If rising atmospheric carbon dioxide is the primary driver of recent warming, we should expect relatively uniform warming across the months of the year. Carbon dioxide is a well-mixed gas; its radiative effect does not change strongly from one month to the next. The world’s longest continuous instrumental temperature record tells a different story.
The Data and The Method
The Central England Temperature (“CET”) series, maintained by the UK Met Office, runs from 1659 to the present and is the longest continuous instrumental temperature record available. For each calendar month, I calculated the linear trend over two periods – 1659–2025 and 1850–2025 – and expressed the result as degrees Celsius (°C) per century. This produces twelve independent rates of change that can be compared directly. Because the same month is compared with itself across centuries, many of the practical difficulties that affect long temperature series (instrument changes, site moves, time-of-observation bias) are held constant.
Full record: 1659–2025
When the monthly rates of change are plotted for the entire 365-year period, a clear pattern appears. March is one of the strongest warming months, roughly 0.45–0.5°C per century. June shows the weakest, around 0.05°C per century – more than eight times slower. Winter months (December–February) and October also warm relatively quickly; late spring and early summer warm far more slowly.
