Explanation:

 ? Cirrus ice clouds extend their thin and wispy tendrils in the upper troposphere, at altitudes of about 5-15 kilometres. Researchers say that these thin clouds are known with confidence to have a net warming effect on our planet, since their ice crystals re-emit infrared radiation absorbed from the sun-warmed surface back down towards ground. So if we can make cirrus thinner still, we’ll let out more heat and cool the globe.
? BBC: One of the largest Geo engineering experiments, known as Lohafex, was an Indian-German experiment in 2009 which involved dumping six tonnes of an iron solution into the South Atlantic to encourage plankton to bloom – trapping carbon which would then be sent to the seabed when the organisms died. Results showed limited success.
? Large volcano eruptions inject ash particles and sulphur-rich gases into the troposphere and stratosphere and these clouds can circle the globe within weeks of the volcanic activity. The small ash particles decrease the amount of sunlight reaching the surface of the earth and lower average global temperatures. The sulphurous gases combine with water in the atmosphere to form acidic aerosols that also absorb incoming solar radiation and scatter it back out into space.