The period between the most likely date of composition at 829 C.E and the age of the manuscript which is likely to be the oldest recording (A: c.1100 C.E.) was probably a period of considerable climatic change. Current thinking is broadly that there was a "Roman Warm Period" ending in ~100 B.C.E., followed by a "Dark Ages Cold Period" ("DACP") ending ~900 C.E.* and then a "Medieval Warm Period" ("MWP" : Summary) ending ~1250 C.E.* before the "Little Ice Age" (Summary) which we climbed out of in the ~1850s.
Because of its position as a possible analogue for future climate change, and its display of naturally-induced warming, the MWP has received far more attention than the DACP, though the existence of both has generally been the source of much controversy*****. Recent work has generally proved their existence, and whether** or not** the MWP was warmer than today, it seems likely the DACP was somewhat cooler*.
Work on multiple sources (including manuscript records and even vinyard locations) by Lamb* suggested that the DACP lasted long enough to bring a series of severe winters to Britain in 830-870 C.E. with cyclonic weather dominant, before the MWP at 1000 C.E. He suggests the MWP saw dominantly anti-cyclonic weather over Europe (but with milder winters for Britain than expected) and warm summers, with wet periods 900-1060 C.E. and after 1300.
Subsequent researchers have always been impressed by Lamb's research, but somewhat more circumspect about his conclusions, based, as they largely are, on contemporary accounts rather than "scientific" data. However, the broad trend of a period of colder than present annual conditions followed by a period at or just above current temperatures, particularly in the Summer*, seems accepted. It seems like there may be a longer-term 1500 year cycle of temperatures in the North Atlantic (at least), though causes suggested range from fluctuations in the ocean-atmosphere system* to changes in solar inputs and volcanic eruptions**.