I understand that that is the "official" view of climate science, but that doesn't mean it's right. I don't think we're going to resolve that sort of dispute here, but if your only argument is from authority--"climate science says so"--then that's not enough for me. I don't think climate science as a field has exercised sufficient care to be just taken at their word.
If you're going to say "well what if there's a long cycle" - then well, what is it exactly?
Nobody knows for sure; we don't understand how the climate works well enough for that. But the fact that we don't have a theory that explains the cycle doesn't mean there isn't one.
Where is all this additional heat retention in the atmosphere coming from?
Actually, the amount of heat being retained in the atmosphere is miniscule on a planetary scale. The significant heat sink is the oceans. You have to be careful distinguishing between heat and temperature.
As for where it's coming from, as above, we don't understand all the causal factors at work; the fact that we have studied one intensively (CO2) does not mean that one must be the only significant one. (The IPCC report itself lists a number of significant causal factors as having a "low" level of scientific understanding.)
The data for example, does not show a 1000 year cycle.
Whose data? I understand that the "Hockey Stick" does not show a 1000 year cycle, but the "Hockey Stick" is not data; it's a (questionable at best) reconstruction of temperatures from data. We do not have direct temperature data from 1000 years ago, and as far as indirect data, there is plenty to indicate that there was indeed a Medieval Warm Period and a Little Ice Age, as I said.
That's what anthropogenic climate change is - since the only effect that suitably explains warming is increased eCO2 in the atmosphere.
Absent (effective)CO2 increases, you can't explain the warming trend. There shouldn't be one as pronounced.
If you're going to say "well what if there's a long cycle" - then well, what is it exactly? Where is all this additional heat retention in the atmosphere coming from? There are no multi-decadal phenomena on Earth which can explain the temperature record.
EDIT: The data for example, does not show a 1000 year cycle. You're inferring there's a cycle, by doing what you accuse others of - pointing to a graph, and declaring that because it looks "roughly" cyclical it is.