During El Niño, New Zealand tends to experience stronger or more frequent winds from the west in summer, typically leading to drought in east coast areas and more rain in the west. In winter, the winds tend to be more from the south, bringing colder conditions to both the land and the surrounding ocean. In spring and autumn south-westerly winds are more common.

El Niño is a natural feature of the global climate system. Originally it was the name given to the periodic development of unusually warm ocean waters along the tropical South American coast and out along the Equator to the dateline, but now it is more generally used to describe the whole "El Niño - Southern Oscillation (ENSO) phenomenon", the major systematic global climate fluctuation that occurs at the time of an "ocean warming" event. El Niño and La Niña refer to opposite extremes of the ENSO cycle, when major changes in the Pacific atmospheric and oceanic circulation occur.

Future climate changes will be affected, amongst other things, by how much extra greenhouse gas goes into the atmosphere. For this reason, NIWA has looked at likely changes in New Zealand across six greenhouse gas emission scenarios from the Intergovenmental Panel on Climate Change (IPCC), but focusing on the mid-range. These scenarios do not include major efforts to reduce greenhouse gas emissions.

Greenhouse gases have continued to increase in the atmosphere. This is due largely to human activities, mostly fossil fuel use, land-use change, and agriculture. About 63% of the warming effect of greenhouse gas increases over the last 200 years is due to carbon dioxide.

The Earth's climate has exhibited marked "natural" climate changes, with time scales varying from many millions of years down to a few years. For example: