Weather forecasting centers around the world have now declared that an El Niño, the most powerful fluctuation in Earth's climate system, has begun in the tropical Pacific. El Niño is a warming of the Pacific Ocean as part of a complex cycle linking atmosphere and ocean. An El Niño can raise global temperatures, says the UK's Met Office. A El Niño event brings a higher risk of more, and more severe, tropical cyclones. Impacts on the biosphere are expected, such as coral bleaching and widespread movements of marine life.