P.J. Gregory, J.S.I. Ingram Agriculture, Ecosystems and Environment 82 2000 3–14 5
most 81 of the increased production in developing countries is expected to come about through intensi-
fication via increased yield of individual crops 69 and intensified use of existing land 12. In South
Asia there is little new land that can be brought into cultivation so that almost all of the increased produc-
tion will be through increases in yield and crop inten- sification e.g. more rice Oryza sativa crops per year
or rice grown in rotation with an upland crop such as wheat Triticum aestivum or a legume. In Central
and South America, a large amount of new land is still available for cultivation and a substantial percentage
29 of the projected increases in production might be met in this way although the costs of introducing
infrastructure such as roads is high. In Africa too, there is potential for expansion of the cultivated area
but the land is often far from centres of population and infrastructure is poor. Yields will still need to
increase substantially on land currently under crops.
The principal conclusion of many such analyses is that yields per unit area will need to increase substan-
tially to meet demand. In fact, this will be a continua- tion of the past trend which has seen the area
harvested per capita decline from about 0.24 ha in 1950 to 0.125 ha in 1993, while global cereal yield
has increased from 1.2 to 2.8 Mg ha
− 1
over the same period Dyson, 1996. Even if the increased produc-
tion is shared between extensification and intensifi- cation, global average cereal yields will need to rise
from their current 2.9 to about 4.2 Mg ha
− 1
by 2025.
3. Global change as a complicating factor in meeting demand
It is well realised that the changes in the major global change drivers Walker and Steffen, 1999
will affect different aspects of production systems in different ways, sometimes beneficially, sometimes ad-
versely. Changes in climate and climate variability, in nitrogen deposition and in atmospheric composition
are of primary interest as these all have a direct bear- ing on agriculture and forestry. While some aspects
of the direct impacts of individual drivers acting on components of production systems are now relatively
well understood e.g. elevated CO
2
on plant physi- ology, or temperature on insect pest dynamics, the
interactive aspects and their effect on global change
Fig. 1. Annual variation in rice yields in Japan
d
and South Korea
h
from 1969 to 1995; from Hayashi and Jung 2000.
impact is still far from clear. This makes the devel- opment of strategies to adapt to, and therefore cope
with, global change impacts difficult. For example, it is well known that temperature can affect the develop-
ment, growth and yield of many crops but quantifying how a particular climatic sequence will influence pro-
duction in the field is very difficult. Hayashi and Jung 2000 sought to explain the observed variability in
rice yields in Japan and South Korea Fig. 1. Linear trend analysis indicates a 2.4 per annum increase
for Japan and a 4.9 increase for Korea for the period 1969–1996. However, the unusually cool summers of
1980 and 1993, when summer temperatures were up to 3.7
◦
C cooler than the mean, caused considerable deviation from this trend. Hayashi and Jung 2000
analysed anomalies of summer temperature to produce a statistical regression with yield that explained 68
of the variance. However, while such relations can be used retrospectively to explain yields, their ability to
predict is often poor and the current uncertainties in global climate models for predicting future climates
at temporal and spatial scales appropriate to cropping systems mean that predicting the consequences for
future production is imprecise. Reilly and Schim- melpfennig 1999 reviewed global climate change
impacts on agriculture and concluded that the current uncertainties about future climate change mean that
there is little ability to predict which regions will benefit or lose beyond the generalisation that tropical
regions are likely to suffer disproportionately.
Even under constant environmental conditions, in- creasing production to meet the growing demand will
6 P.J. Gregory, J.S.I. Ingram Agriculture, Ecosystems and Environment 82 2000 3–14
be a complicated and challenging task. It will be com- plicated still further because global change means that
optimum crop and forest management will be difficult to plan.
4. Environmental consequences of increased production
