Quantification of the radiative forcing of contrails embedded in cirrus clouds – Nature

Report on the Radiative Forcing of Contrails Embedded in Cirrus Clouds

Executive Summary and Relevance to Sustainable Development Goals

This report details the findings of a study quantifying the climate impact of aircraft condensation trails (contrails) that form within pre-existing cirrus clouds, referred to as “embedded contrails.” The aviation sector’s contribution to climate change is a critical concern for achieving Sustainable Development Goal 13 (Climate Action). While CO2 emissions are well-documented, non-CO2 effects, such as contrails, represent a significant and less understood component of aviation’s climate footprint. This research addresses a key knowledge gap by providing the first large-scale, observation-based estimate of the radiative forcing from embedded contrails. The findings are essential for developing targeted mitigation strategies and promoting innovation within the aviation industry, directly supporting SDG 9 (Industry, Innovation, and Infrastructure) and fostering the global collaboration required by SDG 17 (Partnerships for the Goals).

Key Findings

H3>Local and Global Radiative Forcing

The study analyzed approximately 40,000 cases of embedded contrails by combining aircraft positional data with spaceborne lidar observations from 2015 to 2021. The primary findings on their climate impact are as follows:

• Local Warming Effect: An annual mean local net radiative forcing (warming effect) of 60 mW m^-2 was identified for individual embedded contrails.
• Global Impact Estimate: When scaled to a global level, the annual global mean net radiative forcing is estimated to be on the order of 5 mW m^-2.
• Relative Contribution: This impact corresponds to approximately 10% of the current estimated climate forcing from conventional line-shaped contrails, establishing embedded contrails as a non-negligible factor in aviation’s overall climate impact.

H3>Temporal and Diurnal Variations

The radiative forcing of embedded contrails is not constant and varies based on time of day and contrail age.

• Diurnal Cycle: Daytime observations, which constituted 62% of cases, often showed a cooling effect due to the reflection of solar radiation. However, the strong warming effect of the 38% of cases occurring at night (450 to 490 mW m^-2) dominates the overall daily average, resulting in a net warming.
• Contrail Age: Younger contrails (less than 15 minutes old) were observed to have a larger warming effect compared to older, more persistent contrails, which tend to spread and mix with the surrounding cloud.

H3>Influence of Air Traffic Density

The study observed a significant change in atmospheric conditions and contrail effects during the COVID-19 lockdown period in 2020, which saw a drastic reduction in air traffic. This period provided a unique opportunity to observe the atmosphere in a state closer to pre-industrial conditions. The largest cooling effects from individual contrails were observed during this time, suggesting that a heavily perturbed atmosphere may respond differently to additional emissions. This finding is critical for understanding the non-linear effects of aviation and informing strategies under SDG 13 for managing air traffic to minimize climate impact.

Methodological Approach

The quantification of radiative forcing was achieved through a systematic, multi-step process:

1. Data Integration: The study matched aircraft position and waypoint data with height-resolved cloud observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite.
2. Case Identification: Over 40,000 intercepts were identified where an aircraft passed through an existing cirrus cloud between 5 and 30 minutes before the satellite observation.
3. Comparative Analysis: For each case, lidar profiles were divided into a “perturbed” region (containing the embedded contrail) and an “unperturbed” background region. Cloud properties such as optical thickness, ice water content, and ice crystal effective radius were compared between these regions.
4. Forcing Calculation: The local net radiative forcing was calculated as the difference in the radiative effect between the perturbed and unperturbed cloud regions, using established look-up tables that account for solar position, cloud properties, and surface conditions.

Discussion and Implications for Sustainable Development

H3>SDG 13: Climate Action

This research provides a crucial piece of the climate change puzzle by quantifying a previously unaccounted-for warming effect from aviation. By incorporating this data into global climate models, scientists can produce more accurate projections of future climate change. These findings directly support Target 13.3 by improving education and awareness of a specific anthropogenic climate impact. Furthermore, the data can inform the development of climate-aware air traffic management systems that could reroute flights to avoid atmospheric conditions where high-warming contrails are likely to form.

H3>SDG 9: Industry, Innovation, and Infrastructure

The confirmation that embedded contrails have a non-negligible climate impact places greater responsibility on the aviation industry to innovate. This study reinforces the need for investment in:

• Sustainable aviation fuels (SAFs) that may alter contrail properties.
• Advanced engine designs that reduce emissions of water vapor and soot particles.
• Modernized air traffic infrastructure capable of implementing climate-optimized flight paths.

By understanding the full spectrum of its climate effects, the aviation sector can better align its development with the principles of sustainable and resilient infrastructure.

H3>SDG 17: Partnerships for the Goals

This study is a testament to the power of international scientific collaboration. It relied on the integration of data from multiple sources, including U.S. and European space and aviation authorities. Making the data and methodologies publicly available promotes transparency and enables further research by the global scientific community, fostering the partnerships necessary to tackle the global challenge of climate change effectively.

Analysis of Sustainable Development Goals in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

The article’s focus on the climatic impact of aviation emissions connects directly and indirectly to several Sustainable Development Goals (SDGs). The primary connections are to climate action, industry and infrastructure, and sustainable transport systems.

• SDG 13: Climate Action

  This is the most directly relevant SDG. The article is entirely focused on taking “urgent action to combat climate change and its impacts” by quantifying a specific, previously under-researched component of aviation’s climate effect. The study investigates how aviation-induced contrails contribute to global warming by measuring their radiative forcing. The abstract explicitly states the goal is to understand “aviation’s impact on climate,” which is the core of SDG 13.

• SDG 9: Industry, Innovation, and Infrastructure

  The article examines the environmental consequences of the aviation industry, a key component of global infrastructure. By highlighting a “non-negligible contributor to the climate impact of aviation,” the research implicitly calls for innovation and the development of more sustainable industrial practices and technologies within the aviation sector to mitigate its environmental footprint. This aligns with the goal of building resilient infrastructure and fostering sustainable industrialization.

• SDG 11: Sustainable Cities and Communities

  Aviation is a critical transport system that connects cities and communities worldwide. While the article focuses on upper-atmosphere phenomena, the findings are relevant to making human settlements inclusive, safe, resilient, and sustainable. Specifically, addressing the climate impact of air travel is part of creating sustainable transport systems, which is a key aspect of sustainable urban and inter-urban development.

2. What specific targets under those SDGs can be identified based on the article’s content?

Based on the article’s scientific investigation into aviation’s climate impact, several specific SDG targets can be identified:

1. Target 13.2: Integrate climate change measures into national policies, strategies and planning.

   The research provides critical data on the warming effect of embedded contrails. The article’s conclusion that their effect is “on the order of 10% of the effect of line-shaped contrails” provides a quantitative basis for policymakers to develop and integrate specific climate change mitigation measures into aviation policies and regulations.

2. Target 9.4: By 2030, upgrade infrastructure and retrofit industries to make them sustainable… and greater adoption of clean and environmentally sound technologies and processes.

   The article’s quantification of non-CO₂ warming effects from aviation highlights the unsustainability of current processes. This scientific evidence supports the need for the aviation industry to innovate and adopt technologies or operational changes (e.g., flight path adjustments to avoid contrail-forming regions) to reduce its overall climate impact, directly addressing the call to make industries more sustainable.

3. Target 11.2: By 2030, provide access to safe, affordable, accessible and sustainable transport systems for all…

   The study contributes to the “sustainable” aspect of this target by improving the understanding of the full environmental cost of air transport. For aviation to be considered a truly sustainable part of the global transport system, its climate impacts, including both CO₂ and non-CO₂ effects like contrails, must be understood and mitigated.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

Yes, the article explicitly mentions and relies on several quantitative indicators that can be used to measure the climate impact of aviation and thus track progress towards the identified targets.

• Net Radiative Forcing (RF)

  This is the central indicator used throughout the article. RF measures the change in energy balance in the atmosphere due to a specific factor, with positive values indicating warming. The article quantifies the “annual global mean net radiative forcing of embedded contrails on the order of 5 mW m⁻².” This metric is a direct indicator of climate impact and can be used to measure the effectiveness of mitigation strategies.

• CO₂ Emissions

  The article explicitly states that “Aviation leads to the emission of CO₂ but also exerts non-CO₂ effects on climate.” While the study focuses on the non-CO₂ effects, CO₂ emissions remain a fundamental indicator for measuring the climate impact of any industry, including aviation. This aligns with global greenhouse gas emission inventories used to track progress on climate targets.

• Cloud Microphysical Properties

  The article implies that changes in specific cloud properties serve as secondary indicators of aviation’s impact. It measures how embedded contrails lead to “an increase in cloud optical thickness (COT)” and changes in “ice water content (IWC)” and “ice crystal effective radius (ICER).” Monitoring these properties in high-traffic air corridors can provide a way to assess the atmospheric effects of aviation in near real-time.

4. Table of SDGs, Targets, and Indicators

Source: nature.com