全球变化知识资源中心(globalchange): Complementing carbon prices with technology policies to keep climate targets within reach

Economic theory suggests that comprehensive carbon pricing is most efficient to reach ambitious climate targets¹, and previous studies indicated that the carbon price required for limiting global mean warming to 2 °C is between US$16 and US$73 per tonne of CO₂ in 2015 (ref. 2). Yet, a global implementation of such high carbon prices is unlikely to be politically feasible in the short term. Instead, most climate policies enacted so far are technology policies or fragmented and moderate carbon pricing schemes. This paper shows that ambitious climate targets can be kept within reach until 2030 despite a sub-optimal policy mix. With a state-of-the-art energy–economy model we quantify the interactions and unique effects of three major policy components: (1) a carbon price starting at US$7 per tonne of CO₂ in 2015 to incentivize economy-wide mitigation, flanked by (2) support for low-carbon energy technologies to pave the way for future decarbonization, and (3) a moratorium on new coal-fired power plants to limit stranded assets. We find that such a mix limits the efficiency losses compared with the optimal policy, and at the same time lowers distributional impacts. Therefore, we argue that this instrument mix might be a politically more feasible alternative to the optimal policy based on a comprehensive carbon price alone.

To limit the mitigation costs and risks of achieving the 2 °C target, it is essential to start comprehensive climate policy as early as possible^{3, 4, 5, 6, 7}. Recent studies have shown that pledged reductions are not consistent with cost-efficient emissions pathways reaching the 2 °C target^{8, 9}. Furthermore, a continuation of climate policy at the current ambition level will not lead to a stabilization of climate change^{3, 6, 10, 11}, and the delay of more stringent mitigation actions will significantly exacerbate the challenge of reaching long-term climate policy objectives^{3, 4, 5, 6}. Current policies fail to induce the transformation of the energy system to the extent required by long-term climate targets and lead to further lock-in into carbon-intensive infrastructure. Not only do too much emissions occur in the near term, but also mitigation later on is rendered more difficult^{12, 13}. It is an important question whether technology policies can reduce such lock-in and mitigate the impacts of delay. Although a few studies based on global energy–economy models have considered single packages of technology policies in their analysis of twenty-first-century mitigation pathways^{3, 11, 14}, none of them explored this question.

The environmental economics literature has also not focused on the scope of technology policies for overcoming deficiencies in carbon pricing. In this strand of scholarly work, technology policies have mainly been analysed as means to cure market failures beyond the pure pollution externality, for example, due to learning spillovers, information asymmetries and so on^{15, 16, 17}. In contrast, here we analyse their complementary role under sub-optimal carbon pricing. There is wide agreement that market-based instruments pricing the externality of emissions have an advantage in terms of efficiency¹. At the same time it is debated whether or not setting a price (carbon tax) or a quantity of tradable permits (cap-and-trade) is preferable^{18, 19, 20}. Some authors find that the interaction with other instruments favours the price instrument²⁰, a finding that our study extends to the case of sub-optimal carbon pricing combined with technology policies.

This study is the first to assess which mix of emission pricing and technology policies is effective in avoiding further lock-in and initiating the transformation required for limiting warming to 2 °C. We thus fill an important gap in the literature by informing the ongoing climate policy debate, which so far revolves around modest approaches to carbon pricing and various forms of technology policies in several countries around the world, tantamount to a lack of comprehensive emissions pricing in line with the 2 °C limit.

Our analysis identifies a policy mix that—based on the positive effects of technology policies under sub-optimal carbon pricing—keeps ambitious climate targets within reach and is possibly easier to implement politically. It does so by addressing two crucial questions: (1) how weaker-than-optimal carbon pricing schemes and additional technology policies interact, and (2) which combination can best reduce the adverse effects of sub-optimal carbon pricing.

To this end, we employ the energy–economy–climate model REMIND (refs 21, 22) for analysing a variety of scenarios with combined carbon pricing and technology policies in the initial period of 2015 until 2030, followed by pricing-only policies for the remainder of the century designed to be consistent with the 2 °C climate target. Table 1 provides an overview of the considered policies along the two dimensions pricing and technology, including the definitions of the scenario components Opt, Cap, Tax, Zero, noT, CM, LCS and C&L. To enable a meaningful comparison, the two pricing policies are chosen such that they coincide in the case without additional technology policy and with reference energy demand assumption. The corresponding greenhouse gas (GHG) emissions level of 60.8 GtCO₂ in 2030 represents a lenient extrapolation of the Copenhagen pledges²³ and falls short of optimal mitigation action with respect to a 2 °C target in each of the nine models participating in the AMPERE study⁴.

Table 1: Description of medium-term policy options considered in the scenarios.

Full table

We use the integrated energy–economy–climate model REMIND (refs 21, 22) to assess the long-term implications of different short-term climate-related policies. REMIND is an inter-temporal general equilibrium model of the global economy with a technology-rich representation of the energy supply system. It differentiates 11 world regions and runs on 5-year time steps. The model usually operates with perfect foresight over the full modelling time frame 2010–2100. Thus, learning externalities are internalized. Here, we construct two-stage scenarios with sub-optimal policies until 2030, followed by first-best policies that limit global warming to 2 °C. Before 2030, the model does not anticipate the later tightening of emission policies. This leads to an overinvestment into carbon-intensive capital and underinvestment into the scale-up of low-carbon technologies.

REMIND captures crucial aspects of system inertia and path dependencies, as vintage capital stocks of more than 50 energy-conversion technologies as well as technological learning of wind, solar and electro-mobility technologies are represented explicitly. All technologies are subject to cost mark-ups in the case of fast upscaling. Furthermore, the model considers existing final energy taxes and subsidies²⁹ and the scarcities and constraints driving resource prices.

It has to be stressed that all long-term modelling of the future evolution of the global economy has considerable limitations. The scenarios described in this paper should therefore not be interpreted as predictions, but rather as means for analysing interactions between different policy instruments and energy system developments. Despite the explicit representation of second-best near-term policies, the scenarios still assume idealized conditions in many aspects, for example, optimal saving and investment decisions and full regional cooperation.

Until 2030, two different carbon pricing policies and four different technology policies are combined (Table 1). We define the policies on the global level. Thus, the scenarios establish a benchmark against which national climate policy proposals can be compared. In Cap scenarios, an upper bound on global GHG emissions of 60.8 GtCO₂ in 2030 is prescribed; hence, CO₂ prices until 2030 vary depending on the technology policy scenario. In Tax scenarios, in contrast, the tax rate is fixed across scenarios but GHG emissions in 2030 differ (Fig. 1). We chose the tax rate such that without additional technology policies, the Cap and the Tax scenarios are identical. The path of the tax rate starts at US$7.3 in 2015. In both variants, CO₂ prices until 2030 increase by 5% p.a., jump to the optimal level in 2035 and then increase by the endogenous time-variable interest rate in the model of 5–7%.

In the first technology policy option, coal moratorium (CM), no new freely emitting coal-based conversion plants for the production of electricity, liquids and gaseous fuels can be built. To represent the projects under construction, a global total of 150 GW coal-fired electricity plants with technical lifetimes of 35–40 years can be built until 2020. The only freely emitting channel for coal that can be expanded is thus the use of solid coal in industry and for heating purposes.

The second technology option, low-carbon support (LCS), foresees a dedicated push for certain low-carbon options, implemented as a lower bound on their global deployment. For some technologies, such as wind (globally 1.6 TW in 2030), solar photovoltaics (900 GW) and concentrated solar power (18.5 GW) as well as electric light-duty vehicles (27 million vehicles), the implied market developments represent a continuation of market growth observed in the past years (Supplementary Fig. 7). This market growth was the result of policy support such as, for example, feed-in-tariffs. In the model, the extra costs for wind and solar are financed by a premium fee applied to electricity usage. The two additional technologies supported in the LCS scenarios, natural-gas-based electricity generation with CCS and biofuels conversion with CCS are financed out of the general budget. Here, technology policy in the real world has to be ramped up compared with observed policies to foster research, development, demonstration and deployment. The lower bounds in 2030 are 1.4 million barrel oil-equivalent per day for biomass refineries and 50 GW for gas power plants combined with CCS.

The third technology policy variant, coal moratorium and low-carbon support (C&L), is a combination of the other two, with an additional change of final energy taxes and subsidies. Whereas in all other scenarios, final energy taxes stay constant and consumer subsidies are phased out linearly until 2050, C&L scenarios foresee a faster phase-out of subsidies until 2030 and a convergence of transport fuel taxes to a level of ~US$0.4 l⁻¹.

From 2035 on, comprehensive optimal carbon pricing limits the cumulative 2000–2100 CO₂ budget to 1,500 GtCO₂. This implies a 50–60% probability of keeping the increase in global mean temperature in 2100 below 2 °C compared with pre-industrial levels³⁰. Other forcing agents are priced equivalently, on the basis of 100-year global warming potential values⁴. Further details on the methods can be found in the Supplementary Information.


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