In the following passage there are blanks, each of which has been numbered. These numbers are printed below the passage and against each, five words/phrases are suggested, one of which fits the blank appropriately.
Greenhouse gases are only (21) of the story when it comes to global warming. Changes to one part of the climate system can (22) additional changes to the way the planet absorbs or reflects energy. These secondary changes are (23)climate feedback’s, and they could more than double the amount of warming caused by carbon dioxide alone. The primary feedback are (24)to snow and ice, water vapour, clouds, and the carbon cycle.
The most well (25) feedback comes from melting snow and ice in the Northern Hemisphere. Warming temperatures are already (26) a growing percentage of Arctic sea ice, exposing dark ocean water during the (27) sunlight of summer. Snow cover on land is also (28) in many areas. In the (29) of snow and ice, these areas go from having bright, sunlight-reflecting surfaces that cool the planet to having dark, sunlight-absorbing surfaces that (30) more energy into the Earth system and cause more warming.
Find out the appropriate word/phrase in each case.