman	Borlaug,	the	Father	of	the	Green	Revolution,	who	estimated	in	an	Editorial	(18)	that	1.2	billion	hectares	of	land	had	been	spared	from	cultivation	between	1950	and	2000	because	of	yield	increases	over	that	period.	Waggoner	(173,	174)	also	had	proposed	the	same	idea	in	the	1990s	in	an	article	titled	“How	much	land	can	10	billion	people	spare	for	nature?”.			While	the	idea	that	agricultural	intensification	could	promote	nature	conservation	by	land	sparing	originally	came	from	agricultural	scientists,	it	was	picked	up	by	conservation	biologists	in	the	2000s.	In	a	widely	known	article	(66),	Rhys	Green,	Andrew	Balmford,	and	colleagues	proposed	a	theoretical	model	to	examine	the	tradeoffs	between	food	production	and		 29	biodiversity	conservation.	Their	model	suggested	that	the	nature	of	the	tradeoff	is	determined	by	how	the	densities	of	wild	species	and	crop	yields	respond	to	intensification.	Their	proposal	has	been	widely	criticized	since	(53,	65,	170).		One	major	criticism	of	land	sparing	is	that	agricultural	intensification	does	not	actually	result	in	land	sparing	in	practice,	because	intensification	generally	results	in	more	farmers	adopting	the	practice,	resulting	in	increased	(and	not	decreased)	clearing	for	cropland	(7).	Two	studies	(47,	130)	conducted	global	empirical	assessments	and	found	no	evidence	for	land	sparing	in	practice.	However,	neither	study	constructed	a	proper	counterfactual	to	examine	what	might	have	happened	in	the	absence	of	the	Green	Revolution	(75).	Two	recent	studies	that	used	an	economic	modeling	framework	to	construct	a	counterfactual	concluded	that	the	historical	Green	Revolution	did	result	in	land	sparing	(75,	147).	However,	whether	the	spared	land	actually	resulted	in	nature	conservation	remains	an	open	question	(86,	113).	Land	sparing	initiatives	need	to	be	coupled	with	appropriate	policies	to	ensure	that	conservation	actually	takes	place	(86,	113).			7.3.	Genetic	engineering	versus	organic	farming	Another	widespread	and	passionate	debate	in	the	scientific	community	is	on	the	role	of	Genetically	Modified	(GM)	foods	versus	organic	farming	in	navigating	pathways	to	sustainable	food	systems	(55).	As	labeling	of	GM	foods	is	still	not	common	in	most	countries,	organic,	by	expressly	prohibiting	GM,	has	set	itself	up	as	the	only	product	that	ensures	that	consumers	can	have	non-GM	food.	There	is	especially	a	wide	gap	between	scientific	and	public	perceptions	of		 30	GM	foods	(112).	While	both	approaches	could	have	important	roles	to	play	in	different	circumstances	(129),	the	two	communities	have	continuously	clashed.			7.4.	Sustainable	diets	Until	recently,	the	predominant	focus	of	agricultural	science	was	on	supply-side	solutions	to	meeting	the	sustainable	food	security	challenge.	But	a	spate	of	recent	papers	have	pointed	to	the	necessity	and	enormous	leverage	of	demand-side	solutions	(e.g.,	28,	43,	54,	145,	157).	For	example,	Erb	et	al.	explored	500	different	future	scenarios	for	feeding	the	world	in	2050	that	would	also	avoid	further	deforestation.	They	found	feasible	or	probably	feasible	biophysical	options	in	nearly	two-thirds	of	their	scenarios,	but	all	required	either	cropland	intensification	or	a	shift	to	plant-based	diets.	There	was	no	scenario	that	permitted	low-yielding	agriculture	along	with	meat-based	diets.	Cassidy	et	al.	estimated	that	shifting	the	current	mix	of	crops	away	from	biofuels	and	animal	feed	would	itself	increase	global	calories	by	70%.	They	also	calculated	that	this	is	roughly	equivalent	to	all	the	yield	gains	seen	in	maize,	wheat,	and	rice	during	1965-2009;	in	other	words,	shifting	to	vegan	diets	would	be	as	powerful	for	increasing	food	availability	as	was	the	historical	Green	Revolution.	Relative	to	scenarios,	less	extreme	shifts	toward	reducing	meat	consumption,	waste,	and	the	demand	for	non-food	agricultural	products	(e.g.,	cotton)	could	greatly	reduce	the	environmental	impacts	of	the	food	system	(54).		8. CONCLUSIONS:	IMPLICATIONS	FOR	PLANT	BIOLOGY	Humans	have	fundamentally	transformed	global	landscapes	and	shaped	the	distribution	of	plant	life	on	Earth	through	agriculture.	While	advances	in	production	of	food	over	recent		 31	decades	have	kept	pace	with	human	population	growth,	these	advances	have	come	at	a	cost	to	both	the	environment	and	human	health.	Direct	negative	feedbacks	to	agricultural	systems	from	environmental	degradation	now	threaten	long-term	agricultural	productivity.	Coordinated	research	programs	are	needed	to	steer	humanity	into	a	safe	operating	space	for	agriculture.	This	will	require	conjoined	efforts	across	many	different	disciplines	within	plant	biology,	and	collaborations	between	subject	areas	that	have	to	date	remained	pedagogically	and	ideologically	separate.		A	major	challenge	facing	plant	biologists	is	the	joining	of	modern	breeding	approaches	(including	genomic	selection	and	genomic	engineering)	with	agro-ecological	farming	practices.	The	‘package	deal’	of	seeds,	fertilizers	and	energy	that	enabled	the	Green	Revolution,	was	a	massive	success	for	increasing	production	of	a	few	key	crops.	New	modern	varieties	from	industry	have	been	successful	in	increasing	shelf	life	and	catering	to	consumer	tastes	and	preferences.	We	now	need	a	new	package	deal	for	the	future	–	that	is	optimized	across	different	environmental,	social,	and	health	outcomes.	This	will	require	investment	into	understanding	how	diversified	farming	systems	can	be	made	financially	competitive	with	the	current	monocultures	that	dominant	large	swaths	of	the	planet,	not	only	by	developing	plant	materials	that	assist	in	producing	sustainable,	multi-functional	agriculture,	but	also	by	developing	appropriate	policies	that	promote	positive	environmental,	social,	and	health	outcomes.				 32	Breeding	for	agro-ecological	farming	practices,	including	intercropping,	perennial	systems,	and	increased	soil	biodiversity,	should	be	directed	toward	multifunctionality	in	cropping	systems	(e.g.	simultaneous	yield	stability,	microclimate	control,	erosion	control,	water	use	efficiency,	nutrient	use	efficiency,	reduced	pollution,	and	increased	pest	control).	There	is	a	further	need	to	join	these	efforts	with	modern	innovations	in	breeding	of	climate	smart	seeds,	in	improving	photosynthetic	efficiency	(introducing	C4	metabolism	into	C3	crops),	nitrogen	fixation,	increasing	nutritional	content	(i.e.	for	improved	protein	and	micronutrient	supply	to	humans	and	livestock),	and	disease	resistance.	Such	innovations	can	help	avert	yield	stagnation,	adapt	to	changes	in	the	growing	season	and	extreme	weather	events,	close	the	micronutrient	gap,	and	decrease	food	waste.			Co-ordinated	efforts	are	required	to	bring	together	diverse	research	programs	in	agro-ecology	and	plant	breeding,	reduce	agriculture’s	negative	impacts	on	the	environment,	and	ensure	food	security,	at	local,	to	national,	regional	and	global	levels.	This	will	require	re-orientation	of	public	and	private	funding	to	support	the	R&D	needed	for	sustainable	agriculture.	It	will	also	require	input	from	farmers	and	consumers	to	design	systems	to	be	socially	relevant	for	effective	knowledge	transfer	and	adoption,	and	maximum	impact.	History	provides	the	proof	that	this	is	possible	–	the	Green	Revolution	brought	coordinated	international	efforts	across	governments	and	research	institutes	to	increase	productivity,	fundamentally	shaping	human	civilizations	and	the	functioning	of	the	planet	as	we	know	it.	It	is	time	plant	biologists	use	the	lessons	learnt	from	the	historical	trends	and	outcomes	of	agricultural	land	use	to	design	the	next	wave	of		 33	research	geared	towards	developing	both	productive	and	sustainable	agricultural	systems	in	the	future.			ACKNOWLEDGMENTS	Huge	thanks	to	Larissa	Jarvis	for	making	the	figures	that	are	part	of	this	manuscript.	Figure	6	was	a	product	of	Dr.	Chad	Monfreda’s	master’s	thesis,	and	thanks	to	him	for	allowing	us	to	use	a	version	of	it	here.	This	research	was	supported	by	an	NSERC	Discovery	Grant	to	N.	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The	World	Bank	Development	Data	Group		02550751001700 1800 1900 2000Year% of global land areaCropland Forest/Woodland Pasture Savanna/Grassland/Shrubland Other landFigure 1. Global land cover trends from 1700 to 2007. Estimates of cropland and pasture area are based on historical reconstructions using methods described by (27) and (2). Cropland and pasture area were overlaid on a map of global potential natural vegetation (27) to estimate changes in the other land cover categories.OceaniaNorthern AmericaLatin America & CaribbeanEuropeAsiaAfricaCropland area0510151700 1800 1900 2000YearPasture area010201700 1800 1900 2000YearFigure 2. Regional trends in cropland and pasture area from 1700 to 2007. Estimates of cropland and pasture area are based on historical reconstructions using methods described by (27) and (2).Million km2Middle East − North Africa Sub−Saharan AfricaAsia Latin America1970 1980 1990 1998 1970 1980 1990 199802550751000255075100% area planted to modern varietiesMaizeWheatRiceProtein CropsRoot CropsOther CerealsFigure 3. The adoption of modern varieties around the world. Figure adapted from (35), using data presented in (36).OceaniaNorthern AmericaLatin America & CaribbeanEuropeAsiaAfricaTotal area equipped for irrigation0. 1980 2000YearBillion hectaresFertilizer consumption0501001502001960 1980 2000YearMillion tonnes of nutrientsFigure 4. Regional trends in irrigated area and fertilizer consumption from 1961 to 2014. Data were downloaded from FAOSTAT (4). FAOSTAT reports fertilizer data for 1961−2002 separately from the more recent (2002−2014) data. Data was harmonized by calibrating the historical data to match more recent data based on the ratio between the two in 2002. This correction was made by region and nutrient.●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● ●●●●●●●●●●●●●●●●●●0.0 1.0 2.0 3.0 4.0 5.0 6.0| | | | | | |−−−−−WheatRiceBarleyMaizeRyeOatsMilletSorghumCassavaSugar caneBeans, dryChick peasCow peasPigeon peasSoybeansGroundnutsCoconutsOil, palm fruitOlivesSunflowerRapeseedCottonHarvested area ratio (2014/1961)Yield ratio (2014/1961)●●●●●●●●●●CerealsFibre CropsFruitOilcropsOtherPulsesRoots and TubersSugar cropsTreenutsVegetablesFigure 5. Trends in global harvested area and yields from 1961 to 2014. Figure adapted from (160). Vertical axis shows the 2014/1961 yield ratio, while the horizontal axis shows the 2014/1961 harvested area ratio. In cases where crops were absent in 1961, the ratios were calculated using the earliest year with non-zero values. Size of the circle represents crop harvested area in 2014, while color represents major crop groups. Crops above the dotted curve experienced increases in total production from 1961 to 2014, while production declined for crops below the curve.OceaniaNorthern AmericaLatin America & CaribbeanEuropeAsiaAfricaCattle and Buffaloes0. 1980 2000Poultry Birds051015201960 1980 2000Sheep and Goats0. 1980 2000Figure 7. Regional changes in livestock numbers from 1961 to 2014. Data were downloaded from (34).Billion head


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