The author of today's post, Bill Chameides, is Chief Scientist at Environmental Defense.
Last week in our Suggestion Box we got this question:
I've compiled a NCDC state by state average temperature map and trends from 1895 thru 2006. 1998 was the warmest year, but years since then are showing either the same as 1998 or cooler in most states. How can I explain to people why average temps haven't been warming EVERY year since 1998 instead of going up and down?
This is a good question, and one I get fairly often, so let me try to explain.
First a general comment about the data you're looking at. Global warming is about long-term global trends, so you have to be careful when you look at state-wide data. Some states (particularly in the southeast) haven't warmed that much, perhaps due to growing aerosol concentrations. But that's only one region, not a global trend.
Many factors can influence global temperatures besides greenhouse gases. Probably the largest influence year-to-year is a phenomenon called "El Niño", which is related to ocean circulation in the South Pacific.
Most of the time, winds over the South Pacific blow from the east to the west, and warm surface water tends to pool in the western part of the South Pacific.
Every few years or so, an opposing westerly wind arises and pushes the warm pool of water eastward. This changes the weather over a huge stretch of ocean and land. It alters precipitation patterns over the South Pacific, affects ocean productivity and fishing off the coast of South America, and even changes drought and precipitation patterns here in the U.S.
The Spanish term "El Niño" refers to the "Christ Child", and was given to this phenomenon because the warm waters typically arrive off the South American coast near Christmas time. There is also a phenomenon called La Niña ("the girl") – the extreme opposite of El Niño – where easterly winds are especially strong and the warm waters of the South Pacific are confined to the western edge of the ocean. (For more on El Niño and La Niña, visit the NOAA Web site, or read this article on the RealClimate Web site.)
What does El Niño have to do with global warming? When El Niño occurs and the warm water moves eastward, the Pacific Ocean releases excess heat to the atmosphere, causing global temperatures to spike. If you look at a year-by-year graph of temperature, you’ll see that 1998 was unusually warm – the warmest on record. It turns out that 1998 was also a year with one of the strongest El Niño’s on record. Most scientists believe that El Niño was the cause.
Source: NOAA. Click to enlarge.
Interestingly, 2005 tied for 1998 as the warmest on record, and that year did not have an El Niño. The warmth in 2005 was most likely due to the slow rise in global temperatures from greenhouse gas pollution.
A new, relatively weak El Niño formed in September 2006, and persisted through February 2007. That might help explain why temperatures over land in the winter of 2006-2007 were the warmest on record. In fact, 2007 may break the global temperature record of 1998 and 2005, at least in part due to El Niño.
Some scientists believe that El Niños are becoming more frequent due to global warming. My own opinion is that this idea has not been put on a firm scientific footing. So we’ll have to wait and see on that one.
The study of inter-annual temperature variations is fascinating, and it's important to try to understand them. But predicting temperatures on a year-to-year basis will always be problematic. Longer-term trends are much easier to see, and the trend is that temperatures are increasing. After all, long-term trends are what global warming is about.