Path dependency means decoupling takes too long.

Another objection to the studies thus far is that they do not show that a reduction in all relevant environmental pressures is possible to an extent that would comply with the planetary boundaries. Most of the studies focus on CO2 emissions, but this is merely one of the boundary values. The most comprehensive study from an ecological perspective, UNEP 2011a, takes the ecological footprint as the relevant ecological indicator. Nonetheless, in the UNEP scenario, not even the CO2 emissions are reduced sufficiently enough to limit global warming to 2 °C (Victor and Jackson 2011): according to the most ambitious UNEP G2 scenario, CO2 emissions will decrease by 35% between 2011 and 2050 (UNEP 2011a: 514). Rogelj et al. (2015) argue, however, that they must decrease by more than 50% within this time frame in order to comply with the peak values that are compatible with meeting the 2 °C target. A similar objection applies to the calculations in Calderon et al. (2014) if they are used to support the claim that an adequate decoupling will occur. Calderon et al. (2014) identify measures that can be taken to reduce GHG emissions to around 50 Gt CO2e (range 44–54 Gt) by 2050. With emissions at 50 Gt CO2e by 2050, however, climatological models calculate a low probability that the 2 °C target will be met (Rogelj et al. 2015). The range of GHG emissions in 2050 for which climatological models simulate a 66% probability of meeting the 2 °C target is between 99 and 26 Gt CO2e; the range for meeting the 1.5 °C target is between 4 and 19 Gt CO2e. The respective simulations thus far, however, do not answer the question of which measures should be taken to achieve these ambitious reduction targets, what these measures cost, and what effect they will have on economic output.

Some proponents of the green growth strategy also admit that it is not possible to provide a reliable estimation of the costs of transforming economies in compliance with the planetary boundaries. This is because the necessary increases in decarbonisation rates and resource efficiency require disruptive or even radical technological change (Hepburn and Bowen 2013, Aghion et al. 2014, Zenghelis 2016). Green growth proponents put forward various considerations in order to support their claim that over the next few years the necessary four- to ninefold increase in the rates of reduction in GHG intensity will be achieved: Hepburn and Bowen (2013) point to the potential in the photovoltaics industry (its substantial cost reductions thus far, and its upcoming potential technological developments), while Zenghelis (2016) points to the disruptive potential of electricity storage technologies. There is empirical evidence that economic agents tend, because of existing path dependencies, to stick with the technologies they have been using thus far, and not to invest in resource- or GHG-saving technologies (overview in Aghion et al. 2014). Green growth proponents conclude from this that if economic agents are persuaded away from existing path dependencies through political interventions (subsidies, incentives, institutional changes), this can escalate reductions in GHG intensity (e.g. Zenghelis 2016). This conclusion and the empirical and narrative evidence show that the necessary technological leaps are, in principle, possible. The evidence presented, however, does not prove that these technological leaps and their widespread diffusion and adaptation are at all realistic. They enable no reliable prediction that if the path dependencies are removed and GHG-saving innovations are promoted via other measures, this will bring about the reductions in GHG and resource intensity required in order to meet the ecological targets. This is because we cannot rule out the possibility that the rates of reduction in GHG intensity may nonetheless increase only 3-fold if all the proposed green growth measures are implemented consistently.