This claim is that humanity only has 12 years before global temperatures will rise to a point where the Earth can no longer support human life. This claim comes from an article published on October 8th, 2018. Based on the content of the article provided, this headline is thoroughly misleading. The main claim in the article linked is that if global temperatures were to rise above an average of 1.5 degrees celsius per year, drought, floods, and extreme heat could contribute to worsening quality of life.
An article published by the New York TImes, by Gernot Wagner and Constantine Samaras
https://www.nytimes.com/2019/09/19/opinion/climate-change-12-years.html
This article summarizes the point that the concept of 12 years was simultaneously a real timeline, but also somewhat misleading. The IPCC report which the original claim referenced essentially outlines how if climate change continued at the same trajectory, after 12 years the world would have to cut its greenhouse gas emissions by 50% by 2030. There are underlying timelines at play, making it difficult to mark 12 years as some definite range which the world had to make a change by.
This article published by the Scientific American, by Kate Marvel
https://www.scientificamerican.com/blog/hot-planet/thinking-about-climate-on-a-dark-dismal-morning/
Refutes the claim and argues that it is mostly nonsense. The New York Times article above, quotes Marvel briefly, while making a similar point. A longer quote from her article helps summarize the primary claim she makes. “Most of the coral reefs are going to die, and many of the glaciers will melt. Climate change is here, leaving grubby human fingerprints on parched, burned, flooded and melted landscapes. But we don’t have to settle for dystopia.” She claims that climate change and its effects are indisputable, but they have not pushed the planet to a point of no return or imminent doom.
An article published on BBC, by Matt McGrath
https://www.bbc.com/news/science-environment-48964736
McGrath did not argue in this article that the world will end in 18 months if plans to cut global emissions were not enacted. What he did argue is that the 18 months following the IPCC report marked the most important time to take action before climate change reached an irreversible point.
In general, this claim is false, the idea of irreversible climate change is not false. There is not immediate risk that the planet will become unlivable in the immediate future if climate change restrictions are not cut.