Global Temperature Change Potential
Global Temperature Change Potential (GTP) goes further than GWP and integrated RF in describing the effects of emissions: it estimates the change in global mean temperature for a selected year in the future. In other words, this metric tries to answer the question:
What will the temperature change be in year X in response to the radiative forcing of certain GHG emissions?
This metric is more complex because it calculates climate response and not just radiative forcing (see Figure 1). GTP is based on RF. Yet in order to model and calculate GTP, we also need to know the time scale of the climate response: because of Earth’s thermal inertia, there is a lag between when the emissions occur and when they cause warming. In other words, GTP accounts for Earth’s thermal inertia. In addition, the models need to include the Earth’s climate sensitivity (see How to measure climate impacts). This means that GTP calculations are more complicated and are less certain than simple radiative forcing calculations. Although uncertainty is increased, relevance is also increased since it is more useful for policy makers to know what the actual temperature change will be than only the amount of energy that has been added to the system.
GTP can be calculated using a pulse emission or sustained emissions (see Integrated Radiative Forcing for a detailed explanation). Furthermore, climate efficacy can be integrated into formulas that calculate GTP.
As with other approaches discussed here, the chosen time frame greatly influences the results. For example, if we choose to evaluate temperature change after 100 years, the effects of short-lived GHGs are de-emphasized, and changes of temperature in between the time of emission and the evaluation year are not captured.
Figure 7 shows the net future temperature change from a 1-year pulse of current emissions for different transport modes for four future time horizons (20, 40, 60, 100 years). The difference in results between time horizons is starkest for shipping. The emissions pulse led to cooling in year 20 because of the high sulfate emissions associated with shipping, but a warming effect begins to become apparent in the second graph (year 40) because shorter-lived sulfates (cooling) have disappeared while longer-lived CO2 (warming) is still in the atmosphere. The error bars for aviation in the 20-year time frame are very large. This is because of uncertainties surrounding the effects of contrails and cirrus clouds.
Figure 7: Contribution to Net Future Temperature Change (in milli-Kelvin) From a 1-year Pulse of Current Emissions for Different Transport Modes for Four Future Time Horizons (20, 40, 60, 100 years).
The error bars express uncertainties primarily in the effect of contrails and cirrus clouds.
Rail D refers to direct emissions (e.g. fossil fuel) and Rail I refers to indirect emissions (e.g. electricity) associated with rail travel. (Source: Berntsen and Fuglestvedt, 2008)
Two different GHGs with equal GTPs describe the same temperature change at the end of a chosen time horizon, though not at specific points within this time horizon. In other words, two different emissions that give the same temperature effect at a chosen time can have different paths (Berntsen and Fuglestvedt, 2008). That means that total climate impact of these two gases might be very different, yet GTP does not reflect these differences.
GTP can be used to express future climate responses to current aviation emissions. As with Global Warming Potential, the chosen time horizon greatly influences the results: short time horizons include the warming due to short-lived emissions, whereas longer time horizons exclude those effects.