CIRRUS CLOUDS AND CLIMATE CHANGE
Beautiful to look at, and often almost transparent, the high wispy clouds known as cirrus play a surprisingly significant role in climate change.
These thin fibrous clouds are the most common type of cloud, with almost a third of the Earth covered with cirrus at any given time. They form high up, with their base heights starting around 6km (20,000ft) above the surface of the Earth. Formed in regions where the temperatures are below −20 to −30◦C (−4 to −22◦F) they are composed almost entirely of ice crystals. At their center is the particle that all clouds need in order to be able to form. In cirrus clouds, these seed particles (or “ice nuclei”) include desert dust, soot, organic matter, sulfuric acid particles from aircraft exhausts, bacteria, pollen, fungal spores, salt crystals, volcanic ash and mineral dust.
Cirrus clouds are particularly interesting in terms of climate change. Researchers have long recognized the complex role cirrus plays in balancing the “energy budget” of our planet’s climate, the difference between incoming energy from the sun and outgoing energy from the earth’s surface and atmosphere.
Lower thicker non-cirrus clouds, composed primarily of water droplets are widely considered to cause a net cooling effect at the surface of the Earth (despite their greenhouse effect), by reflecting incoming sun’s rays before they even reach the surface. Furthermore, these low clouds have an additional cooling effect. They absorb longwave radiation primarily in the form of thermal emissions (heat) emitted from the Earth’s surface. Some of these thermal emissions are then subsequently radiated back out from the cloud through the upper layers of the atmosphere and out to space, reducing the heating effect at the surface. According to the laws of thermodynamics, more heat is radiated out from a warmer object, in this case the cloud.
The ability of cirrus clouds to warm or cool the planet, however, is strongly dependent on the internal physical properties of ice crystal size, shape and concentration. The altitude of the cirrus clouds above the surface of the Earth is also significant. At their given altitude where the temperatures are very cold, cirrus clouds have much lower temperatures, and therefore their thermal emissions back out to space are smaller than lower, hotter clouds nearer the surface of the Earth. Observations of the composition of cirrus clouds show that their physical properties vary considerably depending on their location around the globe. In particular, tropical cirrus have different ice crystal structures to Arctic, Antarctic and midlatitude cirrus.
They each have unique ice crystal shapes and concentrations determined by the nature of the cloud’s formation, air temperature, relative humidity, turbulence within the cloud and gravitational effects. The ice crystals take many intricate forms including slender needles, plates, solid and hollow columns, bullet rosettes and more complicated aggregates. Each individual shape, and indeed the orientation of each crystal shape within the cloud, determines its effectiveness at reflecting, absorbing or transmitting either the incoming sunlight or outgoing thermal radiation.
When quantifying how much climate will change, the term ‘cloud radiative forcing’ is an important one. It can be defined as the difference between the radiative fluxes (in other words, energy) at the top of the atmosphere in clear and cloudy conditions (note that this includes all types of clouds, not just cirrus) and quantifies the amount by which Earth either warms or cools because of clouds. Cirrus clouds pose a significant measurement challenge since they are the most difficult type of cloud to identify on satellite imagery. However, their effect on the Earth’s radiation budget can be significant. Advances in satellite observations and climate modeling in recent decades have greatly improved the accuracy with which this overall cloud radiative forcing amount is calculated.
Due to their physical structure, which contains ice crystals, cirrus clouds are effective at trapping the infrared thermal emissions from the earth’s surface and lower atmosphere, and preventing those emissions from escaping out to space. In other words, they enhance the Earth’s greenhouse effect. This does not appear to be the case however when the clouds contain very small ice crystals of the order of a few micrometers. The ice crystals then act as minute mirrors and have a stronger ability to reflect the solar rays than to absorb and then radiate thermal emissions.
In a changing climate, what effects are we seeing on the amount, type and physical properties of naturally occurring cirrus? A warmer atmosphere leads to cirrus clouds forming at increasingly higher altitudes where temperatures are low enough for them to form. The higher (and therefore composed of ice) clouds are more effective than lower clouds at trapping the thermal emissions from the surface, and re-radiating them back to the surface, with less thermal energy emitted out to space.
Non-naturally occurring cirrus must also be considered, the most common type being contrail (condensation trail) cirrus, cirrus clouds formed by the water vapor from the exhaust fumes of high-flying aircraft. Soot and sulfuric acid particles also emitted by the aircraft act as the ice nuclei (or seed particles) and precipitate the generation of cirrus clouds in air where the relative humidity would be too low for cirrus to occur naturally. Current estimates show that 0.1% of the Earth’s surface is covered by aircraft contrail on an annually averaged basis, although with much higher values locally over regions with high aircraft traffic. This is a relatively new area of research with data available only for the last five decades or so. Longer-term observations are required for more accurate analysis and conclusions.
Further studies into the seed particles of cirrus indicated that over certain regions of Northern and Central America, they are mainly composed of metallic and dust particles. While mineral dust occurs naturally over arid regions and can be blown across the globe through atmospheric winds, these particles, accurately identified using laser spectrometry, resulted from industrial and combustion sources. Since these seed particles have a significant effect on the ability to produce cirrus, it is important to take them into account when understanding the factors leading to climate change.
One challenge faced by research scientists includes the identification of cirrus cloud signals from satellite data. In visible light they can often be transparent, or are obscured by lower and thicker cloud layers beneath them. However, alternative imaging channels such as water vapor and infrared channels, together with the increased use of satellite radiometers focusing on ice crystals rather than water droplets, has greatly improved the accuracy and availability of global cirrus cloud data.
Accurate cloud modeling also presents a challenge, due to current limitations in the computing power required to analyze the volume of data arising from both modeling microscopic particles within a single cloud, and the fact that clouds can span many tens or hundreds of kilometers. Today’s cloud models therefore rely on theoretical estimates which lead to uncertainties about the effect of clouds on climate.
Nonetheless, regardless of the ability of the theoretical models to accurately model the future, the changes we are now seeing at the Earth’s surface and lower atmosphere are real and quantifiable. Effects that scientists predicted would result from global climate change are now occurring: glacier shrinkage, loss of sea ice, accelerated sea level rise and longer, more intense heat waves.
Predicting the future with respect to climate may be challenging, but observing and measuring the many changes that we see happening at the surface in the present climate is not. Cirrus clouds add yet another complex factor which must be taken into account.