**6. Conclusions**

Improving sustainability in every aspect of our lives is vital to safeguard a durable future for the next generation. Manufacturers ought to pay special attention to sustainability because what they produce, how they produce, and where they produce has a substantial impact on the carbon footprint. The extant literature on sustainable operations management mainly focuses on the consumption side of product life cycles, with an intention to extend the product lifetime and reduce household waste. However, significant inefficiencies exist on the production side of product life cycles, whereby manufacturers overutilize the Earth's limited resources and generate carbon emissions when producing products in excessive amounts. A significant number of these products may never reach consumers. Prominent examples reported in the media include Amazon destroying thousands of unsold TVs and laptops in one of its warehouses [27].

In this research, we aim to fill the gap in the literature by demonstrating how operational flexibility can help organizations achieve sustainability at source. We focus on three different operational-flexibility strategies: (1) lead-time reduction, (2) quantity-flexibility contracts, and (3) multiple sourcing. Our results indicate that lead-time reduction has the highest potential to reduce waste while improving the profits of companies. Therefore, operational-flexibility strategies that promote local production are key to reducing waste and improving sustainability.

In particular, in our numerical analysis, where ordering time varies between 0 (start of planning horizon) and 1 (beginning of sales season), we show that lead-time reduction can result in a profit increase when orders are placed with onshore suppliers at a time *t<sup>s</sup>* greater than 0.29, compared with being placed with an offshore supplier at time *t<sup>l</sup>* = 0. The profit increase can go up to around 40% when orders are placed at *t<sup>s</sup>* = 0.9, even if ordering costs are higher. Waste in terms of excess unsold inventory also decreases, even when *t<sup>s</sup>* = *t<sup>l</sup>* = 0 when the onshore supplier is used, and results in a 20% waste reduction compared with ordering from an offshore supplier as a result of the higher ordering costs, which result in lower order quantities. The waste decreases as *t<sup>s</sup>* increases and finally approaches zero when orders can be placed during the selling season, when *t<sup>s</sup>* = 1.

The results of our research offer some useful insights regarding the development of effective environmental policies. Because lead-time reduction is the most effective strategy, environmental policies should target cutting lead times, not only for inbound but also for outbound logistics. Increasing the import and export tariffs and imposing trade barriers would force countries to promote local production, which in turn leads to shorter lead times and lower waste. Although such policies conflict with the free trade and economic development ideas, we envision that environmental concerns would be highly dominant in the near future and governments would incrementally pass some regulations to reduce the volume of imports and exports. One of the side benefits of local production would be to establish a close connection between local manufacturers and local authorities such that recycling and remanufacturing can be easily implemented near the market bases. Therefore, local production may also help increase the product life cycle along the stages of the closed loop supply chains (Figure 1).

One of the limitations of our research is that we mainly focus on the dynamics on the production side of the product life cycle without connecting it with the consumption side. We mainly consider a newsvendor setting such that the buyer sells the products in the market at a certain price. The classical utility theory [28] suggests that the life cycle of a product is positively associated with the price paid for it. We believe that there is a need for empirical research that investigates the relationship between price and the length of a product's lifetime in the retail industry for different categories. We envision that this would be an interesting avenue for further research.

Another direction for future research is how digital transformation could contribute to lead-time reduction. Recent research focuses on lean manufacturing in the Industry 4.0 era [29]. Studies on lean manufacturing show that lead-time reduction is an important factor that enhances reliability and flexibility while decreasing inventory carrying costs and that integrating lean management and collaboration in the supply chain has important social, environmental, and economic benefits [30]. Therefore, we foresee that studies of the effect of Industry 4.0 on decreasing waste and increasing profit through lead-time reduction are a potential field for future work.

**Author Contributions:** Conceptualization, I.B.; methodology, Y.K.E. and I.B.; software, Y.K.E. and I.B.; validation, Y.K.E. and I.B.; formal analysis, Y.K.E. and I.B.; investigation, Y.K.E. and I.B.; resources, I.B.; writing—original draft preparation, Y.K.E. and I.B.; writing—review and editing, Y.K.E., I.B. and R.W.S.; visualization, Y.K.E. and I.B.; supervision, I.B. and R.W.S.; project administration, I.B. and R.W.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
