Aridity Trend in Northern China

The following sections are included:

• PREFACE
• CONTENTS

The aridity trend in northern China’s arid and semi-arid regions is one of the most severe climate and environmental problems which deeply impact the sustainable development of the region. This chapter introduces the general features of the landscape and the vulnerability of climate and ecosystems in these regions and then discusses the major issues of the regions’ sustainable development in association with its aridity trend.

This paper reviews climatic changes of the arid and semi-arid areas of northern China on different geological timescales, based mainly on the information derived from the dust records of the Chinese Loess Plateau. In the last 3.5 Ma or so, a loess-red clay sequence in the northernmost part of the Loess Plateau has documented an overall increase in the aridity of northern China, with dramatic increases at about 2.6 and 0.7 Ma. This tectonic timescale change in aridity is roughly consistent with the evolution and development of ice sheets in the northern polar area. On orbital timescales, the arid and semi-arid areas of northern China greatly expanded during glacial periods relative to interglacial periods, and analyses of proxy records revealed that at least in the last 1.8 Ma, climate in the arid and semi-arid areas has varied in similar periodicities with global ice volume variability. During the last glaciation, millennial-scale climatic events (D-O cycles and Heinrich events) can often be detected in high-resolution loess records, which are well documented in Greenland ice cores and the stalagmite δ¹⁸O records in southern China. It is proposed that the evolution and variability of the arid and semi-arid areas of northern China may have been forced mainly by climate change in the polar region of the Northern Hemisphere.

The dry-wet oscillation on decadal to centennial scales during the Holocene was investigated using the high-resolution proxy data of precipitation and temperature collected mainly from historical documents and natural recorders such as lake sediments, pollen records, cave records, tree rings, ice cores, and speleothems. All proxy data showed a general increasing aridity trend in northern China during the last two millennia, especially in the last 100 years. During the second half of the 20th century, the higher than ever frequency of light rain and increases of severe drought events provided significant evidences for the aridity trend over this region. Both observational analysis and numerical modeling studies demonstrated great increases in aridity extent and intensity, which was most significant in the semi-arid regions of East Asia. There were various strong evidences for longterm changes in large-scale atmospheric circulation, such as the poleward shift and strengthening of westerly winds. Regional climate trends, however, can be very different from the global average with specific characteristics in response to global change. Variations of the North Atlantic Oscillation (NAO) and the North Pacific Oscillation (NPO), which were teleconnected with anomalous largescale atmospheric circulation and climate variability on interannual and inter-decadal scales, played a key role in decadal to inter-decadal drought or flood in North China. Their impacts were mainly responsible for the wet period of 1951–1964 and the dry period of 1980–1997 in northern China.

In this chapter, global dry-wet trends were studied based on observational data and numerical modeling results, and the dry-wet decadal characteristics and their differences among the continents of the world under global warming were analyzed using the calculated surface wetness index (SWI) data from 1951–2002. The effects of large-scale climate on global dry-wet variation were explored by analyzing the correlation between decadal changes of SWI and the North Atlantic Oscillation (NAO), between the Pacific Decadal Oscillation (PDO) and the Southern Oscillation (SO). Finally, the effects of decadal SST changes on global dry-wet variation were explored using the integrated simulations of five numerical models.

Observational analyses show that the Hadley cell has expanded by 2–5° latitudes over the past three decades. This expansion is also a robust feature of general circulation model (GCM) simulated climate changes in response to the increase of greenhouse gases but with a much slower rate. The Hadley cell widening has important implications for shifts in precipitation patterns that lead to the expansion of world dry land, and for the cloud feedback to the climate system. Reconciling the discrepancy in Hadley cell widening between the GCM simulations and observations and understanding the mechanisms responsible for this aspect of global climate change represent new challenges in climate research.

This study discusses the long-term variation of the principal modes of the Hadley circulation (HC), one of the most important atmospheric circulations, during the boreal winter (DJF) and summer (JJA) for the period 1951–2008, and explores the causes of the formation of these modes, variations, and their associated climatic effects. It was found that the principal mode of the HC is characterized by an equatorially asymmetric mode (AM) in both DJF and JJA, with the ascent branch in the Southern and Northern Hemisphere, respectively. This mode represents the interdecadal variation of the HC in both DJF and JJA, and explains a large portion of the variance. The robust warming trends of sea surface temperature (SST) over the Indo-Pacific warm pool (IPWP) play an essential role in the variations of the HC, contributing to the interdecadal variation of the AM. This point is further supported by the numerical model, suggesting the temporal variations as well as the spatial structure of AM are largely due to the variations of SST over IPWP. The anomalous rising south of the equator in both DJF and JJA is due to the inhomogeneous warming of SST over the IPWP. SST within the IPWP in the Southern Hemisphere shows larger warming trends than that in the Northern Hemisphere. The position of the anomalous convergence associated with SST variations over the IPWP is aligned with the maximum meridional gradient of zonal mean SST, resulting in an equatorially asymmetric meridional circulation. This point is further corroborated theoretically. Under this scenario, the accompanied descent in the subtropical regions (i.e., in the DJF: 30°–20°S and 10°–20°N; in the JJA: 25°–15°S and 15°–25°N) are enhanced, implying a likelihood of frequent aridity in these regions in the corresponding season.

Based on the long-term measurements at two CEOP reference sites, Tongyu and Yuzhong, and 3-year measurements in a coordinated observing network, together with the applications of land surface and climate models, land-atmosphere interaction processes over the arid and semi-arid regions of northern China were characterized. It is the first time that surface fluxes in these regions were measured continuously during a multi-year period at the stations representing an extended zone. Surface fluxes exhibited wide diurnal ranges with latent heat varying over 50–350 w/m², sensible heat 50–180 w/m², and CO₂ flux –0.1–1.1mg/m²s for different land cover types. A set of dynamic parameters, such as surface albedo, roughness length, and bulk transfer coefficients, were estimated for degraded grassland and cropland, and their seasonal and inter-annual variations in semi-arid regions of northern China were quantified for the first time. On average, surface roughness length (Z_0m) ranged from 2.0 × 10⁻³m to 9.2 × 10⁻³ m over grassland and from 8.0 × 10⁻³m to 208.1 × 10⁻³ m over cropland. The momentum drag coefficient (C_d) for degraded grassland and cropland was 3.1 × 10⁻³ and 4.7 × 10⁻³, respectively. The mean heat bulk transfer coefficient (C_h) for degraded grassland and cropland was 2.5 × 10⁻³ and 2.0 × 10⁻³, respectively. All these parameters showed significant inter-annual variations. Several methods were developed to optimize parameters of LSMs using measurement data. Land surface and climate models were validated by ground measurement for their application in arid/semi-arid regions. To study the climatic change in northern China, a 30-year dataset of surface fluxes was constructed using station-based observations combined with NCEP reanalysis data. Finally, a local feedback process in the development of the aridity trend in degraded grassland in response to a warming climate was proposed: warmer temperature → more evapotranspiration and sensible heat flux → drier soil → less latent heat flux → drier air → less precipitation → drier soil → less NDVI → less latent heat flux → drier air → less precipitation.

The current status of land use and cover change (LUCC) and its impact on regional climate over arid/semi-arid China is assessed. The integrated effects of biophysical and biochemical processes of LUCC on regional aridity are presented and discussed.

Due to the large global extent of arid and semi-arid regions, mineral dust in the atmosphere and its interactions with clouds and precipitation can have a substantial impact on regional and global climate. Dust aerosols can influence the radiative energy budget directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN) and ice nuclei (IN). Dust aerosols can also inhibit or enhance precipitation to modify the hydrological cycle. Furthermore, dust aerosols can absorb solar radiation, contributing to adiabatic heating in the atmosphere that often enhances cloud evaporation. In this chapter, we attempt to provide a benchmark for our present understanding of the effects of dust aerosols on cloud and precipitation, primarily in Asian arid/semi-arid regions.

Ecosystem processes are key to understanding how terrestrial ecosystems respond to climate change. This chapter discusses ecosystem processes in semi-arid regions from the impacts of interactive temperature, water, and CO₂ content on ecosystems, the impacts of environmental changes on water use efficiency of plant species, and the relationship of carbon and nitrogen with their environmental regulation in plants under global change. Changes of biodiversity, geographical distribution, and net primary productivity of terrestrial ecosystems were examined as a result of changing ecosystem processes. Carbon budgets were evaluated for Chinese temperate grassland ecosystems. The major factors controlling carbon budgets of temperate grasslands were analyzed, and future tasks of ecosystem process studies were suggested for semi-arid regions.

Climate change and human intervention, two driving forces in the dynamic Earth at present, are converging to create a water crisis. Processes and elements of the hydrological cycle have changed dramatically in China due to both upper boundary (climate change) and lower boundary (landuse/ cover change, population growth, increasing water supply, socioeconomic development activities). These factors increase hydrological uncertainty. In semi-arid regions, hydrological processes are complex, and the effects of climate change and human activities on hydrological processes are significantly more sensitive. Research on hydrological process in semi-arid regions can help understand and solve local water crises and water disaster problems. This chapter focuses on the response mechanism and modeling methodology of hydrological processes in semi-arid regions in changing environments. Four aspects are addressed. Firstly, basin-scaled evapotranspiration (ET) was estimated by using MODIS images and validated by a hydrological model. Secondly, runoff parameterization schemes with spatial heterogeneity were compared across different temporal scales. Thirdly, the authors calculated re-direction of blue water into green water owing to land use and land cover (LULC) changes. Finally, we quantified the effects of climate change and human activities on runoff using three different methods. Four conclusions can be drawn from the research: 1) ET values estimated from MODIS images based on the surface energy balance scheme and the modified Priestley-Taylor equation are applicable and operational for mapping daily actual ET over the study area; 2) in semi-arid regions the hybrid mechanism of runoff generation is better than the single mechanism of runoff generation over a short-term timescale; 3) LULC changes lead to redirection from blue flow to green water; 4) climate change and human activities have made a great reduction in runoff production over the Laohahe catchment, where human activities are the main factors. These conclusions will hopefully be a reference for the awareness of hydrological changes in semi-arid regions in China.

Food security, sustainable water supply and land use under the current aridity trend must be guaranteed for the sustainable development of society and economy. It is crucial to find solutions for these urgent scientific and social issues, both for China and other parts of the world. In this chapter, impacts of the aridity trend on water resources, land use patterns, society, and economy are analyzed. Then the corresponding adaptive strategies and comprehensive adaptation methods are presented. Finally, detailed case studies on two river catchments are provided.

The following section is included:

• SUMMARY