Throughout the late summer and fall of 1995 the hurricane trackers were kept very busy. It was the worst hurricane year for several decades. There was speculation that the increase in the number of hurricanes was linked to global warming, a worldwide increase in temperatures caused by increasing atmospheric concentrations of so-called “greenhouse gases,” primarily carbon dioxide.
Such speculation was premature. There have always been more hurricanes in some years than in others. There were more than average in the five-year periods 1891-95, 1931-35, 1946-50, 1951-55, 1961-65, and 1966-70, but since then there have been fewer. Some scientists suspect the frequency of tropical cyclones rises and falls in a cycle of 30 to 40 years, in which case the large number in 1995 may mark nothing more than a return to a high-frequency stage of the ordinary cycle. Unfortunately, it was during the period of low hurricane frequency that so many people moved into coastal areas of the southeastern United States.
Just because a speculation is premature it does not follow that it is wrong, only that it is too soon to be sure. Most scientists believe that releasing certain gases into the atmosphere may affect the global climate, and some think they have detected the first signs of warming (though others disagree). Over the last 100 to 130 years the average temperature is believed to have increased by 0.54, 1.08 degrees F and the 1980s and 1990s have been the warmest periods on record. If the world does become a warmer place, there may be more tropical cyclones and their effects may be more severe.
So far, the changes are small. Average temperatures vary naturally from one year to another and even during the warm years of the 1980s and 1990s they remained within the limits of natural variability. The average temperature in 1995 was only 1.44 degrees F warmer than the average for 1861-90 and 0.7 degrees F warmer than the 1961-90 average. Such a small change is difficult to detect and even more difficult to interpret. It is suspicious that a sequence of warm years continued so long, and it is the kind of warming scientists expect if the global climate is changing as predicted, but the warming will need to go on for another 10 or 20 years before anyone can be certain it is due to the “greenhouse effect.”
In a greenhouse, the glass allows solar radiation to enter, but prevents heat from escaping (mainly by trapping the warmed air inside), so air grows much warmer inside the greenhouse than it is outside. This is how the “greenhouse effect” acquired its nickname. The nickname is not really accurate, because although the result is similar, the reasons for it are quite different.
Most of the radiation we receive from the Sun is at short wavelengths, concentrated in the waveband we see as visible light. The atmosphere is almost completely transparent to this radiation, but some is reflected back into space from the tops of clouds and light-colored ground surfaces such as snow. Most of the radiation is absorbed, however, and warms the land and sea. The surface radiates some of its heat upward and air in contact with the surface is warmed and rises by convection. As it cools, the rising air also radiates heat back into space. Heat radiation, from the Earth and its atmosphere, is at long (infrared) wavelengths.
The atmosphere consists mainly of nitrogen (about 78%) and oxygen (about 21%). These gases are transparent to radiation at all wavelengths, but the air also contains very small amounts of other gases, which are not. Their molecules are larger than those of nitrogen and oxygen and, depending on their size, they absorb radiation at particular infrared wavelengths. Water vapor is the most important of these gases. Others include carbon dioxide, methane, CFCs (chlorofluorocarbon compounds, use of which is now being phased out because of their effect on the ozone layer), ozone, nitrous oxide, and carbon tetrachloride (a solvent formerly used in dry-cleaning, which is also being phased out). These are the “greenhouse gases.”
The contribution each gas makes to the total absorption of infrared radiation is calculated as its global warming potential (GWP) and carbon dioxide is given a GWP value of 1. On this scale, methane has a GWP of 11, nitrous oxide 270, and CFCs and related compounds from 1,200 to 7,100.
Molecules of these gases absorb long-wave radiation, each at certain wavelengths. It warms them and they start radiating their own heat. They radiate in all directions. Some radiation goes upward, into space, but most does not. It goes sideways, eventually to be absorbed by other greenhouse-gas molecules and radiated in all directions again, or downwards. The overall effect is to warm the lower part of the atmosphere, which is where these gases occur.
The gases are rather like a blanket, and like a real blanket, they allow some heat to escape through “windows,” which are wavelengths at which no molecules absorb infrared radiation. A blanket on your bed keeps you warmer than you would be without it, but it does not keep your temperature rising indefinitely until your body cooks. In the same way, greenhouse gases keep the air warmer than it would be without them, but they do not make temperatures continue to rise until the oceans boil and the rocks melt.
People think of the “greenhouse effect” as a threat, but without it life on Earth would be very difficult and probably impossible. If the air contained no naturally occurring greenhouse gases, the average temperature at the surface would be about 5 degrees F. Plants would not grow at this temperature and most of the oceans would be covered by ice.
The threat arises because we release greenhouse gases into the air, so the air contains more of them than it did naturally, and the concentration is increasing. If this continues, more long-wave radiation will be trapped in the atmosphere, causing the average temperature to rise. This is “global warming.” However, the situation is far from simple. Carbon dioxide is the most important of the greenhouse gases we emit, not because it is more absorptive than the others, but because we release it in much larger amounts. It is produced whenever we burn anything containing carbon, because burning involves oxidizing the carbon to carbon dioxide (C + O2, - CO2). All plant material, peat, coal, natural gas, and oil contain carbon. Of the carbon dioxide released by burning, however, only about half accumulates in the atmosphere. Scientists are uncertain about what happens to the remainder. Some dissolves in the oceans and some is taken up by plants in the process of photosynthesis, but a large amount cannot be accounted for.
Much still remains to be learned about the effect of warming on the oceans. They transport heat from low to high latitudes, so they have a very important effect on climates, but no one is really sure what will happen if the oceans grow warmer than they are now. Nor can scientists predict just how and where clouds will form. Some clouds reflect incoming solar radiation, others absorb outgoing infrared radiation, so it is very important to know how warming may affect cloud formation.
If temperatures rise, more water will evaporate from the surface, so cloudiness will increase and there will be more rain and snow, at least in some parts of the world. This might have several consequences. The polar ice caps might thicken, for example, because more snow would fall on them, so that rather than the ice caps melting and raising sea levels, sea levels might remain much as they are now, or even fall. More rain and snow falling in high latitudes, leading to an increased flow of fresh water into the sea from rivers and more falling on the sea itself, might reduce the density of the surface layer of the sea, because fresh water is less dense than salt. Were this to happen in the North Atlantic, it might interrupt the “Atlantic conveyor”, in which case the warm North Atlantic Drift might cease to break away from the Gulf Stream. That would make the climate of northwest Europe much colder than it is now.
As it is, the warming observed so far is not spread evenly over the world. There is no clear evidence of warming in Polar Regions. Temperatures over the northwestern North Atlantic have risen less than those elsewhere, and have fallen in some places, and over the parts of the continents of the northern hemisphere the warming has been due to an increase in minimum nighttime temperatures, not in maximum daytime temperatures.
This is believed to be due to sulfur dioxide. In the air, sulfur dioxide (SO2) attracts atmospheric water vapor and dissolves to form sulfurous acid (H2SO3) and then oxidizes to tiny droplets of sulfuric acid (H2SO4). These reflect incoming solar radiation and in moist air more water vapor condenses onto them, so they help form clouds. In both cases, by reflecting incoming radiation and increasing cloud formation, sulfur dioxide has a cooling effect. Volcanoes and several biological processes release sulfur dioxide, but it is also released when fuel containing sulfur, for example certain types of coal and oil, is burned. There is much more industrial activity in the northern hemisphere than the southern, which may be why the northern hemisphere has warmed more slowly. It may also explain why nighttime minimum temperatures have risen in the northern hemisphere, but not daytime maximum temperatures.
During the day, sulfuric acid droplets and clouds cool the surface by reflecting incoming radiation. Clouds also reflect outgoing heat radiated from the surface and at night, when there is no incoming radiation and the ground cools, this reduces the rate at which heat is lost. The combined effect is to make the days cooler and the nights warmer. Atmospheric sulfur dioxide has also been found to alter the track of the jet stream, leading to colder winds over the North Atlantic and North Pacific.
Most climatologists believe that by 2100 the average global temperature will have risen by 1.8-6.3 degrees F. Sea levels have risen by 4-10 inches over the past century because water expands when it is warmed. Some scientists expect them to rise a further 6 inches by 2100, and others say they may rise an average of 13 inches with an outside chance they will rise 40 inches.
Studying the global climate is very difficult. Measurements of pressure, temperature, humidity, cloudiness, and so forth are now made in many parts of the world, but conditions in some large, sparsely populated areas are not reported so regularly. Reliable records of past weather conditions are even more scattered and none of them are continuous for very long. The longest is the “Central England” record, consisting of regular monthly reports since 1698, but it says nothing about the weather anywhere else. More records have been kept regularly since the last century, but most of them are local. It is hard to tell whether the climate is growing warmer or cooler without reliable records of past temperatures to make comparisons.
Predicting how warming on a global scale will affect particular areas is even more complicated. Scientists use the most powerful supercomputers in the world to calculate what is likely happen, but the results are meant to give only a general impression. They are not like the evening weather forecast.
It is important to remember the uncertainties, but few scientists doubt that the “greenhouse effect” is real and important and that global warming presents a threat to which we should respond by reducing the amounts of greenhouse gases we discharge into the air. If global warming does occur, an increase in the number of hurricanes may be one of its consequences, and the effects of the hurricanes may be more severe.
Although the sea is cooler in the North Atlantic and North Pacific, it is possible that average sea-surface temperatures have already risen in many parts of the tropics. Temperatures are predicted to rise more in high latitudes than in low ones, with little or no change at the equator itself, but there may well be some warming near the edges of the tropics, around latitudes 10, 20 degrees N and S, inside the belt where tropical cyclones form. Tropical cyclones can form only where the surface temperature of the sea is warmer than 80 degrees F, and the frequency with which they occur today shows that this temperature is often reached. Only a small overall warming would be needed to increase, perhaps greatly, the number of days when sea-surface temperatures were suitable.
At the same time, temperatures over the continents would also increase. There warming might increase convection, leading to more thunderstorms. As these disturbances drift over the ocean they trigger the easterly waves that can develop into tropical storms and then cyclones. A quite small warming might be enough to increase the number of tropical cyclones forming each summer and fall. If so, the 1995 season may indeed become typical, although it is far too soon yet to tell whether it was due to global warming or merely part of a regular cycle.
Higher temperatures will make sea levels rise as the warmed water expands. Scientists do not expect the polar icecaps to melt within the next century. As always with hurricanes, water is as dangerous as the wind and even a small rise in sea level would make storm surges bigger.
It is possible, therefore, that global warming might make tropical cyclones more frequent and more destructive.
This is the last article from a five-part series on hurricanes. The others can be found at the links below.