Yesterday, I referred to Stuart Staniford’s blog post entitled ‘Odds of Cooking the Grandkids’. (His analysis, in turn, was based on a scientific paper published in the prestigious journal the Proceedings of the National Academy of Sciences.) What shocks me most is that when Staniford talks about cooking the grandkids he means it literally:
The bottom line conclusion is that there is a small – of order 5% – risk of global warming creating a situation in which a large fraction of the planet was uninhabitable (in the sense that if you were outside for an extended period during the hottest days of the year, even in the shade with wet clothing, you would die).
I like to think of climate change risk as the likelihood of bad stuff happening to my family or myself due to global warming. Well ‘death’ certainly comes under the category of ‘bad stuff’ and a 5% probability is not negligible, and certainly far higher than the kind of probabilities we usually insure against.
However, to literally cook the grandkids, we would need to see 5 or 6 degrees Celsius of warming, and that is not likely to occur before the end of century in anything but the most dire scenarios. Easier to focus on is whether we are in danger of warming the kids to a degree that transforms their life outcomes (and potentially the old age comforts for any adult under 60).
Now we already discussed whether dangerous climate change (warming of 2 degrees above pre-industrial revolution levels) and extremely dangerous climate change (3 or 4 degrees of warming) would do just that. Simplistically, the former would likely lead to economic disruptions (see here) and the latter would add on socio-political disruptions (mass migration, revolution, war—that kind of thing–see here).
In the previous post we also noted that the international community has decided that 450 parts per million of CO2 equivalent (450 CO2-eq) is the level at which we are likely to get 2 degrees of warming. But how likely is likely?
Stabilisation at 450 ppm CO2 equivalent would only limi warming to 2°C if the climate sensitivity to a doubling of CO2 were 3°C or lower. However, as the IPCC AR4 found, the climate sensitivity is quite uncertain, and whilst IPCC’s best estimate is 3°C there is roughly a 50:50 chance that it is actually higher. Taking into account this uncertainty, global warming may well exceed 2°C for stabilisation at 450 ppm CO2 equivalent.
The paper goes on to note that if the global community wished to play things safe and a) aim for a maximum of only 1.5 degrees of warming and b) limit the probability of this degree of warming being exceeded to one in three, then a more appropriate atmospheric CO2 -eq target would be 350 ppm (of course, we are already over 380 ppm CO2-eq). The probabilities associated with different warming targets and CO2 levels are shown below:
Incidentally, the 350 number was also reached by NASA’s Jim Hansen by a somewhat different methodology and forms the corner stone of the campaigning climate change organisation 350.org headed up by a personal hero of mine Bill McKibben.
In a landmark article in the journal Nature, Meinshausen and his co-authors then took the probabilistic approach further by assigning a carbon budget. Concentrating on the 450 CO2-eq target they asked the question:
“How much CO2 can we emit with only a 25% chance of going over the 2 degree tipping point and how much can we emit is we take the riskier option of only having a 50% option of keeping within the target?”
Using the year 2000 as a base year, the paper calculated that 1,000 Gt CO2 could be released between 2000 and 2050 such that there was only a 25% chance of missing the 2 degree target—or, put another way, a 75% chance of achieving the target. (After 2050, the paper assumed that CO2 emissions would by then be negligible and compensated for by land-use related CO2 absorption.) For the 50% figure (a more risky bet), they arrived at a larger budget of 1,440 Gt CO2.
OK, so you may now be saying “What the hell is 1,000 Gt of CO2? That number doesn’t mean anything to me!”
Well the only reason is doesn’t mean anything to you is that you are not familiar with it. You are already familiar with interest rates and foreign exchange rates, know where to find the relevant numbers, and know how to put them in context. Such knowledge makes you financially literate.
But basic climate literacy is not all that difficult. To first get a quick and dirty take on the risks we and our families face we just need basic math (addition, and perhaps a bit of division/multiplication) and a link to the right data sources. We also already know the theory: the flow of causation from carbon emissions, to atmospheric CO2 concentration to temperature. We also now have three benchmark numbers to work off: 1,000 Gt of CO2 for our budget of carbon emissions between 2000 and 2050, 450 ppm of CO2-eq for our danger level for greenhouse gases and 2 degree Celsius of warming for dangerous climate change.
The next stage is to access the CO2 emissions data. The Carbon Dioxide Information Analysis Center (CDIAC), an organisation within the US government’s Department of Energy, maintains a database of both fossil fuel carbon emissions and land use change carbon emissions. The fossil fuel related data series goes back to 1751 and can be found here, while their land-use change related data series starts from 1850 and can be found here.
Let’s start with the land use change numbers, which CDIAC reports in tera grams (tera as in ten to the power of 12). Shifting these into giga tonnes (Gt, ten to the power of 9), we see that land use change is resulting in approximately 1.4 Gt of carbon emissions per year and for the 10 years through 2010 we would likely have seen emissions from this source of 14 Gt of carbon.
The fossil fuel related emissions have been less stable, that is on a rising trend up through 2008. They are reported by the CDIAC in millions of tonnes. For the years 2009 and 2010 we actually have preliminary emission numbers from the CDIAC here. Their latest advance numbers show a dip in emissions due to the global recession in 2009, but a jump to a new record in 2010. In total, for the 10 years through 2010 (including the two advance estimates), 77 Gt of carbon was emitted from fossil fuel sources.
Putting the fossil fuel and land use numbers together, we get an aggregate 91 Gt of carbon emissions for the decade just ended. However, the unit the CDIAC is using is carbon, while the Meinhausen’s carbon budget approach uses CO2. Deep in your memory you may recall that the atomic mass of carbon is 12 and that of oxygen 16, giving an atomic mass of 44 for the CO2 molecule. Accordingly, to move from carbon to CO2 we need to multiple by 44/12 or rather 3.667. So 77 Gt of carbon translates into roughly 282 Gt of CO2. In other words, of our carbon budget of 1,000 Gt of CO2 we’ve already used up about 28%.
As an aside, and in my humble opinion, one major reason why the educated general public has been unable to get to grips with basic climate change science has been the dog’s dinner of units and base years with which each data point is presented. The figures in no two press releases appear directly comparable, leading to confusion and ultimately disengagement from the debate.
Now let’s try and do a quick and dirty estimate of when we will use up the remaining portion of the budget. To commence with, land use change emissions have been pretty stable recently, so let us just assume they carry on at a rate of 1.4 Gt per annum. And let us take a best case estimate for fossil fuel emissions, that is, they will flat line also at 9 Gt per annum (a pretty conservative assumption since this would mean that global GDP growth slumps). Translate those numbers from carbon to CO2 and we get total emissions of 36 Gt of CO2 a year. At that run rate, we will have used up the budget by around 2030.
At this point, and after some very simple math, I hope you will get a sense of the risk. To me, it looks extremely unlikely that the world will come off fossil fuels at the rate required over the next two decades. Therefore, there is a high risk that the world will push through the 450 CO2-eq barrier, and global mean temperature will move 2 degrees above pre-indusrial revolution levels. As such climate change will loom large as an economic factor, one that generally will act as a perpetual drag on growth.
As to whether temperature will rise by 3 of 4 degrees going forward—the sort of level that will lead to the failure of sovereign states—the likelihood is certainly there and it is absolutely not alarmist to discuss it. Indeed, ignoring those possible extreme climate outcomes is a pretty reckless thing to do from a risk perspective, even at a personal and family level.