This literally means managing some of the planet's natural systems for controlling greenhouse gases. The best known idea is to spray sulphur dioxide particles high into the atmosphere. This would mimic what big volcanoes do when they blast gases and dust high into the atmosphere. They have a cooling effect because they reflect some of the sun's rays back into space, rather like a sunshadeSulphur sunshadesPeople would have to be pretty desperate to do this. It doesn't seem to be a great idea to put more acid-forming gases into the atmosphere. They make acid rain and the oceans are already having trouble with too much acidity. And once you've started doing this, you can't really stop because the greenhouse gases will still be increasing, meaning that if you stop making the 'sunshade', warming will dramatically increase as it catches up to where it would have been without the sulphur dioxide screen.
Another geoengineering route, probably a lot safer, is to 'fertilise' the oceans - which are mostly rather barren - with iron particles. This again mimics the natural system where dust storms in the Sahara desert loft enormous clouds of iron-rich dust way out into the Atlantic Ocean. The iron means it becomes possible for tiny floating plants called phytoplankton in the surface layers of the ocean to photosynthesise. The phytoplankton absorb CO2 on a vast scale and convert it into solid carbon compounds (like their tiny skeletons) and burp out oxygen. The plankton eventually die - or get eaten - and sink (with their carbon) to the ocean floor where they are buried and eventually turned into rocks. The unwanted carbon is then safely locked away. This could have the useful knock-on effect of increasing the number of fish and other marine animals: tiny zooplankton eat the phytoplankton and they then get eaten by fish, penguins and even whales. It could mean more fish for people to eat too.