El Niño and La Niña episodes, for example, result from rapid changes in the sea-surface temperature in the equatorial Pacific Ocean. They influence weather patterns around the world through the subsequent large-scale interactions and transfers of heat in the coupled ocean-atmosphere system. Other patterns affect the climate by strengthening or weakening high-altitude air currents known as jet streams.
The closely related Arctic Oscillation and North Atlantic Oscillation often affect the northern hemisphere winter. Since the 1990s, these two oscillations have remained mostly in a positive phase, which is associated with warmer and wetter winters in northern and central Europe and the eastern USA, drier winters in the Mediterranean and cold, dry conditions over northern Canada and Greenland.
Unlike these natural back-and-forth oscillations, human-caused climate change is trending in just one direction. This is because atmospheric concentrations of carbon dioxide, methane, nitrous oxide and other greenhouse gases are increasing steadily, due to human activities. According to the WMO Greenhouse Gas Bulletin, global-average atmospheric concentrations of carbon dioxide rose to 389 ppm in 2010 (an increase of 39 per cent compared to pre-industrial times), methane to 1 808.0 ppb (158 per cent) and nitrous oxide to 323.2 ppb (20 per cent).
This changing composition of the atmosphere is causing the global average temperature to rise, which, in turn, exerts a significant influence on the hydrological cycle and leads to other changes in climate and weather patterns.