Mathijs Harmsen

Scenarios towards limiting global mean temperature increase below 1.5 °C

The 2015 Paris Agreement calls for countries to pursue efforts to limit global-mean temperature rise to 1.5u2009°C. The transition pathways that can meet such a target have not, however, been extensively explored. Here we describe scenarios that limit end-of-century radiative forcing to 1.9u2009Wu2009m−2, and consequently restrict median warming in the year 2100 to below 1.5u2009°C. We use six integrated assessment models and a simple climate model, under different socio-economic, technological and resource assumptions from five Shared Socio-economic Pathways (SSPs). Some, but not all, SSPs are amenable to pathways to 1.5u2009°C. Successful 1.9u2009Wu2009m−2 scenarios are characterized by a rapid shift away from traditional fossil-fuel use towards large-scale low-carbon energy supplies, reduced energy use, and carbon-dioxide removal. However, 1.9u2009Wu2009m−2 scenarios could not be achieved in several models under SSPs with strong inequalities, high baseline fossil-fuel use, or scattered short-term climate policy. Further research can help policy-makers to understand the real-world implications of these scenarios.Scenarios that constrain end-of-century radiative forcing to 1.9u2009Wu2009m–2, and thus global mean temperature increases to below 1.5u2009°C, are explored. Effective scenarios reduce energy use, deploy CO2 removal measures, and shift to non-emitting energy sources.

Alternative pathways to the 1.5 °C target reduce the need for negative emission technologies

Mitigation scenarios that achieve the ambitious targets included in the Paris Agreement typically rely on greenhouse gas emission reductions combined with net carbon dioxide removal (CDR) from the atmosphere, mostly accomplished through large-scale application of bioenergy with carbon capture and storage, and afforestation. However, CDR strategies face several difficulties such as reliance on underground CO2 storage and competition for land with food production and biodiversity protection. The question arises whether alternative deep mitigation pathways exist. Here, using an integrated assessment model, we explore the impact of alternative pathways that include lifestyle change, additional reduction of non-CO2 greenhouse gases and more rapid electrification of energy demand based on renewable energy. Although these alternatives also face specific difficulties, they are found to significantly reduce the need for CDR, but not fully eliminate it. The alternatives offer a means to diversify transition pathways to meet the Paris Agreement targets, while simultaneously benefiting other sustainability goals.Scenarios that constrain warming to 1.5u2009°C currently place a large emphasis on CO2 removal. Alternative pathways involving lifestyle change, rapid electrification and reduction of non-CO2 gases could reduce the need for such negative emission technologies.

A multi-model assessment of the co-benefits of climate mitigation for global air quality

We present a model comparison study that combines multiple integrated assessment models with a reduced-form global air quality model to assess the potential co-benefits of global climate mitigation policies in relation to the World Health Organization (WHO) goals on air quality and health. We include in our assessment, a range of alternative assumptions on the implementation of current and planned pollution control policies. The resulting air pollution emission ranges significantly extend those in the Representative Concentration Pathways. Climate mitigation policies complement current efforts on air pollution control through technology and fuel transformations in the energy system. A combination of stringent policies on air pollution control and climate change mitigation results in 40% of the global population exposed to PM levels below the WHO air quality guideline; with the largest improvements estimated for India, China, and Middle East. Our results stress the importance of integrated multisector policy approaches to achieve the Sustainable Development Goals.

How climate metrics affect global mitigation strategies and costs: a multi-model study

In climate policy, substitutions metrics are used to determine exchange ratios for different greenhouse gases as part of a multi-gas strategy. The suitability of the metric depends on the policy goals and considerations regarding its practical use. Here, we present a multi-model comparison study to look at the impact of different metrics on the mitigation strategies and global climate policy costs. The study looks into different Global Warming Potentials (GWP) and the Global Temperature change Potential (GTP). The study shows that for all the models, varying between GWPs - from different IPCC reports, with different integration periods: 20 or 100xa0years - has a relatively small influence on policy costs (< 2.2xa0% spread across scenarios with a 2.8xa0W/m2 target) and climate outcomes. Metrics with a constant low substitution value for methane (effectively reducing its abatement), in contrast, lead to higher-cost mitigation pathways (with an average cost increase of 32.8xa0% in a 2.8xa0W/m2 scenario). If implemented efficiently, a time-varying GTP leads to a limited cost reduction compared to GWP. However, under imperfect foresight in combination with inertia of CH4 abatement options, or if implemented sub-optimally, time-varying GTP can result in higher costs than a 100-year GWP. At the same time, given a long-term radiative forcing target, a time-varying GTP results in slightly higher maximum global temperature change rates.

Early action on Paris Agreement allows for more time to change energy systems

The IMAGE integrated assessment model was used to develop a set of scenarios to evaluate the Nationally Determined Contributions (NDCs) submitted by Parties under the Paris Agreement. The scenarios project emissions and energy system changes under (i) current policies, (ii) implementation of the NDCs, and (iii) various trajectories to a radiative forcing level of 2.8xa0W/m2 in 2100, which gives a probability of about two thirds to limit warming to below 2xa0°C. The scenarios show that a cost-optimal pathway from 2020 onwards towards 2.8xa0W/m2 leads to a global greenhouse gas emission level of 38 gigatonne CO2 equivalent (GtCO2eq) by 2030, equal to a reduction of 20% compared to the 2010 level. The NDCs are projected to lead to 2030 emission levels of 50xa0GtCO2eq, which is still an increase compared to the 2010 level. A scenario that achieves the 2.8xa0W/m2 forcing level in 2100 from the 2030 NDC level requires more rapid transitions after 2030 to meet the forcing target. It shows an annual reduction rate in greenhouse gas emissions of 4.7% between 2030 and 2050, rapidly phasing out unabated coal-fired power plant capacity, more rapid scale-up of low-carbon energy, and higher mitigation costs. A bridge scenario shows that enhancing the ambition level of NDCs before 2030 allows for a smoother energy system transition, with average annual emission reduction rates of 4.5% between 2030 and 2050, and more time to phase out coal capacity.

How well do integrated assessment models represent non-CO2 radiative forcing?

This study aims to create insight in how Integrated Assessment Models (IAMs) perform in describing the climate forcing by non-CO2 gases and aerosols. The simple climate models (SCMs) included in IAMs have been run with the same prescribed anthropogenic emission pathways and compared to analyses with complex earth system models (ESMs) in terms of concentration and radiative forcing levels. In our comparison, particular attention was given to the short-lived forcers climate effects. In general, SCMs show forcing levels within the expert model ranges. However, the more simple SCMs seem to underestimate forcing differences between baseline and mitigation scenarios because of omission of ozone, black carbon and/or indirect methane forcing effects. Above all, results also show that among IAMs there is a significant spread (0.74xa0W/m2 in 2100) in non-CO2 forcing projections for a 2.6xa0W/m2 mitigation scenario, mainly due to uncertainties in the indirect effects of aerosols. This has large implications for determining optimal mitigation strategies among IAMs with regard to required CO2 forcing targets and policy costs.