I spent some time this week explaining to my undergraduate class the ideas of thermal equilibrium (loosely speaking, the point at which the planet’s incoming solar radiation and outgoing blackbody radiation are in balance) and climate sensitivity (loosely speaking, how much warmer the earth will get per doubling of CO2, until it reaches a new equilibrium). I think some of my students might prefer me to skip the basic physics, and get on quicker to the tough questions of what solutions there are to climate change, whether geo-engineering will work, and the likely impacts around the world.
So it’s nice to be reminded that a good grasp of the basic science is important. A study produced by the Argentinean group Federacion Ecologia Universal, and published on the American Association for Advancement of Science website, looked at the likely impact of climate change on global food supplies by the year 2020, concluding that global food prices will rise by iup to 20%, and some countries, such as India, will see crop yields drop as much as 30%. The study claims to have used IPCC temperature projections of up to 2.4°C rise in global average temperatures by 2020 on a business-as-usual scenario.
The trouble is the IPCC doesn’t have temperature projections anywhere near this high for 2020. As Scott Mandia explains, it looks like the author of the report made a critical (but understandable) mistake, confusing the two ways of understanding ‘climate sensitivity’:
- Equilibrium Climate Sensitivity (ECS), which means overall eventual global temperature rise that would result if we double the level of CO2 concentrations in the atmosphere.
- Transient Climate Response (TCR), which means the actual temperature rise the planet will have experienced at the time this doubling happens.
These are different quantities because of lags in the system. It takes many years (perhaps decades) for the earth to reach a new equilibrium whenever we increase the concentrations of greenhouse gases, because most of the extra energy is initially absorbed by the oceans, and it takes a long time for the oceans and atmosphere to settle into a new balance. By global temperature, scientists normally mean the average air temperature measured just above the surface (which is probably where temperature matters most to humans).
BTW, calculating the temperature rise “per doubling of CO2” make sense because the greenhouse effect is roughly logarithmic – each doubling produces about the same temperature rise. So for example, the pre-industrial concentration of CO2 was about 280ppm (parts per million). So a doubling would take us to 560ppm (we’re currently at 390ppm).
To estimate how quickly the earth will warm, and where the heat might go, we need good models of how the earth systems (ocean, atmosphere, ice sheets, land surfaces) move heat around. In earth system models, the two temperature responses are estimated from two different types of experiment:
- equilibrium climate sensitivity is calculated by letting CO2 concentrations rise steadily over a number of years, until they reach double the pre-industrial levels. They are then held steady after this point, and the run continues until the global temperature stops changing.
- transient climate
sensitivityresponse is calculated by increasing CO2 concentrations by 1% per year, until they reach double the pre-industrial levels, and taking the average temperature at that point.
Both experiments are somewhat unrealistic, and should be thought of more as thought experiments rather than predictions. For example, in the equilibrium experiment, it’s unlikely that CO2 concentrations would stop rising and then remain constant from that point on. In the transient experiment, the annual rise of 1% a little unrealistic – CO2 concentrations rose by a less than 1% per year over the last decade. On the other hand, knowing the IPCC figures for equilibrium sensitivity tells you very little about the eventual temperature change if (when) we do reach 560ppm, because when we reach that level, it’s unlikely we’ll be able to prevent CO2 concentrations going even higher still.
Understanding all this matters for many reasons. If people confuse the two types of sensitivity, they’ll misunderstand what temperature changes are likely to happen when. More importantly, failure to understand these ideas means a failure to understand the lags in the system:
- there’s a lag of decades between increasing greenhouse gas concentrations and the eventual temperature response. In other words, we’re always owed more warming than we’ve had. Even if we stopped using fossil fuels immediately, temperatures would still rise for a while.
- there’s another lag also decades long, between peak emissions and peak concentrations. If we get greenhouse gas emissions under control and then start to reduce them, atmospheric concentrations will continue to rise for as long as the emissions exceed the rate of natural removal of CO2 from the atmosphere.
- there’s another lag (and evidence shows it’s also decades long) between humans realising climate change is a serious problem, and any coordinated attempts to do something about it.
- and yet another lag (probably also decades long, hopefully shorter) between the time we implement any serious international climate policies and the point at which we reach peak emissions, because it will take a long time to re-engineer the world’s energy infrastructure to run on non-fossil fuel energy.
Add up these lags, and it becomes apparent that climate change is a problem that will stretch most people’s imaginations. We’re not used to having to having to plan decades ahead, and we’re not used to the idea that any solution will take decades before it starts to make a difference.
And of course, if people who lie about climate change for a living merely say “ha, ha, a scientist made a mistake so global warming must be a myth!” we’ll never get anywhere. Indeed, we may even have already caused the impacts on food supply described in the withdrawn report. It’s just that it’s likely to take longer than 2020 before we see them played out.