Powered by winds, clouds have been circulating around the globe like magic carpets carrying weather on their back. Understanding them not only holds the key to decoding changing weather patterns, it is also crucial for predicting a change in climate.
What matters to climate is how the behaviour and extent of clouds have changed over 250 years or will change in the future, says Neil Donahue, Lord Professor of Chemistry at Carnegie Mellon University, Pittsburgh. To understand these changes climate scientists, armed with constantly improving technologies and weather satellite data, are busy untangling the journey of clouds—right from the level of microscopic aerosols, which act as the seed of cloud droplets, till they become the most crucial player in the climatic system.
But clouds keep changing and reorganising themselves. Their character has become even more uncertain in a changing climate. And this has made the job of scientists more challenging.
Aerosols: tiny particles, big impact
One of the many puzzles that climate scientists are trying to unravel is whether aerosols released by human activities, such as vehicular and industrial emissions, affect the formation and behaviour of clouds.
Going by the conventional theory, high aerosol loading in the atmosphere results in more cloud droplets, and that make the sky appear cloudy. Since the concentration of aerosols would have been low before the industrial era began in the 1800s, it is believed that the pre-industrial atmosphere must have been less cloudy than what is observed now.
Cloud droplets also reflect part of the solar radiation back into space and thus have a cooling effect on earth. By this logic, clouds in the industrial era would have a higher cooling effect on earth. Some speculate that this cooling effect masks the warming effect of greenhouse gases (GHGs) released into the atmosphere in the industrial era. “Our best estimate is that about one-third of the extra warming that would have been caused by carbon dioxide has been masked by that pollution, and we do not really know exactly how much hotter it will get if and when we get rid of that,” says Donahue.
The uncertainty is due to the lack of reference data on atmospheric observations from the pre-industrial era.
Scientists at Switzerland’s European Organisation for Nuclear Research, known as CERN, tried to address this uncertainty early this year through a high-tech experiment, aptly named CLOUD (Cosmics Leaving Outdoor Droplets). They simulated the pre-industrial era atmosphere under highly controlled conditions and found that in the absence of GHG emissions, natural aerosols or biogenic vapours released by trees were capable of supporting the cloud cover, similar to the present-day situation.
CERN researchers are now studying the effects of cosmic particles in solar radiation on cloud formation to understand how present-day atmosphere is different from that of the past.
Then there are studies that are trying to bring new insights into the formation of cloud droplets. Scientists from the University of Bristol in the UK and the ETH Zurich University in Switzerland in 2012 debunked the conventional wisdom about cloud formation. They said droplet formation on organic aerosols, which are usually byproducts of combustion processes, is not a uniform process, and that it could take anywhere from less than a second to several hours depending on the viscosity of the aerosol particle. The less soluble an aerosol, the longer it would take to form droplets, said the researchers.
In March this year, researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory also tried to understand the micro processes involved in cloud formation by using a proxy mix of organic and inorganic aerosols to emulate the atmosphere. They observed that more than the solubility of aerosols, what mattered most is the interaction at the interface of the aerosol and condensed water vapour. Organic aerosol particles effectively push down the surface tension of water to facilitate efficient formation of bigger cloud droplets, they concluded.
While specific connections between the chemistry of aerosol particles and the formation of cloud droplets are yet to be ascertained, the findings hold tremendous value for understanding the micro processes that decide the brightness or precipitation potential of clouds.
Clouds of dual nature
Clouds are a key component of the climate system because they help regulate the planet’s temperature. Clouds are responsible for both heating up and cooling down the planet, depending on their type and where they are located. For instance, when located at lower altitude, clouds typically contribute to the cooling effect by shielding the planet and reflecting around half of the sunlight that strikes them. It gives them the white glow. It is estimated that if cloud cover were absent and all the water in the clouds existed as water on the surface of the earth, the planet would have absorbed about 20 percent more heat than what it does currently.
This would have made the earth warmer by about 12oC. But when situated higher up in the atmosphere, they trap the infrared radiations bouncing off the earth’s atmosphere, thus warming the planet to the tune of about 7oC. Understanding this dual nature of the cloud is important because it shows how changes in clouds will affect the energy balance and radiation budget of the planet.
“Clouds have a net cooling effect in the current climate,” says Joel Norris, Professor of Climate and Atmospheric Sciences at the Scripps Institution of Oceanography in San Diego, California. This is because the cooling effect of clouds outweighs their warming effect. However, there is a confusion. No one is sure if the net cooling effect will become stronger or weaker as global warming progresses, he adds.
However, advanced remote sensing technology and satellite imagery over the past three decades have helped scientists make some headway on understanding how clouds have been changing and reorganising themselves.
Researchers from the University of Washington and University of Arizona in the US have compiled meteorological data for the period of 1954-2008 from weather ships to map the changes observed in clouds over oceans. They have also created a map of cloud cover changes over land by using weather station data between 1971 and 2009. The maps indicate significant changes in their extent.
While cloud cover over oceans shows a subtle reduction, it is fast receding over land. Besides, mid-latitude cloud cover is reducing over the tropics, while all kinds of clouds are expanding over the sub-tropics and high-altitude clouds are migrating towards the poles. NASA confirmed the observation in May this year.
The migration of high-level clouds towards the poles is worrisome. Considering the massive amounts of water stored at poles, the increase in warmth could ultimately contribute to sea level rise around the world. The catastrophic results are already visible. The world’s second largest ice sheet, situated in Greenland, is melting rapidly due to the blanketing effect of clouds which are raising temperatures in the region by 2-3oC. It is estimated that a third of the total global sea level rise currently is being forced by this melting.
Researchers at the Scripps Institution of Oceanography say clouds are not only moving polewards, they are moving higher up in the atmosphere. Norris, the lead author of the paper, says an expansion of the subtropical dry zone will lower the reflection of solar radiation back to space by clouds and allow the earth to absorb more solar radiation. This would result in a warmer climate. “I have also found a rise in the height of the highest cloud tops. This is expected to occur with global warming,” Norris adds. As the lower atmosphere warms up and upper atmosphere cools down, it will allow buoyant clouds to rise higher in the sky. These clouds would trap heat and this would result in a considerable warming of the climate.
Still a puzzle
Research on clouds has no doubt picked up considerable speed in recent years. Several data-gathering projects are simultaneously using ground observations, remote sensing, weather balloons, specially equipped planes and satellite imagery to try and produce a coherent picture of how clouds are responding to climate change.
Despite this, the problem of assimilating clouds in weather and climate models remains. “Clouds have not been incorporated in climate models very well because the resolution in models is crude. They have to be deduced, and this is imperfect science,” says Kevin Trenberth, senior scientist at the Climate Analysis Section, the US National Center for Atmospheric Research.
“Climate models cannot handle the physics of clouds directly and parameterise (inspired guesswork) their properties,” says Roger Davies, Professor of climate physics at the University of Auckland in New Zealand. Neither can they handle the heterogeneity of real clouds that affect their radiative properties, and work with average cloud properties. Since the radiative and moisture properties of clouds average differently, model clouds tend to be either too bright or too dry, says Davies, adding that modellers choose to make clouds too dry.
Davies says most confusion about cloud change is due to paucity of quality satellite observations. Global coverage is essential to get enough samples, and satellites are the only option. The early satellites focused more on weather imagery than climate measurements. “We have only had about two decades of quality measurements, and it will take several more decades to draw strong observational conclusions about cloud changes. In the meantime, the conclusions are subject to change as more data becomes available,” Davies explains.
While we wait for more information on how cloud behavioural changes manifest on the ground and how adverse effects can be mitigated, it seems safe to say that unusual and extreme weather
patterns are here to stay. DOWN TO EARTH
(The views expressed are strictly personal.)