On the relationship between metrics to compare greenhouse gases – the case of IGTP, GWP and SGTP

Abstract

Abstract. Metrics for comparing greenhouse gases are analyzed, with a particular focus on the integrated temperature change potential (IGTP) following a call from IPCC to investigate this metric. It is shown that the global warming potential (GWP) and IGTP are asymptotically equal when the time horizon approaches infinity when standard assumptions about a constant background atmosphere are used. The difference between IGTP and GWP is estimated for different greenhouse gases using an upwelling diffusion energy balance model with different assumptions on the climate sensitivity and the parameterization governing the rate of ocean heat uptake. It is found that GWP and IGTP differ by some 10 % for CH 4 (for a time horizon of less than 500 yr), and that the relative difference between GWP and IGTP is less for gases with a longer atmospheric life time. Further, it is found that the relative difference between IGTP and GWP increases with increasing rates of ocean heat uptake and increasing climate sensitivity since these changes increase the inertia of the climate system. Furthermore, it is shown that IGTP is equivalent to the sustained global temperature change potential (SGTP) under standard assumptions when estimating GWPs. We conclude that while it matters little for abatement policy whether IGTP, SGTP or GWP is used when making tradeoffs, it is more important to decide whether society should use a metric based on time integrated effects such as GWP, a "snapshot metric" as GTP, or metrics where both economics and physical considerations are taken into account. Of equal importance is the question of how to choose the time horizon, regardless of the chosen metric. For both these overall questions, value judgments are needed.

Simple emission metrics for climate impacts

“…(1), the impact is temperature, and the discount function is no discounting for t < H and full discounting for t > H . The iAGTP has been discussed indirectly by some authors (O'Neill, 2000;Gillett and Matthews, 2010), but in more detail in Peters et al (2011a); Azar and Johansson (2012). Preliminary work on the GWP was also based on integrated temperature change (Wuebbles, 1989;Derwent et al, 1990), but the link to temperature did not make it into the First Assessment Report (IPCC, 1990).…”

“…Preliminary work on the GWP was also based on integrated temperature change (Wuebbles, 1989;Derwent et al, 1990), but the link to temperature did not make it into the First Assessment Report (IPCC, 1990). Peters et al (2011a); Azar and Johansson (2012) investigated whether the GWP was similar to the iGTP and found close agreement for a wide range of time horizons, but not for very SLCFs like BC. The similarity is since AGWP represents the total energy added to the system and iAGTP/λ the energy lost from the system.…”

“…The integrated absolute Global Temperature change Potential (iAGTP) for species i is the integral of the AGTP i (Peters et al, 2011a;Azar and Johansson, 2012), (Forster et al, 2007;Archer et al, 2009;Joos et al, 2013). In terms of Eq.…”

“…One can conceptualise "climate models" as spanning from simple analytical models to complex general circulation models or earth-system models (IPCC, 2001;Held, 2005). Reisinger et al (2010), Tanaka et al (2009), andAzar andJohansson (2012) are examples of studies that use reduced complexity carbon cycle and energy balance models. In general, the more complex the models are, the better they represent the processes in the climate system, but at the cost of increasing computational time making them unsuitable for many common metric applications.…”

Variation in emission metrics due to variation in CO&lt;sub&gt;2&lt;/sub&gt; and temperature impulse response functions

“…The GWP compares the radiative forcing (RF) integrated from the time of emission until a specified time, the so-called time horizon (Shine et al, 1990), while the GTP compares the globalmean temperature change at a certain time after the emission (Shine et al, 2005). More recently, an integrated version of GTP was presented (iGTP), and this compares the temperature change integrated from the time of emission until a time horizon (Gillett and Matthews, 2010;Peters et al, 2011;Azar and Johansson, 2012). It is found that iGTP and GWP are 268 D. J. L. Olivié and G. P. Peters: Variation in emission metrics similar, with one quantifying the energy added to the system (GWP), and the other quantifying the energy lost (iGTP) (Peters et al, 2011;Azar and Johansson, 2012).…”