Further Look at Growing Season Length

In Tebaldi et al. (2006) growing season was defined in terms of "thermal" characteristics. Obviously, though, moisture and precipitation changes will influence greatly the ability of cultivating crops in areas where structures for irrigation are absent, or water resources are subject to competing demands.

In this section we modify the definition of growing season by including conditions that are related to the available moisture. In addition to requiring mean temperature to be above 5°C we consider a climatology of daily values of the ratio between actual precipitation and potential evapotranspiration. Before taking the ratio, we compute a 10-day moving average of the daily values for each quantity. The growing season starts after the first 5 consecutive days with ratio greater than 0.8 and ends when encountering eight consecutive days with a ratio of less than 0.5 (Thornton et al., 2006). To compute potential evapotranspiration we adopt the formula from Hamon (1961), which uses daily temperature and day length (fraction of hours in the day with sunlight, function of the calendar day and the latitude) as input.

Once the climatological values of the ratio are computed for both present-day climate and future climate, the growing season start- and end-dates are identified and a number of statistics can be computed. Changes in growing season length are of immediate interpretation, but other aspects of a changing climate within the season, like frequency of temperature extremes or dry spells within the growing season, may be evaluated and compared.

Here we comment on some differences between the results based on a thermal definition of growing season length, which suggest a generalized expansion of the growing potential over the calendar year, and the moisture-based definition, which delivers opposite results at least in those regions that are projected to warm the most. In these regions, in the absence of significant changes in precipitation, the increase in potential evapotranspiration causes a moisture deficit that limits the extent of the growing season.

We choose to present some results as area averages over a set of regions of specific relevance for agriculture. They are

In Table 3.2 we list means (and ranges in parentheses) for projected changes in growing season length under scenario A1B. We list changes by mid-century and end of the century, and for the definition based on evapotranspiration ("ET-based") of growing season, together with the more traditional thermal definition. Units are in number of days.

Table 3.2 Mean number of days (ranges in parentheses) for projected changes in growing season length under scenario A1B, using ET and thermal based definitions (see text for details)

ET-based

ET-based

Thermal

Thermal

Region

2046-2065

2081-2100

2046-2065

2081-2100

SAF

-22 (-57,-8)

-29 (-46,-14)

0 (0,1)

0 (0,1)

EAF

2 (-18,18)

4 (-16,25)

0 (0,0)

0 (0,0)

SAH

-10 (-27,12)

-17 (-45,4)

0 (0,0)

0 (0,0)

NIN

5 (-33,41)

11 (-46,62)

9 (3,15)

13 (6,21)

ECH

-5 (-26,23)

-8 (-56,20)

16 (7,23)

22 (8,35)

USC

-4 (-68,41)

-4 (-58,57)

28 (9,62)

41 (18,82)

WEU

-2 (-22,32)

-2 (-30,43)

17 (5,27)

23 (8,37)

AUS

-6 (-43,68)

-8 (-31,15)

0 (0,3)

0 (0,3)

Clearly, including moisture in the definition of growing season changes drastically the nature of future projections for this set of regions. We see a shift from optimistically positive numbers (or no change) across the board to predominantly negative numbers in the mean projections, but with a considerable range of uncertainty spanning both sides of 0 in the individual models' projections. Evidently the effect of temperature is no longer that of simply prolonging mild conditions, conducive to growing crops, but in these instances of exacerbating moisture deficits. The precipitation change signal is not positive enough to balance this off. This is particularly true of areas like Australia, where the distribution of individual model projections lies mostly over negative values by the end of the century, the Sahel and South Africa. All these regions were projected to see either no substantial change or positive changes under the more traditional definition of growing season. Other regions, all seeing an increase under the thermal definitions are now showing large uncertainties, due mainly to a steady increase in average temperatures accompanied by a contradictory set of projections for precipitation change.

Another facet of projected changes potentially affecting the growing season are monthly statistics of extremes of temperature and/or precipitation that may endanger the health of crops in these regions. For example, we may want to extract from the GCM ensemble changes in the number of days when maximum temperature exceeds physiologically critical thresholds (35°C and 40°C) or changes in the average length of dry spells within each month. Figure 3.5 shows an example of this kind of result, for the region of Western Europe, WEU, and three metrics: changes in the number of days with maximum temperature above 35°C, changes in the number of days with maximum temperature above 40°C, and changes in the average length of dry spells. The spaghetti plots show individual model projected changes over the 12 months of the year. There are two sets of lines, dashed and solid, the former showing changes by mid-century, the latter showing changes by end of the century. As we have already pointed out there may be large variability in the numbers projected by each model, but the set of trajectories indicate significant lengthening of dry spells over most of the years, and growing in length the farther in time the projections are. Similarly, large changes in the number of very hot days are projected for the summer months and both time frames.

The results for all the eight regions are described qualitatively in Table 3.3.

J FMAMJ JASOND

J FMAMJ JASOND

months change for dry w/Tmax>35C in WEU

J FMAMJ JASOND

months

J FMAMJ JASOND

months change for days w/Tmax>40C in WEU

_Uate-century change - - mid-century change

_Uate-century change - - mid-century change

months

Fig. 3.5 Spaghetti plots for three metrics of extremes relevant to growing season characteristics in Western Europe (WEU). From top to bottom: changes in the average length of dry spells for each month of the year; changes in the monthly number of days with maximum temperature above 35°C; changes in the monthly number of days with maximum temperature above 40°C. Each line in the plot corresponds to a GCM. Dashed lines show changes by mid-century (2046-2065 vs 1981-2000), solid lines show changes by end-of-the-century (2081-2100 vs 1981-2000). The scenario is A1B. Thirteen GCMs are represented, all those contributing daily maximum temperature and precipitation to the CMIP3 archive for this scenario

Table 3.3 Summary of changes in growing season characteristics for selected cropping regions

Region Dry spells length Max temperature above 35°C Max temperature above 40°C

Table 3.3 Summary of changes in growing season characteristics for selected cropping regions

Region Dry spells length Max temperature above 35°C Max temperature above 40°C

EAF

Large uncertainty for most of the year, mostly increase

Increase over most of the year

Increase over most of the

in the summer months

year

SAF

Some uncertainty in the summer months but mostly

Increase in the spring and summer

Increase in the spring and

increase in the fall and winter months

months

summer months

SAH

Mostly increase in the first six months of the year, larger

Large increase over most of the year

Large increase over most of

uncertainty in the latter part

the year

NIN

Large uncertainty throughout the May through October

Increase over most of the year

Increase over most of the year

months, very small changes during the rest of the year

ECH

Uncertain sign of the change

Increase in the summer months

Increase in the summer

months

use

Uncertain sign of the change

Large increase in the summer months

Large increase in the summer

months

WEU

Increase all year long

Increase in the summer months

Increase in the summer

months

AUS

Uncertain sign of change in the summer months, increase

Increase in the spring and summer

Increase in the spring and

in the fall and winter months

months

summer months

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