Apologies for the large gap since my last post; the result of me relocating from one country to another.
A recurring theme of this blog is how to assess the risk of climate change to one’s family. Stuart Staniford over at Early Warning once characterised this as the ‘Odds of Cooking the Grandkids’. As such, his post plugs into the central theme of this blog: the badness of the potential outcomes (the cooking of the grandkids) and the fact it should be viewed in terms of probability (the odds).
To cook the grandkids you basically need to see around 6 degrees Celsius of warming. But focussing on the grandkids is putting the cart before the horse. Before we get to the stage where we cook the grandkids, we need to parboil our own kids. And the cooking process will begin when we get above 2 degrees of warming (see here) and get progressively worse at 3 to 4 degrees (here).
To extend the analogy, in the kitchen you have two main variables: 1) the intensity 0f the heat and 2) the duration for which the heat is applied. In climate change you have two principal variables as well: 1) the sensitivity of temperature change to an increase in atmospheric CO2 and 2) the amount of CO2 we pump into the atmosphere. If you are able to understand these two variables—and follow them as they evolve through time—then you will get a better idea of the odds of cooking the kids.
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Let’s start with 2 degrees of warming from pre-industrial levels, the level at which the scientific community believes we will get dangerous climate change. In an attempt to forge a global agreement, the international community has chosen the figure of 450 parts per million (ppm) of CO2 equivalent in the atmosphere (C02 plus non CO2 green house gases, usually denoted as CO2-eq) as a line in the sand—that is, a level under which we would be protected from 2 degrees of warming. (A short explanation of the 450 target is given by the International Energy Agency here).
To put this in context, atmospheric C02 concentration has risen from the pre-industrial revolution level of 278 ppm to its current level of roughly 390 ppm (the latest monthly figure can be found here) and is rising at a rate of around 2 ppm per annum. Through happenstance, the CO2 concentration and CO2 equivalent concentration is almost the same at the present time. While there are a number of substances in the atmosphere like methane (CH4) that add to global warming, there are some, like aerosols, that have a cooling effect (for a good explanation see here). However, while the warming and cooling effects of non-C02 gases and particles currently cancel out, this may not be the case in the future.
Against the background of the 450 CO2-eq target, each nation is expected to restrict its emissions such that, in aggregate, the maximum amount of CO2-eq in the atmosphere should not exceed 450 parts per million in the future. In support of this goal, a number of nations have published future emissions targets. Among developed Annex 1 countries (roughly equivalent to the OECD nations), such targets have generally come in the form of reductions from current emission levels. In developing countries, the targets are frequently couched in terms of a slower pace of CO2 emission expansion, with an implicit understanding that cuts will come a decade or so down the road when the countries in question have greater wealth (the ‘get dirty and clean up’ approach to development).
Politics is a messy business, and it is understandable that the creation of a simplistic 450 ppm target was necessary in order to sell an idea both within and outside of each country’s governing elites. But unfortunately little in life can be reduced to a binary outcome. The morbidly obese, chain smoking sedentary alcoholic whose family has a history of heart disease may have a life expectancy far lower than the athletic health fanatic whose grandparents lived into their 90s, but it is possible that it is the latter who could keel over first with a heart attack—not likely, but possible. So if Mr Fitness is prudent and risk averse, he will still take out life insurance to protect his family.
Similarly with the 450 ppm target. Keeping within this goal will reduce the probability that global mean temperature rise will not rise beyond the magic number of 2 degrees, but it will not eliminate the possibility. Or going back to our original analogy, it is not a question of our kids or grandkids being cooked or not cooked, but rather the percentage probability of them being cooked that we need to plan around.
So what is the source of the uncertainty. Well first it comes down to the fact that we do not have a reliable point estimate for the sensitivity of temperature to a rise in atmospheric CO2. We have a best estimate, which the IPCC in its last report, AR4, put at 3 degrees Celsius of warming for a doubling of atmospheric CO2. But this is just an average of all outcomes—and the span of these outcomes is a much wider 2 to 4.5 degrees. A doubling of CO2 from preindustrial levels will take us from around 280 ppm to around 560 ppm (for simplicity’s sake I am ignoring the non-CO2 greenhouse gases). A move from 280 ppm to 450 is only a 60% rise, but if the sensitivity of temperature comes in at the higher end, that is close to 4.5 degrees, then we will still break through the world’s 2 degrees of warming line in the sand.
For an individual planning for his or her family’s future, the temperature sensitivity spread itself is so wide as to be a source of a large amount of risk by itself: the risk of whether we get into dangerous climate change outcomes or even extremely dangerous outcomes. So is it possible that scientists will give us a better handle on this source of risk going forward? Well the answer is ‘yes’ as new data comes in (by how much and how quickly that band of uncertainly will narrow is hotly debated). The bad news is that by the time we have narrowed the range of sensitively sufficiently, the global warming outcome will already be locked in.
As a metaphor, consider a 30 year old 40-a-day smoker (from the age of 16) visiting a doctors office in the 1940s with a persistent cough. The man asks the doctor whether he should give up smoking. The doctor replies:
“Heavy smokers in my practice appear to have greater health problems, but there is a lot of uncertainty. Too much uncertainty for me to have a scientific opinion; that is I can’t get even close to a 95% confidence interval. However, there is a long-term study going on at the moment. Let’s wait to the results of the study come in. Then we will have something statistically significant to work with and I can give you some proper advice.”
The man continues his 40 a day routine, and twenty years on goes back to the doctor for a health check. The doctors runs up to him excitedly.
“The results of the survey I talked to you about 20 years ago have just been published. My hunch was right. There is a strong association between heavy smoking and a whole range of illnesses like cancer, emphysema and heart disease. Indeed, there is less than a 5% chance that there is no actual link with smoking. You should definitely quit. Of course, most of the damage will already have been done by now.”
But it is actually worse than this. An individual with a bad smoking habit principally hurts himself, with perhaps a lesser impact on his spouse and kids through side-stream smoke. He does not commit yet unborn generations to blackened lungs. And if these future generations adopt healthy habits then they can control, to a degree, their morbidity risk. Thus, our current action—or rather inaction—over climate change has the potential to raise a variety of risks for our children and grandchildren regardless of the purity of the lifestyles they come to lead.
So this leaves us with a degree of uncertainty and some very bad potential outcomes associated with extremely dangerous climate change—outcomes that are for all in intents and purposes are locked in for future generations. But there is a degree of uncertainty associated with the very bad outcome of being hit by a run-away bus every time we walk out the door, but we still venture out every day as the risk is deemed sufficiently small.
So can we put a few numbers—preferably some kind of probability—onto various climate outcomes? The answers to that is “Yes” and rests on the important work of climate scientists such as Malte Meinshausen. It is Meinshausen’s ‘carbon budget’ approach that I will turn to in my next post. And once we get a handle on the available budget, it is possible to get a better idea of the odds that family members will be warmed mildly at best, or cooked well through at worst.
The Rational Pessimist 