kw: opinion, global warming, deep time

Reading State of Fear recently prompted me to order my thinking about climate change, the greenhouse effect, and human effects on the atmosphere. My thoughts about the book itself are found in the prior post, or here. I tend to look at the largest possible picture first, and here is what I see:

• We don't know much at all. We can barely detect a possible human-caused "warming" amid the noise of the global long-term climatic cycles caused by continental configurations, orbital changes, the sunspot cycle, and oceanic cycles such as ENSO ("El Niño"-Southern Oscillation).
• Averaged over the past two centuries, the level of CO₂ is lower than it has been for at least the prior 700 million years, and about 8x lower than was typical of the first three billion years of the existence of Earth.
• The Sun is 40% brighter (hotter) now than it was four billion years ago and 15% brighter (hotter) than it was 1.5 billion years ago when the first animals evolved.
• Higher Carbon Dioxide, faster rotation (leading to stronger weather systems), and a more pronounced solar cycle were all factors in keeping the oceans from freezing solid in the distant past.
• Most plant species use C3 photosynthesis, which works best when CO₂ is above 2,000 ppm (0.2 percent), and works poorly when it is less than 250 ppm.
• Only during ice ages has the CO₂ level been as low as 200-250 ppm.
• Prior to the Miocene, its level was always more than 1,000 ppm.
• C4 photosynthesis became widespread in the Miocene.
• C4 photosynthesis plants, mainly grasses, can use CO₂ levels as low as 40 ppm, and large grasslands tend to drive down CO₂.
• The present configuration of the continents together with low CO₂ levels seem to have triggered two million years of ~20 ice ages. We are presently in an "interglacial" period between ice ages.
• I envision: Lotsa grass, down goes CO₂. Ice spreads. Glaciers flatten temperate grasslands. CO₂ goes up. Climate warms. Ice recedes. Grasses begin to recover. Repeat.
• I have no doubt that humans are contributing a hundred ppm or so of CO₂ that would not be there, by burning fossil fuels. We're also staving off the next ice age.
• Soon enough, we'll run out of fossil fuels. Here comes the ice!
• It is well reported that higher CO₂ helps plants grow better. Greenhouse growers add CO₂ to the air in their greenhouses to get plants to market size sooner.
• My last point. Yeah, we're having some warming effect on the climate, but I think it is helpful more than harmful.