350ppm a realistic goal?

[Nuclear]

To reach the 350ppm climate stabilisation goal, emissions would have to peak very soon and steeply decline after that. By the end of the century, the world economy would have to essentially be carbon-negative to remove the excess carbon already in the atmosphere.

The question is: was 350ppm ever a realistic goal? I personally have a hard time seeing world leaders agree to such an aggressive emission reduction scheme. Wouldn't it be more sensible to set less ambitious, but politically more acceptable goals? What about 450ppm, 550ppm or 650ppm? The world's big economies, in particular those still industrialising and relying heavily on fossil fuels to do so, would be far more likely to agree to moderate emission reduction targets. Isn't some progress better than none?

[Grant]

350 ppm is probably not a realistic goal but James Hansen and company insist anything much above that will have a disastrous effect on human civilization. I'd like to see a serious discussion on the matter by a broad range of experts. For the time being here is their recent assessment.

[Nuclear]

What are those "disastrous effects"? The problem I have with the current climate discussion is that too many people, even climate scientists, resort to hyperboles when calling for political action on climate change.

I have yet to see evidence that climate change would pose an existential risk to human civilisation as a whole (well, if you ignore outlandish scenarios like a runaway greenhouse effect leading to Venus-like temperatures on Earth). Thanks to technological advances and economic growth, we are beyond that stage at least what our dependence on the weather is concerned. We don't need a Holocene climate to sustain our civilisation. Cities innundated by the rising sea can be rebuilt further inland. Crop growing areas can shift further north and south and advances in technology allow us to make previously marginal land more productive.

Yes, the consequences of climate change will without any doubts be very costly terms of money and human lives, especially in impoverished third-world countries which don't have the resources required to adapt to a changing climate.

The case for climate action lies in a thorough cost-benefit analysis. I think it is pretty evident that after a certain threshold is crossed, the costs of mitigation are lower than the costs of adapatation. But even under more rapid warming scenarions, the cost-benefit-analysis of poorer countries may still lead to them burning more fossil fuels, at least in the short term. After all, development and industrialisation are the best way to increase a society's resilience to weather extremes.

A sensible climate policy needs to take into account what is realistic, what is politically and economically achievable. 350ppm is not. 450ppm looks out of reach too. 550ppm may still be a viable goal even the developing countries would support, if the industrialised countries (US, Europe) commit to strong short-term emission reductions (80% by 2050) and the developing world feels it gets the time and the means to properly industrialise and catch up to the rest.
One thing to take into account is that in a 550ppm world, geoengineering technologies to draw carbon dioxide from the atmosphere may have to be deployed in the very long run. That is the price we will probably have to pay for delayed mitigation.

[Roger Clifton]

I agree with Grant, 350 ppm (with a 2° asymptote) is unrealistic, already out of date, now that we are (Dec 2013) at 396 ppm and rising at 2 ppm/a (*). It might have been achievable decades ago when Hanson first proposed it, but now even his solution, of massively increasing the forests, is questionable. The 1° of warming that has already been delivered has made classic forests obsolete, and around the world they are conflagrating one by one, made vulnerable by heat and drought.

However, the story of the 350 ppm target is worth keeping alive so that it can be quoted back to world leaders when world opinion eventually reacts, probably in horror at a series of particularly savage climatic disasters.

As far as expert discussions go, future projections are routinely presented at the "Greenhouse Conferences" in Australia. Given by invited experts to an audience of their peers and humbler delegates, they are as authoritative as you can get. I remember there was a workshop in the 2005 conference, on the question of whether we could tolerate 2° or 3° of increase. Here are the presentations for 2011.

(*) Exceeded 400 ppm in Dec 2015, see graph for more recent CO2