Future challenges

In much of this book, we have focused on current farm practices: how they affect our air and how, in turn, the changing atmosphere affects them. We have summarized estimates and processes that describe current agroecosystems. But we know that agricultural systems are always evolving; that many of the systems we have struggled to understand here may be obsolete just years from now. Thus, it is important to at least point to some impending changes and speculate about their possible effects.

One important factor is the continuing drive for higher agricultural productivity. As global population climbs, demand for farm products increases. Moreover, the economic survival of farms often depends on ever-higher output of products. The resulting gains in productivity may have some benefits; for example, they may help to build soil C by producing more crop residue. At the same time, however, the higher yield targets may require more fertilizers and other inputs that could release more greenhouse gas.

Economic factors are another consideration. As cost of inputs and price of products change, farmers alter their farming systems to maintain profits. Consequently, the area of land devoted to certain crops changes from year to year, which affects the release of greenhouse gases and other emissions. Perhaps the most dramatic example is the recent shift toward livestock-based systems. This change has far-reaching implications. On the one hand, higher livestock numbers usually mean more land in forages, which reduce atmospheric CO₂ by storing more C in soil. At the same time, however, increased livestock numbers can lead to more release of CH₄, N₂O, and NH₃. If the trend toward higher numbers of farm animals continues, then many of our current emission estimates will need to be revised and new measures of reducing emissions may be needed.

But it is not only the farming systems that will change. Environmental conditions that affect farms will themselves change over the next decades. Many scientists believe that climate will be noticeably altered by the greenhouse effect over the next decades; even small changes in temperature or precipitation would affect Canadian farms. Another important environmental characteristic has already changed measurably: the CO₂ concentration, already about 30% higher than in pre-industrial times, will likely double within the next century. Since CO₂ is the raw material for photosynthesis, this increase may have important effects on crop yield. Some even predict an increase in yields through "CO₂ fertilization." Other environmental conditions may change as well, including concentration of ground-level O₃ in populated areas, and the intensity of UV-B radiation. These changes, some of which are not easily predictable, may affect the way we farm in the next century. As well, they will alter the emissions from farms, thereby continuing the cycle between farms and the atmosphere.