Newsflash: The first six months of 2008 have been the coolest of the last five years. Does that mean global warming is over? Not likely.
For the past 100+ years the atmosphere has been warming. How do we know? Well, there are the analyses of land-based, ocean-based, and space-borne temperature measurements by a host of independent scientific teams. There is also the simple fact that the globe's glaciers and sea ice are melting. You don't have to be a climate scientist to know that if ice is thawing, there's some heating going on.
However, the warming has not been continuous. Some decades have warmed more rapidly than others. And there has even been a period in the middle of the 20th century -- from about 1940 to the mid-1960s -- when temperatures actually decreased somewhat. But the long-term trend over that 100+ year period has been upward. The net change has been about 0.8 degrees centigrade or about 1.5 degrees Fahrenheit (see graphic ).
The ten warmest years on record in descending order are: 1998, 2005, 2003, 2002, 2004, 2006, 2007, 2001, 1997, 1995. As you can see, most of these have all occurred in the past 10 years. Within this trend, though, there are clearly some short-term ups and downs. The year 1998 was especially warm -- in fact it's the warmest year on record. By contrast, 1999 and 2000 were relatively cool.
The point here is that there are short-term trends that span a few years to a decade or more, and there are long-term trends of multiple decades. When we speak of a climatic trend, like global warming, we refer to the long-term trend not short-term variations.
Which brings us to 2008. The first half of the year has been the coolest of the last half decade, as reported last week.
Despite the fact that the whole year is still expected to be warmer than average, as this news spreads, you'll probably hear proclamations like "Aha, global warming is over." Similarly, some have argued that because 1998 remains the warmest on record, global warming has stopped. Don't be fooled by either argument -- you cannot infer a climatic trend from such short-term fluctuations. The fact that one year is warmer than another is not climatically significant.
What is significant is the fact that temperatures over the past century have increased. Also significant is the fact that the past 10 years have been consistently warmer that any other 10-year period in the record.
So what about those year-to-year fluctuations? Do we have any idea what's going on? The answer is yes.
The tropical Pacific Ocean is like a huge bathtub with water slowly sloshing back and forth from east to west. In one phase of the slosh, called El Niño, the ocean sloshes eastward toward South America bringing warm surface waters with it. In the other phase, La Niña, the ocean sloshes westward and surface ocean temperatures in the eastern Pacific fall. The term El Niño, Spanish for "little boy," refers to the Christ child because El Niños often begin around Christmas time. La Niña, "little girl," connotes the oscillation's opposite phase.
El Niños' warm sea surface temperatures allow heat to escape from the ocean to the atmosphere. As a result, El Niño years tend to be relatively warm. In fact, the warmest year on record -- 1998 -- was a year with an exceptionally intense El Niño.
La Niñas are just the opposite -- they enhance absorption of heat from the atmosphere to the ocean and the result is relatively cool atmospheric temperatures. This explains why 2008 is so cool -- it has been a La Niña year. Not so mysterious after all. Since La Niña began to weaken in the spring, the rest of 2008 is expected to return to a warmer pattern.
But what about that period of cooling in the middle of the twentieth century? Was that caused by a super La Niña? Did scientists really think an ice age was imminent as some claim? Tune in on Wednesday to find out.