Biological Productivity and its Influence on Cloud Formation. (by Diego Fdez-Sevilla)
Recently I have found a couple of articles and press releases which have captured my attention based on sharing a common ground that I believe has a noteworthy relevance still to be fully understood in environmental and climatic assessments, “the influence of biological productivity on cloud formation.”
This subject is linked with some of my previous posts due to the nature of land use and land cover impact, the effect of solar activity in biological productivity and the impact of extending GM crop farming due to its enhanced biological productivity.
Cloud droplets form on aerosol particles that can either be directly emitted, such as evaporated sea spray, or else form through a process known as nucleation, in which trace atmospheric vapours cluster together to form new particles that may grow to become cloud seeds. Around half of all cloud seeds are thought to originate from nucleated particles, but the process of nucleation is poorly understood.
New light on cloud formation
In an open access paper published in the journal Science, CERN’s CLOUD experiment has shown that biogenic vapours emitted by trees and oxidised in the atmosphere have a significant impact on the formation of clouds, thus helping to cool the planet. These biogenic aerosols are what give forests seen from afar their characteristic blue haze. The CLOUD study shows that the oxidised biogenic vapours bind with sulphuric acid to form embryonic particles which can then grow to become the seeds on which cloud droplets can form. This result follows previous measurements from CLOUD showing that sulphuric acid alone could not form new particles in the atmosphere as had been previously assumed.
Sulphuric acid is thought to play a key role, but previous CLOUD experiments have shown that, on its own, sulphuric acid has a much smaller effect than had been assumed. Sulphuric acid in the atmosphere originates from sulphur dioxide, for which fossil fuels are the predominant source. The new result shows that oxidised biogenic vapours derived from alpha-pinene emitted by trees rapidly form new particles with sulphuric acid. Ions produced in the atmosphere by galactic cosmic rays are found to enhance the formation rate of these particles significantly, but only when the concentrations of sulphuric acid and oxidised organic vapours are relatively low. The CLOUD paper includes global modelling studies which show how this new process can account for the observed seasonal variations in atmospheric aerosol particles, which result from higher global tree emissions in the northern hemisphere summer.
These clouds are almost certainly a result of evapotranspiration. The clouds are distributed evenly across the forest, but no clouds formed over the Amazon River and its floodplain, where there is no tall canopy of trees. While water may evaporate from the Amazon River itself, the air near the ground is too warm for clouds to form. Trees, on the other hand, release water vapor at higher levels of the atmosphere, so the water vapor more quickly reaches an altitude where the air is cool enough for clouds to form. When water vapor condenses, it releases heat into the atmosphere. (NASA image courtesy Jeff Schmaltz, MODIS Rapid Response at NASA GSFC )
“The reason why it has taken so long to understand the vapours responsible for new particle formation in the atmosphere is that they are present in minute amounts near one molecule per trillion air molecules”, explains Jasper Kirkby. “Reaching this level of cleanliness and control in a laboratory experiment is at the limit of current technology, and CERN know-how has been crucial for CLOUD being the first experiment to achieve this performance.”
Biogenic vapours join another class of trace vapours, known as amines, that have previously been shown by CLOUD to cluster with sulphuric acid to produce new aerosol particles in the atmosphere. Amines, however, are only found close to their primary sources such as animal husbandry, whereas alpha-pinene is ubiquitous over landmasses. This latest result from CLOUD could therefore explain a large fraction of the birth of cloud seeds in the lower atmosphere around the world. It shows that sulphuric acid aerosols do indeed have a significant influence on the formation of clouds, but they need the help of trees.
Measuring Biological Productivity. When plant cells photosynthesize, part of the energy they produce is emitted as fluorescent light.
Rainforests, whether in the Amazon, Southeast Asia, or Central America, are hotspots of organic productivity. Fueled by abundant rain and a reliable stream of nutrients, the Amazon blooms year-round. For a brief period each summer, however, the ingenuity of humankind trumps even the mighty rainforests at biological production. A group of researchers, including Christian Frankenberg and Joanna Joiner have determined that during peak growing season, the Midwest U.S. Corn Belt is the most productive land on Earth. In other words, there’s more photosynthesis going on here than in the Amazon.”
Rainforests, whether in the Amazon, Southeast Asia or Central America, are hotspots of organic productivity, teeming with life. Fueled by abundant rain and a reliable stream of nutrients, the Amazon blooms year-round. For a brief period each summer, however, the ingenuity of humankind trumps even the mighty rainforests at biological production. At the peak of the growing season, says NASA, the Midwest U.S. corn belt is the most productive place on Earth—there’s more photosynthesis going on here than even in the Amazon.
When plant cells photosynthesize, part of the energy they produce is emitted as fluorescent light. By measuring the strength of this fluorescence from space, scientists can get a measure of plant productivity—as they did in a recent study. NASA has a video explaining in more detail the process of fluorescence, and how the image above was put together:
The difference between the corn belt productivity’s and the Amazon’s is the incredible amount of inputs that go into creating growth in the U.S. We have to draw on vast resources to power unnatural temporary growth in a concentrated area. But, for a short period of time, it means we can produce far beyond what natural ecosystems like the Amazon can muster.
The effect of atmospheric aerosols on climate sensitivity.
Drew Shindell, a climatologist at NASA’s Goddard Institute for Space Studies in New York, hinges on a new and more detailed calculation of the sensitivity of Earth’s climate to the factors that cause it to change.
“Shindell’s focuses on improving our understanding of how airborne particles, called aerosols, drive climate change in the Northern Hemisphere. Aerosols are produced by both natural sources – such as volcanoes, wildfire and sea spray – and sources such as manufacturing activities, automobiles and energy production. Depending on their make-up, some aerosols cause warming, while others create a cooling effect. In order to understand the role played by carbon dioxide emissions in global warming, it is necessary to account for the effects of atmospheric aerosols.
While multiple studies have shown the Northern Hemisphere plays a stronger role than the Southern Hemisphere in transient climate change, this had not been included in calculations of the effect of atmospheric aerosols on climate sensitivity. Prior to Shindell’s work, such calculations had assumed aerosol impacts were uniform around the globe.
This difference means previous studies have underestimated the cooling effect of aerosols. When corrected, the range of likely warming based on surface temperature observations is in line with earlier estimates, despite the recent slowdown.
One reason for the disproportionate influence of the Northern Hemisphere, particularly as it pertains to the impact of aerosols, is that most man-made aerosols are released from the more industrialized regions north of the equator. Also, the vast majority of Earth’s landmasses are in the Northern Hemisphere. This furthers the effect of the Northern Hemisphere because land, snow and ice adjust to atmospheric changes more quickly than the oceans of the world.”
Absence Of Clouds Caused Pre-human Supergreenhouse Periods
In a world without human-produced pollution, biological productivity controls cloud formation and may be the lever that caused supergreenhouse episodes during the Cetaceous and Eocene, according to Penn State paleoclimatologists.
In general, the proxies indicate that the Cretaceious and Eocene atmosphere never exceeded four times the current carbon dioxide level, which is not enough for the models to create supergreenhouse conditions. Some researchers have tried increasing the amount of methane, another greenhouse gas, but there are no proxies for methane. Another approach is to assume that ocean currents changed, but while researchers can insert new current information into the models, they cannot get the models to create these ocean current scenarios.
According to the researchers, changes in the production of cloud condensation nuclei, the tiny particles around which water condenses to form rain drops and cloud droplets, decreased Earth’s cloud cover and increase the sun’s warming effect during supergreenhouse events.
Normal cloud cover reflects about 30 percent of the sun’s energy back into space. Kump and Pollard were looking for a scenario that allowed in 6 to 10 percent more sunlight.
“In today’s world, human generated aerosols, pollutants, serve as cloud condensation nuclei,” says Kump. “Biologically generated gases are dominant in the prehuman world. The abundance of these gases is correlated with the productivity of the oceans.”
Today, the air contains about 1,000 particles that can serve as cloud condensation nuclei (CCN) in a cubic centimeter (less than a tenth of a cubic inch). Pristine ocean areas lacking human produced aerosols are difficult to find, but in those areas algae produce dimethylsulfide that eventually becomes the CCNs of sulfuric acid or methane sulfonic acid.
Algae’s productivity depends on the amounts of nutrients in the water and these nutrients come to the surface by upwelling driven by the winds. Warming would lead to ocean stratification and less upwelling.
“The Cetaceous was biologically unproductive due to less upwelling in the ocean and thermal stress on land and in the sea,” says Kump. “That means fewer cloud condensation nuclei.”
When there are large numbers of CCN, there are more cloud droplets and smaller droplets, consequently more cloud cover and brighter clouds. With fewer CCN, there are fewer droplets and they are larger. The limit to droplet size is 16 to 20 microns because the droplets then are heavy enough to fall out as rain.
“We began with the assumption that what would change was not the extent of clouds, but their brightness,” says Kump. “The mechanism would lead to reduced reflection but not cloudiness.”
What they found was that the clouds were less bright and that there were also fewer clouds. If they lowered the production of biogenic CCNs too much, their model created a world with remarkable warming inconsistent with life. However, they could alter the productivity in the model to recreate the temperature regime during supergreenhouse events.
“The model reduces cloud cover from about 64 percent to 55 percent which lets in a large amount of direct sunlight,” Kump says. “The increased breaks in the clouds, fewer clouds and less reflective clouds produced the amount of warming we were looking for.”
CERN experiment sheds new light on cloud formation. 22 May 2014.
Under the Summer Sun, the Corn Belt Is the Most Biologically Productive Place on Earth, Smithsonian Magazine. April 8, 2014
Absence Of Clouds Caused Pre-human Supergreenhouse Periods. April 11, 2008