Since 1998, global temperatures have dropped. Is this a sign that global warming has stopped?
In 1998 the world saw its hottest year on record up to that point, as measured by average global air temperatures. This has led some to falsely conclude that world has stopped warming ever since. Global warming has not stopped. Here are the facts.
First, climate change (including global warming) is defined as long-term changes in the average parameters of the climate, not shorter year-to-year variability. Air temperatures were somewhat cooler in the years following the extremely hot year in 1998, largely due to a natural effect called La Niña (see breakout box). But to say that this represents a halt to global warming is like saying that just because we have a cool summer day it is not summer any more.
Second, when averaging over the decadal time scales that scientists use to study climate change, the past decade was not only warmer than historical averages, it was the hottest on record. In fact, 8 of the 10 hottest years on record have occurred in the decade after 1998.
Finally, the atmosphere (air) in which we live contains only a very small fraction of the total heat associated with the surface of the earth. The vast majority of the heat, about 85% of it is contained in the oceans, and observations show that ocean heat content has been rising over the past decade.
What is El Nino and La Niña?
El Nino is a driver of natural climate variability, and occurs when the temperature of the surface of the central to eastern Pacific Ocean is significantly higher than normal. This recurs about every three to eight years. In the opposite portion of the cycle, called La Niña, these portions of the Pacific Ocean are cooler than normal.
Because the oceans are a large heat reservoir, El Niño cycles affect weather around the world, including winds, rainfall and air temperatures in complex ways.
In Australia, the occurrence of an El Niño event is usually associated with an increased probability of drier conditions. The Bureau of Meteorology reports that in Australia, La Niña phases tend to have a stronger effect on temperatures than El Niño phases; that is, temperatures are much cooler than average during La Niña events than they are warmer than average during El Niño events.
The Bureau of Meteorology (http://www.bom.gov.au/) can provide further information on El Niño and La Niña (http://www.bom.gov.au/info/leaflets/nino-nina.pdf) and other phenomenon that impact on Australia’s climate.
 Levitus, Antonov and Boyer (2005), Geophysical Research Letters, Vol 32, L02604, ftp://ftp.nodc.noaa.gov/pub/data.nodc/woa/PUBLICATIONS/grlheat05.pdf