Green IoT (G-IoT) is the adoption of energy-efficient procedures (hardware, software, communication, or management) and waste reduction methods (energy harvesting and recycling of e-waste) to conserve resources and reduce waste (including pollutants like carbon dioxide) produced by the IoT ecosystem from the design, manufacturing, deployment and operation of IoT systems from the IoT devices to IoT cloud computing data centres. Green IoT is an emerging field within the IoT ecosystem that is aimed at raising awareness of the sustainability problems that may result from the massive deployment of IoT applications in the various sectors of society (health care, agriculture, manufacturing, intelligent transport systems, smart cities, supply chains, smart homes, and smart energy systems) and exploring ways to address those challenges. These challenges include the increase in energy consumption, which increases the IoT industry's carbon footprint, and the amount of e-waste created resulting from discarding electronic components of IoT devices, especially IoT batteries, as they need to be replaced after a few years.

Although energy-efficient strategies have been developed to minimise the energy consumption of IoT devices, the energy consumption of billions or trillions of IoT devices will be enormous. The amount of traffic generated by IoT devices is increasing exponentially, and it is predicted that by 2024, IoT traffic will constitute about 45% of the total Internet traffic \cite{Alsharif2023}. A rapid increase in the amount of traffic generated by billions to trillions of IoT devices and transported through the Internet to cloud computing platforms will significantly increase the energy consumption of the Internet network infrastructures, especially with the dense deployment of 5G base stations and IoT wireless access points to service IoT devices. Also, a huge amount of energy is consumed by data centres to process or analyse the massive amount of data collected using IoT devices.

Much attention is often focused on the energy consumed by IoT devices, networks, and computing platforms. However, less attention is given to the energy consumed by manufacturing and transporting IoT devices and other ICT systems used in the IoT ecosystem. The carbon footprint of the IoT industry can be traced from mining the minerals required to manufacture IoT devices, the manufacturing process, and the supply chains involved. To realise the green IoT goal, energy efficiency and sustainable practices should be designed to ensure that the mining, manufacturing and supply chains are environmentally friendly or sustainable.

The design and implementation of energy-efficient strategies may significantly reduce the energy consumption of IoT systems. However, the rapid increase in the use of IoT to address problems and increase efficiency and productivity in other sectors of the economy will result in a significant net increase in the energy consumed by these systems. Another approach to enforcing green IoT is using renewable resources such as renewable energy sources to continuously recharge IoT batteries, reducing the maintenance cost of replacing IoT batteries and increasing the amount of e-waste created by the IoT industry.

Another Green IoT strategy is to reuse and recycle IoT components and resources. It will significantly reduce the amount of waste produced by the IoT industry and optimise using natural resources to manufacture IoT devices. Hence, reusing and recycling IoT components and resources is a green IoT strategy to increase the sustainability of the IoT industry.

An effective green IoT strategy should span the entire IoT product lifecycle from the design to production (manufacturing) to the deployment, operations and maintenance, and recycling. The primary goal in each stage is to reduce energy consumption, adopt sustainable resources (e.g., harvesting energy from sustainable energy sources, using sustainable materials) usage, minimise e-waste and other pollutants, and adopt recycling of resource or waste. Therefore, a shift toward Green IoT (GIoT) emphasises the need to adopt energy-efficient practices and processes prioritising resource conservation, waste reduction, and environmental sustainability^[1].

Green IoT strategies can be grouped into the following categories: green IoT design, green IoT manufacturing, green IoT manufacturing, green IoT applications, green IoT operation, and green IoT disposal ^[2].

Green IoT design: Designing IoT hardware, software, management systems, and policies considering the requirement of minimising the energy consumption, carbon footprint and environmental impact of IoT systems. One of the design goals should be to implement energy-efficient strategies to reduce energy consumption and to develop strategies to minimise the amount of e-waste produced from the IoT systems and infrastructures. Green IoT design techniques include Green hardware, green communication and networking infrastructure, green software, green architecture, green software, energy-efficient security mechanisms, and energy harvesting.

Green IoT Operations: Deploying, operating, and managing IoT systems in such a way as to minimise energy consumption and to minimise waste. One such strategy is to switch off idle networking and computing nodes, applying radio resource optimisation mechanisms (e.g., control of the transmission power and the modulation), energy-efficient routing mechanisms, and software energy optimisation mechanisms (improving software code to be energy-efficient and using software optimisation algorithm to minimise energy consumption).

Green IoT applications or use cases: Using IoT applications to reduce energy consumption (or the carbon footprint) and to conserve resources to ensure sustainability in other industries, for example using IoT to reduce energy consumption, water consumption, and the use of chemicals (fertilisers, herbicides, fungicides, insecticides etc) in the agricultural industry. IoT can reduce energy consumption, carbon footprint, waste production, and the over-utilisation of resources in the various sectors of the economy, including manufacturing, energy production, mining, health care, and transportation. Therefore, the massive deployment of IoT in these sectors to address efficiency and productivity challenges should be done in such a way as also to address sustainability issues.

Green IoT waste disposal and management: Reducing the waste created from deploying and operating IoT systems. Renewable energy sources should be used to recharge IoT batteries to reduce the amount of IoT battery waste generated and dumped in landfills. Recycling IoT components and resources should be adopted and promoted to reduce the amount of e-waste generated by the IoT industries and dumped in landfills, which may increase significantly with the large-scale adoption and deployment of IoT systems in the various sectors of the economy.

Green IoT manufacturing: Energy-efficient manufacturing infrastructure for IoT hardware. With the expectation to connect hundreds of billions or trillions of IoT devices to satisfy the demand for IoT to improve various sectors or industries in the evolving tech-driven economy, the carbon footprint from factories manufacturing IoT devices will be enormous. Also, the manufactured IoT systems should be energy efficient.