How much Land Can Ten Billion People Spare for Nature in an Uncertain Climate

Paul E Waggoner
The Connecticut Agricultural Experiment Station, New Haven

Thank you for inviting me to deliver the second Kuehnast lecture in 1994 and attend Don Baker’s graduation.

Population, the context of climate change

I garden because I like to dig.  Farmers farm to feed over 5 billion people now, and ahead in an uncertain climate‑‑feed ten billion.  The world's farmers and I share this: We want to do something.  So I shall push beyond Malthusian gloom to hope.  I shall push on to doing something about feeding multitudes, coping with climate change and sparing Nature.

Near the time Jefferson sent Lewis and Clark along your border with Dakota, Malthus predicted starvation or other disaster would limit population.  The world of Malthus held 1 billion, of Earl Kuehnast's war 2 billion, and of today 5 to 6 billion.  Despite slowing multiplication and Cairo resolutions, 10 billion is a round number for the population to accompany doubled CO2 and a changed climate during the 21st Century.[1]  The context of climate change? Ten billion people.

Ten times the 1 billion that made Malthus gloomy triggers the cry, "Mobs will crowd out Nature".

Jeremiads in the Old Testament condemned sinners to wilderness, and medieval folk feared bandits and trolls lurking in wilderness.  But before the French revolution, Rousseau wrote of the nobility of unspoiled children and Nature.  In the era of Lewis, Clark and then Mankato massacre, the poetry and essays of Wordsworth, Thoreau, and Emerson exalted Nature.  Darwin gave wild animals standing, as a judge would say, arguing they are our relatives.  Modern people add arguments of ecological services, preserving genes, and holding carbon in trees.  Malthus was gloomy about food, and we are gloomy about Nature.

Jefferson's Imperative

Gloom caused a speaker at the AAAS meeting last winter to advocate a world population of 2 billion, requiring 3 billion of today's 5 billion depart.

In a gentler mode, legislatures proclaim and patrons purchase nature or wilderness reserves.  But proclamations from Geneva or Washington and deeds on paper in London or San Francisco scarcely hinder squatters driven by unemployment and hunger.

Representing the American states in France in 1785, Thomas Jefferson followed the French court to Fontainebleau.  After talking with a poor woman along a path, Jefferson wrote James Madison,

I asked myself what could be the reason that so many should be permitted to beg who are willing to work in a country where there is a very considerable proportion of uncultivated lands? These lands are kept idle mostly for the sake of game. ...

Whenever there is in any country, uncultivated lands and unemployed poor, it is clear that the laws of property have been so far extended as to violate natural right.  The earth is given as a common stock for man to labour and live on.  If we [allow land to be appropriated], we must take care that other employment be furnished to those excluded  ... If we do not, the fundamental right to labour the earth returns to the unemployed. ...

The French soon restored the fundamental right with the guillotine.  Reserves persist only when land can be spared.

Spared causes me to write what I call Jefferson's Imperative. Let area cropped be A, Population be P, Calories per capita per day be F for food and feed, and Yield per area per year be Y.  Then

A = P * F * 365 days/yr

The residuum after A is cropped will be spared for other uses, including Nature.

10% fewer people, 10% fewer calories per capita or 10% more yield per hectare (ha) spares 10% cropland.  Changing one factor is as good as another is.

How much cropland?

In 1990, roughly 30% of the world's land was forest, 30% grass, and 30% the miscellany called Other.  Crops grew on 10%.  Crops cover about twice the area of the 48 United States or the Amazon basin.

Wilderness accounts for the 27% without such development as crops, settlements or roads.  The World Resources Institute (WRI) estimates that Wilderness, which overlaps the classes of the pie, occupies between two and three times as much land as crops.

For three centuries cropland advanced and forest retreated. Multiplication of people between 1980 and 1990 lowered the Food and Agricultural Organization (FAO) estimate of cropland per person to 0.27.  Pessimists call less cropland per person, narrowing the resource base; optimists call it high yields sparing land.

Food per capita

Jefferson's Imperative says fewer calories per capita also spares land.  Are people getting fewer calories?

In fact, FAO reports calories per capita rising about 0.4% per year in rich countries.  In poor countries, they report twice as high percentage rise in calories per capita.

What improved the food supply despite less cropland per capita?  World farmers raised yields even faster than people multiplied, simultaneously sparing land and improving food supply per capita.  A happy demonstration of Jefferson's Imperative.

Ten times the population that frightened Malthus, twice today's population living in an uncertain climate, however, makes the middle of the 21st Century a new game. Given 90% of global land stands uncultivated, people will expand cultivation if they are unemployed and poor.  Jefferson's Imperative, enforced by the guillotine, says it is their natural right.  A climate change that lowers yields as population grows will expand the acreage tilled, sparing less for Nature.

Cropland stands near 1.5 billion ha now, yielding an average slightly less than 2 tons or 8 million calories per ha in food, feed and fiber. In tons/ha yields range from 1 for arid African wheat to 8 for Irish wheat and U.S. corn.  The curves are Jefferson's Imperative for 3,000 and 6,000 cal/day for ten billion people. The upper, green curve depicting 3,000 calories per day passes through zero cropland spared because today's yields of 2 on today’s 1.5 million ha would furnish ten billion people each 3,000 cal/day.

Although 3,000 on the plate sustain people, the curve pertains to all crop production, and we use crops for many things besides food.  For example, we feed some to animals, sip some as coffee and wear some as cotton.  Because today's cropland grows about 6,000 original cal/day for today's population, diet and uses, I concentrate on the lower, red curve of 6,000 cal/day for tomorrow's ten billion.

Jefferson's Imperative says that a yield of 2 tons/ha and 6,000 cal, the red curve, will cause cropland to expand to nearly 3 billion ha, taking 1.5 billion from Nature and elsewhere.  Lower yields will take more.  At yields near 4 or twice today’s, however, the red line crosses into positive sparing.  Yields of 4 on today’s 1.5 billion ha cropland would feed tomorrow’s ten billion 6,000 calories.  As farmers lift yields higher, more and more of today’s 1.5 million ha cropland is actually spared for Nature. 


The Big Question.

Will changing climate lower yield and expand cultivation?  I remember my promise to push on to doing something (Yes, doing something) about feeding multitudes, coping with climate change and sparing Nature.

A panel of CAST (The Council on Agricultural Science and Technology)[2] kept the spotlight on doing something by asking this Big Question:

For a warmer planet with more people, more trade, and more CO2 in the air, can U.S. farming and forestry prepare within a few decades to sustain more production while emitting less and stashing away more greenhouse gases?

The first phrase begins with the warming of climate change.  It implies rearranged precipitation.  It emphasizes the simultaneous changes of more people and more trade.  NAFTA, GATT and technology will accelerate trade.  "More CO2 in the air" reminds that the greenhouse gas CO2 is the stuff of photosynthesis and crop yields.

The second phrase, "Can U. S. farming and forestry prepare within a few decades" sets a deadline for adaptation matching the period visualized for climate change.

"Sustain more production" quits scenarios of farmers and scientists dumbly suffering.  By the time climate changes, the production of today will be irrelevant.  To maintain and improve the human condition, the farmers of 2050 AD must more than double the present crop.  And they must keep it up.

The final phrase is "While emitting less and stashing away more greenhouse gases". Farming does emit the greenhouse gases CO2, methane and nitrous oxide.  Burning fossil fuels dwarfs the climate forcing by U.S.   But all the same, regulators might order farmers to emit less.  Also, they might order farmers and foresters to stash away in plants and soil carbon from the air.  Policies might encourage farmers to grow bio fuel to displace fossil fuel.  For a reality check, recall that in 1919 a quarter of cropland fueled horses and mules.

The Big Question asked by CAST brings home that the search for adaptations will be harder and cope with more than climate.  The search will never end.  But hardships are reasons for prompt action‑‑not transfixed dismay.

The active ingredient is water.

Returning to the first phrase, I ask whether warmth, water or CO2 will be the preeminent factor affecting crops in a new climate in the mid 21st Century.

I begin with CO2.  Optimists see a stimulation of photosynthesis from more CO2, and pessimists see little benefit or even harm.  Their disagreement commands newspaper headlines.

That more CO2 in the air enhances growth has been known for nearly two centuries, and growers enrich CO2 in greenhouse air about three fold to speed growth.

But will more CO2 outdoors help ten billion spare more land for Nature?  For a few weeks after the CO2 around Alaskan tundra was raised, photosynthesis sped up but subsequently slowed to that in normal CO2. In contrast, the speeding of photosynthesis by extra CO2 in a Chesapeake wetland continues.

A major force in resolving how CO2 affects crops is Idso, protege of Don Baker.  When Idso doubled CO2 around orange trees in Arizona, he tripled growth, and the effect continues unabated.

While research sorts out the effects of CO2 on a global scale, the best refuge continues to be the speeding of photosynthesis by CO2 as shown by Akita and Moss from Minnesota. In present CO2, photosynthesis is faster in the C4, or maize, class than in the C3, or wheat, class.  Above 300 ppm, raising CO2 1% speeds the photosynthesis of the wheat class 0.4% and of the maize class 0.2%.

Carbon dioxide in the air likely will continue rising 1 to 2 ppm per year or one third while population grows to ten billion.  Measurements of photosynthesis suggest that this will increase photosynthesis and perhaps crop growth some 10% in crops such as wheat and 5% in those such as maize.  Although rising CO2 may not materially augment, it will certainly not limit food production for ten billion people nor lessen the land that they can spare for Nature.

What will warming cause?  Bjorkman observed two C4 plants, one adapted to cool and one to warm temperatures.  A few degrees warming harms these species little.  In fact, warming helps the photosynthesis of the adapted species up to 45 C or 113 F.

Physiologists worry that warming will lower yields by speeding crops through their cycles.  Certainly warming speeds up mathematical models that employ degree‑days.  Although questions will continue whether faster passage through life will prevent exploiting the advantage of longer warm seasons, warming affects photosynthesis little and effects of warming pale beside the greater effects of water.

Frank and colleagues dried and re-wet wheat during the heading of winter wheat.  As the water potential fell below the wilting percentage at -15 bars, photosynthesis promptly fell to a quarter.  Watering above the wilting percentage recovered photosynthesis, eventually to about 3/4 of the initial rate.

One can call the jump in photosynthesis through the threshold of the wilting percentage a non-linearity.  Put one way, if climate change passes through a non-linearity, it will affect matters profoundly.  Put another way, discovering non-linearities far exceeds the importance of precise quantitative observation between non-linearities.

So while global warming grabs headline, the active ingredient of any climate change will be water supply, locally.

Uncertain climate.

Because the active ingredient of climate change is water, scenarios of water supply in real fields are essential.  The water that controls photosynthesis is not a global average but instead that around the roots in a specific field. Although average global warming may be easier to project, only projections of local water supply matter.

Puzzled by a patient's complaints a doctor said, "Why don't you have fits.  I can cure fits."  Reckoning average global warming is as futile as asking the patient to have fits because the doctor can cure fits.

Three world class computers projected changes in summer precipitation.  For your Great Lakes region, computer GISS projected 8% wetter while GFDL projected 20% drier and OSU no change.  For your neighbors in the Great Plains, projections varied from 28% drier to 10% wetter.

And so it goes for other seasons and locales.

Beyond the variation among computers, I offer two more demonstrations of uncertainty.  In their 1983 appraisal the National Academy of Sciences (NAS) saw so little progress in computation that they simply repeated a 4-year-old appraisal: Doubling CO2 would warm the global average 2.5 to 3.5 C.

In 1992 the NAS again appraised computations.  Instead of narrowing the range of uncertainty after nine years, they broadened it from 2.5 to 3.5 out to 1 to 5 C.  This month an IPCC report says 1.5 to 4.5 C. 

The second demonstration of uncertainty comes under the heading of crying wolf.  During the severe drought of 1988, computers of global warming proclaimed before Congress and in the press that global warming had arrived.  Five years later, in 1993, rain inundated you.

Before answering the Big Question of how agriculture should prepare for changing climate, the panel of CAST concluded that the heart of the issue was uncertainty.  So when I come to action, I shall propose action, not for a neat and naive scenario of climate, but for climate that is uncertain.

Dumb farmer scenarios

A challenge in weighing impacts is compounding climate with other changes.  To grasp the changes that next century might accompany climate change and 10 billion people, look back to 1919 when Earl Kuehnast appeared on an Iowa farm only 4 years before I appeared on another.  A year would pass before hybrid corn would be invented at The Connecticut Agricultural Experiment Station and decades before Iowans were fully weaned off Reid's Yellow Dent.

A book from Iowa entitled “In No Time at All” shows horses pulled 1919 plows, farmers walked behind, and fuel for horses and mules occupied a quarter of cropland.  The environmental issue was flies, and a quarantine sign nailed to the front door arrested scarlet fever’s spread.  High tech meant a cow tester in a buggy, and a cream separator on the back porch.  Yes, in no time at all.

The ice on Minnesota lakes grows thick enough to hold a fisherman, it melts and lilacs bloom in the cycle Earl Kuehnast observed.  But in no time at all, farmers farmed in new ways.  And in no time at all they will adopt still newer ones.

Changes since 1919 teach that we must go beyond computation of the effect of 1 to 5°C warming on today's corn.  We must foresee farmers' adaptations to warming amidst a flood of technological, social and economic changes.  If we ignore these adaptations, we shall write a "dumb farmer scenario".  Imposing the climate near the middle of the next century on the activities of 1994 implies dumbly ignoring new circumstances for decades.

Simulating effects of future weather on present farming does provide a worst case scenario, the case of dumb farmers.  The CAST report on climate change cited two climate change scenarios for doubled greenhouse gases along with more CO2 in the air.  All climate scenarios specified in the underlying Environmental Protection Agency (EPA) study called for warming--but different precipitation.  Compared to the present yields, the scenarios produced yields ranging from mild gains to severe losses.  Again, the uncertainty of the impacts of climate change, even on imaginary farmers dumbly suffering.

Encouraged by report of the Panel on Policy Implications of Greenhouse Warming (NAS) report naming adaptation among the options for global warming, investigators have turned their minds to adaptation, to farmers like the real, smart ones.  The so-called MINK study, MINK for Missouri, Iowa, Nebraska and Kansas, is a landmark about adaptation.  Investigators at Resources for the Future led by Rosenberg used the drought of the Dust Bowl, a real if temporary climate change, as an analog of climate change.

During 1951-80 unirrigated corn yielded near 8 tons/ha.  The MINK investigators envision yields rising to about 13 tons by 2030 if climate doesn't change.  A yield of 10 is their estimate of what the analog climate, the Dust Bowl,  would do to unadapted farming growing corn in 300 ppm CO2.  Adding the benefit of more CO2 produces 11.  Finally, they envision CO2 plus the adaptations on the right would raise yields over 12, higher than now and about the same as 2030 without climate change.

Recently, Rozenzweig and Parry reported an ambitious, international estimate of the effect of a warming of 4 to 5 C, at the top end of IPCC's 1.5 to 4.5 C.  They asked agronomists around the world to opine what adapted farming would suffer.  Although the authors warned that poor countries would likely suffer, they estimated this severe warming would change global production little.

Nordhaus and others at Yale sought a measure of real rather than conjectured adaptation.  Arguing that land prices reflect the best adaptation farmers can make, Nordhaus related land prices rather than yields in U.S. counties to climate.  For a U.S. value, the impact on counties must be weighted.  Weighted by crop area, 2 C warming lowered value of land by 4 to 5% of annual revenue.  On the other hand, weighted by crop value rather than area, warming has little effect.  That is, with real adaptation, warming harms extensive crops somewhat, but affects valuable crops little.

The generality is: For each uncertain scenario, someone can compute its impact on fictitious farmers who would dumbly suffer the change without adapting.  And someone can speculate about a world of real, adaptable farmers.

The portfolio

I, however, promised to do something about feeding multitudes, sparing Nature and coping with climate change.

Answering their Big Question, the CAST panel found the crux of climate change is uncertainty.  They proposed a prudent nation will hedge for the risks of uncertainty by assembling a diverse portfolio of ten agricultural assets and assure flexibility to use them.  The assets:

Land, Water, Energy, Physical infrastructure, Genetic diversity, Research, Information systems, Human resources, Institutions, Market.

First I illustrate diversity and flexibility with the assets of land, trade and institutions.  Then I turn to water--the active ingredient of climate change, and to research--your business.

In land, diversity brings diverse climates.  Good weather and improving climate in one region can balance losses in another.

Trade exchanges the fruit of diverse climates.  As the invisible hand that coordinates adaptation, the world market is a priceless climate change asset.  Freeing trade imparts the essential flexibility.

Governments exemplify institutions.  If their policies are harmonized, their subsidies of, say, declining crops or water will not hobble adaptation.  Neither will their disaster relief discourage adaptation to a new climate or water supply.


Now, water.  Water already limits cropping in places, other demands are cutting into the water left for irrigation, and irrigated area is shrinking in all U.S. regions but the East.  Also, worldwide irrigated area barely expands 2% annually. So the possibility that the climates of some regions may become drier heightens the urgency to grow more value in crops per unit of water.

Ahmad found Pakistani farmers fertilize to raise the yield from three levels of irrigation.  Lifting the yield from each level of irrigation fertilizer improved water use efficiency.  Ahmad wrote, "Water cannot be considered to have become a real constraint to meeting the world food supplies as long as there is the scope for manipulation of the various underlying factors for further increasing the agricultural production".


Water use efficiency is the kg of crop grown per cubic meter of water evaporated.  Observations assembled by Jensen’s showed the gilt edge of research in employing the asset of water.  From the 1960s into the 1980s, evapotranspiration ET from Texas grain varied little because the sparse foliage of the early years evaporated about as much water as the abundant foliage of later years did.   So rising yield from the 1960s into the 1980s raised water use efficiency.  Raising yield raises water use efficiency.

CAST wrote that on an inflexible globe, research is the gilt-edge investment that will be asked to do much of the work of adaptation.  The right response to an uncertain future is technical change widening the possibilities of substitution among natural resources and between resources and technology.

Profitable research

Given this central importance of research and their responsibility for knowledge of the landscape and farming the agricultural experiment stations in 1994 published “A guide for agricultural experiment stations to the questions and research anticipating global climate change”.  The guide focuses on weather, its relations to crops and exchanges of energy, water and gas between crops and atmosphere.  It puts central questions about climate change that Stations have a comparative advantage for answering.

I report five of the questions, beginning with a micrometeorological one.

How much does cutting or planting forests change the energy budget and so the climate of a region?

Lists of activities affecting greenhouse gas condemn deforestation and urge reforestation.   By and large, however, they overlook the full effect of vegetation on the energy budget of the earth.

Tall forests lessen the reflection of solar radiation.  Gash and colleagues computed that less reflection raises by a quarter the energy for evaporation and heating.  Other scientists computed that cutting boreal forests and exposing more reflective snow overwhelmed the expected warming from the carbon released from the forest. Tall, dark trees evaporating water drawn by deep roots may affect climate in more, surprising ways than foreseen by a single-minded focus on carbon in their trunks.

Agricultural meteorologists hold a comparative advantage for removing uncertainties about the energy budgets of forests versus short vegetation.

Although the label global warming focuses attention on temperature, the lever of climate change that harms or helps crops and forests will be water. So the question

How will changed climate and CO2 plus changed vegetation alter evapotranspiration?

An open question is whether more CO2, which is feared as a greenhouse gas, will narrow foliar pores enough to change evapotranspiration.  In the long-run outdoors? Up, down or inconsequentially?  Plant physiologists need professional help--from meteorologists.

Can the microclimatic models for the soil-plant-atmosphere continuum of plots be put together to simulate regions in general circulation models (GCMs)?

The tools for anticipating climate change by calculation, GCMs, must simulate processes from the small scale of evaporation from fields to the great ocean currents.  Having invested years in models of the soil-plant-air continuum, agricultural meteorologists should now realize a return by furnishing an underpinning for GCMs that is realistic enough to reflect the physics of the landscape but simple enough to put in GCMs.

The NAS Panel on Policy Implications of Greenhouse Warming wrote, “Monitor climate, forecast weather”.  They hoped to relieve the obsession with forecasting the climate a century from now while neglecting to observe whether climate is changing.  So the question:

Are climate, radiation and pollution changing in the countryside?

Agriculture employs many of the nation's environmental observatories in the countryside.  The countryside is exactly where global changes must be detected.  It is where crops and forests grow.  So agriculture has an advantage--and responsibility--to assure the comparability and continuity of their observations of weather, radiation and pollution.  Starting the St Paul Climatological observatory in 1960 and--even harder--keeping it going, Don Baker has done the Lord's work.

The second phrase of “Monitor climate, forecast weather” means a farmer must pick a variety to plant this spring or pesticide to apply this week.  The climate of the next century matters little, but the weather of the coming week or season matters greatly.  Monitoring the excursions of weather during the evolution of a new climate and forecasting them even shortly beforehand will make adaptation logical rather than haphazard.  The question:

How can monitoring, forecasts and timely information mitigate the harm or heighten the benefits of climate change?

Although the National Weather Service regularly publishes medium to long range weather outlooks, Kuehnast Lecturer Changnon found agricultural managers use them infrequently.   They don't use today's predictions in quantitative decisions.  Paradoxically, they call for forecasts with accuracy and lead-time already available.

Growing computer power plus knowledge of such things as El Nino will surely improve forecasts for weeks and seasons.  Agricultural meteorologists should invent ways for farmers to exploit the improving forecasts.

The analysis and dissemination of weather information to make it profitable are deficient, whether the information is about climate, current weather or forecasts.  Being close to farmers, scientists at Stations have a special advantage, even a responsibility, for remedying this deficiency that could hinder warning and adapting to climate change.  Minnesota's preeminence in combining and using information in soil specific farming gives it a special responsibility.

Removing the stumbling block that inhibits the profitable use of present information is as important as more accurate forecasts.

Spread across the climatic zones of NAFTA from arctic to tropics plus a bent to unite theory with practice imparts advantages for research in climate change to U.S., Canadian and Mexican Stations.


So in the end, I kept my promise.  I give you something to do about feeding multitudes, sparing Nature and coping with climate change.  Jefferson's Imperative says: If cultivation is not to expand, crowding out Nature, farmers must lift yields despite uncertain climate.

Malthusians fear we have exhausted our ingenuity, that the gap between average and maximum yields is closing.  The record of farmers winning the Iowa Masters Contest, however, shows otherwise. The world average corn yields at the bottom are not closing on the Iowa average in the middle.  And the Iowa average is not closing on the winners of the Iowa Masters contest at the top.  The opportunity to raise yields remains open.

Other pessimists fear that growing susceptibility to weather will nullify research to improve yields.  Duvick, however, found otherwise.  He grew corn cultivars from old Reid's Yellow Dent to the newest hybrids. He grew them together in dry 1991, flooded 93 and favorable 92.  The advantage of the new varieties was as big in bad years as good.  Weather did not cancel what breeding accomplished.

I give you this final, Indian encouragement that Jefferson's Imperative rules: Yield really spares land.


The orange area at the bottom shows the actual ha of wheat grown in India after about 1960.   The sum of the two orange and green areas is the ha actual production would have taken if yields had not risen.  So rising yields since 1960 spared the top area, far more than Minnesota’s 19 million ha of non-Federal land and 64 times Yellowstone’s land.[3]

To feed ten billion and spare nature in an uncertain climate, hope makes a better partner than despair and action a better response than paralysis.  We have things to do.


Further  reading

Ahmad, M. 1987. Water as a constraint to world food supplies. In W R Jordan (ed.) Water and water policy in world food supplies. College Station TX: Texas A&M Univ. Press. p 23-27.

Akita, S, and D N Moss. 1973. Photosynthetic responses to CO2 and light by maize and wheat leaves adjusted for constant stomatal apertures. Crop Sci 13:234-237.

Bjorkman, O, M Badger and P Armond. 1980. In N C Turner and P J Kramer (eds.) Adaptations of plants to water and high temperature stress. Wiley Interscience, NY. P. 231.

Council for Agricultural Science and Technology (CAST). 1992. Preparing U. S. agriculture for global climate change.  Ames IA: Council on Agricultural Science and Technology. Report 119.

Duvick, D N. 1996. What is yield? In Edmeades, G O, M Banziger, H R Mickelson and C B Pena-Valdivia (eds.).  Developing drought- and low N-tolerant maize. El Batan Mexico:CIMMYT. #784 tough varieties.

Easterling, W E, M McKenney, N J Rosenberg, and K Lemon. 1990. A farm- level simulation of the effects of climate change on crop productivity in the MINK region. DOE/RL/01830T-H8 (TR052D). Washington, DC: U. S. Department of Energy, Carbon dioxide Research Program,

Experiment Station Committee on Organization and Policy. 1994.   A guide for agricultural experiment stations to the questions and research anticipating global climate change.  Griffin, GA:  Georgia Experiment Station.  59 pp.

Frank, A B, J F Power and W O Willis. 1973. Effect of temperature and plant water stress on photosynthesis, diffusion resistance, and leaf water potential in spring wheat. Agron. J 65:777-780

Gash, J H C, and W J Shuttleworth. 1991. Tropical deforestation: albedo and the surface-energy balance. Climatic Change 19:123-133.

Hamilton, C. 1994. In no time at all. Iowa Heritage Collection. Ames: Iowa State Univ. Press.

Panel on Policy Implications of Greenhouse Warming. 1992. Policy implications of greenhouse warming. Washington DC: National Academy Press.  1992. Adaptation, pp. 499-657.

Waggoner, P E. 1994. How much land can ten billion people spare for Nature? Ames IA: Council for Agricultural Science and Technology. Report 121. 64 p.  On line at

[1] On August 20, 2002 the New York Times reported, “United Nations demographers who once predicted the earth's population would peak at 12 billion over the next century or two are scaling back their estimates. Instead, they cautiously predict, the world's population will peak at 10 billion before 2200, when it may begin declining.” (Added on August 22, 2002).


[2] At the end of this manuscript, I list “Further  reading”.  The booklet referenced here is “CAST. 1992. Preparing U. S. agriculture for global climate change.”

[3] The diagram of Indian wheat sparing cropland was first published in “How much land can ten billion people spare for Nature?” in 1994.  In August 2002, the version above was updated to 2001.