Large-scale human engineering of the Earth's climate to prevent catastrophic global warming would not only be ineffective but would have severe unintended side effects and could not be safely stopped, a comparison of five proposed methods has concluded.

Science academies around the world as well as some climate activists have called for more research into geoengineering techniques, such as reflecting sunlight from space, adding vast quantities of lime or iron filings to the oceans, pumping deep cold nutrient-rich waters to the surface of oceans and irrigating vast areas of the north African and Australian deserts to grow millions of trees. Each method has been shown to potentially reduce temperature on a planetary scale.

But researchers at the Helmholtz Centre for Ocean Research Kiel, Germany, modelled these five potential methods and concluded that geoengineering could add chaos to complex and not fully understood weather systems. Even when applied on a massive scale, the most that could be expected, they say, is a temperature drop of about 8%.

The potential side effects would be potentially disastrous, say the scientists, writing in Nature Communications. Ocean upwelling, or the bringing up of deep cold waters, would cool surface water temperatures and reduce sea ice melting, but would unbalance the global heat budget, while adding iron filings or lime would affect the oxygen levels in the oceans. Reflecting the sun's rays into space would alter rainfall patterns and reforesting the deserts could change wind patterns and could even reduce tree growth in other regions.

In addition, say the scientists, two of the five methods considered could not be safely stopped. "We find that, if solar radiation management or ocean upwelling is discontinued then rapid warming occurs. If the other methods are discontinued, less dramatic changes occur. Essentially all of the CO2 that was taken up remains in the ocean."

Even the foresting of deserts on a massive scale could prove disastrous if the irrigation needed to grow the trees were stopped, they say. "The desert regions would eventually return to desert and the carbon that was stored in the plant biomass and soil would slowly be returned to the atmosphere through decay and respiration," says the paper.

Each of the five climate engineering methods considered has advantages and disadvantages but individually they are all limited, say the authors. "If CO2 emissions remain high, the climate engineering methods … should not be solely counted on to prevent warming. Our results suggest that CO2 mitigation seems the most effective way to prevent climate change. Climate engineering does not appear to be an alternative option, although it could be possibly used to complement mitigation," say the authors, who do not look at the ethical, economic, legal, political or technological feasibility of the five methods.

"The paper sounds a timely warning about the abject stupidity of relying upon climate engineering solutions when reducing our reliance on carbon-based energy systems is the only sensible option," said Dr Matt Watson, a lecturer in geophysical natural hazards at Bristol University.

"The paper … highlights the urgent need to action approaches to climate change that increase mitigation and adaptation efforts, while simultaneously performing rigorous studies of proposed climate engineering methods. Although some climate engineering approaches, including air capture, may prove useful, they cannot be relied on as a 'silver bullet'," said Dr Tim Fox, Head of energy and environment at the Institution of Mechanical Engineers.