The Warmest Decade on Record in London: The 2020s

Updated: 2026-08-07

For a resident of London, the numbers reflect a tangible change in everyday experience. Winters that rarely produce sustained frost, springs that arrive earlier, and summers that push the urban heat island effect to its limits are not anecdotal impressions — they are the statistical signature of a capital city whose decadal mean temperature has climbed from 10.8°C in the 1910s to 12.9°C in the current decade. The difference between those two figures, a span of 2.1°C across roughly a century, is precisely the kind of shift that climate scientists point to when they talk about the scale of change now underway.

The data used for this analysis comes from the Met Office station at London St James's Park (station ID 00697), one of the most reliable and continuous urban climate records in the United Kingdom. The series analysed here runs from 1 January 1912 to 31 December 2025, with only verified observations included. The 2020s decade is marked as the warmest in this complete series, and its dominance over every previous decade is unambiguous.

Decade-by-Decade Mean Temperatures (1912–2025)

The table below shows the average annual temperature for each decade from the 1910s through to the 2020s. The progression is not perfectly linear — the 1940s, 1930s and 1980s all reached 11.2°C, for example — but the overall trend is unmistakable, with the four warmest decades all occurring since 1990.

What stands out most clearly in this series is not just the peak at 12.9°C, but the sustained elevation of the last four decades. The 1990s and 2000s both averaged 12.0°C — a level never seen before in the preceding eight decades, where the mean fluctuated between 10.8°C and 11.2°C. The 2010s then pushed the average to 12.2°C, before the current decade surged ahead to 12.9°C. In other words, the four warmest decades on record have all occurred in the last 35 years, and the gap between the warmest and coolest decades in the series now exceeds 2°C.

Ranking of the Ten Warmest Decades

Looking at the ranking of the top ten warmest decades, the pattern is striking. The 2020s lead with an average temperature of 12.9°C, followed by the 2010s at 12.2°C, and then the 1990s and 2000s tied at 12.0°C. The remaining six places in the top ten are occupied by earlier decades, but all of them sit at or below 11.2°C — a full degree or more cooler than the current decade.

For context, the maximum temperatures (daytime highs) also tell a consistent story. The 2020s have averaged a mean daily maximum of 16.6°C, compared to 15.9°C in the 2010s and 15.5°C in both the 1990s and 2000s. Even the warmest earlier decades, the 1940s with 14.9°C and the 1930s with 14.7°C, fall well short of what the 2020s are delivering. This represents a dramatic increase in daytime warmth across the capital, with implications for everything from energy demand to public health.

The inclusion of the 1940s at 11.2°C and the 1930s at 11.2°C in the top five is notable, as it shows that mid-20th-century warmth was not negligible. However, those decades, which historians associate with the height of the industrial-era warming before the mid-century stabilization, never approached the levels seen since 1990. The 1930s and 1940s were warm for their time, but they are now 1.7°C cooler than the 2020s — a gap wider than the entire difference between the coolest and warmest decades prior to 1990.

The 2020s decade is still in progress, with data running through 31 December 2025. If current trends continue, the final figure for the complete decade could rise further before 2030. Even with the first six years already counted, the average of 12.9°C is already the highest of any decade in the 113-year series, and it is not a marginal lead — it surpasses the 2010s by 0.7°C, a margin that exceeds the warming seen between any other consecutive decades in the entire record.

What This Means for London

The climate of London has changed in a generation. For anyone who grew up in the 1970s or 1980s, the city's average temperatures have risen from around 11.1°C to 11.2°C in those decades to 12.9°C today. That is a shift of more than 1.5°C in roughly 40 years — a rate of change that outpaces anything in the earlier part of the 20th century, when temperatures fluctuated within a narrow band of 10.8°C to 11.2°C for eight consecutive decades.

This warming has practical consequences. Higher mean temperatures translate directly into more frequent heatwaves, milder winters with fewer frost days, and a longer growing season for urban vegetation. The mean daily maximum of 16.6°C in the 2020s means that, on average, daytime highs during the decade have been more than 2°C warmer than they were in the 1930s or 1940s. For city planners, public health officials, and residents alike, these are not abstract statistics — they are the new baseline against which all future weather will be measured.

As the decade progresses towards 2030, the data from St James's Park will continue to be a key reference point for tracking whether this warming trend accelerates, stabilizes, or — in the unlikely event of a series of exceptionally cool years — reverses. Based on the evidence from 1912 through 2025, the 2020s have already secured their place as the warmest decade London has experienced since systematic meteorological observations began well over a century ago.

Source: Met Office — London St James's Park station, period 1912–2025. Data from the Met Office MIDAS archive, held at CEDA (Centre for Environmental Data Analysis), under the Open Government Licence (OGL v3.0). All figures are based on verified observations: 1912-01-01 to 2025-12-31.

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© 2026 HistoClima · data: Met Office (OGL v3.0) · not affiliated with Met Office · citation permitted with source.