Study On Global Semi-arid Climate Change

Semi-arid regions cover about 15% of the Earth’s land surface and are home to 14.4% of the world’s population. Semi-arid regions are characterized by a dry climate and low nutrition content and water conservation capacity of soil, and precipitation is scarce and typically unpredictable as well. In addition, the ecosystems over semi-arid regions are fragile and sensitive to human activities and climate change. Studying on the characteristics and the dynamics of climate change in semi-arid regions under global warming will give us a better understanding of how it will change in the future in such regions, and provide some scientific knowledge for the purpose of policy making. Therefore, it is very important to further investigate the semi-arid climate change and its impacts.We calculate the potential evapotranspiration (PET) using the Penman-Monteith algorithm and the aridity index (AI) by using the observed monthly temperature and precipitation datasets from 1948 to 2008 developed by Climate Prediction Center (CPC) and the reanalysis data. According to UNEP’s definition, semi-arid regions are defined as regions with 0.2≤AI<0.5 (AI is defined as the ratio of annual precipitation to annual PET). Nine semi-arid subregions are selected, i.e., East Asia, Central Asia, northern Africa, USA, Mexico, South America, southern Africa, western Australia, and eastern Australia. We systematically examine the areal changes and the spatial and temporal characteristics of climate change in these nine semi-arid subregions. We also quantify the contributions of precipitation and PET to the dry/wet variations, and illustrate the dynamics of semi-arid climate change globally. The main results are as follows:(1) The global semi-arid regions cover 2.2×107km2, accounting for 15% of the land surface. Semi-arid regions are mainly located in the central and western United States, most of Mexico, the west coast and northeastern corner of South America, Sahel in northern Africa and southern Africa, Central and East Asia, western India and a large portion of Australia outside of the central desert regions. The land cover type is primarily grassland, which cover 41.6% of the semi-arid regions.(2) Global semi-arid regions have expanded over the last 61 years (1948-2008), with the rate of 0.24×106 km2/decade. The area of semi-arid regions in the last 15 years (1994-2008) is 1.4×106 km2 (7%) larger than that in the period 1948-2008. The expansion of semi-arid regions is mainly contributed by the transition from subhumid/humid to semi-arid, which accounts for 75% of the newly-formed semi-arid regions. The expansion of semi-arid regions across the continents in the Eastern Hemisphere is 1.2×106 km2, which is six times higher than that across the American continents (0.20×106 km2). Although expansions of semi-arid regions occur in both areas, the shifting patterns of the expansion are different. The newly formed semi-arid regions are shifted from arid regions over the American continents, where the climate becomes wetter. Conversely, in the continents of the Eastern Hemisphere, the expansion of semi-arid regions replaces the sub-humid/humid regions, where the climate becomes drier.(3) Semi-arid regions have expanded in East Asia, northern/southern Africa, western/eastern Australia and South America subregions, whereas shrank in the other 3 subregions, i.e., Central Asia, USA and Mexico. The expansion of semi-arid regions in East Asia subregions is the most significant and severe, which is 0.78×106 km2, accounting for 55% of the global expansion of semi-arid regions, and the boundary of semi-arid regions in China has extended eastward over Northeast China by about 6° of longitude (～480km) and by about 2° of latitude (-220km) to the south along the middle-to-lower reaches of the Yellow River..(4) Semi-arid regions in East/Central Asia Mexico, northern/southern Africa and eastern Australia subregions became drier. Except for Mexico and eastern Australia (where PET slightly decreased, and the decreasing trend is not statistically significant), the regional averaged precipitation and PET showed decreasing and increasing trends, respectively, thus AI decreased. Statistically significant decreasing trends of AI at the 99.9% confidence level occurred in East Asia and northern Africa, which indicates that the drying trend is severe. Semi-arid regions in USA, South America and western Australia subregions became wetter. The increase of precipitation and decrease of PET over USA and western Australia have led to the decrease in aridity. In South America, both precipitation and PET showed increasing trend, thus precipitation plays a dominate role in the wetting trend of South America. Temperature showed warming trend in all nine semi-arid subregions, except for Mexico, South America and western Australia where the trend is not statistically significant, statistically significant warming trends at the 99.9% confidence level occurred in the other six subregions, especially in East Asia, the warming rate reaches 0.31℃/decade.(5) In all nine semi-arid subregions, the sings of contributions of precipitation to AI changes and AI changes are the same, whereas the contributions of PET to AI changes and AI changes are opposite signs in South America and eastern Australia (the same signs in the other six subregions). Except for the East Asia and central Asia, where the contributions of precipitation and PET are comparable, precipitation plays a dominant role in the drying/wetting trends of the other six subregions, i.e., USA, Mexico, South America, northern/southern Africa and eastern/western Australia.(6) The two moderate semi-arid regions in North America and East Asia show different climatic patterns. The distinct behaviors of semi-arid regions in North America and East Asia reveal the different responses of semi-arid regions to the decadal variability in the westerlies and monsoon systems under global warming. The climate change in the drying semi-arid region over East Asia is mainly dominated by a weaker East Asian summer monsoon, whereas the wetting semi-arid regions over North America are mainly controlled by enhanced westerlies.