Scientific Understanding

ICAO Standards and guidance material on environmental issues are always founded on scientific consensus. As a consequence, in support of a data-driven decision making process, ICAO is constantly monitoring the evolution of scientific knowledge related to the impacts of aviation on the environment.

This is in line with provisions from the ICAO Resolution A41-20 (2022), where the ICAO Assembly invites States and international organizations to provide the necessary scientific information and data to enable ICAO to substantiate its work in this field; and encourages the Council to continue to cooperate closely with international organizations and other UN bodies on the understanding of aviation impacts on the environment and on the establishment of policies to address such impacts.

"White papers" containing the state of science on the environmental impacts of aviation were provided in the latest versions of the "ICAO Environmental Report", as follows:

IPCC Reports


Specifically regarding aviation impacts on Climate Change, a comprehensive assessment concerning aviation's contribution to global atmospheric problems is contained in the Special Report on Aviation and the Global Atmosphere, which was prepared at ICAO's request by the Intergovernmental Panel on Climate Change (IPCC) in collaboration with the Scientific Assessment Panel to the Montreal Protocol on Substances that Deplete the Ozone Layer and was published in 1999. The conclusions of this report include:


  1. that aircraft emit gases and particles which alter the atmospheric concentration of greenhouse gases, trigger the formation of condensation trails and may increase cirrus cloudiness, all of which contribute to climate change; and
  2. that aircraft are estimated to contribute about 3.5 per cent of the total radiative forcing (a measure of change in climate) by all human activities and that this percentage, which excludes the effects of possible changes in cirrus clouds, was projected to grow.

    The Report recognized that the effects of some types of aircraft emissions are well understood, revealed that the effects of others are not, and identified a number of key areas of scientific uncertainty that limit the ability to project aviation impacts on climate and ozone.

    IPCC Assessment Report 4 (2007) and Assessment Report 5 (2014)

    At ICAO's request, the IPCC included an update of the main findings of the 1999 report in its Fourth Assessment Report (IPCC AR4) published in 2007, and on its Fifth Assessment Report (IPCC AR5) published in 2014.

    The new findings related to aviation emissions in IPCC AR4 included:
  • Due to developing scientific knowledge and more recent data estimates of the climate effects of contrails have been lowered and aircraft in 2005 are now estimated to contribute about 3.0 % of the total of the anthropogenic radiative forcing by all human activities;
  • Total CO2 aviation emissions is approximately 2 % of the Global Greenhouse Emissions;
  • The amount of CO2 emissions from aviation is expected to grow around 3-4 per cent per year; and
  • Medium-term mitigation for CO2 emissions from the aviation sector can potentially come from improved fuel efficiency.
  • However, such improvements are expected to only partially offset the growth of CO2 aviation emissions.


The new findings related to aviation emissions in IPCC AR5 are summarized in the White Paper On Climate Change Aviation Impacts On Climate: State Of The Science, published in the ICAO Environmental Report 2016.


This white paper also highlighted that the understanding and confidence in aviation climate forcings would be enhanced by a new international scientific assessment.


Supplement to the IPCC 1999 Report

While a new international scientific assessment is not done, ICAO has compiled a list of supplemental references that are connected with the IPCC 1999 Report topics. This compilation is provided in the following website:

Supplemental references on the IPCC 1999 Special Report on Aviation and the Global Atmosphere.

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