Actually, our goal should be to reduce atmospheric CO2 levels to those before the industrial revolution, but 350 ppm is clearly better than 450.  Where I part ways from both Jonas and the conservative Hansen is that humans must simplify their lifestyles and live more naturally, especially those humans living in technologically advanced societies.  There will be no magical technological solution to global warming, and the sooner humans grow up and realize that, the more likely this problem will be solved.

The fact that you seriously (I assume) ask what is more simple and more natural is a problem per se.  If you really don't know, nothing I can tell you in a few words could possibly explain it.  All I can say is, if you really want to know, go live in nature for awhile with some people who still live relatively naturally, like the Dine or Hopi in northeast Arizona or the hunter-gatherers who still live in rainforests.  I can give you a strong hint of what simple and natural is definitely not, which is a society based on things like driving and other harmful technologies.  The less technology and use of resources you need, the more simply and naturally you're living.

But I suspect by your post that you're not at all interested in what simple and natural means.  Your priorities are very anthropocentric and you show no concern for the natural world, except as it can be exploited by humans.

And BTW, I watched an hour long interview with Zizek and was not at all impressed.  His thinking is very myopic and anthropocentric, as are those of all leftists, and he understands nothing of the natural world.

Is has been said that scientific revolutions do not occur through logic, but by the Old Guard dying off.   Hansen's retirement would be the greatest Renaissance in Earth and Atmospheric science in modern history.   Finally Naturo-Genic Global Warming theory, Earth Expansion, and other new paradigms would no longer be under the boot of Hansen.

For "ordinary" folks it would mean CO2 neutral.
For those really caring, CO2 negative.

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What is Earth Expansion?

Wolverine, I am not convinced by your simplistic call to go 'back to nature'.

There simply aren't enough caves to turn 6.4 billion people back into cavemen.

But perhaps you are advocating the mass-slaughter of people so that the limited number of caves is no longer an obstacle to the realisation of your naive dream?

My alternative would be a radical denaturalisation of mankind. Build hyper-efficient high-rise cities that are self-sufficient in food and energy and that have an ultra-low impact on their surroundings; make all productive activities part of a cradle-to-cradle cycle. And leave nature to fend for itself.

Just imagine 6.4 billion people returning to villages or bands of hunters-gatherers. That would be environmentally catastrophic. It would mean the death of all forests and its wild animals. It would mean the wholesale destruction of all more or less conserved ecosystems. It would cause mass social warfare.

Jonas,

Instead of my view being "simplistic" as you call it, it is a view that is shaped by a strong reverence for the natural world, which you obviously lack.

First and foremost, I guess you haven't seen any of my posts on the subject, but overpopulation is one of the twin root causes of all serious environmental harm, the other being overconsumption.  However, I've never advocated killing anyone.  The way to address this problem is through birth control, specifically a one-child-family policy like that of China, which has worked so well that the average number of children per family there is now 1.4 in urban areas, where most of the people are.  Abortion and birth control methods should be completely unrestricted and free on demand, and both carrots and sticks need to be used for the one-child-family policy.  That's the way you reduce human population, not by killing people.

Second, of course I don't advocate people becoming hunter-gatherers at this point in human history, mainly for the reasons you identified.  Humans have been causing serious ecological harm to the Earth for 10-12,000 years, so these problems are not going to be completely corrected anytime soon.  But the goals worked toward, whether those goals are set in the near or far future, determine present actions.  Returning to a far more natural way of living must be our ultimate goal, albeit one that transcends the hunter-gatherer lifestyle that humans have known to this point.  What humans should NOT be doing is seeking more, and more advanced, technologies, because that puts us on a path to ever greater destruction of the natural world.

Third, while your idea of all humans living in an extremely dense skyscraper environment and leaving nature alone is a quite tempting one, there are two major problems with it.

First, humans society would be far from leaving nature alone, because the immense waste created by and consumption of natural resources needed in order to sustain your type of community would wreak havoc on the natural world.  These communities would also be ecological dead zones just because of the ridiculously dense human population.  So while I'd support your idea as one that prevents humans from further harming the natural world, it unfortunately would not come close to accomplishing that goal.

Second, this would be a very unpleasant way to live, despite its purported advantages for the natural environment.  I live in San Francisco, where I find the extreme density of humans to be despicable.  The constant industrial noise and feeling of living like a rat in a cage packed with other rats is overwhelming.  (The only reason I live here is because we have a rent controlled apartment and can't afford to move.  Otherwise, I'd be in Berkeley, a much more sane city re human density.)  You're always talking about what's practical; well, this idea clearly isn't, as if even a city boy like me doesn't want to live in such a dense environment, most others won't either.  

Long after Hansen passes from this world, we'll be stuck with his alarmist's claims that politicians are already using to craft policy (see the climate security act debacle a few weeks back). And that is really the rub: his prophesies are so vague, so 'far' off in time, he'll be long gone and all this hero-worship will be for naught. I foresee a future where we look back at our own hubris and gullibility for putting any credence in a government-paid hack who uses his position to garner power like so many carbon offsets.

How many scientists of yore do we already look back upon as disastrously wrong with their theories and predictions and chuckle that people at one time actually believed the then-current snake oil? As late as 1850, current medical theory (the consensus) believed that too much childhood education would lead to over stimulation, causing nervous disorders and insanity (hence a far shorter school day and calendar than we have today). This and other examples far outweigh the number of times men of learning actually got it right. History can and will repeat itself, especially with people like Hansen at the helm, if only temporarily.