In the third part of a four-part series, Eloise Gibson talks to climate scientists about what they don't know, what they wish they knew and how they can find out more.

Car and smoke-stack pollutants shunned since the 1970s for causing acid rain may be taking some heat out of global warming, but there is uncertainty about their exact effect.

Some aerosol particles that rise from the oceans, industry and transport are thought to deflect sunlight from the Earth's atmosphere but some, such as soot, absorb heat and have a warming effect.

Despite years of work, scientists do not know exactly what the net effect of the particles is and must work within huge margins of error, holding back efforts to make better projections of climate change.

The problem was one of four singled out by the journal Nature as the top areas of climate change uncertainty globally, with three others - regional predictions, rainfall, and palaeoclimate data.

Dr Mike Harvey, a principal ocean and atmospheric emissions scientist at Niwa, said aerosol particles were thought to have a cooling effect on the Earth "but there is quite a big error bar around the effects".

Scientists needed a better picture of what aerosols were being produced and how the quantities were changing, he said.

If efforts to curb greenhouse gas emissions fail, answers about the exact role of aerosols may determine whether some of the seemingly far-out suggestions for staving off global warming could work.

Scientists have suggested pumping sea water into the atmosphere from ships to reflect sunlight off the sea salt particles, while others suggest sending aircraft to drop sulphuric acid into the stratosphere, where it would form cooling sulphate aerosols.

Dr Harvey said there were worries that the latter could harm the ozone layer and undo efforts to heal the Antarctic ozone hole.

There were also questions about how well aerosols - which can be flushed out of the atmosphere by rain in a month or less - could fight long-lived warming effects from carbon dioxide.

"The CO2 we are putting into the atmosphere now, some of that could still be around in millennia ... whereas the aerosols are quite shortlived," said Dr Harvey.

He said including the role of sulphates in computer models used to project climate changes had already greatly improved them and the models should get better as further advances were made.

Scientists at Niwa's Lauder base in Otago are working as part of an international network to detect what aerosols are in the atmosphere by shooting laser-light about 100km up.

Sulphate aerosols - which have a cooling effect - are emitted by heavy industry and shipping and trucking fuel among other things but around New Zealand they mostly come from plankton living in the ocean.

Dr Harvey and a team of New Zealand and international scientists hope to take a trip to test the air and waters near Chatham Rise, east of New Zealand, on the research vessel Tangaroa in 2012.

That could help understanding of how the ocean is contributing to atmospheric aerosols.

Sulphate emissions from people dropped sharply in Europe and the United States after clean air acts were brought in to fight acid rain in the 1970s.

But Dr Harvey said sulphate emissions from China and other developing economies were on the rise.