More sulphur trails from the stratosphere
Tiny aerosol particles in the atmosphere are perhaps the second most important way - after greenhouse gas emissions - in which human activities are changing the Earth's climate.
But changing it how?
The last major Intergovernmental Panel on Climate Change (IPCC) report in 2007 concluded that aerosols "remain the dominant uncertainty in radiative forcing" - partially because of the direct effects the particles have, such as reflecting and scattering the Sun's energy, and partly because they can affect cloud formation.
Since that report was compiled, researchers have gone deeper into the known unknowns of aerosols - one of the main aims being to build a picture of how they behave naturally, in order that any man-made perturbation can be more accurately assessed.
The satellite era has made things easier, with instruments on board orbiting platforms such as Calipso deploying lidars and other instruments capable of sensing how much material is present at what kinds of altitudes.
But still, obtaining a global picture is not easy - and to make things worse, observations have begun in earnest during the very period in which human activities are changing the atmosphere's aerosol load.
But scientific groups are on the case.
A couple of weeks ago, we had some insights into how increased coal-burning - notably from China - could be pumping the stuff into the atmosphere so quickly as to ameliorate the temperature rise from greenhouse gas emissions.
Last week, a different research group unveiled new findings on the impact of volcanic eruptions, which pump vast amounts of particles into the air, some rising as far as the stratosphere.
The conclusion was that volcanoes may have a greater cooling effect than previously recognised.
And this week, in Science, comes a fresh analysis of aerosols in the stratosphere itself, and their impact on global temperatures.
The top line message is that the increase in concentrations over the "noughties" has partially masked temperature rise - similar to the conclusion of the research released two weeks ago, but using a radically different line of inquiry.
And climate models, the researchers say, mostly understate the impact of aerosols in the stratosphere, so may be projecting that temperatures will rise faster than they actually are.
Volcanoes have massive impacts on the stratospheric load of these tiny sulphate particles. But they're fairly short-term, producing stark spikes that decay away after a few years.
But the background aerosol concentration is also changing, the researchers find - for reasons that are not immediately obvious.
When I called one of the researchers, Ells Dutton from the National Oceanic and Atmospheric Administration (Noaa), he described the top line message as "relatively mild".
"When you account for this additional aerosol that's not been accounted for previously, that weakens the overall net forcing that existed over the 2000s.
"So the positive forcing was partially negated by this negative forcing, so you'd expect warming to have been less."
But there are also things it doesn't tell us.
The sources could be entirely natural; "but if you then ask whether some of the Chinese stuff is getting into the stratosphere, it could be - we can't sort that out."
Another question is this; if all or most of the sources are natural, what are those sources, besides vulcanism?
So, lots more to research - and one potential implication outside the realm of understanding climate change.
The cheapest of all geo-engineering techniques yet conceived involves putting multiple tonnes of this stuff into the stratosphere, where it would reflect sunlight back into space, cooling the Earth's surface.
Controversial, of course; but a number of eminent people such as UK renewable energy pioneer Stephen Salter suggest that research should begin in earnest, because they don't believe our society is going to curb greenhouse gas emissions at anything like the rate needed to slow global warming.
Clearly, if this is ever to be attempted, understanding how the particles will behave in the upper atmosphere would be of paramount importance.
David Keith from the University of Calgary, one of the leading lights in the field, told me this: "I don't see any strong implications for solar geo-engineering, [but] it may help to dispel naive assumptions that solar geo-engineering would be an all or nothing affair.
"In fact, if it ever did make sense to try it, it would make sense to try incremental additions of sulphur coupled with an intense observation campaign that looked for problems."
It's interesting to note by comparison how another geo-engineering option - iron fertilisation of the oceans - has been investigated in practice.
The mass of evidence generated over more than a decade of experimentation has told researchers a lot about the various things that are likely to determine how well it works - and the biggest trial of all, the German Lohafex project, suggested it might not work at all.
By comparison, our knowledge about sulphate aerosol injection is tiny.
The title of the Science paper - The Persistently Variable "Background" Stratospheric Aerosol Layer and Global Climate Change - is well-chosen in that it indicates both that significant research has been done, and that the findings it's produced are somewhat frustrating - and honestly given as such.
Earth experiment could buy precious time
As the UK's Royal Society prepares to publish its conclusions on whether geo-engineering can help combat climate change, physicist Alan Gadian argues that geo-engineering techniques, in particular cloud whitening, must be properly tested - and soon.
Planet Earth has become a huge science experiment, and the consequences will affect all of us.
"Global warming poses a greater threat than world terrorism and agreement must be reached within two years to mitigate global warming and minimise environmental catastrophe," Sir David King, the UK's chief scientist, wrote in 2007.
On 8 July this year, the G8 proposed a 50% reduction of global emissions of carbon dioxide by 2050. It remains to be seen whether this will be fast enough.
And carbon dioxide is not the only problem.
Methane is a greenhouse gas 21 times more potent, and the wastelands of Siberia are now releasing fountains of methane as the permafrost melts, adding to the greenhouse warming effect.
So how can geo-engineering help?
I define geo-engineering as man-made environmental change; and I would include in its definition the unprecedented burning of fossil fuels that has pumped large quantities of carbon dioxide into the atmosphere, and the massive increase in the number of farm animals with consequential methane production.
Changes in agriculture, in Africa for example, have resulted in the felling of large areas of forest and have removed the water storage capacity of the land. This has led to the rapid advance of the desert in the Sahel region.
The planet has been and is warming further. This will also lead to significant changes in precipitation, and flooding will remove prime agriculturally productive land.
In a 1990 paper in the journal Nature, John Latham, a scientist at the National Center for Atmospheric Research in Colorado, US, suggested that increasing the number of droplets in maritime layer clouds (stratocumulus) could significantly increase their reflectance.
These clouds cover a third of the ocean.
The water droplets in clouds reflect solar radiation back to space. And the numbers of droplets they contain are largely controlled by the number of cloud condensation nuclei (CCN), such as specks of dust.
Many of these nuclei are produced over the land. Land-locked clouds therefore contain many hundreds of cloud droplets per cubic centimetre, whilst clouds that form over the sea contain substantially fewer.
Generally, the more droplets that are present in a cloud, the smaller they are.
For a given mass of water in a cloud, clouds with smaller droplets tend to be whiter. This was illustrated by the Edinburgh University scientist Stephen Salter's example of glass beads in a jar - the smaller the beads, the whiter they appear.
So the proposal is to inject a fine spray of sea salt from the ocean surface into the clouds; to artificially increase the number of drops, reduce their size and increase the reflectance of the clouds, making them whiter.
This one-off increase in reflectance - and the resulting cooling - could buy us precious time; maybe as much as 25 years.
But we need numerical models and field experiments to determine the ideal size of the sea-salt nuclei.
Results from climate models show that a modest increase of nuclei in marine stratocumulus clouds could produce the desired cooling.
Further research is required, but preliminary results suggest this could compensate for up to a doubling of atmospheric carbon dioxide from pre-industrial levels.
Initial results suggest that the biggest cooling would occur in the polar regions, which is consistent with theory, and is exactly the place where cooling is most needed.
The big advantages of this scheme are that it uses sea water spray, a naturally occurring substance, and that it can be turned off immediately if there are any undesirable consequences.
Professor Salter has even suggested a design for a fleet of about 2000 of wind-powered yachts, which incorporate a sophisticated spray mechanism that is now being developed.
My colleagues and I propose to carry out detailed research of the scheme and provide an answer on its viability within five years.
There are four elements to this research:
•cloud physics modelling; there are many questions about the optimal size of sea-salt CCN and how the clouds will respond to their increased numbers
•further climate modelling
•developing and building Stephen Salter's test yachts
•a field experiment; a limited-area field experiment is needed in a region of stratocumulus clouds, and we already have advanced-stage plans with potential collaborators in the US
Initial estimates suggest that we could complete the research for approximately £6m ($10m), and produce a result that will determine if the proposed scheme is viable or not.
The research needs to be carried out, otherwise we will not know, five to 10 years from now, if we could have done anything to slow down the warming and the irreversible change in the Earth system.
It is an insignificant sum compared to the cost of doing nothing.
As James Lovelock states: "There have been seven disasters since humans came on the Earth, very similar to the one that's just about to happen."
He argues that billions of people are likely to die in the ensuing famine. "Enjoy life while you can.
"Because if you're lucky it's going to be 20 years before it hits the fan".
We can do something to provide a breathing space. That something should start now.
Dr Alan Gadian is a senior research lecturer in the School of Earth and the Environment at the University of Leeds, UK
Dr Gadian would like to extend his thanks to collaborators Alan Blyth, John Latham and Stephen Salter
The Green Room is a series of opinion articles on environmental topics running weekly on the BBC News website
Climate 'technical fix' may yield warming, not cooling
By Richard Black
Environment correspondent, BBC News, Vienna
Whitening clouds by spraying them with seawater, proposed as a "technical fix" for climate change, could do more harm than good, according to research.
Whiter clouds reflect more solar energy back into space, cooling the Earth.
But a study presented at the European Geosciences Union meeting found that using water droplets of the wrong size would lead to warming, not cooling.
One of the theory's scientific fathers said it should be possible to make sure droplets were the correct size.
Cloud whitening was originally proposed back in 1990 by John Latham, now of the University Corporation for Atmospheric Research in Boulder, US.
It has since been developed by a number of other researchers including University of Edinburgh wave energy pioneer Stephen Salter, joining a number of other "geoengineering" techniques that would attempt either to reduce solar radiation reaching earth or absorb carbon dioxide from the air.
One version envisages specially designed ships, powered by wind, operating in areas of the ocean where reflective stratocumulus clouds are scarce.
The ships would continually spray fine jets of seawater droplets into the sky, where tiny salt crystals would act as nuclei around which water vapour would condense, producing clouds or thickening them where they already exist.
It has not yet been trialled in practice, although proponents say it ought to be.
But Kari Alterskjaer from the University of Oslo in Norway came to the European Geosciences Union (EGU) meeting in Vienna with a cautionary tale.
Her study, using observations of clouds and a computer model of the global climate, confirmed earlier findings that if cloud whitening were to be done, the best areas would be just to the west of North and South America, and to the west of Africa.
But it concluded that about 70 times more salt would have to be carried aloft than proponents have calculated.
And using droplets of the wrong size, she found, could reduce cloud cover rather than enhancing it - leading to a net warming, not the desired cooling.
Continue reading the main story
The trouble is that clouds are very complicated; as soon as you start manipulating them in one way, there are a lot of different interactions”
University of Leeds
"If the particles are too small, they will not brighten the clouds - instead they will influence particles that are already there, and there will be competition between them," she told BBC News.
"Obviously the particle size is of crucial importance, not only for whether you get a positive or negative effect, but also whether particles can actually reach the clouds - if they're too large, they just fall to the sea."
The possibility of this technique having a warming impact has been foreseen by cloud-whitening's developers.
In a 2002 scientific paper, Dr Latham wrote: "... the overall result could be a reduction in cloud droplet concentration, with concomitant reductions in albedo and cloud longevity, ie a warming effect".
But, he argued, this possibility could be eliminated by careful design of the spray system.
Contacted after the presentation in Vienna, Professor Salter took the same line.
"I agree that the drop size has to be correct and that the correct value may vary according to local conditions," he said.
"However, I am confident that we can control drop size by adjusting the frequency of an ultrasonic pressure wave which ejects drop from micro-nozzles etched in silicon.
"We can test this at very small scale in the lab."
Professor Salter is working with engineers in Edinburgh to produce extremely fine yet robust nozzles from semiconductor sheets.
In an era when many climate scientists are frustrated by slow progress in reducing greenhouse gas emissions, cloud whitening has sometimes been held up as an example of a technology that could make a real difference, at least to "buy time".
The technique's prospects depend crucially on how droplet size affects reflectivity
It has been calculated that a fairly modest increase in the reflectivity of these marine clouds could balance the warming from a doubling of carbon dioxide in the atmosphere - although even proponents admit it would do nothing to combat the other major consequence of carbon emissions, ocean acidification.
One scientist at Ms Alterskjaer's presentation, having heard her outline why it might not work, commented that it was the most depressing thing he had heard in a long time.
And Piers Forster from the UK's University of Leeds, who is leading a major UK project on geoengineering techniques, suggested more research would be needed before cloud whitening could be considered for "prime time" use.
"The trouble is that clouds are very complicated; as soon as you start manipulating them in one way, there are a lot of different interactions," he said.
"We need real-world data and we need modelling that tries to simulate clouds on more appropriate scales, and that means less than 100m or so, because if you look at a deck of stratocumulus it's not one big thing, it has pockets and cells and other features.
"Far more uncertain is the idea that you'd inject a particular drop size, because it won't stay that size for long - it will spread out, and that would be uncertain."
Professor Salter, too, believes more research needs to be done, including building a prototype injector ship and studying how it works in practice.
Interviewed by the BBC late last year, he said that such research was urgently needed because there was little sign of real cuts being made in the world's greenhouse gas emissions.