Hansen on climate change over centuries

Following my recent post, a number of commenters suggested that I ought to respond more directly to the arguments of James Hansen and others for a CO2 target of 350 parts per million, as opposed to the 450 ppm that forms the basis of much current policy discussion. I’m using this paper as a basis, and take the following two points as its central claims

* To avoid unacceptable risk of passing a point of no return beyond which explosive feedbacks (icecaps melting etc) are inevitable, we should aim to reduce CO2 concentrations to 350 ppm by 2100. This is below current levels and won’t be achieved simply by ending net emissions
* We can achieve part of this (maybe a reduction of 60 ppm) through reforestation, biochar and similar measures
* Further reductions will require expensive technological solutions, estimated cost $200/tonne or $20 trillion to remove 50 ppm. Given a maximum point around 450 ppm and 50 ppm from reforestation, that’s about the amount required.

What then should we do? In particular, how much should we be willing to pay now, to avoid high costs in the second half of this century?

It’s important to note a big shift in focus here. Most of the discussion so far has been along the lines “What do we need to do by 2050 to avoid unacceptable damage to the climate later this century”. Looking ahead for a century is challenging, to put it mildly. But the questions raised by Hansen shift the time-scale for action out another 50 years, and the consequences are centuries into the future. That means there are huge uncertainties that are difficult to reason about. As a starting point, I’m going to follow Hansen and co-authors in treating the problem as if it were deterministic, with a known target of 350 ppm and costs as stated.

With these drastic simplifications, the problem is not all that hard, and can be made a bit simpler with the right choice of parameters. The question is, how much would we pay (in $/tonne) today (I’ll say 2010) and around 2050 (I’ll say 2045) to avoid a cost of $200/tonne in 2080 (all in constant value dollars). With a 2 per cent discount rate (I’ve argued at length that this is a good choice), the answer is given by the rule of 70: values double every 35 years. So, we ought to be willing to pay $50/tonne now and around $100/tonne in 2045.

I’ll come back to this a bit later and discuss less simplified estimates. The main point that would suggest a higher current price is a lower trajectory with the same endpoint implies less residence time for CO2 in the atmosphere and therefore less risk.

152 thoughts on “Hansen on climate change over centuries

  1. Glenn– Those who actually understand Control Theory know what I am talking about. The analysis is shown. You learned what integration means in engineering school. I did the integration numerically in time increments of a year. The references to data sources are all given.

    It is history that the LIA and Maunder Minimum happened during the same time period. Google it.

    A complete quote from my pdf might help. “Thus a rather simple energy relation can be developed where the energy received by the earth is proportional to the integral of the sunspot activity and the energy leaving the earth is proportional to the fourth power of the absolute temperature of the earth. This was done…”. Instead of saying “This was done…” Perhaps this would be more clear to you ‘I did this and then determined a proportionality constant that resulted in approximately level results for the first part of the curve as shown here.’ Did you not understand that “proportional to the fourth power” meant proportional to the time integral of the fourth power? That is what I did.

    “…integral of the sunspot activity” It is well known that low planet temperature has coincided with low sunspot activity. If you are unaware of that, you might review this http://en.wikipedia.org/wiki/Maunder_Minimum and this http://solarscience.msfc.nasa.gov/SunspotCycle.shtml . If you reject this indication of causation between sunspots and planet temperature change then what follows isn’t for you and you will continue to wonder why the average global temperature trend continues down.

    The rate of absorption by the planet of energy from the sun is power. The time integral of power is energy. The radiation from the earth is also power so the time integral of that is also energy. From the first law of thermodynamics the difference between these is the change in energy of the planet which is proportional to change of average global temperature. That is what the ordinate of the graph is about.

    The name of the second graph is apparently mysterious to anyone but me. I had put it there while doing the work as a reminder that this was the chart that had radiation from the planet proportional to the fourth power of its absolute temperature and then neglected to delete it.

    The AGW mistake itself is of little interest to me since with time, as the added atmospheric carbon dioxide level continues to increase and the average global temperature doesn’t, it will become obvious that the IPCC and many Climate Scientists got it wrong. The black eye on all science will eventually be forgotten. However, the AGW mistake is heavily influencing policy and economics and taking away freedom and prosperity. I am against that.

    The science community (including the journal peer reviewers) apparently do not understand Control Theory which is a graduate level engineering subject. If they did they would recognize from the paleo temperature trend reversals that there is no significant positive feedback from average global temperature. Some journals refuse to even review papers from reputable Climate Scientists if the Climate Scientists are known to disagree with AGW. Challenging the bias of the journals would be hopeless although the continuing substantial increase in atmospheric carbon dioxide (the increase since 2000 is 18.8% of the increase from 1800 to 2000) with no increase in temperature trend must surely have caught their attention.

    The last time the sun was this quiet (average number of sunspots less than 4) this long (31 months and counting, August had none) was around 1913 (41 months). Some of us know what this portends.

  2. Doesn’t Hansen say that best evidence for AGW comes not from modeling, which supports it, but the historical temperature and CO2 records? Climate scientists be reading those records differently from Control Theory advocates, which seems like a rather slim thread in denialism. Also, turned up this:


    Also, might it not matter what is doing the forcing in a Control Theory analysis? I realize the contradiction with using correlation as I described in previous paragraph, but if you’re not having it that way, how about this way?

    Really enjoying this blog, and totally sympathize with Glenn. At times like this, I wonder if we need a command economy. Just research and build renewables and storage as fast as we can. Put everyone to work. Anyway, didn’t the commanding heights of capitalism collapse in 2008? It’s likely that will happen again soon, the way things are going. Next time, I don’t think we should be doling out credit simply to prop up the pre-existing distribution of wealth, but rather to build what humanity really needs.

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