Gleick thinks that 390 parts-per-million carbon dioxide can swing the 999,610 ppm atmosphere.
In his latest hand-wringer in Forbes (“Climate Change, Disbelief, and the Collision between Human and Geologic Time“), Peter Gleick writes,
… Human-caused climate changes are different. As the planet’s population has grown to 7 billion people, and as we have learned how to mobilize vast quantities of carbon-based fossil fuels (ironically, created over geologic time scales) to satisfy our short-term energy demands, we have become powerful enough to overwhelm slow geological cycles. We are, for the first time in the 4+ billion year history of the Earth capable of altering the largest geophysical system on the planet – the climate – and we are doing it on a human time scale of years and decades, with consequences we are only just beginning to comprehend. And ironically, our effect on the climate is still slow enough for policy makers, climate contrarians and skeptics, and those simply not paying attention to either actively deny it or to just look the other way, committing the planet to more and more change. [There are other examples of human influences on a global scale: our construction of dams and storage of massive quantities of water behind reservoirs has literally, albeit modestly, altered the rotation of the planet. But none are as significant as our effect on the climate.]
Some will never be able to accept this, no matter the evidence. They will continue to conflate geologic and human time scales and assume that what is occurring today must be what has always occurred in the past — natural. But the inability to comprehend the planetary influence of humans isn’t based on reviewing and rejecting the scientific evidence, which is clear to 97-98% of climate scientists publishing in the field. It is based on ignoring or disbelieving it, just as some dogmatically refused to abandon their belief in a geocentric universe for reasons that had nothing to do with science. And alas, these modern-day dogmatists are unlikely to change their minds, at least not on a human time scale.