In http://news.discovery.com/earth/global-warming/million-year-old-bubbles-reveal-climate-snapshot-150518.htm we see the key quote: "for the past 1 million years, carbon dioxide levels never rose above about 300 ppm". This means we're in uncharted territory and even if we didn't know anything about the probable effects we should still stop pumping CO2 ASAP. But we do know some highly probable effects: climate change and surface ocean acidification (more H+ ions, less OH- ions, even though staying on the basic side of neutral).
Two other items remind us of things that nature might throw at us.
The 20th century had few major geological events. The 21st century has started with two huge earthquakes. What might happen if we have some big volcanic eruptions. Recent analysis of ice cores and tree rings is very interesting: http://www.dri.edu/news/dri-news-and-press-releases/5102-volcanic-eruptions-that-changed-human-history.
There is also the possibility of a maunder minimum in the sun's activity. Some modellers claim (http://www.independent.co.uk/environment/climate-change/mini-ice-age-coming-in-next-fifteen-years-new-model-of-the-suns-cycle-shows-10382400.html) to have an improved model of the sun's sunspot cycle, and that model predicts a maunder minimum in the next 20 years. The last one was associated with, and may have been a contributing cause of, the Little Ice Age (around 1700).
It would be interesting to see the climate models of a couple of big volcanoes going off close to each other in time, during a maunder minimum of solar activity, combined with a lot of open ocean in the Arctic pumping moisture into the atmosphere to fall as snow...
These scientific investigations act as an antidote to the impression non-scientist Greens try to give that all bad things come from humans, and all natural stuff is good. But they don't effect the imperative to stop pumping CO2. Maybe we've lucked out by making it a bit warmer before a cold snap hits, but after the cold snaps we'll race quickly to the trend line of warming. And, of course, acidification will be added stress to ocean surface life during any cold snaps.
The way to prepare for nature's cold snaps is to have lots of spare capacity of cheap carbon-free energy, so that we can do a lot of farming indoors with artificial light. The way to do that is with advanced nuclear power. Needless to say: solar power is not going to work in the next year without a summer.