Of course, as one might expect, the reality lies somewhere in between.
Does human activity affect climate variation? Certainly. Does climate vary in the absence of human forcing? Most certainly. What does this mean for human civilization and the future of the Earth's environments? We-e-e-ell, it's hard to say. Prediction is a tricky business, especially with respect to the future.
As an archaeologist, I view climate variation from a different time scale than that discussed by most climate change enthusiasts on both sides of the argument. 17, 20, 50, 100 years is an eye-blink in geologic time, even in the evolutionary history of Homo sapiens. When we look at long series data, such as the Vostok ice core records, it's clear that the current warming phase is the most recent warming in a cycle that goes back 1.5 million years. We're about to the top of the cycle, ready for the long plunge into the next "ice age" that will bottom out in about 20,000 years.
Coupled with Peak Oil and the incipient decline in fossil fuel resources, human contributions to Global Warming will decline within the next 100 years. If human activity has delayed the onset of the next cooling period, this will certainly come to an end within the lifetime of many people now alive. That doesn't mean glaciers will start marching across Fargo, North Dakota anytime soon. Rather, it means that Anthropogenic Global Warming will not continue indefinitely, atmospheric CO2 levels will decline as the cooling ocean absorbs the excess, and the Earth will slide gently into the next cycle of Global Cooling.
What does that mean for humans alive on this Earth right now?
Given the overwhelming force of natural climate change exhibited in long term climate cycles, there's nothing humans can do to forestall the ultimate descent into the next cooling phase. At present, science, politics and public policy is focused on mitigation of observed increases in global average surface temperature, perceived to be caused by anthropogenic CO2. While this is ultimately futile, the economic and technological changes necessary to reduce anthropogenic CO2 will have the unintended consequence of also decreasing some human caused pollution, habitat loss and biodiversity reduction caused by fossil fuel exploitation.
However, reduction in fossil fuel use must be accompanied by increases in "renewable" energy resources such as wind, solar, tidal and geothermal, which have their own demands on natural resources, critical natural habitats and the species that live therein. There's no "clean" energy source to replace fossil fuels that will eliminate negative consequences to non-human species.
Whether the coming changes in human societies be aimed at reducing global warming, preparing for the inevitable global cooling, or learning to live as a part of the Earth, not apart from the Earth, humans must develop resilient societies capable of accommodating to a variable environment shared by myriad other living beings. This means a drastically smaller human population, consuming considerably less energy and natural resources than we do today.
Rather than fighting among ourselves over the last bite on the plate, and wasting billions of monetary units from our declining economies, why not just do the right thing that we should have been doing all along? That is, reduce our impact on the natural world so as to increase the chances of all species surviving indefinitely into the future.
Now would be a good time to start.