Here's the reality of the situation: we do not know the effect of mankind on the climate and the ecology. However, we do know that certain activities *have* an impact.
Is that a positive or a negative impact? If we don't know the effect, then how can we possibly say whether it is for our benefit or to our detriment?
Do not mess with the ecology of the planet without understanding the consequences.
Yes. Let's roll back the last 10,000 years or so of agriculture, as it "messes" with ecology. I'm not so sure the ~7 billion people on Earth today would be sustained by this, but it's more ecologically sound not to farm, so let's do it. Those whose childrens' children who manage to survive will thank us for our sacrifice.
Don't get me wrong - I'm all for not taking more than what I need in the interest of sustainability for others or eating fish without toxic levels of mercury, but inconvenient half-truths deserve to be called out as such.
Ecology is about the relationship of organisms to the environment. Do humans have an effect? Of course, so do koala bears. My point is that it's impossible not to mess with the ecosystem in which you live. Living implies having an impact on said ecosystem. Understanding that impact won't prevent it. It is a cold, hard fact that a population cannot outgrow its food supply.
It's probably safe to say we're headed towards a mass extinction event, and none of the previous ones (generally believed to be 5 of them, AFAIK) weren't caused by humans, as humans hadn't existed yet. That leaves external stimuli, such as the Sun as a strong potential contributor to climate change. Forgetting that humans have never caused a mass extinction before, let's entertain the idea that we have the ability to cause one. How long can the current rate of consumption of resources & population growth continue before something bad happens? Do we have levers that can effectively extend that time? What if Canada and Siberia warm up enough to support even MORE farming than we can today? Perhaps the food supply can grow enough that the population explosion can be supported.