Daniel* It might surprise you, but the 1991-2000 summers average 637.5 hours - just 9 hours more than the 1991-2020 figure at Heathrow. The 90s had good summers but a few which will drop off were sub-600 hours (1991, 1992, 1997 and 2000)
Essentially, as long as 2021-2030 average more than 637.5 hours it will rise on the next 30 year mean.