El Niño vs. Greenhouse Warming
Judging from the conflation of El Niño and human‐induced global warming, you might think the two were one and the same, or maybe even, as Vice President Al Gore intimated, that one caused the other.
Like many of his reaches, there was a bit of truth in the stretch, but only a bit. Global warming didn’t cause El Niño in any appreciable sense, but the two were related: It was a very good El Niño year, and it was a very, very warm year.
El Niño is natural. Just because scientists discover something, or because we, as taxpayers, shell out tens of millions of dollars to research something, does not mean that something new has happened. Chemicals existed before chemists, DNA existed before its discovery won a Nobel Prize, and El Niño ebbed and flowed long before the first climatologist was born.
|El Niño is a commonly occurring slowing (or even reversal) of the northeasterly and southeasterly trade winds that dominate the tropical Pacific Ocean (Figure 1). When the trades are blowing, they move vast amounts of oceanic water northwestward and then westward from the coast of South America. In doing so they drag the warm water off the surface, and much colder water “upwells” in replacement. So the eastern equatorial Pacific Ocean is relatively cool for a tropical ocean.
No one knows why the trades suddenly slow or even reverse, piling warm water up against South America. Reid Bryson, the modern founder of the very true notion that climate does change in ways that are important to people, believes this reversal is mainly an effect of some other large‐scale physical fluctuation. During an El Niño, a large portion of the tropical Pacific is much warmer than average — as much as 8oC (14.4oF) — and this heat eventually disperses through the atmosphere.
|Figure 1. Schematic representation of the conditions that occur during an El Niño event as compared with a La Niña event.|
Heat is energy, and an El Niño shows up both as warming and as motion. Its reach extends into the tropical Atlantic, where it suppresses — yes, suppresses — hurricanes. Rain, often absent for years, falls in the ultradeserts of Peru and Argentina. And the global temperature warms.
When the trade winds return, the cold upwelling reappears. This is La Niña. It stands to reason that the more the cold water is suppressed, the greater the amount that eventually bubbles up, so a big El Niño warm spike may mean a big temperature fall in the months thereafter.
|As our daily satellite data show (Figure 2), the lower atmospheric temperature peaked around April, and was in rapid decline through the rest of the year. As of this writing (mid‐March 1999) it continues to head south faster than an Internet stock with a bad earnings report.
We’re totally confident that 1998’s big warming spike was a result of El Niño, and not dreaded “global warming” — that is, a human product. We know because the stratosphere tells us so.
The human version of global warming is caused by increasing amounts of “greenhouse” gases in the lower atmosphere. These compounds absorb the heating radiation that results from the sun’s warming of the earth’s surface, and reemit that radiation either downward, resulting in additional atmospheric warming, or out to space. If these compounds weren’t there, the radiation would pass directly outward.
|Figure 2. Daily measurements of the global temperature anomalies as observed by satellites show that the anomalies peaked in the month of April 1998 and have been declining ever since.|
|Increasing the greenhouse effect, then, warms the lower atmosphere but, by virtue of the “recycling” of warming radiation in the lower atmosphere, cools the stratosphere that lies above. The heat from El Niño, on the other hand, burbles up through it all.
So what we should see from the increasing greenhouse effect is a lowering trend for stratospheric temperature. And El Niño should temporarily stop that trend, at least for a year or so. Figure 3 shows that 1998 was indeed one of the warmest years in the stratosphere in the last two decades and is testimony to El Niño as the cause.
|Figure 3. The rather steady decline in stratospheric temperatures was abruptly interrupted in 1998 — a strong sign of El Niño.|
We were besieged with news reports about how El Niño (and, by not‐so‐subtle extension, global warming) would cause terrible agricultural disasters. Who can forget the miles of CNN footage showing tractors mired in the Georgia mud, or network reels of browned corn in Texas?
Well, some folks did poorly, and some folks did well. That happens every year. About the best way we know of to settle the overall score is in the wheat, corn, and soybean markets. When there’s a big supply, the price goes down. Demand fluctuates some too, but a perpetually increasing population has a way of ensuring more mouths to feed.
By late 1998, the price of U.S. wheat stood, after adjusting for inflation, at its lowest level since reliable records began in 1915. Fluctuations in America’s massive supply of agricultural products, more than anything, dictate the global price.
|Turns out all that rain in the winter — so ugly on television — was quite salutary for the major food and feed crops, especially wheat. Figure 4 shows the U.S. historical wheat yields, and there’s little doubt that 1998 gets the prize.
Many agricultural economists and a few climatologists have made careers of studying the influence of global weather patterns on crop yields. Moisture at planting time and in the winter before harvest is the major determinant — by far — of winter wheat yield. Winter wheat is planted in the early fall, requires moisture to germinate, and then, when spring springs, is really poised to take advantage of wet soil. In addition, yields are positively influenced by above‐normal winter temperatures.
|Figure 4. Historical yields of wheat in the United States (inset: since 1970). The El Niño year of 1998 holds the record. Notice that yields were also very high during the last great El Niño year, 1983.|
Undoubtedly, the climate of 1998 led to the record yields. But there was another factor as well: Increased carbon dioxide in the air increases yields and makes crops much more efficient in their use of moisture. As Sylvan Wittwer, former head of the Board on Agriculture of the U.S. National Research Council wrote,“Overall, it has been conservatively estimated that global crop productivity has risen by approximately 2.5 to 10 percent, and possibly as high as 14 percent from the current increase in atmospheric CO2 over pre‐industrial levels.”
We’ve Seen Fire and We’ve Seen Rain
Two other prominent newsmakers this year included the spate of overland fires in Florida during the early summer, and Mitch, a real son‐of‐a‐gun of a flood, but really not much of a hurricane by the time it hit land.
|We were tempted to say,“Now there you go again, Al,” about Florida, when he said the fires offered a “glimpse of what global warming may mean to families across America.” In fact, the natural Florida ecosystem (something pretty hard to find with all the Disney Worlds, Homosassa Springs and Kissimmees dotting the landscape) is attuned to fire. Judging from their writings, the early Florida explorers found the burning of the peninsula perhaps its most impressive aspect.
That didn’t stop everyone from blaming all this on El Niño and global warming, but the fact is that, in general, there is no relationship between summer dryness in Florida and the existence of an El Niño during the previous winter. That’s because El Niño makes it rain during the winter greening season. In the summer, there’s precious little documentation that El Niño does anything at all to Florida weather. Of course, we could blame Florida’s high temperatures this year on global warming, but that would mean ignoring the fact that changing the greenhouse effect warms up the coldest air masses a lot more than it heats up the warmer tropical ones.
|Figure 5. The history of the Palmer Drought Severity Index for central Florida during summer. The filled circles indicate those years with a strong El Niño in the preceeding winter. There is no relation between El Niño and summer droughts in Florida.|
|Hurricane Mitch was a tragedy, but unfortunately, not a singularity. Mitch started as a Category 5 (that’s the worst kind) hurricane in the western Caribbean. These are not all that uncommon in that part of the world. Edith (1971) and Janet (1955), for example, come to mind (Figure 6). Because it was a slow mover, as Mitch interacted with the mountains of Central America, the winds dropped to Category 2 status, but the rains were extreme. Precipitation totals of more than 50 inches brought tremendous flooding and loss of life. Although the actual number of deaths remains quite elusive, it was clearly in the ten‐thousand range. Speaking in Argentina at a meeting designed to strengthen the United Nations climate treaty, State Department Spokesman J. Brian Atwood told U.S. networks that Mitch was a “typical greenhouse effect.”||Figure 6. Six Category 4 or 5 hurricanes have occurred in the same vicinity as Mitch since 1950. That’s about one every eight years.|
That was inflammatory nonsense. A 1974 hurricane named Fifi, which was also a Category 2 at landfall, took much the same track and killed 7,500. Janet and Edith had much more powerful winds and wreaked tremendous havoc. Perhaps the most interesting comparative aspect with Mitch is that a tropical storm named Claudette, in 1979, also produced 50 inches of rain and resulted in nine deaths (that’s about 9,990 fewer than Mitch caused) when it hit Texas. Perhaps infrastructure and poverty, not global warming, created the tragedy named Mitch. Maybe, just maybe, allowing us to save our money for investment in developing nations like Honduras and El Salvador is a better idea than taking it away in an attempt to stop something that would happen anyway.
El Niño and hurricanes do share at least one common trait. They have been around for a long time and the biota of the world, thanks to the nature of evolution, likely take advantage of them. In California, rains of the magnitude that associate with El Niño are required to make the desert bloom. Just any old storm isn’t enough, even though the ground gets wet. In that environment, many seeds have to be scarred by the motion of overland movement of water before they’ll even germinate.
Long before banging the climate‐disaster gong became the key to career advancement, federal climatologist George Cry calculated the percentage of normal rainfall that comes from all tropical cyclones, including tropical depressions, storms, and hurricanes in the eastern United States. Figure 7 shows the result for September.
In most of the areas with high values, American agriculture has adopted a double‐crop system that plants one early, fast‐maturing crop, and then replaces it with an October harvest crop, mainly soybeans. Late August and September rain can be very important determinants of final yield. It’s pretty clear that years in which amounts are below normal because of lack of tropical cyclones are those in which yields are in jeopardy.
|Figure 7. The percentage of September rainfall in the eastern United States that comes from tropical systems. The regions that normally receive more than 15 percent are shaded.|
People adapt to their climatic environment. The biota of the world take advantage of change, and so does our agriculture: One of the biggest El Niños in recent centuries produced a glut of food. That’s the lesson of 1998.
But the climate hype of 1998 also has portents. If this past year is any guide, when global warming becomes a major focus of the Y2K presidential campaign, the amount of distortion, exaggeration, scare stories and fear‐mongering we’re sure to witness will be a real climate disaster.
Cry, G.W., 1967. Effects of tropical cyclone rainfall on the distribution of precipitation over the eastern and southern United States. ESSA Professional Paper, 1, U.S. Dept. of Commerce. Washington, D.C.
Michaels, P.J., 1979. Atmospheric anomalies and crop yields in North America. Ph.D. Dissertation. University of Wisconsin.
Wittwer, S., 1995. Food Climate and Carbon Dioxide. CRC Press, Boca Raton, Fla., pp. 236.