03. Overclouding

A Study in Infra-Red

Part three – Overclouding


“It is a capital mistake to theorize before you have all the evidence. It biases the judgment.” Sherlock Holmes


In part 2 we eliminated Carbon dioxide and soot emitted by industrial activity outside of the Arctic circle as suspects in the disproportionate warming concentrated in the Northern hemisphere. We established that soot emitted inside the Arctic circle, primarily by means of gas flaring carried out by industrial activities within the region but also soot produced by wildfires, may account for a significant portion of it.

In this part, we shall turn our attention to another major factor in the climate, the role played by aerosols in altering the make-up of clouds. First, however it is important to understand how clouds themselves affect global temperatures.



Do clouds warm or cool the climate?


Around 70% of the earth is covered by cloud and this has a profound influence on the climate. Solar radiation is reflected from above and Infra-red radiation is reflected from below. The balance between the two is crucial and hinges on the type of cloud and the proportions of those cloud types to one another.

  • The lowest clouds below 6000 feet cover about 28% of the earth. They reflect more solar radiation than the infrared they trap.  Their warm tops also radiate heat into space.

  • From 6000 to 20,000 feet are the mid-level clouds, covering 7%. Also net coolers, they reflect more heat than they trap.

  • The high level clouds above 18,000 feet cover about 35% of the planet. These clouds have an overall net warming effect, they trap more Infrared radiation than the Solar radiation they reflect out to space. 


  • The warming effect of clouds (cloud blanket) as a whole is estimated to be around:

+7 °C

  • The cooling effect of clouds (albedo) as a whole is estimated to be around:

-12 °C

  • Thus, overall, clouds cool the earth by about:

-5 °C










How clouds affect earth’s temperature





It follows then, that a relatively small change in the distribution and frequency of cloud types can cause a significant shift in global temperature.


Has there been such a shift in cloud make-up to correlate with the unprecedented global warming we have experienced since the mid-seventies?


Clouds from 60°N to 60°S


In the regions 60°N to 60°S, lower clouds have a net-cooling effect, this being more pronounced the further one moves towards the equator, whilst upper clouds have a net-warming effect, this being less pronounced the further one moves towards the equator.


We will now establish whether there has been an increase in global cirrus cloud coverage and a decrease in global cumulus cloud coverage to correlate with the unprecedented global warming we have experienced since the mid-seventies.


The High Resolution Infrared Radiometer Sounder (HIRS) polar orbiting satellite data as reported by Wylie et al (1994) and Wylie and Menzel (1999) found that in the region 60°N to 60°S, over the period from 1985 to 2001, the high clouds, which have an overall warming effect, had increased globally by 1.95% on average, per decade whilst the lower, with an overall cooling influence, had decreased by 1.7% per decade.










Trends in Global Cloud Cover in Two Decades of HIRS Observations




The geographical locations of changes in all-cloud and high-cloud frequency between the first and last 8 yr of this study

(1994–2001 minus 1985–92).


This shift in the ratio of net cooling to net warming clouds is likely to have contributed towards significant shifts in global temperature in the last 35 years or so, potentially greater than the effects of greenhouse gases.

The heat that builds up in this fashion finds its way eventually to the poles by means of the atmospheric convection cells and also the major ocean currents. Recall that 90% of warming goes into the giant heat sink that is the ocean. A warming ocean is a major contributor to arctic ice melting.


How Clouds Form


Clouds form when water vapour, in the air, condenses onto tiny particles known as cloud condensation nuclei (CCN), to form liquid water droplets which in turn, form clouds. The process is similar for upper level clouds only that water vapour freezes onto the particles, known as ice nuclei (IN), to form ice crystals.

These tiny particles can be of natural origin such as sea salt or desert dust, or they can be of anthropogenic origin.








Suspect 4 – Anthropogenically Aerosolized Clouds


In the regions excluding the Arctic and Antarctic, 60°N to 60°S, lower clouds have a net-cooling influence, whilst upper (cirrus) clouds have a net-warming influence. These require particles that act as ice nuclei (IN).


Cirrus Clouds


Although there are numerous kinds of particle, specific mineral and metal particles act as the majority of the nuclei (ice nuclei) that lead to the formation of ice crystals that make up cirrus clouds. These favoured particles make up 61% of the ice nuclei. These two types of seed account for 1% of the particles found at the altitude where cirrus form. The two types in question were those found in mineral dust and various metals including lead, zinc, tin, copper and silver. 

Feathery cirrus clouds have a cold metallic heart

Lead, in particular, has been shown, in addition to being an ice nucleus itself, to have the effect of “supercharging” pre-existing particles, making even more highly efficient nuclei.

Mineral dust is mainly composed of the oxides (SiO2, Al2O3, FeO, Fe2O3, CaO, and others) and carbonates (CaCO3, MgCO3) that constitute the Earth’s crust. 

Inadvertent climate modification due to anthropogenic lead


As we know, the anthropogenic aerosols that lead to cloud formation are, like clouds, found in abundance in the Arctic but not the Antarctic.




Researchers used an electron microscope to capture these images of black carbon attached to sulfate particles. The spherical structures in image A are sulfates; the arrows point to smaller chains of black carbon. Black carbon is shown in detail in image B. Image C shows fly ash, a product of coal-combustion, that's often found in association with black carbon. While black carbon absorbs radiation and contributes to warming, sulfates reflect it and tend to cool Earth. Credit: Peter Buseck, Arizona State University


Aerosols may drive a significant portion of global warming.


Some of these aerosols are more efficient than others as cloud condensation nuclei (CCN).


Pure black carbon particles (50nm) are hydrophobic and do not act as CCN. Black carbon coated, for example with sulphate, can act as moderately effective CCN for lower clouds. Black carbon, coated or uncoated is not a significant contributor to levels of ice nuclei (IN), the seeds for cirrus clouds.


Ice nucleation and droplet formation by bare and coated soot particles


Sulphate particles (1 micron), converted from Sulphur dioxide act as moderately efficient cloud CCN for lower clouds but relatively few ice crystals condense around them at the elevations of upper clouds despite their abundance.


Another product of coal combustion and often found in association with black carbon is coal fly ash. Coal fly ash particles (100nm) are very efficient CCN for both lower and upper clouds.

Other anthropogenic aerosols that act as excellent CCN are mineral dust from land use and metal oxides from industrial smelting.


“At very high altitudes, water vapor can freeze around aerosols, creating ice-containing clouds known as cirrus. Lab experiments suggest that mineral dust from land use, metal oxides from industrial smelting, and so-called “fly ash” from coal combustion are excellent at seeding ice crystals.

Seeing Beyond the Clouds

Of interest is the fact coal fly ash particles, which are very efficient CCN, are very difficult to distinguish from mineral dust and tend to be coupled with it by scientists. One study on cirrus cloud conditions found, using Single Particle Mass Spectrometry, that 33% of the ice crystal residues were “mineral dust/fly ash”. Then they used electron microscopy to show that 20 % of the particles in this category had a high degree of sphericity which indicated that they were fly ash. Therefore, a significant quantity of “mineral dust” particles that form the ice nuclei for cirrus clouds are actually fly ash.


Ice nucleation by combustion ash particles at conditions relevant to mixed-phase clouds

That’s around 7% of cirrus clouds formed on fly ash. The singular importance of this fact will become clear in due course.

In addition to coal combustion, industrial smelting and land use, another source of anthropogenic aerosols is jet engine emissions from aircraft. These include sulphates, soot and metal particles in addition to carbon dioxide, water vapour, nitrogen oxides (NOx), carbon monoxide, and hydrocarbons such as methane.

Despite the switch to unleaded fuel on the surface, it appears that jet fuel emissions still contain this heavy metal which “supercharges” other ice nuclei as outlined previously.

Aluminium, and other toxic heavy metals such as barium are also included in the additives put into jet fuel. 

The impact of aircraft on cirrus cloud formation is of central importance, as we shall see.


Next we will observe the impressions left behind on the climate by the clouds formed in the Arctic.






















Arctic Clouds





Generally, greater amounts of lower clouds, which reflect more sunlight than the heat energy they trap, will have a greater cooling influence. This applies the closer the lower clouds are to the equator. However, the further pole-wards the lower clouds occur, the greater the shift towards net warming until, over the Arctic, lower clouds have a warming effect overall. This applies throughout the year, except, briefly, during the summer.

The upper level, cirrus clouds have a net warming effect at all latitudes, though particularly pronounced in the Arctic.

In summary, particulates act as cloud condensation nuclei for lower and upper clouds, both of which have a net-warming effect in the Arctic.


Clouds amplify warming in Arctic, study finds


In the Arctic, 60° - 90° N, the IPCC showed in 2001 that, for actual observed values, average year round cloud cover was about 70% whilst, curiously, in the Antarctic it was about 3%.



Source: IPCC, Third Assessment Report: Climate Change, 2001



This correlates well with the record for temperature change by latitude for the period 1980 to 2001. Take note that the northern latitudes are on the left hand side in the above graph and on the right hand side in the graph below.





Looking at the change in Arctic cloud cover in spring, we can see that there has been a roughly 10% increase from the period 1980 (73%) to 2005 (83%). This linear change has overridden the effects of the Arctic oscillation.




Time series of seasonally averaged cloud fraction

Time series of seasonally averaged cloud fraction over the arctic seas in spring (March, April, May). Provided by Axel J. Schweiger.


spatial distribution of trend in cloud cover


Spatial distribution of trends in cloud cover over twenty years. Provided by Axel J. Schweiger.


Climate Indicators - Clouds


“Correlations with temperature and sea ice extent are strongest during spring and autumn when the cloud longwave effect dominates. It is shown that low clouds have a strong positive relationship with temperature during these seasons. However, the trend of Arctic aerosols has gone in the opposite direction, as Quinn et al. (2007) have observed with the decreasing sulfate aerosols since the mid-1990s at surface stati