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Here is James Hansen's original paper published on the topic suggestion 350ppm as a safe point. Earth used to be about 260-270ppm before civilized human life thrived. That number is considered the natural atmospheric CO2 levels for earth. We are currently at levels of 387ppm CO2. If we reach levels any higher (and we WILL given current trends), we will face complete loss of ice caps on the planet and we will see species pass.
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Please don't blast me for saying this, but I have a bone to pick with Hansen's science. While I do not doubt that global warming is real and it is, to some extent, anthropogenic, I have doubts about how bad
moderate warming is. Why?
First, 90% of the past 400,000 years have been characterized by ice age. Ice ages are, to put it mildly, going to be detrimental to food production for humans and other species. Let's say that we bring down CO2 to Hansen's 260-270ppm level. Well, during the "Little Ice Age" (approx. 1550-1850AD), CO2 levels hovered around the 280ppm level. The twelve thousand year Holocene era, which we live in, has also been warmer - between 2-5ºC warmer! That warm time period, about 7000 - 5000BC, was associated with the rapid transition from nomadism to the development of agriculture and sustainable human culture. Humanity also thrived during the Medieval Warm Period, ~800 - 1300AD, where temperatures were ~1ºC warmer than today.
I have two points here: a) It is not the hottest it's ever been and b) Warmer temperatures are not historically linked to humanity fairing poorer.