The latest United Nations IPCC Report projects that every increment of global warming—now 1.1 degrees Celsius above pre-industrial levels and rising—will intensify local impacts of climate change, from drought to wildfires to flooding. This finding bolsters the need to meet the Paris Agreement’s long-term goal of capping global warming at 1.5°C, ideally by the end of this century. Achieving that goal means that the planet’s total greenhouse gas emissions will eventually need to decline to net-zero: the sum total of greenhouse gases released into and removed from the atmosphere must be zero. To that end, about 140 countries have announced or are considering net-zero emissions targets, most with a target date of 2050.
Despite the focus on 2050, there are different timelines for meeting a net-zero target that could be consistent with achieving the 1.5°C goal. In a study in the journal Climate Change Economics, researchers at the MIT Joint Program on the Science and Policy of Global Change explore the energy, environmental and economic implications of meeting a global net-zero-emissions target by 2050 versus choosing other pathways of limiting global warming to 1.5°C.
Applying a coupled human/Earth-system model to scenarios in which the coverage of net-zero targets (applying to all countries vs. just the U.S. and E.U.) and participation in international emissions trading were varied, the researchers find that for all scenarios, meeting a net-zero emissions target will require a two-pronged approach. This approach combines the deployment of (1) zero-to-low-carbon technologies to reduce released emissions with (2) “negative emissions” technologies/processes to offset persisting released emissions that are difficult to eliminate. The former include wind, solar, hydro, bioenergy, nuclear, and carbon capture and storage (CCS); the latter include bioenergy with CCS (BECCS), direct air capture with carbon storage, and nature-based solutions such as reforestation, afforestation and agricultural practices that sequester carbon in soils.
In all scenarios, the researchers find that the policies are met by utilizing large amounts of negative emissions—from afforestation in the short term and BECCS in the long term. The negative emissions offset ongoing emissions from hard-to-abate sectors such as iron, steel, cement, chemicals, trucking, aviation and agriculture. In particular, they enable the continued use of oil as a fuel source for commercial transportation.
When every nation achieves net-zero emissions by 2050, a more rapid scale-up of BECCS becomes necessary, incurring much higher costs at midcentury (more than twice as high) than if some countries are allowed to continue producing some emissions in the second half of the century. In the latter scenario, the costs for those countries that do meet net-zero targets are reduced if they participate in emissions trading with the rest of the world and utilize international credits. Additional cost savings could be achieved through the emergence of novel technological and electrification options to reduce emissions in hard-to-abate industries.
At the same time, when all countries achieve net-zero emissions by 2050, the average global surface temperature slightly overshoots 1.5°C in 2050 but falls to 1.2°C by 2100, making it highly likely (with a 96-percent chance, accounting for uncertainty in the climate system) that the 1.5°C target is met. By comparison, global mitigation with only some countries achieving the net-zero-emissions-by-2050 target can result in a 50-percent chance of limiting temperature to 1.5°C by 2100, but with an upper limit on temperature as high as 1.85°C.
“Our research shows that achieving global net-zero emissions by 2050 is not necessarily required in order to keep global warming at or below 1.5°C, and would add considerable policy costs, especially at midcentury,” says Jennifer Morris, a principal research scientist at the MIT Joint Program and the lead author of the study. “That said, meeting the 2050 deadline—ideally utilizing international emissions trading to reduce policy costs—would essentially guarantee the achievement of the 1.5°C target.”
The model used to produce the study’s results, the MIT Integrated Global Systems Modeling (IGSM) framework, consists of the MIT Earth System Model (MESM) and MIT Economic Projection and Policy Analysis (EPPA) model. The IGSM framework incorporates all major economic sectors and greenhouse gases along with their sources and sinks, and enables the development of emissions pathways consistent with different 21st-century temperature outcomes.
Photo: Under a scenario in which every nation achieves net-zero emissions by 2050, a very rapid scale-up of bioenergy with carbon capture and storage (BECCS) becomes necessary, incurring much higher short-term costs than if some countries are allowed to achieve net-zero emissions in the latter half of the century. (Source: Flickr/CenUSA Bioenergy)