Short answers, pulled from the story.
GPU mining is the use of graphics processing units to perform computationally intensive hash calculations that verify transactions on proof-of-work cryptocurrency networks such as Bitcoin. Miners who successfully complete those calculations receive cryptocurrency as a reward. Graphics processors are suited to the task because of their efficiency at the specific mathematics involved.
GPU mining first became possible in October 2010, when software was created to configure graphics processors for mining tasks. Bitcoin itself launched in early 2009, but GPU mining was not technically feasible at that point.
A Bloomberg report suggested that cryptocurrency miners spent $15 billion on GPUs during the mining craze that began in 2021. That demand contributed to a worldwide GPU shortage that was not resolved until production caught up in 2023.
Bitcoin GPU mining produced more than 85 million tons of CO2 during 2020 and 2021. During those same years, 67% of the electricity powering Bitcoin mining came from fossil energy sources.
GPU mining became unprofitable due to falling cryptocurrency values, Bitcoin's built-in reward halvings, and the shift of major networks away from proof-of-work. Bitcoin's block reward fell to 6.25 bitcoins per block after the May 2020 halving, and Bitcoin's price dropped from roughly $46,000 to around $20,000 around March 2022. Ethereum's transition to proof-of-stake in September 2022 removed one of the largest networks from the proof-of-work space.
After Ethereum completed its proof-of-stake transition in September 2022, miners began offloading their graphics cards onto the second-hand market in large numbers. Some switched to other proof-of-work coins such as Ethereum Classic, but those alternatives proved equally unprofitable. By early January 2023, some mining companies had begun repurposing their hardware for AI computation.