5. Atmospheric measurements

Measurements of the atmosphere improve the performance weather forecasts, underpin our understanding of Earth’s changing environment, and help inform climate models that are used to look at future climate scenarios. Measuring the atmosphere is non-trivial and expensive so why do we bother? ‘Atmospheric measurements’ considers the necessity of both atmospheric modelling and data collection. By combining information from both, more realistic results will be produced. It then describes some of the numerous processes involved in taking measurements—from fixed, ground-based observations to weather balloons, research aircraft, and satellite technology that allows the collection of data hundreds of kilometres above the Earth’s surface.

4. Atmospheric composition

Nitrogen, oxygen, and argon represent more than 99.9% of the air we breathe. But Earth’s atmosphere hasn’t always had that composition—it is on at least its third distinctive atmosphere. ‘Atmospheric composition’ provides a brief history of Earth’s atmosphere, before considering the two most important regions of the atmosphere for human survival—the stratosphere and troposphere. The stratospheric ozone layer shields harmful ultraviolet-B light penetrating to the surface, thereby protecting humans and ecosystems from harmful ultraviolet radiation. The troposphere is where billions of people live and breathe. It is also where air pollutants are emitted, wildfires burn, vegetation grows, and where the oceans exchange gases. The impact of atmospheric aerosols and greenhouse gases is also discussed.

2. Atmospheric physics

Earth’s atmosphere is tied closely with the Sun. The Sun emits electromagnetic radiation at a wide range of wavelengths. Radiation is transported through the atmosphere by transmission, absorption, and scattering. ‘Atmospheric physics’ outlines the Earth’s radiation budget—the incoming and outgoing radiation, equilibrium between them, and departures from this equilibrium due to increasing levels of clouds, greenhouse gases, and atmospheric aerosols. It then describes the greenhouse gases that absorb and emit radiation and the thermodynamics of the atmosphere. The importance of water, the dominant atmospheric constituent responsible for the loss of radiative energy to space and hence atmospheric cooling, and the electrical energy stored in the atmosphere are also discussed.

1. What is special about Earth’s atmosphere?

‘What is special about Earth’s atmosphere?’ describes the several interconnected layers that make up Earth’s atmosphere before considering the atmospheres of other planets. Each layer has different characteristics determined by the density of air and their relative proximity to Earth’s surface and outer space. The lower atmosphere consists of the troposphere, which extends from the surface to the tropopause at 10–15 km. The middle atmosphere is comprised of the stratosphere, extending to the stratopause at 50 km, and the mesosphere that stretches to the mesopause at 100 km. Above this is the upper atmosphere divided into the thermosphere, which takes us to 500–1,000 km, and the exosphere, which extends to the near vacuum of outer space.

3. Atmospheric motion

Solar activity is the main driver for Earth’s large-scale atmospheric motion. Due to the Earth’s tilt, lower latitudes receive more energy from the Sun that they emit back to space, while the higher latitudes emit more radiation back to space than they receive directly from the Sun. The Earth is in approximate thermal equilibrium, suggesting that energy is being transported from low to high latitudes. The thermal gradient between the tropics and the poles drives the hemispheric circulation. But Earth is rotating and is composed of land and ocean. ‘Atmospheric motion’ outlines the effects of these factors on the atmosphere’s circulation patterns and describes key features such as jet streams and the Southern Oscillation.

6. Our future atmosphere

There is still much about Earth’s atmosphere we do not fully understand, which limits our ability to predict large-scale changes to the atmosphere. As Earth’s climate changes new scientific challenges will emerge that need to be addressed with new measurements and models. These challenges have implications for assessing the impact of future global economic growth and mitigating humanitarian risks. ‘Our future atmosphere’ outlines some of the future scientific, technical, and philosophical challenges we face. These include our responses to the changes in natural and anthropogenic-driven climate change, facing future unknown challenges, using improvements in technology to address the scientific challenges, and aiming for an international legally binding agreement on atmosphere policy.