How to achieve food security in the face of climate change?

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rates of fossil fuel consumption. The presently unknown balance, andmost importantly its sign, between the large fluxes represent considerable uncertainty for climate security (Table 1).

Soil and Food Security

The late 20th and early 21st centuries have been, for industrialized countries, an unprecedented era of increasingly low food prices (39). There are numerous factors that may reverse this trend— such as increased global demand; climate change (40); and competition for soil by nonagricultural uses, such as biofuels or urbanization. Abundant energy has been the key driving force behind our ability to maintain food production apace with an expanding population that is estimated to reach 11 billion by 2100 (41). Low-cost energy, which led to advanced agriculturalmachinery replacing human labor, is causing migration to urban cen- ters. Energy is used to replace the soil nutrients removed or lost by agricultural perturbations of soil. Energy transforms atmospheric N2 to the bioavailable NH4 fertilizer through the energy- intensive Haber-Bosch process—constituting the first and most important green revolution (42) (Fig. 4A), one that allows us to feed the increas- ing global population (Fig. 4A). Before the indus- trial fixation of N, any increase in food production for a given country was largely due to increased soil used for production (43), and only after the advent of N fertilizer (Fig. 4A) did yields per area of major crops begin their upward trajectory (43).

Last, energy is essential to mine and transport essential plant nutrients, such as P andK, that can only be accessed from limited geological reservoirs. Agricultural soil erosion is one of the most de-

structive human perturbations to soil sustain- ability. Given little opportunity or desirability for further agricultural expansion, stewardship of our existing domesticated soil is essential for sus-

tained human prosperity. Yet despite the impor- tance of soil conservation, the implementation of practices to minimize soil erosion has not fol- lowed apace with the severity of the problem. The most pervasive mechanism of soil erosion is via water. Before European contact and the removal of native vegetation by plowing and cultivation, the geologicalmechanism of soil erosion onmost

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Fig. 3. A cause-effect diagram of the major soil-atmospheric CO2 feed- back processes.The values in parentheses, in the circles, are the approximate pool sizes (Gt) of C. A solid arrow represents a direct response (e.g., as CO2 increases, temperature increases); an arrow with a circle indicates an inverse response (e.g., as temperature increases, soil carbon storagedecreases). Dashed lines are for processes that are less well understood. (A) Environments with adequate moisture. (B) Water-limited environments. In (A), the a-b-c loop is a positive feedback process (even number of inverse relationship arrows), one especially important in regions of melting permafrost.The d-f-c loop is a negative feedback, one with less certain feedbacks between vegetation and soil (f-g loop) and temperature (h-i).The strength of the a-b-c- versus d-f-c loops on soil carbon pools will likely determine whether soil carbon losses in northern latitudes serve

as amajor source of CO2 and CH4 this century, a balance that also hinges on the abilityof the soils to supply nutrients to plants (arrowg) in order to respond to the increases in CO2 (arrow d). In (B), regions with limitedmoisture, the strengths of vegetation response toCO2 (d) and soil carbon response to temperature (b)may be weakened (thinner arrow). In addition, the vegetation response to increasing temperature may become negative. These figures reveal the importance of soil carbon to the global CO2 balance this century, as well as the uncertainties in the strength and direction of important processes. Arrow references are as follows: a, (60); b, (32, 33); c, CO2 loss by respiration is the overwhelming pathway of C removal from soils; d, water efficiency response in (61); f, soil C is the balance between plant inputs and decomposition losses; g, not well constrained, but see discussion in (62); h, (63); i, e.g., (64).

Fig. 4. The post–World War II rise in fertilizer production and cost coincides with the spike in global human populations. The growth in world population in the late 20th century (65) mirrors the increasing use of industrially derived N fertilizer. Before N fertilization, formajor U.S. crops (66),wheat and other grain yields per acre increased only following World War II, coincident with the rise in the use of N fertilizer produced by the Haber-Bosch process. Prices have sharply risen (67) because global demand is straining the supplies.

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