Scientists warned that the window for making sufficient cuts to greenhouse gas emissions is narrowing fast and the globe could warm by four to six degrees this century unless action is taken.

This prompted some Australian scientists to call for a ''war plan'' to urgently cut the amount of fossil fuels being burned. That remains their plan ''A'' - moving faster to introduce renewable energy and other clean technologies.

A plan ''B'' for adapting to some unavoidable climate change is now also being seriously explored, as nations consider how to secure essential resources and strengthen infrastructure against a future of higher temperatures and greater weather extremes. And then there is plan ''C'' - geoengineering, or trying to tinker directly with the planet's climate to slow down global warming and buy more time for emissions cuts.

Geoengineering, with its science fiction overtones, is considered a risky last resort, because while it might be able to slow or deflect some warming, it wouldn't deal with other effects of rising carbon dioxide levels, such as ocean acidification.

Britain's Royal Society commissioned a first detailed look at geoengineering options in 2009, and concluded ''all of the geoengineering methods assessed have major uncertainties in their likely costs, effectiveness or associated risks and are unlikely to be ready for deployment in the … medium term''.

A study released in April by Australia's Office of the Chief Scientist, assessed some options.

Geoengineering can be divided into two types: reflecting more of the sun's heat away from the Earth, and sucking more heat-trapping greenhouse gases out of the air.

The first type includes some wildly ambitious schemes, such as trying to ''shade'' part of Greenland from the sun's heat with space-based mirrors, an idea with no credible backing among the world's scientists.

One option that has found some favour is that of artificial volcanoes, or mechanisms for sprinkling sulphur dioxide across the sky. High in the stratosphere, sulphur dioxide reacts with water droplets to become sulphuric acid, which reflects some of the sun's radiation back into space.

Such a scheme would seek to mimic the effect of real volcanoes, because big eruptions expel huge amounts of sulphur dioxide into the air, causing enough of a cooling effect to show up in global temperature records. Getting millions of tonnes of sulphur into the stratosphere would be difficult, however, with few large planes able to fly high enough to spray their cargo. The most reliable method, according to the chief scientist's study, might be raising huge tubes from the ground with the aid of balloons, and pumping the stuff up.

''You have a downside, of course - acid rain,'' says David Karoly, an atmospheric scientist at Melbourne University and a lead author with the Intergovernmental Panel on Climate Change.

As well as corrosive acid rain, caused by too much sulphuric acid in the atmosphere, there would be human health impacts of increasing air pollution.

''We know that particulates in the atmosphere kill people - that's why decisions were made in Europe and North America to stop particulates being released when coal was burned. So, if there is a way of masking some of the warming by venting particulates into the atmosphere, whoever has to make that decision will have to make a trade-off.''

Researchers at the University of Edinburgh have published a proposal to whiten clouds by using remote-controlled sailing ships to spray a mist of seawater high into the air. Whiter clouds would reflect more heat away from the Earth. But vast new fleets of ships would be required to carry out the task on a big enough scale, and it would be vulnerable to local changes in weather.

As the Royal Society investigation noted, all projects to reflect away the sun's heat all have one drawback - they have to be maintained at a constant level, more or less forever. A break of even a few weeks would cause a sudden surge in warming.

Attempts to absorb more carbon dioxide confront similar snags. Early experiments in ''ocean fertilisation'' have yielded mixed results. The technique involves promoting huge blooms of algae by adding powdered iron to the water, so that when the algae dies and sinks to the bottom it takes some carbon dioxide with it, out of the air and into the depths.

''Ocean fertilisation would only be effective at tackling global warming if significant amounts of carbon were to sink very deep into the ocean [more than about 1 kilometre deep] and remain there for long periods [at least 100 years],'' the chief scientist's report says.

One ingenious method for soaking up CO2 is creating ''fake trees'' - actually large, upright screens of absorbent resin that react with carbon dioxide in the air and turn it into solid form. A prototype ''tree'' developed at Columbia University in the US captured a tonne of CO2 a day in lab tests. But the technology is years away from deployment, and vast ''plantations'' of the fake trees would be required to make any impact on human CO2 emissions, making it costly and improbable.

Real trees offer more hope. While not usually regarded as geoengineering, tree planting and maintenance is one method of taking carbon dioxide from the air in the eyes of Australian climate planners.

''Even doing things like planting trees are teetering towards small scale geoengineering,'' says Mark Stafford-Smith, the science director of the CSIRO's Climate Adaptation Flagship. ''The issue is at the moment we are trying to conceptualise geoengineering wrongly,'' he says. ''As long as we conceptualise it as one or two big technologies that solves the problem that will be disastrous. Because if they don't work, or work for some time and fail, then you will have a worse problem then you would have had before.

''What we should be conceptualising is a whole mish-mash of smaller things applied regionally, which together add up to a significant effect but which gives us much more resilience. So if something goes wrong, you pull out of it but you don't lose everything.''

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eanwhile, BHP Billiton announced last week that it would be upgrading its coal export terminal at Hay Point, in Queensland - to better withstand climate-related extreme weather events. The new coal export terminal at Newcastle was also built up three years ago as modelling showed it could be vulnerable to storm surges and rising sea levels this century.

The trend, being repeated by government and industry around the country, is part of plan ''B'' - climate adaptation, or preparing to live with some dangerous global warming.

Bruce Thom, the president of The Australian Coastal Society, and emeritus professor at Sydney University, says if the globe warms by four to six degrees critical coastal infrastructure would be vulnerable. The key, he says, is to start planning now. ''We have got airports, so the taxi ways at Sydney as well as one of the runways would be affected,'' Thom says.

''You also have infrastructure like the Opera House that has an expected life of at least 200 years and there is other infrastructure that needs to be managed … A lot of the drainage systems are built to levels consistent with current sea-levels. Rising sea-levels would require major retrofitting, so what are the retrofitting costs?''

Michael Nolan, an associate director for sustainability with the consultancy AECOM, says the technical know-how to build climate-resilient structures is available, but in higher temperature scenarios the economic costs of adapting infrastructure would be very high.

''There are very few places on the planet where we can't build infrastructure, we can pretty much design it for any type of conditions,'' he says. ''We are going to end up having to replace a lot of infrastructure early, which is a massive cost … You can always design for conditions, it is just whether you can afford to design for the conditions.''

Some research has looked into food production. For a world that is four to five degrees warmer than pre-industrial levels, Stafford-Smith says, the key to this could be commercially growing algae as feedstock for cattle.

''The whole chain of production would need to change, so you would be going to much more algal production that acts as a feedstock for agriculture perhaps, rather than growing grain for cattle.''

Richard Eckard, the director of Melbourne University's primary industries climate challenges centre, says studying current extremes around the world would hint at the type of changes Australian farmers would have to make if temperatures were four to six degrees higher.

''You start to look at the extreme climates of the world and say, 'Well, what currently survives in the southern Sahara in Africa?' because that is the kind of temperatures we would be looking at,'' he says. ''What do they grow in the Niger and that sort of area, that would be our reality at four to five degrees.

''You could grow winter crops that are normally summer crops in the tropical climates. That would be a possibility and that would probably be the only thing we could be doing.''

Estimates of the cost of plan ''B'', a comprehensive climate adaptation plan for Australia, vary widely. In general, the costs are governed by the principle laid out in the Garnaut Climate Change Review - the longer we wait, the higher the price.

That's why scientists keep returning to plan ''A'' as the only viable option - cutting greenhouse gas emissions, by replacing fossil fuels with renewable energy and other technologies. The longer the world waits, the more the cost of adapting to climate change escalates, and the higher the risks. And, barring an unforeseen technological breakthrough, the geoengineering hopes of plan ''C'' are unlikely to do more than mask a portion of rising temperatures.''This is critical - we really need to see emissions start to come down in the next three to four years,'' says Professor Will Steffen, executive director of the Australian National University climate change institute and a member of the government's Climate Commission.

Professor Jean Palutikoff, the director of the National Climate Change Adaptation Research Facility, says the social cost of not acting should also be taken into account.

''You will adapt, you will survive, yes we will still be alive,'' she says. ''But if you define adaptation as preserving well-being, then I think it gets a little more questionable when you are thinking about 4 to 6 degrees. 'Would you want your children and your grandchildren to be living in that environment, and in that way,' is a question you will have to ask yourself.''

Clarification

Part one in this series, published on Saturday, said a five-degree rise would increase the global average temperature from 14 degrees to 19 degrees. This is correct. It also said this would make the average day 35 per cent hotter. This may be misleading. Using the Kelvin scale, which places absolute zero at -273 degrees , an average day would be 1.7 per cent hotter.

This is a clarification in how the relative increase in temperature is expressed only. It does not affect the rest of the article, which describes what scientists believe a five-degree warmer world may look like and explained how projections of future temperature rises are made.