Freight's 8% of Global Emissions: Breaking Down the Numbers by Mode
TL;DR
Ocean shipping: 2.89% of global GHG (IMO Fourth GHG Study 2020). Road freight: ~4.3% (IEA). Air cargo: tiny volume but 47x the carbon intensity of sea. Here's what the numbers actually mean for Scope 3 reporting.
The IMO's Fourth Greenhouse Gas Study (2020) put international shipping at 1,076 million tonnes CO2 in 2018 — 2.89% of global emissions. Add road freight (ITF: ~1.8 Gt) and you're looking at nearly 3 billion tonnes, or about 8% of the global total. Those aren't abstract round numbers. They come from the most rigorous measurement efforts the industry has produced, and they're the baseline every Scope 3 freight calculation starts from.
I spend most of my time inside the GLEC factor tables and IMO study annexes, and the question I get asked most often is: which mode actually moves the needle? The IEA's 2023 Transport Report confirmed that transport accounts for 37% of CO2 from end-use sectors, and freight's share within that keeps growing. If you're reporting Scope 3 emissions under CSRD, GHG Protocol, or any framework that touches Category 4 (upstream transportation), these mode-level numbers are what your auditors will want to see.
What follows is the mode-by-mode breakdown I use when teams ask where to focus first, built on GLEC Framework v3.2 emission factors — the same standard behind ISO 14083 and the factors that power our calculator engine.
Mode-by-Mode Breakdown: GLEC v3.2 Emission Factors
The gap between transport modes isn't a gentle gradient — it's orders of magnitude. A short-haul air shipment can emit 300x more CO2 per tonne-km than a large container vessel. These are the GLEC v3.2 default emission factors that underpin most freight carbon calculations today.
Sea Freight
Ranges from ~5 g/tkm for the largest container vessels (10,000+ TEU) up to ~20 g/tkm for smaller feeder ships. UNCTAD estimates 80–90% of global trade by volume moves by sea, yet shipping accounts for 2.89% of global emissions (IMO Fourth GHG Study 2020).
Road Freight
A heavy articulated truck runs around 60 g/tkm; a delivery van can hit 580 g/tkm or higher. The IEA attributes roughly 4.3% of global emissions to road freight. That massive range is why "road freight" as a single category can be misleading — a fully loaded 40-tonne truck is a different animal from a half-empty sprinter van.
Rail Freight
Electric rail sits at around 11 g/tkm; diesel rail runs up to 31 g/tkm. Electrified networks in Europe and parts of Asia bring rail close to sea freight in per-tkm efficiency. Where the infrastructure exists, rail is the strongest candidate for modal shift from road.
Air Freight
Long-haul air cargo averages around 608 g/tkm; short-haul flights climb to 1,516 g/tkm because takeoff and climb burn disproportionate fuel relative to distance covered. Air handles under 1% of global freight tonnage, but its carbon intensity is 47x that of sea freight.
The Counterintuitive Finding
Sea freight moves 80–90% of global trade by volume (UNCTAD Review of Maritime Transport) but produces far less CO2 per tonne-km than any other mode. Air is the opposite — under 1% of tonnage but disproportionate emissions because of that 47x carbon intensity gap.
Relative Carbon Intensity (indexed to sea = 1x)
This is why modal choice dominates Scope 3 freight calculations. We've seen a single shipment switch from air to sea cut transport emissions by 97% on the same origin-destination pair — that's not an optimisation, it's a structural difference. For teams building decarbonization roadmaps, mode selection is the single biggest lever available, assuming transit time constraints allow it. If you want to see exactly how the swing falls on your own lanes, the calculator will run air-vs-sea on a real shipment in about 30 seconds.
The Growth Problem
ITF Transport Outlook 2023
Projected freight demand growth by 2050
Freight's current share of global emissions
The ITF's Transport Outlook 2023 projects that global freight demand could grow 2.6x by 2050. That's not a worst-case scenario — it's the central projection, driven by GDP growth, trade expansion, and rising consumption in developing economies.
If the modal mix stays roughly the same and efficiency improvements follow historical trends, freight emissions won't just fail to decline — they'll grow substantially. The IEA puts transport at 37% of CO2 from end-use sectors already. Without deliberate intervention, freight's share of that will keep climbing.
The arithmetic is simple. Trade volume rises 2.6x by 2050. Even with continued efficiency gains of 1–2% per year, that compounds out to roughly 50% higher absolute freight emissions by mid-century — not the reductions the Paris pathway requires. The modal mix is the lever that can change that trajectory; the per-tonne factor is not.
What Per-Tonne-Km Numbers Don't Tell You
Per-tkm numbers flatten a lot of variance. A half-empty truck emits the same CO2 as a full one. A mega-container ship maneuvering near port is dirtier than the same vessel at cruising speed mid-ocean. Load factors, vessel age, and fuel type each shift a shipment's actual emissions by 2–3x in either direction from the default factor.
Load Factor
A truck at 50% capacity has roughly double the per-tkm emissions of the same truck at full load. GLEC v3.2 default factors assume average utilization, but your actual shipments may vary widely.
Vessel Age and Type
A 20-year-old bulk carrier burns significantly more fuel per nautical mile than a modern eco-design vessel. The IMO's EEXI and CII ratings are pushing older ships toward retrofit or retirement.
Fuel Type
Heavy fuel oil (HFO), very low sulfur fuel oil (VLSFO), LNG, methanol — each has different well-to-wake emission profiles. LNG cuts tank-to-wake CO2 by ~20% but has methane slip concerns on the well-to-tank side.
Operating Conditions
Container ships near port — maneuvering, waiting at anchor — burn fuel at different rates than at cruising speed mid-ocean. Urban delivery trucks in stop-and-go traffic emit more per km than highway trucks at steady speed.
This is why the GLEC Framework defines data quality tiers — from Tier 1 (default factors) through Tier 3 (primary data from the carrier). The better your data, the closer your reported number gets to reality. Default factors are a starting point, not the final answer.
Caveat I owe you
The "47x" headline between air and sea is at the indexed-mode level — a long-haul cargo aircraft against a 10,000+ TEU container vessel. On a smaller short-sea feeder against the same long-haul aircraft, the multiple drops to roughly 30x. On a short-haul air shipment against a large bulker, it widens past 300x. The single-number comparison is convenient. The actual answer depends on the specific vessel and aircraft class you compare, and on load factor. We've not yet built a vessel-class-by-aircraft-class matrix into the calculator — that's the next iteration.
What This Means for Scope 3 Reporting
For most manufacturers, retailers, and distributors, freight falls under GHG Protocol Scope 3, Category 4 (upstream transportation and distribution) and Category 9 (downstream transportation). These are often among the largest Scope 3 categories, and regulators are paying closer attention.
Under CSRD, more than 50,000 EU companies will need to report Scope 3 emissions. The EU ETS now covers maritime shipping. The IMO's CII ratings penalize inefficient vessels. CBAM entered its definitive period on 1 January 2026. This isn't theoretical compliance risk — it's live.
Understanding your emissions by mode isn't just a reporting exercise. It's the foundation for identifying which shipments to target first. A company that moves most of its volume by sea but uses air for 5% of urgent orders may find that those 5% of shipments account for 40%+ of its freight carbon footprint. That's where per-shipment calculation becomes useful — it turns aggregate statistics into actionable lane-level decisions.
Sources
- IMO Fourth GHG Study 2020 — International shipping: 1,076 Mt CO2 in 2018 (2.89% of global emissions)
- IEA Transport Tracking 2023 — Transport: 37% of CO2 from end-use sectors
- ITF Transport Outlook 2023 — Freight demand projected to grow 2.6x by 2050 (OECD)
- GLEC Framework v3.2 — Default emission factors by mode (Smart Freight Centre)
- UNCTAD Review of Maritime Transport — 80–90% of global trade by volume is seaborne