In the past few decades, the study of the Earth system has achieved significant progress, to the point where it has now reached a time of transition. Specifically, for the past two decades, our priority has been to understand the functioning of the Earth system and the impact of human actions on that system. Over the next decade, however, the global scientific community must assess the risks humanity is facing from global change and find ways to mitigate, in an effective manner, the dangerous changes taking place and cope with the change that we cannot manage. To meet such a challenge, the 2011 ICSU General Assembly approved the establishment of Future Earth——Research for Global Sustainability as a 10-year interdisciplinary program. This is a collaborative program with an alliance of partners as follows: the ICSU (International Council for Science), ISSC (International Social Science Council), Belmont Forum (funders of GEC research), UNEP (United Nations Environment Program), UNU (United Nations University), UNESCO (United Nations Education, Science and Culture Organization), and WMO (World Meteorological Organization).
Recently, a statement from the "Our Common Future under Climate Change International Scientific Conference", 7-10 July 2015, Paris, France, once again indicated that "Climate change is a defining challenge of the 21st century. Its causes are deeply embedded in the ways we produce and use energy, grow food, manage landscapes and consume more than we need. Its effects have the potential to impact every region of the Earth, every ecosystem, and many aspects of the human endeavor. Its solutions require a bold commitment to our common future. We in the scientific community are thoroughly committed to understanding all dimensions of the challenge, aligning the research agenda with options for solutions, informing the public, and supporting the policy process."
Based on the Future Earth Research Plan and other related documents, the key issues in this new phase will be: (i) What significant climate and environmental changes are likely to happen in the future due to both natural and anthropogenic forcing? (ii) What will be the vision of future development of human society? (iii) How can human well-being adapt to the future change of the Earth system? It is obvious that the final goal of the Future Earth program should be to reach global sustainability through orderly and proactive adaptation to global change.
Earth's climate has been changing ever since it came into existence. As the global climate changes, Earth's plants and animals adapt by continuously evolving, according to Charles Darwin's theory of natural selection (Darwin, 2005). All species are faced with more pressure for "survival" as global environmental change becomes exacerbated. In plants, global warming has led to earlier budding and blooming and narrowed leaves, for instance; and in animals, a major change has been in the timing of migratory activities. This is a manifestation of plants' and animals' adaptation to climate change. However, such adaptation has not taken place through conscious choice. Similar to other species on Earth, humans have also survived and evolved through adapting to climate change. To this end, the history of humankind is a history of human adaptation to climate changes, but mainly in a passive sense. Essentially, passive adaptation involves action taking place when changes occur——and such adaptation actions are taken based on our current understanding of the relationship between climate variables and ecological/social/ economic parameters.
In the above context, it follows that, with knowledge not only of the past and present, but also in terms of likely future changes, a great challenge emerges for adaptation to move from a passive to a proactive stance. In other words, rather than just to react to the change which has already happened, one could proactively adapt by developing strategies based on projections of future climate changes, knowledge of the consequences of future climate changes to ecosystems, societies and economic systems, assessment of the capacity of human society to adapt to climate changes, and risk analysis (through virtual experiments) of the consequences of proposed adaptation actions. Within this paradigm of proactive adaptation, forecasting capability is the scientific foundation, numerical simulations or potential test beds are the technical backbone, and socioeconomic power is the material foundation. There is no match in human history for the current depth of our understanding of climate change and its impacts on global ecosystems and socioeconomic growth, the level of scientific and technological development, as well as economic power. With that, turning passive to proactive adaptation is now technically plausible. Therefore, proactive adaptation should be the future direction for sustainable development of human society, as illustrated in Fig. 3.
Schematic diagram of proactive human adaptation to global environmental change.
As indicated in the award announcement of the 48th IMO Prize, Professor YE's "fundamental contributions to both basic and applied science to meteorology", particularly "the initiation of studies on global change and its relationships with sustainable development, orderly human activities and adaptation to its impacts", during his life time, have paid special attention to the environmental issues faced in China. The aridity trend in northern China is one of these issues.
Because of the long history of human civilization and the particularity of the landscape, the aridity trend over northern China features interwoven short- and long-term effects, as well as global and regional effects. The situation is most serious over the semi-arid region——the transitional zone of climate and ecosystems (Fig. 4). There is an integrated behavior of the climate system and interaction with human activities, and should therefore be studied as a cross-disciplinary issue. To address the problem, a 10-year research project entitled "On the Aridity Trend of Northern China and Human Adaptation", during 1999-2010, was carried out, under the support of the Ministry of Science and Technology of China.
3.5.1. Research framework of the project
The project was designed with six tasks:
Task 1 was to document the history of climate and environmental changes in northern China, from geological, to centennial, to decadal scales, based on high-resolution proxy data as well as modern meteorological and hydrological observation data.
Task 2 was to examine the global patterns of aridity trends and time regimes in transition in the past 60 years in association with the aridity trend of northern China.
Task 3 was to study the atmospheric dynamics and air-sea interactions in relation to the aridity trend, from both an observational and modeling perspective.
Task 4 was to study the impacts of anthropogenic factors, such as increased concentrations of dust aerosols and changing land-surface processes, mainly due to human-induced land-cover changes, on the aridity trend.
Task 5 was to study the ecosystem and hydrological processes in the semi-arid region and assess the impacts of the aridity trend on socioeconomic development in the region.
Trend in the aridity index in China, 1951-2007 [Revised and reprinted from (Fu and Ma, 2008)]. SWI is the surface wet index.
Task 6 was to study human adaptation to the aridity trend in northern China.
The most significant progress in the implementation of this project was the development of field observation sites in the semi-arid region, and a regional integrated environment model system (RIEMS) and observation-data-model fusion system (ODMFS) for studying aridity in northern China. The detail of RIEMS and ODMFS are introduced in Figs. 5 and 6.
Schematic diagram of the Regional Integrated Environment Model System (RIEMS) [revised and reprinted from (Wang et al., 2015)].
An observation-data-model fusion system for studying aridity in northern China.
3.5.2. Main results of the project
The research results of the project are presented in detail in the book Aridity Trend of Northern China (Fu and Mao, 2017). However, the key findings can be summarized more briefly as follows:
(1) The climate over northern China is characterized by wet-dry oscillations on multiple time scales. The aridity trend in the last half-century is part of a composite picture of those oscillations.
Documented changes of aridity over northern China on geological time scales, from the dust records of the Chinese Loess Plateau, in the last 3.5 Ma or so, indicate that there has been an overall increase in aridity over northern China, with dramatic enhancements in aridity at about 2.6 and 0.7 Ma. This tectonic time scale change in aridity is roughly consistent with the evolution and development of ice sheets in the North Polar Region. On the orbital time scale, the arid regions of northern China greatly expanded during the glacial periods, as compared with the interglacial periods. During the last glaciation, millennial-scale climatic events (D-O cycle and Heinrich events) can be detected in most high-resolution loess records, which are well documented in Greenland ice cores. It is believed that the variability of aridity over northern China on that time scale is associated with the climate change in the North Polar Region.
The dry-wet oscillation on decadal to centennial time scales during the Holocene has been documented using high-resolution proxy data, including historical documentary records, lake sediments, pollen records, cave records, tree rings, ice cores, and speleothems. All proxy data have generally shown an aridity trend over northern China during the last two millennia, especially in the last 100 years. During the second half of the 20th century, the aridity trend was demonstrated by the increased frequency of light rain and extreme arid events. In addition, the most significant aridity trend was observed in a semi-arid region.
(2) Large scale atmosphere-ocean interaction of the climate system is believed to be one of the main driving forces for the development of the aridity trend in northern China in the last several decades.
Intensification of meridional SST gradients over the tropical Indian Ocean-western Pacific warm pool region is one of the key modulators of the global subtropical decadal aridity trend, and one that has strengthened in recent decades. Consequently, the Hadley circulation has been weakening in boreal summer and intensifying in boreal winter. As a result, descending motion anomalies have intensified and rainfall has reduced in the subtropics in both boreal summer and winter. The aridity trend in northern China appears to be a part of this story.
Observational analyses suggest that the Hadley cell has expanded by 2°-5° of latitude in the past three decades. Such a widening of the Hadley cell has important implications for shifts in precipitation patterns that lead to expansion of dry land worldwide, including the aridity trend in northern China. This expansion can also be captured in the simulated climate change in response to increased concentrations of greenhouse gases, albeit at a much slower rate. To understand the mechanisms driving the expansion of the Hadley cell, and the discrepancy between GCM-simulated and observed expansion, presents new challenges.
(3) Evidence suggests that anthropogenic factors, such as increased concentrations of dust aerosols and changing land-surface processes, mainly due to human-induced land-cover change, may also have contributed to the aridity trend in arid/semi-arid regions of northern China.
Transported and local anthropogenic dust aerosols can significantly reduce cloud droplet size, optical depth, and liquid water path. Dust aerosols may warm the cloud, enhance evaporation of cloud droplets, and further reduce cloud water path via the so-called semi-direct effect. Such semi-direct effects have played an important role in cloud evolution and exacerbated drought conditions over the arid/semi-arid areas of northwestern China.
Observational evidence and modeling studies suggest that the intensification of the regional aridity trend is directly related to human-induced land-use/land-cover changes. A local feedback process of the aridity trend over degraded grassland under a warming climate has developed as follows: high temperature leads to increased evapotranspiration and sensible heat flux and then to reduced soil moisture and less latent heat flux; Such change of land surface process would further have feedback effect on atmosphere: low humidity and less precipitation. Such changes of atmosphere will affect further the land surface processes and result in dry soil, low NDVI and and less latent heat flux. Finally the aridity trend in the region will further be enhanced. Such a feedback mechanism may also interact with large-scale atmosphere-ocean interaction on decadal and multi-decadal scales. However, there is no quantitative estimation of such effects, because results are very likely data- and model-dependent.
(4) Hydrological processes are complex and variable in semi-arid regions of northern China. Climate change and human activities substantially influence runoff in northern China, with significant regionality. For instance, the annual runoff within the Laohahe Basin, a representative semi-arid region in northern China, has had a significant decreasing trend, with an abrupt change in 1979. The effects of climate change and human activities on runoff from 1980 through 2008 have been simulated using three types of hydrological models. The simulations suggest that human activities are the dominant factor, contributing 89%-93% of the total reduction in runoff, while climate change has contributed 7%-11%.
(5) The aridity trend in the arid and semi-arid regions of northern China has had direct impacts on natural and agricultural ecosystems, as well as the aquatic system, and indirect impacts on agricultural production and people's livelihoods, ultimately affecting economic and social development. Measures need to be taken to mitigate their negative effects. Under the influence of the aridity trend, high temperatures will reduce the positive effect of elevated CO2 concentrations on biomass, while drought stress will decrease the accumulation of carbohydrates and alleviate/eliminate photosynthetic down-regulation. Based on field experiments and numerical simulations, the adaptabilities of plants to drought stress will be enhanced by the combined effects of elevated CO2 and high temperatures. The vegetation distribution will also change, due to the altered water and heat conditions. Since the 1990s, the crop area affected by severe aridity has constituted more than 60% of the total crop-disaster area in China. It is estimated that the national economic loss due to aridity disaster has reached approximately 1.1% of Gross Domestic Product.
(6) The following adaptive strategies are recommended: distinguish regional advantages; adjust the agricultural structure; increase the proportion of forest and animal husbandry; control agricultural irrigation areas; promote water-conserving irrigation and reduce water-use of each unit; implement a water ticket system; and improve the capacity of water conservation and the rainwater resource (Tyson et al., 2001; Fu et al., 2002).