Environmental Engineering Reference
In-Depth Information
A wide variety of approaches have been used in an attempt to quantify
yield losses for different climate scenarios. Some models represent indi-
vidual processes in detail, while others rely on statistical models that, in
theory, should capture all relevant processes that have influenced histori-
cal variations in crop production. Figure 5.1 shows model estimates of the
combined effect of warming and CO
2
on yields for different levels of global
temperature rise. It is noteworthy that although yields respond nonlinearly
to temperature on a daily time scale, with extremely hot days or cold nights
weighing heavily in final yields, the simulated response to seasonal warm-
ing is fairly linear at broad scales (Lobell and Field, 2007; Schlenker and
Roberts, 2009). Several major crops and regions reveal consistently nega-
tive temperature sensitivities, with between 5-10% yield loss per degree
warming estimated both by process-based and statistical approaches. Most
of the nonlinearity in Figure 5.1 reflects the fact that CO
2
benefits for yield
saturate at higher CO
2
levels.
For C
3
crops, the negative effects of warming are often balanced by
positive CO
2
effects up to 2-3°C local warming in temperate regions, after
which negative warming effects dominate. Because temperate land areas
will warm faster than the global average (see Section 4.2), this corresponds
to roughly 1.25-2°C in global average temperature. For C
4
crops, even mod-
est amounts of warming are detrimental in major growing regions given the
small response to CO
2
(see Box 5.1 for discussion of maize in the United
States).
The expected impacts illustrated in Figure 5.1 are useful as a measure of
the likely direction and magnitude of average yield changes, but fall short of
a complete risk analysis, which would, for instance, estimate the chance of
exceeding critical thresholds. The existing literature identifies several promi-
nent sources of uncertainty, including those related to the magnitude of local
warming per degree global temperature increase, the sensitivity of crop yields
to temperature, the CO
2
levels corresponding to each temperature level (see
Section 3.2), and the magnitude of CO
2
fertilization. The impacts of rainfall
changes can also be important at local and regional scales, although at broad
scales the modeled impacts are most often dictated by temperature and CO
2
because simulated rainfall changes are relatively small (Lobell and Burke,
2008).
In addition, although the studies summarized in Figure 5.1 consider
several of the main processes that determine yield response to weather,
several other processes have not been adequately quantified. These include
responses of weeds, insects, and pathogens; changes in water resources
available for irrigation; effects of changes in surface ozone levels; effects of
