The conclusions noted in the previous section point to a climate change future for U.S. agriculture resources that is pretty much a business-as-usual situation at least for the next several decades. As such, there might appear to be no major changes on the climate change horizon that would seriously impact U.S. agriculture on a national scale, even though regional-scale winners and losers can be expected.
Unfortunately, this reasonably favorable scenario is subject to substantial uncertainties due to many factors that are difficult to incorporate adequately into the analyses. Almost every first-level statement on impacts is subject to major qualifications. These qualifications in turn are subject to subsequent qualification--like taking apart a set of nested Russian dolls. Although a more extensive list could easily include more than a score of factors that need more consideration, the following items illustrate where the above estimates may go seriously astray:
- Climate change estimates of global mean temperature and global mean precipitation are widely accepted.
- But almost everyone agrees that regional and local estimates of climate change are highly uncertain.
- Such estimates are obviously critical for determining impacts on agriculture.
- Limited basis for focusing on a 2x CO2 climate (one in which the concentration of carbon dioxide is doubled from today's level).
- We may very well be forced to cope with a 3x to 4x CO2 (or greater) climate scenario over the longer term.
- Nature may not cooperate; the climate system may produce major surprises.
- If ocean currents are disrupted, for example, some part of the world may actually need to deal with a substantially cooler climate.
- Climate may not change in a "linear" fashion.
- This assumed gradualness underlies the assumption that society will have time to adjust.
- Some evidence suggests that climate has changed in a rather abrupt and chaotic fashion on the past; under such circumstances the impact on
agriculture could be severe.
- If extreme weather events were to become more frequent, the outlook for agriculture could quickly become much less favorable.
- GCM simulations of today's (regional) climates are often inaccurate.
- Direct projections of GCM regional climates are seldom used.
- Changes in the perturbed simulations relative to the control run are generally applied to historically observed weather data.
- Simulations are often inconsistent with respect to whether mean precipitation levels will rise or fall on a USDA regional scale.
- Crops respond to increased temperature in complex ways--above a minimum level, response to a rise in temperature tends to be positive up to a characteristic optimum range.
- But when exceeded, crops tend to respond negatively, resulting in a steepening drop in net growth and yield.
- The accelerated growth due to warmer temperatures results in quicker maturation and can actually reduce the yield of annual crops.
- Since many weeds associated with major C4 crops (maize, sorghum, sugarcane, and millet) are C3, the weed/crop competition for such crops may favor the weeds.
- Feeding requirement for insects may increase.
- Higher CO2 may increase the carbon-nitrogen (C:N) ratio in crop leaves, stimulating the feeding of some insects.
- Some insects may need to eat more C-enriched leaves to gain adequate nutrition.
- Impacts depend on how the hydrological regime changes in detail.
- Total seasonal precipitation, its within-season pattern, and its between-season variability.
- Crops are also vulnerable to a rise in the daily rate and seasonal pattern of potential evapotranspiration.