بررسي تأثير تغيير اقليم بر كشاورزي ايران: 1- پيش‌‎بيني وضعيت اگروكليماتيك آينده

Evaluation of Climate Change Effect on Agricultural Production of Iran: I. Predicting the Future Agroclimatic Conditions

گرمایش جهانی و پی‌آمدهای ناشی از آن بسیاری از متغیرهای اقلیمی مؤثر در رشد و نمو گیاهان زراعی را تحت تأثیر قرار می‌دهد و پاسخ گیاهان به این امر تابع شدت و الگوی تغییرات می‌باشد. در این مطالعه تأثیر تغییر جهانی اقلیم بر شاخص‌های اقلیمی كشاورزی ایران برای سال 2050 میلادی (1430 شمسی) براساس سناریوی استاندارد IPCC توسط دو مدل گردش عمومی مورد ارزیابی قرار گرفت. نتایج نشان داد كه میانگین دمای سالانه مناطق مختلف كشور تا سال هدف بین 5/3 تا 5/4 درجه سانتی‌گراد افزایش می‌یابد در حالی‌كه میانگین بارش سالانه بین 7 تا 14 درصد كاهش خواهد یافت. به‌علاوه این تغییرات از غرب به شرق و از شمال به جنوب كشور شدیدتر خواهد بود. افزایش دما باعث طولانی‌تر شدن فصل رشد به دلیل افزایش تعداد روزهای بدون یخبندان می‌شود. البته كاهش بارش طول فصل خشك را از حدود 20 روز در مناطق غربی تا بیش از 30 روز در جنوب كشور افزایش خواهد داد كه این امر به‌ویژه در مناطق كشت دیم از اهمیت بیشتری برخوردار است. افزایش دما همچنین میزان تبخیر و تعرق بالقوه سالانه را در سال هدف بین 18 تا 30 درصد افزایش می‌دهد و در نتیجه تفاوت بین میزان بارش و تبخیر و تعرق بالقوه یا شاخص كمبود بارش به‌طور قابل توجهی افزایش می‌یابد كه عمدتاً به دلیل افزایش میزان تبخیر و تعرق خواهد بود. با وجودی‌كه پی‌آمدهای تغییر اقلیم بر شاخص‌های اگروكلیماتیك تمامی مناطق كشور مؤثر می‌باشد، ولی نتایج این تحقیق نشان داد كه بیشترین اثرات منفی به‌ترتیب در جنوب، شرق و مركز كشور بروز كرده و كمترین اثرات در شمال و شرق كشور ظاهر خواهد شد.

Introduction Future climate change may affect agricultural production through changes in both mean and variability of climatic conditions which in turn could affect crop growth and development. Results of many studies have shown that crop production systems of dry regions are more vulnerable to the predicted climate changes (5) and these impacts are mainly due to the effects of increased temperature on agro-climatic variables (4). During the last decade future changes in agro-climatic variables such as growth degree days, length of growth period and duration of dry season have been studied at regional or national scale with different results depending on studied location (1, 6). However, such information are not available for Iran. In this study different agro-climatic indices of Iran across the country are calculated for the target year 2050 based on business as usual scenario and the results are compared with the current conditions. Materials and Methods Long term climatic data (1965-2005) of 34 stations covering different climates across the country were used as the baseline for predicting future climate as well as current conditions. Two general circulation models (GISS and GFDL) were used for prediction of climatic variables in the selected stations for the year 2050 based on business as usual (A1f) scenario of CERES family (2) and the results were statistically downscaled for higher resolution (Koocheki et al., 2006). Daily temperatures (minimum, maximum and mean) and precipitation were generated from the predicted monthly values. Several agro-climatic indices including potential evapotranspiration, length of growing season (time period between the last spring frost and the first autumn frost), length of dry season (time period where evapotranspiration exceeds precipitation which obtained from ombrothermic curve), and precipitation deficiency index (sum of differences between evapotranspiration and precipitation) were calculated based on daily temperature and precipitation. Same indices were also calculated for the current climatic conditions and the results were compared with that of future. Results and Discussion The results indicated that mean annual temperature for different regions of the country would increase between 3.5-4.5°C. However, increased temperature predicted by GISS model was larger than GFDL model. This increase is significantly higher than average global temperature rise of 1.6-2.5 °C predicted for 2050 under business as usual scenario (2). Annual precipitation would decrease in the range of 7 to 15% by the target year 2050 furthermore both temperature rise and decreased precipitation showed a North-South and West-East gradient. Future temperature rise will led to a longer growing season because of increased frost-free days. The results indicated that extended growth period is mainly due to delayed autumn frost and the highest increase in growing period of 33 days was predicted for the most Northern part of the country. However, lower precipitation results to a higher length of dry season ranging from 20 days at East up to 30 days in the South regions of the country. Extended dry period would lead to unfavorable conditions for rainfed cropping systems due to rapid depletion of soil moisture before crop maturity. Annual increase of potential evapotranspiration (PET) by 18-32% follows the same spatial direction as was predicted for temperature rise with the highest increase for South regions. As a result, precipitation deficiency index, which is the sum of differences between rainfall and PET, will increase drastically over the country, mostly due to increased PET or due to an increase in PET. While all of the studied agro-climatic variables would be affected by the future climate changes, our results showed that the highest overall negative effects would be appeared respectively, in the South, East, and central parts of the country while the North and Eastern regions will experience less vulnerability. Conclusions Agricultural production systems of Iran will be faced with new climatic conditions affecting crop growth and development. The results of this research indicated the future changes of main agro-climatic variables. Prediction of these changes effects on crop productivity at national level could be helpful for designing adaptation strategies.