Global Change and Agriculture

Global warming

Evidence of global warming:

• Temperature records-most of the increase has been in night temperature
• Retreat of glaciers; decreased snow and ice cover
• Measurable rise in sea level
• Increased heat content of oceans
• Increased plant growth (Myneni et al. 1997)

• Increased values of NDVI (normalized difference vegetation index) detected by remote sensing
• Increased biomass deposition in European forests
• Increased recent tree-ring growth in Mongolia
• Upward migration of plants on European mountain tops

The increase in plant growth is likely due to longer growing seasons; high latitude winter temperatures increased up to 4 C in the winter.

Nicholls (1997) attributes 30-50% of the increased wheat yield in Australia since 1952 to decreased frequency of frost.

Presumed causes of global warming:

• Greenhouse gases-CO₂, CH₄, N₂0 (nitrous oxide), CFCs (chloroflurocarbons)
• Land-use changes. Deforestation
• Increased fire frequency

That greenhouse gases have caused global warming as not been "proved", there are still valid disagreements. Robinson et al. (1998, unpublished paper privately distributed) dispute that any global warming has occurred in response to increased CO₂. It is accurate to say that there is currently a strong concensus among scientists that changes in atmospheric chemistry are affecting climate in predictable and understandable ways.

Conclusions of IPCC (Intergovermental Panel on Climate Change) that temperatures are increasing are based on recent long-term studies such as those of Crowley (2000).

• Use proxy data (tree rings; coral growth; isotopes in ice cores) to reconstruct climate for last 1000 years.
• After correcting for effects of variation in solar radiation and volcanism, there is a pronounced 20^th Century increase in temperature consistent with warming by greenhouse gases.

General Circulation Models (GCMs)

Processes modeled include (1) atmospheric circulation and (2) ocean currents. Models describe transfer of energy and moisture between adjacent grid cells.

Grid cell sizes are large:

• Goddard Institute for Space Studies (GISS): ca. 550 cells, 7.83^o Lat. × 10^o Long.
• Geophysical Fluid Dynamics Laboratory (GFDL): ca. 1300 cells, 4.4^o Lat. × 7.5^o Long.
• (United Kingdom Meterological Office (UKMO): ca. 1150 cells, 5.0^o Lat. × 7.5^o Long.

Results:

• Temperatures increase everywhere; models show general agreement.
• Higher temperature increases expected at higher latitudes
• Growing season lengths increase at higher latitudes
• Percipitation increases globally but not everywhere. Models disagree, especially regionally and seasonally, on expected precipitation changes

Uncertainties in models:

• Cloud cover in upper atmosphere
• Feedbacks-positive or negative.

Negative feedback is possible through enhanced plant growth [the "CO₂ fertilization effect"].

A potential positive feedback involves decreased albedo at high latitude which could increase warming; subsequent increased decomposition of SOM would accelerate CO₂ increase further increasing warmer.

More recently, Energy Balance Models (EBMs) have developed which better simulate the exchange of heat between the oceans and the atmosphere, and better account for heat storage in the oceans.

Effects of [CO₂] on Plant Growth

Gross photosynthesis increases and photorespiration decreases.

Stomatal resistance increases (stomates close partially in response to increased [CO₂]), transpiration therefore decreases, and water-use efficiency increases (since stomatal closure affects transpiration rates more than CO₂ uptake rates).

However, Hall (2001) cautions that stomatal effects observed in the well-mixed air of a curvette, with controlled temperature and humidity, may not resemble stomatal responses in a crop canopy.

C3 vs C4 plants: Growth of C3 plants would be enhanced more than that of C4 plants [why?].

Hu et al. (1999) suggest that elevated [CO₂] may increase root exudation, in turn increasing mycorrhizal colonization, root nodulation, and BNF.

Tree-ring studies sometimes fail to demonstrate increased tree growth after the industrial revolution in response to an increase of atmospheric CO₂ from 280 to 360 ppm (DeLucia et al., 1999), suggesting that plants may acclimate to higher atmospheric [CO₂].

Interactions need to be considered:

• [CO₂] and other resources. For example, if N is limiting, increased [CO₂] may not increase crop growth.
• [CO₂] and environmental influences (especially temperature).

Feedbacks between aboveground vegetation and belowground microbes could be positive or negative, and may depend on the temporal scale:

• Initial feedbacks could be positive if C exudates stimulate mineralization and BNF
• Later nutrients could become immobilized in less-available pools creating a negative feedback

Scaling Issues

Scaling refers to the extrapolation of observations or processes from (usually) smaller temporal or spatial scales to larger temporal or spatial scales. For example, extrapolating from instantaneous photosynthetic rates to seasonal crop yields. The problem with scaling is that different processes may be working at different scales.

For Example: Processes controlling temperature at different temporal scales:

• hourly-- rotation of earth on its axis
• monthly-- rotation of earth around sun
• millennial--eccentricities in earth's orbit, solar cycles, greenhouse gases

Example: upscaling (zooming) photosynthesis (A) to yield (Y)

• Units of Y are Mg ha^-1 y^-1
• Units of A are mmol CO₂ dm^-2 s^-1
• Assume Y = kA, that is that we can scale directly from A to Y. What are the units of k? Is k in fact a constant?
• The units of k must be: (Mg crop/mmol CO₂) (dm² leaf/ha^-1) (s crop^-1) (crops y^-1). [Work this out for yourself if you don't see how I arrived at this.]

Looking at the units of k, we can see that k is not a constant:

• (Mg crop/mmol CO₂) would be a crop-specific constant, depending on the chemical composition of the crop biomass.
• (dm² leaf/ha^-1) is LAI, integrated over the growing season.
• (s crop^-1) is the length of the growing season. If increased temperature accelerates crop phenology, the growing season would be shorter. On the other hand,
• (crops y^-1) is the number of crops per year, which, in tropical latitudes, could increase with shorter crop growing seasons.

Therefore we cannot scale directly from A to Y without taking into account how crop biomass (for example, higher or lower protein content), LAI, and growing season might change.

Other ecosystem-scale processes may also be altered (weed growth and competition, insect herbivory, nutrient cycling, soil erosion, etc.) which further complicates scaling from physiological processes to ecosystem responses.

Affects of Global Change on Agriculture

The overwhelming evidence from (short term) experiments with increased [CO₂] (either greenhouse or FACE-free atmosphere carbon dioxide enrichment-studies) is that biomass and/or seed production increases with increasing [CO₂]. These studies are almost always done with (1) no temperature increase, and (2) optimum levels of other resources, especially N and water.

[One interesting conclusion we might draw is that much of the crop yields experienced in the past 50 years must be due to increased [CO₂] and not just breeding and improved management, as usually assumed.]

Predicting the general affects of both changing climates and changing [CO₂] is usually done by modeling; the approach involves:

• Combine GCMs and crop growth models.
• Run Crop Growth Models with gredicted climate based on effective [CO₂] doubling, both (1) without CO₂ fertilization effects, and (2) with CO₂ fertilization effects.

Selected Results:

• Shift in geographic zones (e.g. corn belt could shift northward). Southern hemisphere agriculture is likely to benefit less [Why?].
• Yield declines are expected in the absence of CO₂ fertilization effects; realization of the effects is thus critical to future food production.
• C3 crops usually expected to have yield increases; C4 crops may have yield decreases

Rosenzweig and Parry (1994) provide one of the best analyses:

Methodology:

• Used outputs from 3 GCMs-GISS, GFDL, and UKMO
• Used crop models for wheat, maize, rice, and soybean
• Applied one or more models to crops in 18 countries (just 1 site in Africa!)
• Looked at 2 levels of Grower Adaptations:

Level 1- Shift planting date ± 1 month, increased irrigation where crops already irrigated; switch crop varieties.

Level 2-Larger shifts in planting date; increased fertilizer application, expand irrigation; develop new crop varieties.

Results were linked to a World Food Trade Model

Results:

• Large yield decreases without CO₂ effects for all crops
• With CO₂ fertilization effects soybean and wheat increase for all GCM scenarios but rice and maize decrease for all three-largest decreases for maize
• All crops decrease under UKMO scenario. What actually will happen is critical!
• Changes less severe with adaptation levels. Under UKMO scenario Adaptation Level 2 does not eliminate yield decreases.
• Food production is projected to increase in the developed world; decrease in developing world.

URL: http://ag.arizona.edu/~spmcl/lecturenotes/globalchange.htm