Guest essay by Roger Graves
Predictions of atmospheric CO2 levels at some future time are commonly made, usually with the object of assuring us that beyond a certain point our planet will be fried to a crisp. Most of these predictions are in the form ‘if CO2 levels reach x, the effect will be such-and-such’, without actually indicating why CO2 levels should reach such a value. However, it is possible to make an accurate prediction of future CO2 levels using readily available information.
The graph shown below plots CO2 levels as a function of world population, encompassing the period 1960 to 2015. Note that although each data point represents an individual year in sequential order, time is not explicitly represented on this graph, which merely shows how CO2 levels are related to overall world population.
It is reasonably obvious to the naked eye that CO2 and population seem to move in lockstep. To show this in more analytical terms, we can superimpose a trend line, which is simply a mathematical curve which fits the data. After some experimentation it was found that a trend line consisting of a third-order polynomial provided a very good fit to the data. In statistical terms, the R2 value for this curve is more than 0.999, which indicates that it correlates with the data to an accuracy of better than 99.9%.
One advantage of a trend line is that we can then extend it to make predictions of what the future may hold, which you can see in the chart below. Both axes in this chart have been extended for this purpose.
Of course, the same graph could have been drawn with CO2 levels on the horizontal axis and population on the vertical axis, so that population would be shown as the dependent variable and CO2 as the independent variable. The same correlation between the two would still exist.
The question that now arises is whether population is driving the CO2 level, or CO2 is driving population. There are four possibilities to be considered:
1. There is no connection between the two, the apparent lockstep is just a fluke. Possible, but very unlikely. This possibility can reasonably be ignored.
2. Population causes CO2. This is ‘obvious’ explanation that most people would give. The more people there are on our planet, the more CO2-generating activities there will be, such as electrical power generation, industrial activity, automobiles, cooking fires, and so on.
3. CO2 causes population. Much of the population growth in the foreseeable future will come from sub-Saharan Africa. Population growth in these regions is dependent to a large extent on the food supply, and as we know, crop yields increase with CO2 levels. The greater the food supply, the more children will survive to maturity.
4. The connection between CO2 and population results from some, as yet unspecified, combination of 2 and 3.
My personal view, and this is only an unsupported guess, is that possibility 4 is the most likely.
What conclusions can be drawn from this? First, the population/CO2 curve is a smooth curve. No evidence is shown of any significant decrease in the inexorable rise of either CO2 or population from beginning to end of this curve. We can conclude from this that none of the measures taken by industrialized countries to reduce CO2 output have had any noticeable effect, nor does it appear likely that they will have any significant effect in the foreseeable future.
Second, current UN population projections indicate that we will reach a population of 9 billion by 2038, and 10 billion by 2056, using the medium variant of their three prediction levels. (To follow this further, go to http://esa.un.org/unpd/wpp/Download/Standard/Population/, then download the spreadsheet called Total Population – Both Sexes.) Assuming that the population/CO2 relationship still holds good by then, we can predict a CO2 level of nearly 460 ppm by 2038 and 500 ppm by 2056.
Population growth is a quantity which can be reasonably well predicted for one or two decades into the future. While extrapolating a curve in order to make predictions is always fraught with danger, based on the data so far I am fairly confident that the CO2/population relationship will hold for a few years yet. In the long term of course, your guess is as good as mine, but I think we can reasonably well predict CO2 levels in the range 450-500 ppm by the middle of this century. Whether the world then disappears in a puff of smoke, or enters a new golden age of unsurpassed crop yields, remains to be seen.
Roger Graves is a physicist and mathematician who, much to his chagrin, is not associated with big oil, big coal, or big anything else.