But is it really likely that global temperatures will increase by at least 4 degrees Centigrade by 2100? It seems not. There are still reasons for thinking that the chance of such a rise in temperature is less than 50%.
The alarming headline was a result of a study on what is called climate sensitivity. Climate sensitivity is the issue of how much global warming will be produced by a particular amount of increase in the carbon dioxide in the atmosphere. Scientists debate how much the temperature will rise if the carbon dioxide in the atmosphere doubles from its current level of 400 parts per million, to become as high as 800 parts per million. A post on RealClimate.org says, “The ‘meta-uncertainty’ across the methods remains stubbornly high, with support for both relatively low numbers around 2ºC and higher ones around 4ºC, so that is likely to remain the consensus range.”
The study announced today was taking a position that doubling the atmosphere's carbon dioxide will cause an increase of 4ºC, but other scientists apparently think that the climate sensitivity is only half as much, and if they are right, doubling carbon dioxide will produce a temperature rise of only 2ºC.
Another reason for doubting today's shocker headline is that it is very much uncertain whether carbon dioxide levels will double by the end of the century. It seems as likely as not that they will only rise by a much smaller amount, an increase such as 50%.
Here is an IPCC graph showing projected rises in carbon dioxide under different IPCC scenarios for global warming:
IPCC Projections of Carbon Dioxide Increases
Note that in most of these scenarios, the carbon dioxide only increases to 600 parts per million or less by the year 2100. But today's shocker headline is based on both a worst-case climate sensitivity assumption, and also a very pessimistic projection of carbon dioxide increase (one projecting a doubling of carbon dioxide to 800 parts per million by 2100).
I may note there are fossil fuel resource availability experts (such as Dave Rutledge of Cal Tech) who think that carbon dioxide will not be able to increase to much beyond 500 parts per million by the end of the century, simply because global demand for coal and oil will exceed the supply in a few decades. In this link Rutledge cites a Schmitz model which projects a peak in carbon dioxide levels at 459 parts per million in the year 2067, at a point when 90% of fossil fuels will be exhausted.
So let's do a little probability analysis here. Let's assume that there's a 50% chance that the “climate sensitivity” pessimists are right, and that a doubling of carbon dioxide would produce a 4 degrees centigrade increase in temperature. Let's also assume that there's a 50% chance that experts such as Rutledge are right, and that we won't get anything close to a doubling of carbon dioxide by 2100 (as well as a 50% chance that carbon dioxide levels will double by 2100). That means the chance that we will actually see a 4 degrees centigrade temperature rise by 2100 is roughly 50% times 50%, or roughly 1 in 4.
This is why it is dubious journalism indeed for news sources to be showing headlines like today's headline Planet likely to warm by 4C, scientists warn. A more accurate headline might be 25% Chance of Temperatures Rising by 4 Degrees Centigrade by 2100, but of course, such a headline wouldn't generate as many web clicks.
Does this mean we can be complacent and continue with our wicked ways of profligate conspicuous consumption? Not at all. A 25% risk of a 4C global catastrophe is still reason enough for reducing your carbon footprint. Moreover, the very considerations I have cited about resource exhaustion provide an additional impetus for conserving resources, perhaps one even more powerful than the global warming rationale. If 90% of the world's fossil fuel resources will be exhausted by the year 2067, we have a moral duty to conserve those resources, to minimize the chance of a devastating Energy Crunch later in this century that may leave many millions starving, shivering, or sweating out their lives, while sometimes fondly remembering the days when there used to be regular electrical power.
Imagine a father driving his car on a cross-country trip, and inside the car are his wife and his daughter. The father drives through long empty roads of Nevada at excessive 80 mile an hour speeds. The wife warns the husband, “Don't drive so fast – the engine will overheat and the car will catch on fire.” But the daughter has a different warning: “Don't drive so fast – we must conserve gas or we'll run out in the middle of the desert.” The father represents our culture of runaway consumption. The mother represents the warning of climate experts. The daughter represents the warning of resource reserves experts, who give us an entirely different, but perhaps even more compelling, reason for reducing our runaway consumption.