When summer is defined, not as an astronomical event (the solstice) but as a climate event, the beginning of reliably summer-like weather, this is what some British researches found:
To determine the onset of summer, they looked for the third day of each year when average temperatures reached 14C (about 54 Fahrenheits–ed.). That may sound distinctly chilly for summer, but comfortably allows for daytime temperatures above 20C (about 64 Fahrenheit; remember, this the UK–ed.).
Records show that in the period 1954-1963, the average date for the third such day was 25 May. By the 1990s, it had shifted forwards to 14 May. By 1998-2007, on average, summer arrived on 7 May. The shift is consistent with global warming, Bigg said. “It’s always very difficult to make direct attributions but scientists say global warming is very likely driven by human activity and I think we can say the same thing.” The researchers saw a similar, though smaller, pattern with summer plant flowering. On average, the first flowering date for 1954-1963 was 29 May. By 1991-2000 it was 26 May.