The climate of Tonga is changing. The Tongan Government understands the implications of a changing climate and has considered them as high priorities in its 2009-2014 National Strategic Planning Framework. The present study indicates that the detrimental effects observed now will be further exacerbated by future changes within the global climate system. The study examines the simulations of 18 global models known as CMIP3. The projections indicate that in Tonga: the annual average air temperatures and sea surface temperatures will continue to rise; rainfall will decrease in the dry season and increase in the wet season; and extreme rainfall days are projected to occur more often. Projections also include: a decrease in Tropical cyclones frequency; an increase in Tropical cyclone intensity; continued rise in sea level; and aragonite saturation level reaching 3.5 by 2035, with continued decline thereafter.