Zero-emission shipping and the Paris Agreement: Why the IMO needs to pick a zero date and set interim targets in its revised GHG strategy

On Friday the 13th of April 2018, I was sitting in the back of a hot plenary room at the London headquarters of the International Maritime Organization (IMO), updating ICCT’s real-time analysis of what IMO member states agreed to that day for the initial IMO greenhouse gas (GHG) strategy. Unfortunately, we found that the strategy’s goal of cutting emissions by at least 50% from 2008 levels by 2050 wasn’t aligned with achieving the Paris Agreement temperature goals. Luckily, though, IMO member states have an opportunity to increase their ambition in the revised GHG strategy, which will be agreed to in 2023. Negotiations will start at the 77th session of the Marine Environment Protection Committee (MEPC 77) in November 2021 and I want to talk here about the two main ways in which the initial GHG strategy can be strengthened.

First, we need a zero date. We need to choose a year by which the carbon dioxide equivalent (CO₂e, which includes CO₂, methane, black carbon, and nitrous oxide) emissions from international shipping, measured on a life-cycle basis, are zero. Second, we need interim targets. We need to set absolute emissions reduction goals for years between now and the zero date that get us on the path to full decarbonization now.

Regarding the zero date, my analysis shows it should be not later than 2050. That’s based on updated carbon budgets from the Intergovernmental Panel on Climate Change’s Sixth Assessment Report. If humans pollute the atmosphere with an additional 400 gigatonnes (Gt) of CO₂, it’s estimated that we have a 67% chance of keeping warming below 1.5 °C. At 700 Gt, it’s estimated there’s a 67% chance of limiting warming to 1.7 °C, which I’ll interpret here as “well below 2 °C.” Over the last several years, international shipping has accounted for about 2.5% of anthropogenic CO₂ emissions, according to the Fourth IMO Greenhouse Gas Study. This implies that shipping’s proportional share of the remaining 1.5 °C and well below 2 °C carbon budgets is about 10 Gt and 17 Gt, respectively. (Side note: I know I said we have to get CO₂e emissions to zero, not just CO₂, but the IPCC’s carbon budgets are based only on CO₂. If they were based on CO₂e, the budgets would be larger in absolute terms, but so would the annual emissions from anthropogenic sources, and thus the time we have to decarbonize would remain roughly the same. We’ve written extensively about the climate risks associated with the other pollutants, including methane and black carbon. Cutting these emissions can slow the rate of global warming, and that’s important for keeping below 1.5 °C or 2 °C.)

International shipping currently emits more than 0.9 Gt of CO₂ each year, and that’s growing. Based on the carbon budgets, I’ve figured that reaching zero by 2040 is aligned with limiting warming to 1.5 °C and zero by 2050 is aligned with a well below 2 °C future. But this only works if shipping starts reducing emissions now, and that’s where the interim targets come in.

The red line in Figure 1 shows the current emissions trajectory based on one of the business-as-usual scenarios (SSP2_RCP2.6_L) in the Fourth IMO Greenhouse Gas Study. The gray line shows the 2008 baseline emissions which, coincidentally, are nearly the same as 2018 emissions. The yellow line shows the implied straight-line emissions trajectory to achieve the 2050 absolute emissions reduction goal contained in the initial IMO GHG strategy. The light green and dark green lines show the pathways consistent with the well below 2 °C and the 1.5 °C scenario, respectively.

What policymakers need to understand is that it’s the cumulative emissions between now and the zero year that we’re concerned with, because that’s how the carbon budgets are conceived. The yellow line, which is aligned with the IMO initial GHG strategy’s 2050 target, uses up a cumulative 21 Gt of CO₂ between now and 2050. That’s already twice the 1.5 °C-compatible budget. The zero-by-2040 trajectory results in 9.3 Gt of CO₂ between now and 2040, less than the 10 Gt carbon budget necessary for a chance to keep below 1.5 °C. But this requires shipping emissions to decrease along this straight-line trajectory, including a 50% reduction in absolute emissions relative to 2008 by 2030 and zero emissions by 2040. To keep well below 2 °C requires cutting emissions by one-third by 2030 and by two-thirds by 2040 relative to 2008.

Meeting these interim reductions is critically important. If we exactly follow the zero-by-2050 trajectory, cumulative emissions between now and 2050 would total only 14 Gt, lower than the 17 Gt well below 2 °C budget. Alternatively, the blue dashed line in Figure 2 below shows that if we continue along the current trajectory and only start reducing emissions in 2030, we’ll need to get to zero emissions by 2044 to stay within the well below 2 °C budget. Even scarier, if we continue along the current trajectory, which accumulates 10 Gt of CO₂ between 2020 and 2029, we’ll need to get to zero emissions by 2030 to stay within the 1.5 °C budget. That’s why it’s so important start reducing emissions as quickly as possible. If emissions remain high, we’ll need to cut back even more rapidly in the future and move up the zero date to keep within our carbon budget. No one wants that.

So, while there’s some nuance, if international shipping is to do its part to achieve the Paris Agreement temperature goals, member states should agree to reduce absolute emissions by 33% or 50% by 2030 and 67% or 100% in 2040 relative to 2008. No matter what, the IMO’s revised GHG strategy should aim for zero emissions by no later than 2050.

The 2030 target is probably the most important. Anyone can say what they’ll do in 20 or 30 years’ time, but what are we prepared to do this decade? How do we show that we’re serious about decarbonizing shipping? I’d say it’s the interim targets and the zero date, paired with effective and enforceable regulations, that would show we mean business.

I hope that when the IMO agrees to its revised GHG strategy in 2023 I’ll be back in London, sitting in the back of that hot plenary room, getting ready to report that shipping’s climate ambitions are aligned with the Paris Agreement temperature goals.

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