Climate
Background
Temperature: A significant warming trend of about 0.57°C in the annual mean temperature was observed in the past century from 1901 to 2000 in Pakistan. This increase is less than the mean annual temperature increase of 0.75°C in the past century in the South Asia region.[1]
A more accelerated trend of warming, with the rise of 0.47°C, was observed from 1961 to 2007 in the country. The warmest year recorded until 2007 was 2004, and the highest increase is observed during winter when the temperature ranges from 0.52°C to 1.12°C. This is in agreement with the pronounced rate of warming observed over the South Asia region in the decade 1998–2007, which was attributed to increase in winter temperature and post-monsoon changes (footnote 1).
On a regional basis, the highest increase in winter temperature was observed for Balochistan province, while the northwestern parts of the country showed negative temperature trends in the summer. The annual temperatures in Pakistan increased by 0.87°C (maximum) and 0.48°C (minimum) from 1960 to 2007.
Based on the Pakistan Meteorological Department (PMD) station data from 1951 to 2000, a rising tendency in the annual mean surface temperature was observed throughout the country.[2] In the hyper arid plains, arid coastal areas, and mountain regions of Pakistan, an increase of 0.6°C–1.0°C in the mean temperature was observed, whereas an increase of 0.5%–0.7% in solar radiation was noted over the southern half of the country. In central Pakistan, the cloud cover decreased by 3%–5% with a consequent increase of 0.9°C in temperature. The northern parts of the country outside the monsoon region suffered from expanding aridity during the study period.
Rainfall: During 1951–2000, a decrease of 10%–15% in winter and summer rainfall in arid plains and coastal areas was observed while a rise of 18%–32% in the summer rainfall was observed in the core monsoon region of Pakistan. A decrease of 5% in relative humidity was observed in Balochistan province. Similarly, a decrease of 17%–64% in rainfall was observed during the seven strong El Niño events in the last 100 years. Depressions, storms, and cyclones forming in the Bay of Bengal and Arabian Sea increased in frequency during the last decade of the 20th century, and has been affecting Pakistan as well as other countries in the region (footnote 2).
For a long-term precipitation time series, 18 stations with available data from 1901 to 2007, and five stations with available data from 1914 to 2007 were used. A 10-year moving average showed that rainfall gradually decreased from 600 millimeter (mm) to 400 mm a year from early 1900s to 1940. After 1940, an increase of 133 mm was observed. Annual precipitation increased by 61 mm in Pakistan from 1901 to 2007. Monsoon rains increased by 22.6 mm, and winter precipitation increased by 20.8 mm.
Future Projections
Using the General Circulation Model for future climate change projection, the Global Change Impact Study Centre (2007) modeled annual temperature and precipitation change for future years 2020, 2050, and 2080 under emissions scenarios of A2, A1B and B1: A2 scenario shows the "business as usual" or current situation; A1B shows a balanced scenario, and B1 makes projections for an "ideal world" scenario. Table 4 below maps out the temperature increase per decade for the different regions of Pakistan according to this projection model, and for the "ideal world" scenario, an average temperature decrease per decade for the different regions of Pakistan.
According to the model, by 2080, the temperature increase in Pakistan will be as high as 4.38°C. Regarding regional change in annual temperature, the study further noted that (i) the temperature increase in both summer and winter are higher in northern than southern Pakistan, and (ii) the temperature increases in both regions are higher in winter than summer. For the percentage of precipitation change, no significant change is observed. However, there is some precipitation increase in summer, and decrease in winter in southern Pakistan.
The PMD conducted another significant study that computed temperature and precipitation change for different regions of Pakistan from 2011 to 2050. The climate models show a maximum rise in the northern areas of Pakistan, central and south Punjab, and lower parts of Khyber Pakhtunkhwa Province. However, mixed trends are projected for precipitation over different regions of Pakistan. Table below shows the region-wide changes in temperature and precipitation over Pakistan.
In the Indus Basin area, one important study projects an almost uniform variation in the rainfall distribution over the entire basin region.29 At subregion levels of the Upper Indus Basin (UIB) and Lower Indus Basin (LIB), the models show an increasing trend in the rainfall over the UIB, decreasing in the LIB with a small change in the border zone between the two basin subregions. Winter precipitation is projected to decrease, particularly in the southern part of the basin with greater warming in the winter than in other seasons of the year.
Similarly, UIB will become warmer than LIB. The model simulations also suggest a rise in the total number of rainy days over the basin, but a decrease in the number of rainy days, and an increase in rainfall intensity is projected in the border zone between the upper and lower basins, where the volume of rainfall is highest.
The PMD in May 2015 released a daily gridded downscaled precipitation and temperature time series from 2010 to 2099 climate change scenarios at 10 km resolution for the four different General Circulation Models using the World Climate Research Program-Coupled Model Inter-Comparison Project Phase-5 (CMIP5). The results are high resolution gridded projections of temperature and precipitation for the whole Indus River Basin:
The downscaled results show a 3°C–5°C temperature rise in mean temperature under the Representative Concentration Pathways (RCP) 4.5 emission scenario.30 The mean trend of temperature under RCP8.5 indicates a 4°C–6°C rise by the end of the century with a sharp increase after 2050. The rainfall is highly variable in both spatial and temporal domains. Area average rainfall over Pakistan shows a large inter-annual variability. Sharp rising peaks give some indication of extreme precipitation events while negative peaks indicate droughts. CMIP5 multi-model mean projections of annual average temperature and precipitation changes for 2046–2065 and 2081–2100 under RCP4.5 and 8.5 relative to 1986–2005 are in Figure 7.
According to the model, spatial patterns of temperature and precipitation have similar behavior. Snow-covered areas of Pakistan in the north show a larger increase in mean temperature compared to central and southern regions under both RCP scenarios. However, RCP8.5 shows a more abrupt increase in temperature in the region after 2060 and up to 10°C–12°C, especially in northern Pakistan, whereas RCP4.5 shows a similar increasing trend but with less intensity, i.e., 5°C–6°C (Figures 8 and 9).
