If we can't explain it, It must be human-induce
The fact is, science doesn't understand why these natural climate variations occur, and can not reliably distinguish between natural and possible human influences on global temperatures. So, if scientists have no other natural explanation for a warming trend, they tend to assume that it is manmade.
For instance, you might have heard claims to the effect that no peer-reviewed scientific study has refuted the claim that global warming is manmade. Well, there have indeed been some papers that have at least questioned the theory that our current warmth is manmade....but the publishing of alternative explanations is hindered by the fact that our long-term global climate observations (e.g. of cloud characteristics) are not good enough to measure the small changes that might offer an alternative explanation for our current warmth.
Science can not deal with what we can not measure. But scientists could at least admit to incomplete knowledge -- unfortunately, most of them do not.
I can not overemphasize this -- the theory that our current warmth is manmade is largely the result of not having good enough global observations over a long enough period of time to rule out natural causes. Therefore, the current widespread support for the theory of manmade global warming is NOT because the alternative explanations have been ruled out. It is because our poor understanding of natural climate variability does not yet permit alternative explanations to be investigated thoroughly.
Thus, while it is indeed possible to explain much of the warming over the last 100 years with manmade greenhouse gas increases, this is only one possible explanation -- one that necessarily ignores or minimizes any natural sources of temperature variability.
As a result, our worries that global warming is manmade are directly related to how much faith we have that natural climate variations (for instance, a small decrease in low-level cloudiness) are not substantially contributing to our current warmth. Some scientists who believe in manmade global warming have asked me, "But what else could be causing the warmth?" Note that this is arguing, not from the evidence, but from a lack of evidence.
There is an old saying, "When all you have is a hammer, everything looks like a nail." Well, manmade global warming is our hammer, and so every change (nail) we see in the climate system gets attributed to mankind.