Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 56

4.3.4 Effects on Growing Seasons

Rise in the mean temperature above a threshold
level will cause a reduction in agricultural yields.
A change in the minimum temperature is more
crucial than a change in the maximum tempera-
ture. Grain yield of rice, for example, declined by
10 % for each 1 °C increase in the growing sea-
son minimum temperature above 32 °C (Pathak
et al. 2003 ). The climate change impact on the
productivity of rice in Punjab (India) has shown
that with all other climatic variables remaining
constant, temperature increases of 1 °C, 2 °C, and
3 °C would reduce the grain yield of rice by
5.4 %, 7.4 %, and 25.1 %, respectively (Aggarwal
et al. 2009b ).
Even moderate levels of climate change may
not necessarily confer benefi ts to agriculture
without adaptation by producers, as an increase
in the mean seasonal temperature can bring for-
ward the harvest time of current varieties of many
crops and hence reduce fi nal yield without adap-
tation to a longer growing season.
In areas where temperatures are already close
to the physiological maxima for crops, such as
seasonally arid and tropical regions, higher tem-
peratures may be more immediately detrimental,
increasing the heat stress on crops and water loss
by evaporation. A 2 °C local warming in the mid-
latitudes could increase wheat production by
nearly 10 %, whereas at low latitudes, the same
amount of warming may decrease yields by
nearly the same amount (Fig. 4.5 ). Different
crops show different sensitivities to warming.
It is important to note that the large uncertainties
in crop yield changes for a given level of warm-
ing (Fig. 4.5 ). By fi tting statistical relationships
between growing season temperature, precipita-
tion, and global average yield for six major crops,
Lobell and Field ( 2007 ) estimated that warming
since 1981 has resulted in annual combined
losses of 40 million tons or US$5 billion (nega-
tive relationships between wheat, maize, and bar-
ley with temperature).
Whether crops respond to higher temperatures
with an increase or decrease in yield depends on
whether they are determinate or indeterminate
and whether their yield is currently strongly lim-
ited by insuffi cient warmth. In cold regions very
near the present-day limit to arable agriculture,

One of the most important effects of an increase
in temperature, particularly in regions where agri-
cultural production is currently limited by tem-
perature, would be to extend the growing season
available for plants (e.g., between last frost in
spring and fi rst frost in autumn) and reduce the
growing period required by crops for maturation.
The effects of warming on length of growing
season and growing period will vary from region
to region and from crop to crop. For wheat in
Europe, for example, the growing season is esti-
mated to lengthen by about 10 days per °C and in
central Japan by about 8 days per °C (Yoshino
et al. 1988 ). In general the conclusion is that
increased mean annual temperatures, if limited to
two or three degrees, could generally be expected
to extend growing seasons in high mid-latitude
and high-latitude regions. Increases of more than
this could increase evapotranspiration rates to a
point where reduced crop-water availability
begins to limit the growing season.

4.3.5 Reduction in Crop Yield

Higher growing season temperatures can signifi -
cantly impact agricultural productivity, farm
incomes, and food security. In mid- and high lati-
tudes, the suitability and productivity of crops are
projected to increase and extend northwards,
especially for cereals and cool season seed crops.
Crops prevalent in Southern Europe such as
maize, sunfl ower, and soybeans could also
become viable further north and at higher alti-
tudes. Here, yields could increase by as much as
30 % by the 2050s, dependent on crop (Ewert
et al. 2005 ). For the coming century, large gains
have been simulated in potential agricultural land
for the regions such as the Russian Federation,
owing to longer planting windows and generally
more favorable growing conditions under warm-
ing, amounting to a 64 % increase over 245 mil-
lion hectares by the 2080s. However, technological
development could outweigh these effects,
resulting in combined wheat yield increases of
37-101 % by the 2050s (Ewert et al. 2005 ).