Good question - I’m going to have to look up the sources for that chart in the IPCC report, so I can’t answer it immediately, but I should have some info for you by tomorrow.
The short answer to your first question is that nobody really knows - taking the climate +4°C above 20th century temperatures would put us into an extremely warm interglacial period, possibly the warmest interglacial in 400,000 years or more, so temperatures will not have been that high on average for a very long time and it’s hard to say what feedback would dominate at that point. Will it kick the Earth out of its Ice Age phase that’s lasted for the past 2 million years or so? That could be a crucial “tipping point” and I doubt anyone would say they know what sort of climate mechanisms become import after the tipping point, given that there’s very little data to go by.
In general, we do know that some important negative forcings are aerosols, albedo, and stratospheric ozone.
As Chronos mentioned, Venus-like conditions are impossible for Earth without fast-forwarding to billions of years in the future as the sun expands.
However, people do talk about ‘runaway’ climate change in the context of positive feedbacks far outweighing negative feedbacks. The balance of ecosystems switching from absorbing lots of carbon through photosynthesis (young growing forests, for example, are a net sink of CO[sub]2[/sub]) to releasing lots of carbon through respiration (lots of organic matter sitting in tundra doesn’t decompose rapidly at current temperatures, but if that tundra warms up, a lot more bacteria will be respiring, releasing long-stored carbon as CO[sub]2[/sub].)
At the same time, there don’t seem to be a lot of ecosystems in which CO[sub]2[/sub] is a limiting reagent for photosynthesis, so it’s a good bet the two processes will not stay in balance.
Well there is cause and effect to concider, however when one thinks of nuking the planet, your really bringing back imagry of the old cold war , when at their peak both sides would have had enough warheads stockpiled to even think of this.
But when the rubber meets the road, I dont doubt that Europe would have been turned into slag , from Birmingham to Vladivostok, it gets a bit stretched to think that the rest of the world would be glassed.
Not really important for our discussion , but that is about the only time that the autocrat would have enough warheads to even think of this as an option. Todays Autocrat has other options at his, her , or their disposal. You want a high enough casualty rate, but with short term consequences and Someone needs it within a few hours, there is only so many ways of effecting the total population with those parameters.
Declan
That’s the point, isn’t it? If the carbon was sequestered in the biosphere, that means the biosphere was absorbing carbon in a high-carbon atmosphere, not making the problem worse.
This makes sense. My intent is that the Autocrat has to decide right away, but the method itself doesn’t require an instant effect.
There is more predictive lag than you’re probably used to, since in 2035 they’re much better at predicting things.
Ok, fair enough. Venus-like conditions is off-limits.
Still, in the past the Earth has gone on without ice, chugging away with 250+ more feet of water in the oceans.
Neat. There must be a pre-Venus upper limit of the heat capacity of the atmosphere.
Limestone vs. oil & gas: How do they compare geologically, in terms of total mass of CO2?
Thank you for all the great data!
I suppose kicking us out of the Ice Age phase counts as an ‘alarmist’ scenario, no? It’d be interesting to try to guess what would dominate the climate in that world. Especially interesting if you had a lot of money to invest :rolleyes:
You forgot anthrocidal massacre.
It makes sense that levels can’t run away right off the chart. At some point, all the organic matter will have decomposed and things (might) reach a new equilibrium.
Not sure what you mean. Might photosynthesis overwhelm CO[sub]2[/sub] emmissions then?
Thanks, that’s great info.
It looks like we’ll have a pretty good set of sound scientific predictions in front of us in no time. Presuming the correct answer won’t be implemented, as governments will continue to focus on how to help banks steal all our money, the only thing left to debate will soon be the most acceptable massacre method.
One thing to think about:
We have a pretty good handle on the direct effect of adding CO2 to the atmosphere. We have a MUCH weaker understanding of the long-term feedback effects of that. However, the Earth apparently has some very strong feedback mechanisms to regulate climate, because for hundreds of millions of years the climate has stayed relatively stable (in terms of the atmosphere not freezing out or a runaway greenhouse raising temperatures high enough to kill everything). It has remained relatively stable despite major fluctuations in CO2, major asteroid strikes, periods of intense volcanic activity, fluctuations in the sun’s output, the movement of the earth through dusty lanes in the galaxy, etc.
We learn more about these mechanisms every day, and still make somewhat startling discoveries that could have major impacts on our understanding of how the earth will deal with the increase in CO2 over a timeframe of decades or centuries. For example, we still don’t fully understand exactly how the ocean currents regulate warming. We are just beginning to understand how cosmic rays affect cloud formation.
This is why the error bars are so big around long-term climate models. The IPCC’s estimates for the next 100 years range from warming of 1.1 degrees to 6.4 degrees. 1.1 degrees actually represents the long-term warming trend for the inter-glacial period, and would be a net economic benefit to the planet. In fact, any warming below about 2.5 degrees would have a net economic benefit, as modest levels of warming would be mostly felt in terms of longer growing seasons and warmer winter nights in the temperate regions where the bulk of the population lives.
Nonetheless, there is risk of enough warming to cause serious economic damage and human dislocation. We can’t ignore that. 6.4 degrees C of warming is not to be taken lightly.
But still… If you’re going to use the possibility of 6.4 degrees of warming as your baseline for action, it’s equally reasonable to use 1.1 degrees, which would indicate we should do nothing at all, and welcome the change. That’s why we’re better off debating the issue by using the most-likely scenario, and the median value within it.
The IPCC has a ‘low’ scenario, with the best estimate of 1.8 degree of warming, and a ‘high’ scenario, with a best estimate for warming of 4.0 degrees. I think those are reasonable numbers to use for a range when discussing policy options and environmental impact.
In terms of sea level rise, that represents perhaps 10" to about 17".
For a number of reasons, I think the ‘low’ scenarios are more likely. The SRES Scenarios are the basis for the IPCC 4th Assessment report. What caught my eye about them is that the ‘high’ scenarios assume a constantly increasing population, while the ‘low’ scenarios assume a population peak of about 9 billion, with the population stabilizing or decreasing thereafter.
Right now, the UN Population division median model for population growth most closely matches the ‘low’ scenarios. In fact, the UN ‘low’ scenario for population actually has it crashing to 3.6 billion by 2100. I don’t believe that scenario is even considered in the global warming models, and I’m not sure why. The population projections seem to be taken from the median and high estimates only. That doesn’t seem right. And in fact, the ‘high’ scenarios seem to use a linearly increasing population, which even the U.N’s own high model for population growth doesn’t support.
In addition, none of the models factor in the potential for a rapid move away from fossil fuels - either due to ‘peak oil’, or technological breakthroughs.
For all of these reasons, I think any errors in the current models are more likely than not to be errors on the high side. But regardless, I’m happy to use values between the range between the median ‘low’ value and the median ‘high’ value for purposes of discussing policy options.
What is the actual time frame then, we can switch from nuclear and go right to bio warfare. Assume that the population remains constant and somewhere between six and eight billion, how many do you actually envision living in your brave new world society.
Declan
From: http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-chapter19.pdf
(pages 793-794)
Emphasis in original.
From: http://www.springerlink.com/content/u428134536q00138/fulltext.pdf (may be behind a paywall, look for “Risk estimation of the collapse of the West Antarctic Ice Sheet” in Climatic Change 52, published in 2002. Quote is from pg. 83)
So the upshot of it is:
[ul]At +4°C globally we can expect that the Greenland ice sheet will be essentially gone.[/ul]
[ul]It should take anywhere from 200 years to 2000 years for all that ice to melt, adding 2-7 m (about 6-23 ft) to sea level.[/ul]
[ul]Once that melting starts, limiting GHG emissions will not stop it, and it’s unlikely that it could be stopped at all barring fantastical science-fictiony technology.[/ul]
[ul]We don’t know what will happen with the West Antarctic Ice Sheet. If it does melt, that would likely take 400-6000 years.[/ul]
[ul]It may also be important to note that just because these are slow processes, they are not benign. Rate of sea-level change may be more important for risk assessment than total time to melt.[/ul]
There is considerable uncertainty in this, because there’s a lot we don’t know about how glaciers and ice sheets melt. We should remember that such uncertainty cuts both ways – melting might occur more slowly or faster than estimated.
I hope that helps address your questions. 
Again, you’ve managed to find the most extreme possible interpretation of the data. The IPCC projects that warming of 4 degrees C would result in sea level rises more on the order of 2 feet or less. They don’t have ANY models predicting sea level rises anywhere close to 2-7m.
That’s a great answer.
It is interesting that the models do not account for a lower- or precipitously reduced- population (any chance they are trying to be alarmist?). Also interesting that they didn’t resolve the PPP v MER debate by constructing another set of models and comparing the results. The Autocrat is nowhere to be seen in the SRES scenarios, alas. The closest is probably the B1 scenario, which assumes “An emphasis on global solutions to economic, social and environmental stability.” The Autocrat definitely has that going for him.
There isn’t very much sea level rise predicted. Would it simply continue rising at that pace for the following century, and so on, for millennia? And hit ‘zero-ice’ in about 15,000 years. Does that sound right?
And- given your best-estimate scenarios, the upper limit is still 4 degrees C. I’m being alarmist, so consider: terrestrial ecosystems emit rather than absorb carbon with that increase, so there remains the chance for a ‘runaway’ to a worse situation, even including peak oil or a massive move away from fossil fuels.
You can see I’m answering these somewhat in order…
Wow, great links, thanks
On the one hand, there is a significant predicted probability of a large-scale climactic event. OTOH, the effects do not appear to be immediate or dramatic enough to warrant the Autocrat’s following through with his rapid adaptation plan.
Collapse of WAIS represents 15 feet of sea level rise. If ‘zero-ice’ represents a 250 foot rise, WAIS represents 6% of the world’s ice. Most local glacial retreats are trivial by comparison, but there sure are a lot of them. Though I’m sure the IPCC has considered that, and still predict a relatively non-alarming rate of annual eustatic sea level rise.
So the the policy is: go shopping?
Your thinking is so 2009 
The time frame could be a decade or more.
I suppose the end population would be 4+ billion, accounting for the possibility of sub-disasters arising.
This is considering threat of a catastrophic climate change, with rapid melting to zero ice and the attendant sea level rise, accompanied by cataclysmic weather patterns. The stabilization could be 500 million or even less in this scenario. The negative effects might peak in 50 years or so.
You have to admit, I’ve devised an alarmist case. It just isn’t particularly realistic.
Sam Stone - if you read my post carefully, you’d notice that I cited the IPCC’s 4th Assessment Report.
I believe you may mean ‘The IPCC projects that warming of 4 degrees C would result in isostatic sea level rises more on the order of 2 feet or less. They don’t have ANY models predicting isostatic sea level rises anywhere close to 2-7m.’ The projections out to 2100 don’t include melting ice sheets for the obvious reason (elaborated on in my post) that ice sheet melting will result in sea level rise after 2100, not before.
If you think I’m lying, follow the links in my post.
My solutions:
- Elminate anyone with a disability and/or IQ lower than 125.
- Declare currency worthless.
- Eradicate technology totally.
(Not in any specific order)
So you want to cast humanity into primitive misery ( or quite possibly extinction, with NO technology ), after massacring most of the population according to a eugenics scheme. And you want to unnecessarily declare currency to be useless, when you’ve already made it useless by destroying civilization.
That’s a “solution” that is far worse than the problem.
Living without a TV, or extinction of the human race. Hmm, that’s a toughie.