Rerouting Around Hormuz Adds 6,400 km — Here's What That Means for CO2
The Strait of Hormuz is 33 kilometres wide at its narrowest point. About a fifth of the world's seaborne oil and a quarter of its LNG squeezes through it every day. If that door closes — even for a few weeks — the ships have exactly one alternative, and it is a very long way around.
The detour goes around the southern tip of Africa, past the Cape of Good Hope. For a laden tanker bound from Ras Tanura to Singapore, it's an extra 6,400 kilometres on top of the usual 6,300. Double the distance, double the fuel, and — on most days — double the CO2. That last part is what I want to work through in this piece, because the fleet-level number surprised me when I first ran it.
The one number that matters
A Very Large Crude Carrier — the 300,000-deadweight-tonne ships that do most of the Gulf-to-Asia work — burns 80 to 100 tonnes of heavy fuel oil a day at a typical laden speed of around 12 knots. Rerouting around the Cape adds roughly 12 to 14 days at sea. Pick the middle of both ranges: 90 tonnes a day for 13 days is 1,170 tonnes of extra HFO.
Heavy fuel oil has a tank-to-wake emission factor of 3.114 tonnes of CO2 per tonne of fuel, a figure that comes straight from the IMO's Fourth GHG Study and is used unchanged by the GLEC Framework v3.2. Multiply it out and a single VLCC voyage around the Cape puts roughly 3,640 tonnes of extra CO2 into the atmosphere. That's about the annual footprint of 450 average European households, burned off in two weeks of steaming by one ship.
LNG carriers look better on paper and worse in reality. A modern 170,000-m3 carrier running on boil-off gas has a lower per-tonne CO2 factor — roughly 2.75 — but it burns more fuel per day, around 130 to 150 tonnes equivalent. The net comes out to 2,800-3,500 tonnes of extra CO2 per rerouted voyage. Container ships, which rarely transit Hormuz directly but run feeder services in and out of Gulf ports, are the worst of the three on this particular lane: 14,000-TEU vessels burn something like 180 tonnes a day, and they don't slow-steam around the Cape because liner schedules don't allow it. Call it 3,500 to 4,200 tonnes of extra CO2, per ship, per voyage.
Why the fleet number is the scary one
The U.S. Energy Information Administration tracks Hormuz transits closely. In a typical month, more than 20 VLCCs pass through every day, plus 8 to 10 LNG carriers, and a thicker layer of product tankers and container feeders I'm going to hand-wave at here. Take the VLCC number alone: 20 ships per day, 365 days per year, 3,400 tonnes of extra CO2 per voyage. That's roughly 25 million tonnes of CO2 per year in additional emissions, just from the crude-tanker leg. Stack the LNG and container rerouting on top and the envelope is somewhere between 30 and 40 million tonnes.
For context: that's about the annual CO2 output of Portugal, or roughly what the entire global aviation industry emitted in two weeks before the pandemic. For one rerouted chokepoint. It is, as numbers go, rude.
Two caveats before anyone holds me to the exact figure. First, these are direct voyage emissions only. I'm ignoring second-order effects — coal-switching by power grids that lose LNG supply, scope-3 consequences of delayed cargo, the refining shuffle as regional balances rearrange themselves. Most of those effects push the number up, not down. Second, the range depends on how long the disruption lasts and how many operators choose precautionary rerouting versus tolerating the insurance premium. When premiums jump to 1-2% of hull value — as they did briefly in 2019 and again in 2024 — a surprising number of owners reroute even without a formal closure.
The speed tradeoff makes it worse, not better
Here's the part that trips up people new to this math. When operators want to recover schedule on a longer route, they push the engines harder. Fuel burn on a big ship scales with roughly the cube of speed. A 20% speed increase raises daily fuel burn by about 73%. So the "sensible" reaction to a detour — go faster to make up time — compounds the CO2 penalty rather than absorbing it.
The opposite — slow-steaming around the Cape — cuts the per-day burn but ties up fleet capacity for two extra weeks per voyage. That pulls older, less efficient ships out of lay-up to cover the freed-up cargo. I've never seen a clean analysis of how these effects net out, but in 2023 Clarksons estimated that the Red Sea diversions (which are a shorter reroute than Hormuz) knocked about 1.5 percentage points off the effective capacity of the global tanker fleet. Hormuz would be worse. Much worse.
Does any of this show up in the carbon price?
Increasingly, yes. Since 1 January 2024, maritime voyages into and out of EU ports have been covered by the EU Emissions Trading System, ramping from 40% of verified emissions in 2024 to 100% in 2026. At EUA prices in the EUR 65-80 range, a rerouted container voyage from the Gulf to Rotterdam adds something like EUR 200,000-350,000 in allowance cost that wouldn't exist on the direct route. IMO's Carbon Intensity Indicator, which rates individual ships A through E, is also tracking the consequences: a year of Cape-bound detours is more than enough to push a marginal C-rated vessel to D, and three D ratings in a row trigger a mandatory corrective action plan. If you believe the CII targets tighten by 2% annually through 2030 — and that's the current IMO trajectory — geopolitical rerouting is going to start showing up on individual ships' compliance records in a way it didn't a decade ago. For how the same voyage CO2 rolls into FuelEU Maritime intensity arithmetic, see the FuelEU pool penalty walkthrough.
What this means in practice: the carbon cost of a Hormuz closure is no longer an abstract externality. It's a line item on the operator's P&L. That's new.
What I'd watch for
If you're modelling energy markets, the usual oil-price reaction to Hormuz headlines is the thing to watch. If you're modelling supply-chain carbon, three things matter more: (1) how fast insurance premiums move, because they drive precautionary rerouting even without a formal closure; (2) whether slow-steaming wins out over speed-recovery, which depends entirely on charter terms; and (3) how CII and EU ETS reporting absorbs the detour — specifically whether the affected voyages are logged in MRV data as a single 100%-scope trip (if they start and end in the EU) or as a 50%-scope trip.
The third one is the least obvious and possibly the most consequential. A VLCC diverted from Singapore to Rotterdam via the Cape is now a 100%-scope EU ETS voyage, with the full emissions of that longer route priced into allowances. The same ship making the same detour but discharging at Port Dickson instead is a zero-scope voyage under the ETS. That kind of accounting asymmetry creates a quiet but real incentive to route sensitive cargoes out of EU destinations during a disruption. Whether operators actually optimise for it is another question, but the incentive is there now in a way it wasn't before. For the wider compliance perimeter this connects into — EU ETS, CSRD, CII — see the 2026 freight compliance checklist.
One thing I haven’t solved yet: the second-order chain. The numbers above are direct voyage emissions only. When LNG flows out of the Gulf get displaced, coal-fired power picks up in some grids and pipeline gas in others. When refined products reroute, the regional refinery balance shifts and the marginal barrel moves. I’ve seen rough estimates that second-order effects add another 50–100% on top of the direct voyage figures, but I haven’t found a clean accounting that I’d stand behind. Real-world deviation on the headline 30–40 Mt/year figure could be ±30% in either direction depending on disruption length and which substitutes pick up the slack. Treat the number as the floor, not the ceiling.
Sources and notes
Distance figures for the Persian Gulf-to-Singapore benchmark (via Hormuz vs. via Cape of Good Hope) are calculated from standard routing tables and cross-checked against the EIA's Hormuz chokepoint briefing (updated 2024). VLCC and LNG daily fuel consumption ranges are Clarksons Research benchmarks. Emission factors for HFO (3.114 t CO2/t) and LNG boil-off (2.75 t CO2/t) come from the IMO Fourth Greenhouse Gas Study 2020 and the GLEC Framework v3.2. EU ETS maritime provisions are in Article 3gd of the amended EU ETS Directive (EU Commission, 2023); IMO CII methodology is in MEPC 78. I'm happy to share the back-of-envelope spreadsheet if you email the team — nothing in it is proprietary.
If you want to run your own version of this calculation for a specific vessel, cargo, and route, the EcoFreight calculator uses the same GLEC v3.2 factors. For the broader picture on chokepoint rerouting, see the comparison across Suez, Panama, and Hormuz.