KEY FINDINGS

  • All scenarios that keep warming below 1.5°C demand rapid and sustained reductions in CO2 emissions by 2030 and continuing thereafter.
  • All successful 1.5°C scenarios require net zero CO2 emissions by 2050.
  • The scale of BECCS assumed in successful 1.5 °C scenarios may conflict with other important sustainability goals.
  • Considering climate change mitigation along with other sustainability goals, protection of existing forests and natural afforestation may show advantages over large-scale deployment of BECCS.
  • A Low Energy Demand scenario shows it is possible to stay below 1.5°C warming with limited or no reliance on negative emissions technologies.
OVERVIEW PAPER:

New scenarios for realizing Paris

Joeri Rogelj (IIASA), Detlef van Vuuren (PBL), Keywan Riahi (IIASA) and Colin Jones (U. Leeds)

To address the Paris Agreement, scientists have developed a set of future emission scenarios that aim to limit global warming to less than 1.5°C above pre-industrial values by 2100. Scenarios were tested under a range of assumed societal and political futures, using five shared socioeconomic pathways (SSPs).

If the assumed future development in a given SSP was regionally and socially unequal, very few or no scenarios realizing the 1.5°C limit could be identified. In other, more successful SSPs, the modelled scenarios all temporarily exceed 1.5°C before 2100, although peak warming stays below 2°C. The degree to which 1.5°C is exceeded at some point in the century is heavily dependent on the level of greenhouse gas emission reductions by 2030. All scenarios require net zero CO2 emissions by 2050 (see figure below).

The successful 1.5°C scenarios demand a rapid and sustained shift away from fossil fuel use to renewable energies. Bio-energy becomes increasingly important in the future, but is associated with extensive demand for land.

FURTHER READING:

Global CO2 emissions from a range of SSP scenarios. The full blue line (labelled 1.9Wm-2) represents the main 1.5°C scenario. Light blue lines around the full blue line are different 1.5°C scenarios from 6 modelling groups. Other warmer scenarios are also shown, for example the dark green line (labelled 2.6Wm-2) limits warming to 2°C relative to pre-industrial values. Actual historical emissions are plotted in black.

Future warming could be reduced if the planting of forests and fuel crops was massively increased alongside carbon capture and storage. Carbon emission reductions required to meet the Paris Agreement will require a significant increase in renewable energy production to meet future electricity demand.

Can the biosphere help us achieve Paris?

Lena Boysen (MPI), Anna Harper (U. Exeter), Taraka Davies-Barnard (U. Exeter) and Sonia Seneviratne (ETH)

Plants take up CO2 to build leaves, branches and roots, in the process extracting CO2 from the atmosphere. This can occur on “natural” timescales or be accelerated by bio-energy plantations, where rapid-growth trees are used to generate energy, with any emitted CO2 captured before entering the atmosphere. Such technology is known as Bioenergy with Carbon Capture and Storage (BECCS).

The scale of biomass plantation assumed in scenarios that stay below 1.5°C warming demands extensive use of land, placing BECCS in competition with other societal priorities, such as food security and nature conservation. Studies suggest BECCS could balance up to two thirds of current fossil fuel CO2 emissions up to 2050. Because of the potential impact on other sustainability goals and the CO2 emissions associated with the conversion of land into bio-energy forests, the benefit of BECCS may be overstated compared to protection of natural forests.

While land-based mitigation focuses on removal of CO2 from the atmosphere, some associated non-CO2 changes, such as in surface reflectivity and surface evaporation of water, may be regionally more important for climate. In regions such as the US or central Europe, the choice of BECCS over natural afforestation may increase future risk of heat waves.

FURTHER READING:

Changes in global energy demand may help realize Paris

Keywan Riahi (IIASA) and Asher Minns (UEA)

A new global Low Energy Demand scenario shows the 1.5°C limit can be realized without relying on untested negative emission (Carbon Dioxide Removal) technologies. Low demand for energy delivers greater flexibility and speed for end-use and supply-side decarbonization, as required for the necessary and imminent peak in global GHG emissions. Such a low energy demand future decreases pollution, as well as reducing infrastructure and system costs and delivers on a number of the UN Sustainable Development Goals, unlike most other mitigation scenarios in the literature.

Ultimately, Low Energy Demand depends on major social and institutional changes that reverse the historical trajectory of ever-rising energy demand. How these can be represented in modelling studies remains a critical research agenda that is not commonly represented in global scenario and modelling analysis.

FURTHER READING: