Climate Change
In recent times climate change has become an important worldwide issue.
Factors, such as changes in the concentrations of gases in the atmosphere and changes in the surface of land can alter the energy balance within the Earth’s climate system (Alley et al, 2007). Climate change can occur due to natural changes such as fluctuations in solar radiation and volcanism and due to human impacts (anthropogenic sources) such as changes in land-use and burning of fossil fuels.
The Intergovernmental Panel on Climate Change, (IPCC), was formed by the World Meteorological Organisation (WMO) and the United Nations Environment Programme (UNEP) to assess and present current knowledge of climate change (Houghton, 2009). The understanding of anthropogenic influences on the climate has improved with each IPCC report. The IPCC Fourth Assessment Report (AR4) stated that warming of the climate system is unequivocal and that there is very high confidence that the global average net effect of human activities since 1750 has been one of warming (Alley et al, 2007).
There have been large natural fluctuations in the Earth’s climate in the distant past, for example during ice ages (Houghton, 2009). However, in the last 30 years the warmest temperatures have been recorded since accurate temperature recording began around 100 years ago and the predicted rate of global average temperature change is more rapid than any change in the last 10000 years (Houghton, 2009).
Carbon Dioxide (CO2) is the most important anthropogenic greenhouse gas (GHG). However, significant contributions to climate change also come from other anthropogenic sources such as ozone-forming chemicals, changes in land use, aerosol emissions and aviation contrails (Alley et al, 2007). Continued research is required to accurately assess the contributions to climate change.
FAQ 1.1, Figure 1. Estimate of the Earth’s annual and global mean energy balance. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation. About half of the incoming solar radiation is absorbed by the Earth’s surface. This energy is transferred to the atmosphere by warming the air in contact with the surface (thermals), by evapotranspiration and by longwave radiation that is absorbed by clouds and greenhouse gases. The atmosphere in turn radiates longwave energy back to Earth as well as out to space. IPCC (2007). Source: Kiehl and Trenberth (1997).
The energy from the sun that is not reflected back to space is absorbed by the Earth’s surface and atmosphere. The sum of total that is reflected, amounts to approximately 240 Wm-2. To balance that, Earth must radiate on average the same amount of energy back to space, by emitting outgoing long wave radiation. To emit 240 Wm-2 a surface would have to have a temperature of around -19°C, instead the Earth’s mean surface temperature is about 14 °C, due to the presence of greenhouse gases, which act as a partial blanket for the long wave radiation coming from the surface of the Earth (reference).
Water vapour is the most significant greenhouse gas. The water vapour in the atmosphere is not significantly influenced by human activities. Greenhouse gas concentrations that are modified as a result of human activities include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone (O3).
Metrics, such as radiative forcing (RF), global warming potential (GWP) are used to compare the climate change impacts from different sources and different greenhouse gases. Metrics are required due to factors such as the different lifetimes and distributions of greenhouse gases.
Radiative forcing is used to measure the impact of a factor on the radiation balance of the Earth. A positive radiative forcing tends to warm the Earth’s surface and a negative radiative forcing tends to cool the Earth’s surface (Alley et al, 2007).
Clouds contribute to warming of the atmosphere by trapping outgoing infra-red radiation but also contribute to cooling the atmosphere by reflection of incoming solar radiation.
The amount of CO2 has risen by approximately 35% compared to pre-industrial levels due to human activities (Alley at al, 2007).
The climate impacts of transport is an important area for research as transport activity is increasing globally, particularly in developing countries and this growth is likely to continue into the future. This is in response to economic growth and increased demand for personal mobility (Kahn-Ribeiro et al, 2007). Civil aviation is one of the fastest growing modes of transport (Kahn-Ribeiro et al, 2007). Aviation passenger transport volume grew at an average rate of 5.2% from 1992 to 2005 and continued growth is predicted (Lee et al, 2009).
The interest in the impact of aviation on climate change and ozone depletion dates back some 40 years back to late 1960’s during the US and UK cooperation on developing ideas or a supersonic aircraft. The concerns were raised over stratospheric O3 depletion and the awareness that contrails might modify the climate.
As air transport has continued to grow, the focus has shifted to studying the impacts of subsonic aircraft and the environmental impact of aircraft emissions.
Aviation stands as a unique sector as majority of its emissions are released in 8-12km altitudes, which lead to an increased effectiveness in chemical and aerosol effects relevant to climate forcing. Following the 1999 report on ‘Aviation and the Global Atmosphere’ a comprehensive assessment of aviation’s impacts on climate concluded that aviation represents a small but potentially significant and increasing forcing on climate. The scope of the effects are uncertain due to its non-CO2 effects on the climate – IPCC estimated the sum of non-CO2 RF effects to be 63% of the total radiative effects from aviation in 1992(excluding cirrus cloud enhancement) leading to creation of green aeronautical technologies under the European Commission’s fifth framework programme: PartEmis, NEPAIR, TRADEOFF, INCA, AERO2K, METRIC, SENIC and CRYOPLANE
In the late 1980’s to early 1990’s research initiated into the effects of nitrogen oxide emissions NOx on the formation of troposphere O3 (ozone) and the effects of condensation trails (contrails) from subsonic fleet. 1997 European project Aeronox and Us SASS project investigated the emissions of nitrogen oxides (NOx) from aircraft engines and global air traffic at cruising altitudes.
The increase of CO2 in the atmosphere can be put into perspective by comparing the anthropogenic increase with the natural cycles in the past. The data coming from air enclosed in bubble in ice cores from Greenland and Antarctica, the initial measurements demonstrated that levels of CO2 were significantly lower during the last ice age than over the last 10 000 years of the Holocene. From 10 000 before present, up to 1750, levels of CO2 stayed within the range of 280 ± 20 ppm and during the industrial era, the levels rose roughly to 367 ppm to 379 in 2005.