填空题 Complete the summary below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
Write your answers in boxes 19-23 on your answer sheet.

A range of geoengineering ideas has been put forward, which aim either to prevent the melting of the ice caps or to stop the general rise in global temperatures. One scheme to discourage the melting of ice and snow involves introducing (19) to the Arctic because of their colour. The build-up of ice could be encouraged by dispersing ice along the coasts using special ships and changing the direction of some (20) but this scheme is dependent on certain weather conditions. Another way of increasing the amount of ice involves using (21) to bring water to the surface. A scheme to stop ice moving would use (22) but this method is more likely to be successful in preventing the ice from travelling in one direction rather than stopping it altogether. A suggestion for cooling global temperatures is based on what has happened in the past after (23) and it involves creating clouds of gas.