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A Closer Look at Weather Variability and Its Impact on Agriculture
Paul Bullock, Department of Soil Science, University of Manitoba, Winnipeg, MB R3T 2N2
[email protected]
Introduction
Weather variability is something that makes news headlines on a regular basis because it often has
devastating impacts on people and industries in the locations affected. It can take the form of drought,
flood, tornados, hurricanes, ice storms, snow storms and flash floods to name some of the better known of
these events. With improved newscasting and communication technology, the occurrences of these
events is becoming more widely known in the global population in near real-time and lead many people
to believe that the weather is becoming more variable. These impressions may or may not be correct
because of the limitations of human perception and memory. However, providing a more objective
assessment of trends in weather variability is by no means a simple task. Weather variability can occur
over a range of different time scales with varying spatial extent. Many of them are legitimate
meteorological concerns for agriculture but the diversity of variable weather conditions that can impact
the industry creates a challenge to answer a seemingly simple question about the degree to which weather
variability is affecting agricultural production and how it is changing over time. Climate change research
is now incorporating more insight about changing probabilities for extreme weather events. This could be
very useful information for determining future adaptation strategies for agriculture. However, much
uncertainty remains about the quantification of either past or future weather variability, which is a
challenge for determining potential impacts on agriculture and how best to mitigate the effects.
The Varying Nature of Variable Weather
Climate change research focused initially on changes in long-term mean weather values expected in
upcoming decades. Initially, the variability around the mean was not given much attention and in some
cases the range in weather conditions about the mean was simply assumed to remain the same (Figure 1).
However, the response to climate change concerns, notably within the agriculture industry is that a longterm change in average weather conditions was less of a concern than a change which involved greater
variability around the mean, such as that shown in the second panel of Figure 1. Now it seems that
climate change forecasts are raising more concern not just about changing mean values in the future but
also the likelihood of more extreme conditions, such as shown in the bottom panel of Figure 1.
Extreme weather conditions can occur over vastly different spatial and temporal scales. For the purpose
of this presentation, extreme events have been grouped according to the length of time over which they
exert their influence. Short time-scale events can happen in a few minutes to a few hours and include
hailstorms, tornados and extreme rainfall events. Intermediate time-scale events happen over a few days
to a few weeks and include heat waves and drought. Inter-annual time-scale events refer to the year-toyear fluctuations in weather that challenge many industries in addition to agriculture with drought one
year, flood the next or a warm, mild winter followed by one with bitter cold and heavy snow. In each
case the threat is quite different and requires different approaches in terms of adaptation. Are these events
going to become more frequent in the future or perhaps become even more extreme (i.e. more intense
drought, even heavier rainfall, etc.)?
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Figure 1.
Illustrations of the different ways that climate change could increase a mean weather
value (top), increase the extreme weather values (middle) and increase both the mean and
the extreme weather values (bottom). (Lemmen and Warren, 2004. Climate Change
Impacts and Adaptation: A Canadian Perspective http://adaptation.nrcan.gc.ca/
perspective_e.asp)
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Trends in Extreme Weather
Quantifying weather variation and extreme weather events is not a simple task. One has to figure out how
the long term weather records that exist can be used to isolate the events of interest and then determine
how frequently they have occurred in the past. The same analysis can be applied to the output of general
circulation models to determine the predicted frequency of occurrence for the same events in the future.
For short time-scale events, there is very little information for guidance. Hailstorms and tornados occur
over very short time periods and affect relatively small areas. Weather radars now provide better
information on these events, however, previously, we were dependent upon human observation to
determine the frequency of occurrence. We had a much smaller population with more limited
communication ability a century ago, so we cannot determine whether a lower number of occurrences
reported are because the events actually happened less frequently or because there were fewer people to
report them when they did. Future predictions are also not available. The General Circulation Models
(GCMs) that generate future climate projections operate at time steps and spatial scales that are both too
coarse to capture short, intense, small area events like hailstorms or tornados. So, we have no idea what
trend there has been in the frequency of these events in the recent past nor what the future trends might
be.
Heavy precipitation events have received some attention. The last assessment report from the Intergovernmental Panel on Climate Change (IPCC) stated that in mid to high latitudes of the Northern
hemisphere, there has been a 2-4% increase in the frequency of heavy rainfall events since 1950 (Folland
et al. 2001). However, the trend for extreme precipitation events is far from clear and appears to be quite
inconsistent with different locations. For example Akinremi et al. (1999) found that from 1921 to 1995 in
western Canada, there has been a significant increase in the number of precipitation events, mainly due to
an increase in low-intensity events, which is exactly the opposite to what was reported by Folland et al.
(2001). This is similar to the findings from Salinger and Griffiths (2001) for New Zealand that rainfall
indices displayed no trend towards an increase in extreme intensity or frequency and that there were
significant decreases in frequency, and at a few locations, intensity, over the 1951–1996 period
(Figure 2). However, several studies in Europe (Schär et al 1996, Frei et al 1998, Durman et al 2001) are
suggesting an increase of up to several tens of percentage points in the frequency of 30+ mm/day
precipitation events.
For extreme temperature events the results appear to be more consistent for different locations. Easterling
et al (1997) reported that most of the increase in global temperatures in the last half of the 20th century has
occurred during the winter and at night and that summer maximum temperatures in the Northern
Hemisphere show no significant trend. Salinger and Griffiths (2001) found a similar result and, as will be
reported by Jeff Andresen in this conference, the temperature trends for some weather stations around
Manitoba that he recently analyzed show the same pattern. Kharin and Zwiers (2000) have analyzed
daily temperature projections from the GCMs for the end of the 21st century and find that projected
temperature trends are consistent with those seen in the recent past. The extreme daily maximum
temperature (the 1 in 20 year extreme value) is expected to increase between 4 to 8oC for most of the
world by 2100 but the 1 in 20 year extreme value for daily minimum temperature is expected to increase
by as much as 20oC in some locations. Since the daily minima are increasing more rapidly than the daily
maxima and are expected to continue to do so, we also expect that the daily range in air temperatures will
decrease.
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Figure 2.
Trend in nationally averaged (New Zealand) frequency of daily rainfall exceeding the
1961–1990 mean 95th percentile (extreme frequency) for rain days with rainfall totals of
1+ mm, over the period 1951–1996, for 24 stations (Salinger and Griffiths, 2001)
Drought is a particular concern for western Canada and as well as any other location where it is a regular
feature of the climate. The IPCC report stated there was likely to be increased summer drying over most
mid-latitude continental interiors and associated increased risk of drought (Folland et al. 2001). This is a
real concern for western Canadian agriculture because years with widespread drought are clearly evident
in the long term yield series for most crops. However, inter-annual variability of precipitation makes
drought prediction difficult and complicates the determination of past trends. The results are variable
with location. Zhao et al (2005) reported that the total area of land affected by drought in China by
decade increased from the 1950s to the 1970s but then declined in the 1980s and 1990s. Salinger and
Griffiths (2001) reported that the lengths of the longest annual dry spell in New Zealand have mainly
declined in the last half of the 20th century (Figure 3). Thus, the trend in drought extent is variable.
Inter-annual variability of weather has been clearly evident in Manitoba in recent years. The dry and
warm weather of 2003 was followed by near record cold summer temperatures in 2004 then near record
wet weather in 2005 and back to warm and dry in 2006. The same fluctuations are common in other areas
of the world including China (Figure 4) and Australia (Figure 5). In Australia, drought is very common
and the years of extreme dryness such as 1982, 1994 and 2002 (Figure 5) are the same years with poor
crop yields (Figure 6). Australia is being hit hard by drought yet again in 2006. Therefore, the frequency
and intensity of drought years is a big concern. However, the trends that are evident over the past century
are not necessarily indicating that the situation is becoming worse. In fact, yield trends continue to rise in
many areas of the world including western Canada.
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Figure 3.
Trend in the annual maximum number of consecutive dry days (dry spells) where the
rainfall total is less than 1 mm over the period 1951–1996. The sign of the linear trend is
indicated by +/− symbols. Bold indicates statistically significant trends (95%), with •
symbol indicating trends close to zero (Salinger and Griffiths, 2001).
Figure 4.
Average annual precipitation from 160 Chinese weather stations (Zhao et al, 2005).
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Figure 5.
Eastern Australia average rainfall (Wright, 2005).
Australia Average Wheat Yield
Wheat Yield (T ha-1)
2.5
2.0
1.5
1.0
0.5
0.0
1960
Figure 6.
1970
1980
1990
2000
2010
Average annual Australian wheat yield (USDA, Foreign Agricultural Service, PS&D
Online - www.fas.usda.gov/psd/). Note, especially the low yields in 1982, 1994 and
2002 which correspond to the same low rainfall years shown in Figure 5.
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Summary
Will future weather be more variable? The answer to this question is very uncertain. Small scale, short
time-frame events are completely unknown since we have no past data to use for trend analysis and no
models that can forecast at this scale. Past trends for other weather extremes are providing mixed
messages. Winter and overnight temperature values are increasing fairly consistently around the globe
while daily maximum and summer temperatures are not showing a significant trend. Thus, daily
temperature range is becoming narrower. However, the past trends for extreme precipitation events are
quite variable with increased risk of heavy precipitation in some areas and a declining trend in others.
The risk of increasing drought severity and frequency is also uncertain. Long-term trends indicate that
drought continues to occur periodically, often with a group of occurrences over a period of time followed
by a time frame with relatively few occurrences then back again.
Certainly the extreme weather events associated with variable weather have a significant impact on
agriculture. However, at this time, there is no clear evidence to suggest this will become more serious in
the future. Crop yields are currently increasing and hopefully, will continue to do so as farmers further
improve their management skills. We may want to consider developing varieties with tolerance to a
wider range of weather conditions. However, it is possible that this type of strategy may sacrifice some of
the potential gains during high production years. Varieties with more consistent average yields may
prevent some of the significant yield losses in unfavorable growing seasons but could also display
reduced yield potential in favorable growing seasons.
References
Akinremi, O.O., McGinn, S.M. and Cutforth, H.W. 1999. Precipitation trends on the Canadian prairies.
J. Climate 12: 2996-3003.
Durman, C.F., J.M. Gregory, D.C. Hassell, R.G. Jones and J.M. Murphy 2001. A comparison of extreme
European daily precipitation simulated by a global and a regional model for present and future
climates. Quart. J. R. Met. Soc
Easterling, D.R., B. Horton, P.D. Jones, T.C. Peterson, T.R. Karl, D.E. Parker, M.J. Salinger, V.
Razuvayev, N. Plummer, P. Jamason and C.K. Folland, 1997: Maximum and minimum
temperature trends for the globe. Science, 277, 364-367.
Folland, C.K., Karl, T.R., Christy, R., Clarke, R.A., Gruza, G.V., Jouzel, J., Mann, M.E., Oerlemans, J.,
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Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge
University Press, p. 99–182; [on-line] http://www.grida.no/climate/ipcc_tar/wg1/048.htm
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Schär C., C. Frei, D. Lüthi and H.C. Davies 1996. Surrogate climate change scenarios for regional
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Wright, W.J. 2005. Agricultural drought policy and practices in Australia. In Natural Disasters and
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