【写作思路】
开头段：介绍全球水资源短缺的消极影响。
1. 限制经济发展。
2. 威胁人类生存。
主体段：分析导致全球水资源短缺的原因。
1. 人口增长。
2. 水资源污染严重。
结尾段：总结全文，提出解决水资源短缺的有效措施。
1. 实施全方位节水。
2. 开发污水处理新技术。

【参考范文】

Global water shortage has been a major concern over the last few decades, which has brought about many negative effects on society. Firstly, water shortage will limit economic development. Today’s agriculture and industry are extremely dependent on water resources, and water shortage will hinder agricultural development and lead to industrial shutdowns. Secondly, the deterioration of ecosystems and the destruction of biodiversity caused by water shortage will seriously threaten the survival of mankind.
Why are we facing such a severe water shortage? To begin with, the global population is now growing explosively, leading to an increase in human demand for water resources and thus exacerbating water shortage. In addition, many of the world’s rivers and lakes are polluted by sewage discharges. The indiscriminate discharge of domestic sewage and industrial effluent has seriously polluted water quality, adding to the already critical water supply situation.
In order to solve this problem constructively, we should take effective measures now. First, we should implement all-around water conservation in industry, agriculture, and domestic use to improve the utilization rate of water resources. What’s more, we need to develop new sewage treatment technologies to increase the reuse of water resources. To sum up, it is vital for us to recognize the seriousness of the problem and work actively to protect water resources so that we can create a better future for all mankind.