Short-term Change and El Niño

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.

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7 Comments

  1. Eddie
    Posted July 23, 2007 at 11:45 pm | Permalink

    This one comes up all the time in the blogs I hang out in, so thanks for the tip!

  2. kenzrw
    Posted July 24, 2007 at 11:30 am | Permalink

    Thanks for the explanation. I'm a meterologist (retired) and knew about El Nino's for awhile, but can see your point of looking at the bigger global picture regarding warming. El Nino's also tend to cause more precipitation which may be one reason some regions of the US are cooling slightly.

    Is there any study that shows what the global temperatures would be without El Nino effects? If there would not have been El Nino in 1998, that year would not have been a record warm year, correct? 2005 would have been, is that right? Do you think an El Nino effect caused the heat in the 1930s as well? Does anyone have data on that?

    Anyway, thanks. The next 3-5 years of temperature data will tell all I think. I just hope that data is not somehow altered/filtered to fit a specific long-term forecast trend.

  3. Brian
    Posted July 24, 2007 at 3:55 pm | Permalink

    Nice explanation of El Nino, Dr. Chameides.

    On a (somewhat) related note, what impact do higher daily low temperatures have? Does the global data suggest that daily temperature ranges are contracting (ie, the difference between daily high and low temperatures)?

    It seems like the cumulative effect of a shrinking temperature range would be at least as powerful as rising high temperatures (if not more). A lot more attention is given to high temperatures though.

  4. Posted July 24, 2007 at 4:14 pm | Permalink

    Good question. The trend in the diurnal temperature range, or in the lingo of climate scientists DTR, has been of intense interest to climate scientists. For many years the data appeared to indicate the DTR was decreasing; i.e., nighttime temperatures were increasing more rapidly then daytime temperatures. However, according to the most recent report of the Intergovernmental Panel on Climate Change "updated observations reveal that DTR has not changed from 1979 to 2004… (However), the trends are highly variable from one region to another."

  5. Posted July 24, 2007 at 8:34 pm | Permalink

    It's so great to find a serious scientific take on this issue. People gossip about it all the time but nobody ever seems to come from a position of knowledge.

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