Yesterday I found myself dreaming of clouds at 30,000ft while stuck on the ground waiting for a badly delayed flight. Discover Magazine (April 2012) held an article on new ideas about clouds and how they develop rainfall. A research group in San Diego has developed an aerosol time-of-flight mass spectrometer (ATOFMS) that can be used during flights through clouds to look at the size, chemical composition, and optical properties of single particles in real-time. Their flights have turned up some unexpected results that may impinge upon climate theories.
Why do some clouds form ice crystals while most remain composed of liquid droplets even at -30°C? Liquid water can readily exist under supercooled conditions, and it takes a speck of dirt to initiate ice nucleation or snowflake formation. The view that particles such as soot, dust and pollution are responsible for growing raindrops and snowflakes is still prevalent in the scientific community, however this research, building on work initially done in the 1960s, is challenging that view as more data is gathered using ATOFMS. The new research points to biological particles – microbes – as a possible significant contributor to cloud ice formation. The dominant view is that soot and dust are much more abundant and that microbe concentrations are too low to make much difference. The new research is contoversial because “…dominant views do not die easily“. Where have we heard that before?
I remember the discovery in the 1980s of the ice-nucleating bacterium Pseudomonas syringae and its importance to agriculture. Basically such bacteria produce proteins on their cell surface that align water molecules in a specific configuration, allowing water to freeze readily at just below zero. This was thought to allow the microbe to use ice crystals to penetrate plant cells enabling access to the ‘food’ inside – an evolutionary benefit. For the agricultural industry it allowed a new form of frost protection – using bactericides. I remember also colleagues working on spraying bacteria on frost-tolerant aphid-prone plants; this allowed a light frost to kill the aphids by impaling them with ice crystals and enabled control using Spring frosts to halt the growth in populations and reduce the burden on plants and the use of insecticides later in the year. Other microbial species have been discovered with the same ice nucleating ability and, interestingly, it seems this ability arose only once in microbial evolution – the spread of the trait is a result of gene transfer. But I digress.
The article describes research on the freezing of water droplets in the laboratory, where the frequency and effects of dust and other particles is being investigated. Researchers are also looking at particle frequency in samples of snow. It’s not just the quantity of bacteria that is of interest, but their efficacy:
Lab studies have yet to turn up a common mineral that triggers freezing as effectively as bacteria like syringae do. “These organisms are able to catalyse ice formation at a temperature warmer than any other naturally occurring particle”.
Thanks to ATOFMS and other analyses, bacteria with ice nucleating ability have now been discovered in clouds – both as free microbes and associated with dust; there is a strong association with clouds in which the water droplets are frozen as opposed to supercooled. The dust, it seems, is frequently from North African or Asian desert regions – even in clouds over the western USA or Caribbean.
There’s a whole ecosystem going on in the clouds that’s largely undefined.
…dust from Africa and Asia might carry ice-forming microbes all around the globe.
All told, up to 2 million tons of bacteria may find their way into the atmosphere each year, not to mention 55 million tons of fungal spores and unknown quantities of algae.
That’s a lot of ‘bits’ in addition to dust particles to help water droplets in clouds coalesce and perhaps freeze. The article talks only briefly about using the bacteria in cloud seeding, but I found my mind bouncing to all sorts of links.
When the article mentioned dust I immediately thought of the map above which shows the worldwide natural distribution of (2.5μm) particulates. It shows where natural particulates are generated, tying in nicely with the supposed origin of the cloud dust. But what about the Southern Hemisphere? Is there sufficient dust from Australia, the Atacama and Namibia or do the ocean and DMS (dimethyl sulphide) take over the cloud forming role there? Or, is it the Northern Hemisphere, where we see more cyclic warming from ocean cycles, that really is in the driving seat – clouds and all? How does this fit in the climate jigsaw?
Obviously the CERN CLOUD experiments spring to mind although they were dealing with molecules such as ammonia rather than minerals and bugs. Wouldn’t it be interesting to throw bugs into that mix with cosmic rays? Let’s hope that’s planned.
However, I also thought of Willis Eschenbach’s Thunderstorm Thermostat Hypothesis, specifically with the freezing of water droplets in mind. Here’s an excerpt from his “A day in the tropics” thought experiment (bold mine):
As the temperature continues to rise, as the evaporation climbs, some of the fluffy cumulus clouds suddenly transform themselves. They rapidly extend skywards, thrusting up to form pillars of cloud thousands of meters high in a short time. These cumulus are transformed into cumulonimbus or thunderstorm clouds. The columnar body of the thunderstorm acts as a huge vertical heat pipe. The thunderstorm sucks up warm, moist air at the surface and shoots it skyward. At altitude the water condenses, transforming the latent heat into sensible heat. The air is rewarmed by this release of sensible heat, and continues to rise.
Freezing (fusion) releases relatively little additional sensible heat into the atmosphere. The energy release is only 334 kJ/kg for fusion compared to the 2260 kJ/kg already released by condensation, but that extra 15% is still additional heat transfer and its occurence at higher altitudes than condensation may be in some way significant. Since we can’t use cloud temperature per se as a guide, we don’t even know the percentage of clouds in which freezing occurs.
Sometimes it seems that the more we learn, the less we know.