Impacts of climate change in the troposphere, stratosphere and mesosphere on the thermosphere and ionosphere

An increasing amount of advanced, satellite-based technology, used for both commercial and scientific purposes, operates within the outer regions of the Earth's atmosphere. To safeguard this technology and ensure that we will be able to continue exploiting this environment safely and effectively, reliable predictions of its future state, especially its density, are essential. To make reliable predictions, a clear understanding of the causes of long-term (multi-decadal to centennial) changes that have taken place in the past is needed.

The Earth's upper atmosphere (~90-500 km altitude) is influenced both by processes taking place on the Sun (e.g., so-called coronal mass ejections, which propel high-energy plasma towards the Earth) and by processes taking place in the atmosphere below (e.g., thunderstorms). Unravelling the relative importance of these two categories of drivers is a key challenge in achieving a better understanding of the upper atmosphere. This proposal focuses on quantifying the role of climatic changes in the lower and middle atmosphere (0-90 km altitude) in causing long-term changes in the state of the upper atmosphere.

The lower and middle atmosphere are thought to affect the upper atmosphere mainly via upwardly propagating atmospheric waves. As atmospheric waves travel upwards, their amplitude increases due to the exponential decrease in atmospheric density with height. Very large-scale waves, such as atmospheric tides, therefore constitute an important part of the motion of the upper atmosphere. However, the amplitudes of the waves eventually become so large that they become unstable and break, similar to waves on a beach. When atmospheric waves break, they transfer energy and momentum to the surrounding atmosphere, which drives large-scale, global circulations and causes mixing. Both the characteristics of the waves, and the state of the surrounding atmosphere, determine how far these waves can propagate (in altitude as well as horizontally) and when they break.

There is evidence that, as a result of man-made climate change in the lower and middle atmosphere, both wave generation processes and the wave propagation conditions in the lower and middle atmosphere have changed over the past 4-5 decades, with further changes expected in the future. This has already caused changes in large-scale circulation patterns in the lower atmosphere, and is likely to affect the climate of the upper atmosphere as well.

This project will quantify the importance of man-made climate change in the lower and middle atmosphere in causing long-term changes in the upper atmosphere, both in the past (1950s-2000s) and projected into the future (2050s) according to established emission scenarios. Computer simulations with a state-of-the-art, global, 3-dimensional climate model, extending from the surface up to ~500 km altitude, will be used to do this. Results from these simulations will be compared to observed long-term changes in the upper atmosphere (e.g., in temperature, density) and to contributions made by other known factors. These include the increase in greenhouse gas concentration within the upper atmosphere itself, which has a cooling effect, and changes in the Earth's magnetic field, which cause more complicated patterns of long-term change. Interactions of changes in the Earth's magnetic field and changes in atmospheric tides due to climate change will also be investigated. This will focus at least initially again on the period of the 1950s to 2050s, but this may be broadened to a larger timespan from 850 to the present-day.

In summary, this project will establish how important climate change in the lower and middle atmosphere is in causing long-term changes in the average state of the Earth's upper atmosphere. This will improve our understanding of past change in the upper atmosphere and enable better predictions for the future.

The Earth's upper atmosphere is host to an ever increasing number of satellites in low Earth orbit, which offer services to businesses, governments, and the general public. For instance, the International Space Station is located at an altitude of 350-400 km, within the upper thermosphere. Also many scientific satellite missions (e.g. the Swarm mission of the European Space Agency) operate in this approximate region.

In addition to active satellites, the population of space debris is growing dramatically. Space debris is defined as "all man-made objects, including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional" (United Nations Committee on the Peaceful Uses of Outer Space (UN COPUOS), 2010). Space debris poses a significant hazard to operational spacecraft due to the risk of collisions. Even collisions with relatively small particles can cause major damage because of the high orbital speeds (e.g. Lewis et al., 2011). Monitoring the location of all known space debris objects to avoid collisions, even when no avoidance manoeuvres are needed, comes at considerable cost to satellite operators (Lewis et al., 2011).

All objects in low Earth orbits experience an atmospheric drag which is proportional to the ambient atmospheric density. This drag acts to lower the object's orbit and will eventually cause it to fall back to the Earth's surface. This is the main way in which space debris is lost from the near-Earth space environment. Any long-term change in the density of the upper atmosphere will therefore have a significant impact on the time evolution of the space debris population.

The increase in atmospheric CO2 concentration is causing cooling and contraction of the upper atmosphere, which leads to lower densities at high altitudes and thereby a longer lifetime of space debris. In addition, there may be indirect effects of climate change in the lower/middle atmosphere on long-term trends in upper atmosphere density. This is investigated by the proposed project, covering altitudes up to ~500 km. The project will also provide estimates of thermospheric density in the 2050s, which will be used for space debris population projections through simulations with the Debris Analysis and Monitoring Architecture for Geosynchronous Environment (DAMAGE) model developed by the University of Southampton (Lewis et al., 2011). Such projections are needed to define appropriate space debris mitigation measures to ensure that the space debris population can be maintained at an acceptable level, allowing the continued use of the LEO environment for commercial and scientific purposes in the future.

For the above reasons, the main beneficiaries of the proposed project in a socio-economic sense are those that define space debris mitigation policies and operators of satellites in low Earth orbit, as well as the broader communities that rely on their services, including ultimately the general public. The space debris problem has attracted considerable media attention in recent years, indicating the societal concern there is for this issue. Examples are the BBC Horizon documentary "The Trouble with Space Junk", aired in August 2015, and a BBC article (http://www.bbc.co.uk/news/science-environment-33782943) on the same topic. In addition, a portion of the general public has a keen interest in environmental science, especially in how human actions affect our environment. This project will also provide new information on this topic and will demonstrate the relevance of whole atmosphere science to a wide audience.

References:
Lewis, H.G., A. Saunders, G. Swinerd, et al. (2011), Effect of thermospheric contraction on remediation of the near-Earth space debris environment, J. Geophys. Res., 116, A00H08.
United Nations Committee on the Peaceful Uses of Outer Space (UN COPUOS) (2010), Space debris mitigation guidelines of the Committee on the Peaceful Uses of Outer Space, Rep. 09-88517, N.Y.