My response is to signpost the 2007 IPCC summary for policymakers. The following is an excerpt:
"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations. It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica). During the past 50 years, the sum of solar and volcanic forcings would likely have produced cooling."
It is true that significant climate change has happened in the long-distant past. However, the main threat to humanity now is that, largely driven by human activity and in a much shorter timescale than geological changes, the climate has changed and is continuing to change in ways that the natural world is struggling to survive and adapt to, let alone thrive in. The potential impacts on future crops, let alone human and non-human inhabitants, is very concerning. Like the Easter Islanders, if we don't respond in time to the need to operate self-restraint within a closed ecosystem, disaster eventually awaits ...
The following chart of warming and cooling influences, from "Climate Change - evidence, impacts and choices" is also informative: