Inevitability of geoengineering, part 2

In my previous post, I explained why geoengineering to counteract global warming seems inevitable. To summarize, data suggests that between 2009 and 2050:

• World population will increase by about 45%
  
• Poor people are getting a lot richer, average wealth will increase by 200%
  
• This means they will use a lot more energy than now
• Neither renewables nor nuclear power seem likely to be able to replace them by 2050, at realistic growth rates
• Therefore only plausible source of that energy are fossil fuels, and we already have plenty of examples of countries drastically increasing their fossil fuel use and CO2 emissions extremely quickly.
  
• Reduction in energy use by rich countries is nowhere near big enough to counteract this
  

What changes around 2050?

• World population is expected to more or less flatten out around 2050. We're already adding fewer people each year than during peak growth, so growth is sublinear, let alone expecting it to be exponential. Long term projections need to be taken with a pinch of salt, but it's as safe as it gets to expect much slower growth in second half of 21st century than now.
• While world economy will keep growing, average GDP per capita will be more or less where EU levels are now, and energy use past this level doesn't increase that much.
• In all likelihood, sources of energy other than fossil fuels will be far better developed than now, so responding to any new energy needs by more renewable energy instead of by more coal power plants, will be much more likely.

Now I don't know exactly when we will have peak CO2 emissions, and at what level - somewhere vaguely around 2050, and at levels twice current seems likely, as decades before it will have much higher pressures to increase fossil fuel use than decades after.

The first problem, is that peak CO2 concentrations will happen long after peak CO2 emissions. Here's a random graph with emissions on the top, and concentrations on the bottom, color-indexed. In all scenarios, including almost immediate halving of emissions, peak is somewhere in 21st century, but CO2 concentrations keep growing long past 2300, peaking hundreds of years from now!!!
Now I don't actually believe CO2 emissions will be falling that slowly - once we figure out how to use renewable energy cheaply, we'll be able to switch almost all our use to it, so peak concentrations will be somewhere during 22nd century.

It gets worse - many effects of CO2 concentrations, like temperature change and ocean acidification, lag it significantly. So peak warming and peak acidification will occur much later than peak concentrations. With 2050 emission peak, and 2150 concentration peak, we might have 2250 climate change effects peak. (last figure completely made up, as it depends on too many details, different effects have different lags, and nobody makes serious estimates that go that far into the future). So even with a lot of optimism about alternatives to fossil fuels, we will have to deal with massive effects of global warming.

This gets us back to geo-engineering. No matter what we do now, short of a genocide and collapse of global economy (world economy grew during the recession, and fell only a few percents in the worst hit countries; this isn't the collapse I'm talking about), our choices are geo-engineering and adapting to higher temperatures.

Disclaimer time

Now disclaimer time. I'm making the common mistake that anthropogenic CO2 emissions are responsible for all global warming. This is not true - added CO2 is only 2/3 of it, the rest being methane, nitrous oxide, CFCs. But there are good news here - their lifetimes in atmosphere are much shorter - while we're talking tens of thousands of years for CO2, it's only 12 years for methane, and about 100 for nitrous oxide and CFCs, so peak concentrations will occur relatively quickly after peak emissions. And in fact, peak concentrations of methane and CO2 seem to have already happened or be happening about now. In longer perspective global warming problem is CO2 problem, with only small methane and nitrous oxide contribution.
The second disclaimer is that there are two kinds of geoengineering - solar radiation management (stratospheric sulphides, cloud seeding, albedo changes and such), and carbon capture and storage.

With solar radiation management we let warming happen, and create equal negative effect.

With CCS we could pull more CO2 from atmosphere than we're emitting. If we went this way peak concentrations might happen much faster after peak emissions than otherwise, and due to rapid fall in concentrations peak warming will be earlier and smaller.

I'm mostly talking about solar radiation management, as we already know how to cool down the planet with current technology and for ridiculously small money; while CCS solutions don't seem to be anywhere near scalable enough soon enough.

Any alternatives like fussion power, nanotech, and new kinds of renewables that people love to talk about won't seriously affect emissions until long after 2050 even in the most optimistic scenarios.