Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and oceans have warmed, the amounts of snow and ice have diminished, and sea level has risen.
Global surface temperatures have warmed, on average, by around one degree Celsius since the late 19th century. Much of the warming, especially since the 1950s, is very likely a result of increased amounts of greenhouse gases in the atmosphere, resulting from human activity.
The Northern Hemisphere have warmed much faster than the global average, while the southern oceans south of New Zealand latitudes have warmed more slowly. Generally, continental regions have warmed more than the ocean surface at the same latitudes.
Global sea levels have risen around 19 cm since the start of the 20th century, and are almost certain to rise at a faster rate in future.
Surface temperature is projected to rise over the 21st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions. The ocean will continue to warm and acidify, and global mean sea level will continue to rise.
Relatively small changes in average climate can have a big effect on the frequency of occurrence or likelihood of extreme events.
How the future plays out depends critically on the emissions of greenhouses gases that enter the atmosphere over coming decades.