By Richard Black
Environment Correspondent, BBC News website, Darwin
A three-week experiment to resolve the biggest riddle in climate science begins in Australia on Thursday.
The experiment will use planes to study how clouds form
Scientists will use radar, aeroplanes, weather balloons and a ship to study the life cycle of tropical clouds.
They are searching for details of how clouds form and carry heat high up into the atmosphere.
A better understanding of these crucial processes should lead to computer models that can predict the extent of global climate warming more accurately.
Current projections of global temperature rise, reported in the last assessment of the Intergovernmental Panel on Climate Change (IPCC), see increases by the end of the century that range from 1.4 to 5.8C.
The spread of possible temperatures represents a huge uncertainty - and much of it stems from unknowns to do with how the world's economy will develop over coming decades.
But there is also uncertainty over how the climate will react, and one of the key issues centres on a poor understanding of what goes on inside clouds - how they form, and how they behave.
Over the next three weeks, about 200 scientists drawn from 30 institutions in 10 countries aim to rectify this data deficit.
Clouding the issue
Tropical clouds carry heat and moisture from the Earth's surface high up in the atmosphere, a key process in driving heat around the globe.
"You have these 'hot towers', tropical storm clouds acting like chimneys to carry heat to the upper atmosphere," said Peter May from the Australian Bureau of Meteorology Research Centre, co-chair of the project's organising committee.
"Also, you've got large areas of cirrus clouds which are reflecting a lot of incoming sunlight back away from the Earth; but they're also absorbing infra-red radiation coming back from below," he told the BBC News website.
Scientists need to learn how climate change affects storm activity
"So you've got competing processes going on; and that balance depends on how big the ice crystals are and what the density is, how high they are and so on."
Existing computer models did not reflect these processes accurately, said Tom Ackerman of the University of Washington in Seattle, US, because they typically treated convection and cloud formation as separate processes.
"We're working now to develop a more integrated approach to the convection where these processes are all tied together, and the convection leads directly to the clouds," he told the BBC News website.
"But in order to do that you need to understand this total life cycle of air going into the clouds, condensation, vertical lifting and then cloud being dumped out at other levels."
All of which provides a mandate for a comprehensive and intensive new research programme.
Investigators and funding for the Tropical Warm Pool International Cloud Experiment (TWP-ICE) come from a number of countries, including Australia, the US, Britain, Japan and Canada.
In order to get a better picture of cloud life cycle, they have pulled together a formidable array of technology and manpower.
Radar systems on land and on the Australian research vessel Southern Surveyor will scan the atmosphere around Darwin night and day, producing a three-dimensional picture of water distribution and droplet size.
Aeroplanes will fly above the clouds, providing further information on what they contain.
Ground-based instruments will measure the flow of heat and moisture from land into the lower atmosphere, augmented by sensors on another set of aeroplanes.
Southern Surveyor will also take ocean measurements, looking at the impact of rainfall on heat exchange between sea and atmosphere.
And a barrage of weather balloons, or radiosondes, will measure temperature and humidity.
"We have five sites in a 150km radius ring around Darwin from which we launch those weather balloons," said Christian Jakob from the Australian Bureau of Meteorology.
"We're going to launch eight balloons a day at each site for 23 days - if you do the maths, it's in the order of 1,000 balloons - and these stations are populated by poor students who have to stay out in the heat and rain and launch a balloon every three hours."
In Darwin's intense summer rains, this surely qualifies as suffering for science.
Refining the models
All this makes TWP-ICE one of the biggest climate experiments ever mounted.
Investigations are limited to 23 days, the length of time for which Southern Surveyor is able to remain at sea; but analysing the data and building it into climate models will take much longer.
As well as refining the models and producing better estimates of global temperature rise, this experiment, conducted as it is near the southern boundary of the monsoon belt, should provide better indications of how the monsoon may change as temperatures increase - a key factor for the billions of people who depend on it for water.
The ability to turn global models into regional projections is crucial if nations and regions are to plan for a warmer future.
"The overwhelming majority of the atmospheric science community believe that climate change is with us," said Peter May.
"The next questions are, 'how does it affect peoples' lives; what does it mean on regional scales, what does it mean for tropical cyclones or thunderstorm activity?'
"You absolutely cannot say, as some people tried to, 'Hurricane Katrina was due to climate change' - that's just nonsense. But the question you need to ask is, 'are we loading the dice for more intense storms more often?'"
If the TWP-ICE investigators are correct, we should start gaining answers to these key questions in the years immediately following their 23 days of intense research.