Warm Mean CET years outside the era of Climate Change

[Timmytour]

We often see historical cold periods of several years of low annual CETs and can assign particular reasons to them, such as significant volcanic explosions.

And of course the increase in annual mean CETs over the last thirty years is assigned to climate change and the various contributing factors that sit behind that .

For me, the latter period really kicked in during the late eighties.  Both 1989 (10.22%) and 1990 (10.76%) had annual CETs that were 10% or more above what the rolling thirty year average had been up until the preceding year.

Yet they are the only two years since 1949 to do that. With 1921 there have been four such occasions in the last 120 years.  Yet in the 19th century there were 11 such occasions.

In fact, if you rank each years by Annual CET of Year x  / The 30 year rolling average up to the year X-1 , the only year since 1960 apart from 1989 and 1990 which makes the top 30 is 1997 which scrapes into it at 8.81%.

Now I guess it's harder for years to get that degree warmer than a working average when the working average itself is getting so much warmer all the time.

But what I'm curious about is the years that were relatively so warm in different times.  What was the reasoning behind it, if you like the counter argument to volcanic activity?

For example 8 years within a 50 year period  in the 19th Century which saw mean CET's above 10C at a time when the normal average was more like 9C.

1834 was remarkable.  Only January and May within the year could be regarded as particularly warm (and yet May was 2C down on the previous year!!!) but it rolled in with an annual value of 10.51C, a year that would not look out of place in today's warming climate. In fact it beat the record of 1733 just over a 100 years prior which came in at 10.5C , and not other year came within a tenth of a centigrade until 1921.

Do we know why those years were some warm relative to their time?