So, according to IPCC: "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"
And wiki:
"Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a small cooling effect since 1950"

So if you accept these statements. Human activity had little affect upon increasing global temperatures prior to mid-20th Century. Which means that of the 0.74 ± 0.18 °C of warming of the 20th century about 1/2 of the increase wasn't caused by human activity. Next, according to their statement the increase from mid-20th century [which is about .3 C {with huge error bars}] has more half of it due to human activity.
Therefore from their statement one can conclude that all of human activity may have caused a .2 C increase in global temperature [again with huge error bars].

That seems fairly reasonable. If someone said about 1/3 of 20th Century warming was caused by human activity I would say that is a reasonable guess. But if they said more than 1/2 of 20th Century warming was caused by human activity then they would be in disagreement with the IPCC statement.

And the history of IPCC statements are they have overestimated the affect of human activity upon global temperature and/or climate. And UN in general has not proven itself to be a very good institution. But it's hard to argue that human activity has not had .2 C ± 0.18 °C increase upon global temperature.

Now, if they can get those climatic models predicting the next few years, instead the next 100+ years that would be a great improvement.