• Shipping's CO2 emissions today remain close to 2008 levels, despite seaborne trade growing by around 60% over the same period.  

• Three distinct drivers explain the gap: ships slowed down, got larger, and new vessels were designed to fundamentally different efficiency standards. 

• These changes emerged from a combination of market pressures, incremental regulation, and technology adoption that varied widely across segments and operators. 

• As the IMO enters Phase 2 of its Energy Efficiency Framework review, the historical record shows us that a fortunate alignment of market forces, technology, and incremental regulation has reduced shipping’s emissions intensity over the past 15 years. The opportunity now is to build on this foundation by designing even more effective energy efficiency regulations. 

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The story of shipping's emissions since 2008 is one of the most significant, and least told, efficiency achievements in any hard-to-abate sector. Understanding what drove it matters directly for the regulatory decisions ahead. At MEPC 84, Member States discussed both the IMO Net-Zero Framework and the Energy Efficiency Framework. The choices made at the MEPC will determine whether the gains of the past 15 years are built upon systematically, or left to market conditions that may have reached their limits. 

In its 2023 GHG Strategy, the IMO has set ambitious objectives for international shipping: to reach net-zero GHG emissions by or around 2050, and to reduce total GHG emissions by 20-30% below 2008 levels by 2030 and by 70-80% by 2040.  

Assessing whether the sector is on a credible pathway towards these goals requires a clear understanding of how emissions have evolved over time and what has driven past changes. In this newsletter, we ask a simple but important question: what have energy efficiency measures already achieved in practice when it comes to emissions reductions? 

To answer this question, we explore the evolution of CO2 emissions from international shipping since the turn of the century. We discuss the decoupling of emissions and transport growth since 2008, the role of operational responses and market conditions, and how the implementation of energy efficiency regulations helped to reinforce and sustain this shift.

To understand how shipping’s CO2 emissions have developed over time, we draw on a body of evidence that has expanded significantly over the past two decades (see box below). Taken together and interpreted as trends rather than as precise point estimates, these sources allow a coherent longer‑term assessment of how emissions, shipping demand, and carbon intensity have evolved (Figure 1).

Figure 1. Data sources at a glance. Trend lines are based on CO2 emissions from shipping and are built by combining data from the Second IMO GHG Study, the Third IMO GHG Study, and the Fourth IMO GHG Study, building a projection of emissions, trade growth, and carbon intensity up to 2018. From 2018, we add data from UNCTAD's 2024 Review of Maritime Transport (and previous years) and the IMO DCS, by combining yearly growth changes in emissions to project Fourth IMO GHG Study findings. The trends shown in Figure 1 are therefore projections; more precise data will be available from the IMO upon completion of the Fifth IMO GHG Study, particularly for the period 2018 to 2024.

Looking at Figure 1, one feature stands out immediately: shipping has, since around 2008, decoupled emissions (blue) from growth in maritime trade (pink). 

From around 2000 to 2008, emissions were coupled to seaborne trade, rising almost proportionally to transport work and reaching a peak around the time of the 2008 financial crisis. After 2008, shipping activity resumed its upward trajectory and, over time, grew to levels around 60% higher than in 2008.  

Emissions, however, followed a different path. Instead of continuing to scale with transport demand, they fell in the immediate aftermath of the crisis and have since fluctuated within a relatively narrow range. Carbon intensity (yellow) followed a similar trajectory, declining steadily after 2008 even as trade recovered. By 2024, it had fallen by 31.5% relative to 2008.

In this section, we outline three major drivers that contributed to the decoupling of trade and emissions shown in Figure 1: speed reduction, increased vessel size, and changes in vessel design.

A sharp rise in fuel prices from 2004 onwards made speed expensive. This coincided with a period of fleet expansion, driven in large part by a surge in Chinese shipbuilding capacity. When seaborne trade contracted in 2008-2009 in connection with the financial crisis, the shipping industry was left with significant overcapacity.  

The resulting intense pressure on operators changed how ships were operated. Speeds were reduced across most major segments, power usage fell accordingly, and slow steaming shifted from a marginal practice to standard industry behavior. Evidence from the Third IMO GHG Study shows that average speed reductions in the order of 6-15% translated into much larger reductions in power usage – typically around 20-40% (see Figure 2). Overall, the widespread adoption of slow steaming reduced average daily fuel consumption by roughly a quarter.

Figure 2

The shift to slow steaming forced vessels into off-design operating conditions, spurring retrofits (e.g., bulbous bow modifications, propeller replacements, engine de-rating) that physically locked in lower speed ceilings. It also marked the beginning of more systematic performance monitoring: noon reports evolved from a compliance formality into an operational tool, and dedicated vessel performance teams began to emerge onshore. 

When fuel prices eased after 2014, speeds did not fully recover. Slow steaming had become the new normal, embedded in commercial contracts, scheduling practices, and operational planning across the industry. This normalization also helps explain why, when the Energy Efficiency Existing Ships Index (EEXI) took effect in 2023, most vessels could meet it by simply limiting engine or shaft power. Many ships were already routinely operating at power levels below what was required by the regulation, so compliance required little or no change in day-to-day operations.  

Energy efficiency via speed reduction was not introduced as a climate measure, nor as the result of any regulatory intervention. Rather, it was a byproduct of managing costs under pressure. This early progress was also constrained by a lack of reliable fleetwide performance data – in other words, the basis for operational efficiency targets. Now, the IMO Data Collection System (DCS) can fill this data gap. Given that the sector achieved meaningful efficiency gains without (at that time) any regulatory push and with a limited data foundation, well-designed energy efficiency regulations present an opportunity to achieve much more.

Figure 3

Across all major segments, average ship size increased between 2011 and 2024 (Figure 3). For example, container ships grew by 41% in average deadweight capacity per vessel, and the size of the largest vessels in operation also grew markedly. The largest container vessel operating in 2011 was Emma Mærsk, delivered in 2006 with approximately 15,000 TEU capacity. By way of comparison, the largest container vessel delivered in 2024, OOCL Valencia, has a capacity of approximately 24,000 TEU. 

Larger ships exploit a fundamental physical property of maritime transport: resistance, and therefore energy demand, does not scale linearly with cargo capacity. In other words, a ship that is twice as large does not consume twice as much fuel. This means that concentrating the same trade volumes on fewer, larger vessels produces lower emissions per tonne-mile, even if individual vessels burn more fuel in absolute terms. 

The result was a quiet but significant improvement in fleet-level carbon intensity, achieved through a structural shift in how maritime capacity was organized and deployed.

As lower operating speeds became established across much of the fleet, ship design began to adjust accordingly. These changes happened in interaction with the introduction of regulations aiming to improve the energy efficiency of ships at the design stage.

Vessels ordered in the decade after the financial crisis were no longer designed for the high speeds that characterized earlier generations. Instead, hull forms were optimized for reduced resistance at lower speeds, and propulsion systems were reconfigured to match the new operating profiles. This shift in design philosophy reduced the energy demand of the vessel itself, which in turn allowed – even required – a significant reduction in installed engine power. This pattern carries across different shipping segments (Figure 4).

Figure 4

Within this period, energy efficiency also moved from an implicit outcome of market behavior to an explicit and enforceable regulatory objective. The Energy Efficiency Design Index (EEDI) for new ships, alongside the Ship Energy Efficiency Management Plan (SEEMP) applicable to all vessels, marked the first time that a mandatory energy efficiency standard had been applied across an entire global industry. EEDI phases tightened the required efficiency improvements over time, ensuring that design improvements were not just a trend but an ongoing requirement.

By embedding efficiency requirements directly into the physical characteristics of newbuilds, the EEDI converted short-term operational responses into lasting change. Ships built to lower power specifications will have the potential to operate with lower emissions for their entire service lives of 20-25 years.

The effectiveness of energy efficiency measures, first demonstrated through market forces, provided the empirical basis for regulatory action. The progressive tightening of the regulatory framework, from EEDI through to the Carbon Intensity Indicator (CII), has helped keep the industry on a trajectory of improving efficiency rather than allowing these gains to erode. 

Consider two Suezmax tankers: one representative of what was being built around 2008, the other of what has been delivered in the early 2020s.  

The 2008 tanker was built with around 18.6 MW of installed main engine power, for service speeds of 13-15 knots and with no regulatory efficiency standard – the SEEMP did not yet exist and the EEDI had not yet been conceived. 

The early 2020s tanker looks similar from the outside. It carries roughly the same cargo, sails the same routes, and serves the same commercial purpose. But the engineering underneath is fundamentally different:  

• Installed engine power has been cut to approximately 15.6 MW 

• The hull has been shaped using computational fluid dynamics to minimize resistance at the 10-14 knots where this vessel is expected to operate  

• Energy-saving devices, such as optimized propellers, rudder fins, and air lubrication systems further reduce the power required to move the same cargo at the same speed 

• Waste heat recovery captures energy that would otherwise be lost through the exhaust 

• Critically, this vessel enters service with an attained EEDI that meets Phase 3 requirements: at least 30% better than the 2008 baseline 

A tanker designer working in 2024 could not simply replicate the 2008 design and deliver it to a shipowner. EEDI requirements meant that lower installed power and improved hull efficiency were mandatory, not optional. Market incentives and fuel cost pressures pointed in the same direction, but the regulation ensured that efficiency improvements were locked into the physical design of every newbuild, regardless of which owner or yard produced it.

Shipping's total CO2 emissions today remain close to 2008 levels despite transporting vastly more goods – the result of an industry operating at lower speeds with vessels carrying less installed power and more cargo per trip. These changes have been anchored by an IMO Energy Efficiency Framework that converted market-driven gains into enforceable, lasting requirements.  

Nevertheless, carbon intensity has improved more slowly in recent years, and total emissions have edged upward as trade volumes have continued to grow. The decoupling achieved so far is significant, but it is not a complete transition. 

Today, much of the policy and media attention in maritime decarbonization is being directed to how to bridge the cost gap between sustainable alternative marine fuels and conventional fossil fuels. While maintaining focus on this important challenge, we should not forget the energy efficiency measures that have already demonstrated their ability to deliver emissions reductions.  

The potential of energy efficiency in maritime decarbonization deserves a more deliberate regulatory debate. The framework that produced the gains so far was not designed as a system, but accumulated over time. How can we leverage these existing regulatory instruments to build an even better framework? Phase 2 of the IMO Energy Efficiency Framework is an opportunity to answer this question.  

The historical record shows that energy efficiency measures, whether driven by markets or by regulation, have already reshaped the emissions trajectory of a global industry. If 15 years of largely uncoordinated effort produced increased trade growth with stable emissions, the case for what deliberate, well-designed regulation could deliver is compelling. That is the foundation worth building on, and what this newsletter series will explore in the next issues.  

Authors: Daniel Barcarolo and Athanasia Vasileiou