That goal of keeping CO2 emissions below 350 ppm (parts per million is simply a way of measuring the concentration of different gases, and means the ratio of the number of carbon dioxide molecules to all of the molecules in the atmosphere) was once thought of as a reasonable and reachable one. But greenhouse gases change the climate in ways we have never encountered before. Above the level of 350 ppm of CO2, life for most species on earth, especially humans, is more and more tenuous and at risk.
As of May 10, 2013, the most up-to-date reports about accelerating Climate Catastrophe show that CO2 levels have already reached 400 ppm. This is much sooner than climate scientists once predicted (see both Climate Collision Course: CO2 Levels About to Hit 400 PPM | Common Dreams http://www.commondreams.org/headline/2013/04/29-2 and In 'March Toward Disaster,' World Hits 400 PPM Milestone | Common Dreams http://www.commondreams.org/headline/2013/05/10-5 ), meaning all life on earth will be seriously threatened in the next 30 years.
As the first article linked above states, "The last time the greenhouse gases were at 400 ppm was likely the Pliocene epoch, between 3.2 million and 5 million years ago. The current reading is at 399.72 ppm -- far past the 350 ppm level many, including noted climate scientist James Hansen, have warned is the upper safe limit before the planet hits a tipping point."
This level of CO2 at 400 ppm means much higher average temperatures in summer, more droughts, more melting glaciers and ice caps, more rising sea levels, more intense hurricanes and other extreme weather, more famines, rising food prices, increased scarcity of clean water, more climate refugees, more species extinctions, wider spreads of tropical diseases into temperate zones, and more civil unrest.
Yet, no world leaders are seriously addressing this coming catastrophe, let alone proposing effective policies to mitigate its disastrous consequences. And "mitigate" is the appropriate word, because we are not going to avoid this catastrophe, nor reverse it.
In fact, many leaders, such as President Obama, are extolling more drilling for oil, more oil from tar sands, more gas from fracking, and more mountain-top removal for coal, as though the burning of all this additional fossil fuel will have no effect on Catastrophic Climate Chaos. Their energy advisors and corporate funders, all solidly part of the fossil fuel lobby, ignore science in favor of more obscenely-high profits for themselves and their shareholders, as though more money will somehow insulate them on an unlivable planet. And an unlivable planet, for humans and many other species, is exactly what we are headed for in the next decades.
Most analogies are far from perfect, but indulge this one: all of us, including non-human species, are on a runaway train heading for a cliff's edge. The engineers in the cab (world political and corporate leaders), instead of applying the brakes, are feverishly shoveling more coal and wood into the engine, carelessly ignoring the increasing speed as the train rushes faster and faster to the cliff's edge. Meanwhile, too many of us in the passenger cars are preoccupied and distracted to pay much attention, as the train's P-A system (corporate media) continues to focus our attention on stories about celebrity, political scandals, more products to buy, and "reality" TV and sports. In this way, the accelerating threat to our existence is almost completely ignored.
At levels of CO2 beyond 400 ppm, there is the added danger of permafrost melting. This would release methane, another greenhouse gas, and multiply the already-dangerous effects of high CO2 levels, further speeding up global warming and Catastrophic Climate Chaos. If that were to happen, all hell could break loose in very literal ways on this, our only planet.
The future would then be very bleak, as billions of people would suffer and die, in addition to most other species. For those who managed to survive, life would be extremely difficult and so different from what we now experience people would think they were living on another planet. And, in a sense, they would be. They would be on a planet no humans have never known.
In such conditions, future historians, if there were any, would look back at the last decades of the 20th century and the first decades of the 21st and wonder how and why we allowed this once-avoidable catastrophe to occur. They would also wonder how we could have all but destroyed the futures of our children and grandchildren so carelessly and selfishly.
We already know the answers to those questions. They indict us as a species still arrogant and foolish enough to call itself "homo sapiens sapiens."