Yes, earth infact does have a natural climate shifts opcycles over time. But the change we have seen over the years since the industrialization age is substantial compared to a natural change. Not only polar ice caps, but ice caps on mountain such as Kilimanjaro have continued to melt and disappear. Rise of co2 levels emitted by humans clearly has an correlation to the rise of temperatures. The change in the currents in the ocean leads to a completely different weather pattern around the world.
Yes, humans are substantially responsible for global climate change, as a consensus among the Scientific community has proven. Humanity's usage of vehicles that require fossil fuels, as well as factories that give off greenhouse gases, is significantly affecting the world's climate. As there is provable Scientific consensus, to think otherwise is a bit silly.
Humans are not substantially responsible for global climate change today because climate change has happened for millions of years and is a cycle that would go on even without humans having existed. There is a major impact as far as the ozone layer being affected by pollution, but rising temperatures are not solely dependent upon human influence.