A simple climate change model for the concerned public

The Intergovernmental Panel on Climate Change addresses policymakers with elaborate models and projections of global warming that are difficult for most people to understand. The simplest model is the trend line in the recent global mean annual near-surface warming data, and the simplest projection is to extrapolate the line into the future. Over the 50 years from 1974 to 2023 the trend has been close to linear with a mean rate of increase of 0.20 °C per decade. The 20-year time series (2003-2022) gives the very similar rate of 0.21 °C of warming per decade, but annual updates (2004-2023 and 2005-2024) give rates of 0.24 °C and 0.27 °C per decade, respectively. Amid the high variation from year to year, the trend line is a practical index of the mean near-surface warming at any one time. Projected linearly into the future the trend line for the 2005-2024 time series predicts +1.5 °C and +2 °C in 2029 and 2047, not dissimilar to more complex projections. Such simple exercises are open to almost anyone to understand or to perform for themselves and could help to lessen the large gap between the public perception of climate change and scientific reality.