9 January 2024, NOAA Climate.GOV: What’s in a number? The meaning of the 1.5-C climate threshold. Numbers and the meanings we attach to them can be weird. A number can mean a lot or very little depending on how it is being used, and who is using it. To a Taylor Swift fan, 13 is a lucky number. To many in Western cultures, it is a day of bad luck when falling on a Friday. To others, 13 is just the number that comes after 12. When it comes to climate science and policy, one of our “13s” is the 1.5°C climate threshold, shorthand for global average surface warming of 1.5 degrees Celsius above pre-industrial temperatures. That’s the level of warming that the countries who signed the Paris Agreement have agreed to try to stay below. But what does pre-industrial mean? How do we know when we’ve passed 1.5°C? And what happens if we do? When you read or hear climate numbers, they are often being compared to average. The September 2023 NOAA global surface temperatures, for instance, were 1.44 degrees Celsius above average. That average represents a defined period of time. In this case, September was 1.44°C warmer than the average September of the twentieth century. For the 1.5°C climate threshold, the “average” time range is defined as the “pre-industrial period”, or the period of time before the increase in atmospheric greenhouse gases due to human-emissions began to significantly influence global temperatures. Sounds a bit vague, right? What years make up this “pre-industrial period?” It depends. Most history books define the dates of the Industrial Revolution as between the mid-1700s and mid-1800s, but scientific definitions of pre-industrial typically cover some range of decades between 1850 and 1900. Why? Because that’s the earliest time period with widespread, consistent surface temperature records. Different research groups use different parts of the broader time range. For NOAA data, we currently define pre-industrial as 1850-1900. (Footnote 1) While there can be differences in what counts as “pre-industrial”, it’s important to remember that industrial versus pre-industrial is really about the human signature on our climate and how scientists can distinguish that from natural variability going back thousands of years. (Footnote 2) Read more here