The Nature of Change
DEWPOINTS AND OTHER FINER POINTS ABOUT CLIMATE CHANGE
OVER THE PAST TWO DECADES Virginians have endured the largely federal cant that our climate is going to pot because of the effects of dreaded greenhouse gases, and that its only going to get worse. Readers of this Advisory might recall the "State Impact" report on climate change by the Environmental Protection Agency that was discussed in Vol 21, No. 4. (Winter 1996). That report began with the statement "Over the last century, the average temperature near Richmond, Virginia has increased 0.2° F [!!-eds] and precipitation has increased up to 10% in some parts of the state."
We proceeded to show that the statewide average temperature and rainfall havent changed a lick in the last 100 years. Not surprisingly, our readers have often been bludgeoned by an unequal but opposite point of view, compared to that of the Federal Climatologists, in these pages.
Throughout this little disagreement, time has marched on. And greenhouse gas concentrations in the atmosphere, particularly Carbon Dioxide, have gone up and up. So, perhaps, maybe we ought to hear from a neutral third partysomething called realityabout what our climate really has been doing.
Is Virginias climate changing? Is it changing in a way that is consistent with an enhanced greenhouse effect? If it is, are these changes largely deleterious? How do they compare with past changes? Thats the main subject of this feature. We look at winter vs. summer temperatures, night vs. day temperatures, dewpoints, and precipitation.
*******WINTER vs. SUMMER WARMING
Most people, including us, argue that greenhouse warming is likely to express itself much more as a warming of the winters than the summers, and everyone seems to believe that the winters of the past decade are a lot warmer than they were, say in the 1960-85 period.
Our chart provides some support for this contention. But an inspection of winter temperatures back to the beginning of the modern Virginia record in 1895 shows that winters during the period from 1960-85 were as cold as the winters of the 1990s were warm. Also worth noting is that, while our recent winters have been fairly mild, there hasnt yet been one that can touch the winter of 1931-1932.
The summer record is similar. The 1970s tend to be cool, and the 1980s and 90s are warm. But, despite everything we hear and read, it is clear that the 1930s, on the average, remain the hottest decade.
Thats 70 years and a lot of greenhouse gases ago. One of our Advisory fans has written to us, speculating that IF the weather of the 1930s returned, whether there would be enough space in newspapers to print the horror stories.
In toto, Virginias seasonal temperature history hardly looks like a greenhouse change. True, recent years have been warm, but summers still havent gotten to where they were when my generations parents were children, and theres no winter/summer differential warming.
Comparison of the mean winter (December-February) temperatures for Virginia in two time periods: 1960-1985 and 1986-97.
Virginia mean winter (December-February) temperatures for the period 1896-1999.
Virginia mean summer (June-August) temperatures for the period 1895-1998.
COMPARING VIRGINIA AND THE REST OF THE U.S.
Changes in U.S. Temperatures in three-month periods since the mid-1960's.
Changes in U.S. cloudiness and sunshine duration.
Day and Night, Night and Day...
People have long hypothesized that greenhouse warming should be more accentuated in the winter for a variety of reasons. First, if there is more warming in the winter than summer, the fact that nights are longer in the winter ipso facto places more warming in the night.
In addition, winter air is simply a lot drier than summer air, and the greenhouse changes that humans put in the atmosphere in no small part mimic the effect of water vapor in the atmosphere. But the warming that occurs becomes progressively less for each unit of greenhouse gas. As a result, the first changes in carbon dioxide or water vapor produce the most heating. Dry air should be more sensitive to carbon dioxide changes.
A cautionary note: increasing cloudiness has been noted around the world in the spotty records that exist for the last fifty years. Cloudy days are obviously not as warm, and cloudy nights are not as cold (as the heat of the earth is trapped by the cloud, some of it is returned down towards the surface).
Annual averaged daily high temperatures for the Southern Piedmont region of the U.S. for the period 1901-1987.
Annual averaged daily low temperatures for the Southern Piedmont region of the U.S. for the period 1901-1987.
Clouds could increase by changing the greenhouse effect (a little warming would evaporate a lot of water, which would then condense in the form of more clouds). But they could also increase because of other anthropogenerated emissions. Notable among these is sulfate aerosol, produced by the combustion of coal in power generation, and agricultural and forest burning.
It should be noted that sulfate emissions have been dropping across our region as a result of environmental regulation; nonetheless some of the aerosol is still produced. It is "hygroscopic," meaning that it readily adsorbs moisture onto its surface, and it can serve either as an enhancer of our normal (and substantial) summer haze, or it can serve as a nucleus for moisture condensation that results in cloud formation.
One might argue, then, that a relative warming of the nights compared to the days could result either from greenhouse changes or sulfate aerosol in our region.
But any sulfate effect is regional, compared to the global change in greenhouse gases. It is noteworthy that around the planet, there appears to be a propensity for night (rather than day) warming, especially in the Northern Hemisphere.
Our plots of day and night temperatures are not necessarily harmonious with all of the theorizing of the last several paragraphs. We do see a decline in daytime high temperatures, at least from 1930 through the late 1980s. But there is not a corresponding rise in the night temperature in that period (the net result being a slight decline in overall temperature). Since then there has been a rise that makes the overall net change of the last 70 years a big, flat zero.
One other item of interest is the decline in daily temperature range (difference between high and low) that shows up through most of the century. See our sidebar "land use" for another view!
Score for this point: Doesnt look like much of a greenhouse change, or, if it is, the effect is a lot smaller than expected by some of our friends in the climate business.
Annual averaged daily temperature range for the Southern Piedmont region of the U.S. for the period 1901-1987.
LAND USE CHANGES AND CLIMATE CHANGES
Consider that, if left alone, most of the Mid-Atlantic will revert to a pine-oak forest which maintains itself with occasional, large fires. Thats a surface cover that looks and acts a lot different than, say, what happens when Washington DC is substituted for the woods.
When forest turns to blacktop, the surface gets much hotter. Run this "thought experiment": Take off your shoes and imagine you are walking on the top of a forest on a sunny, hot summer day. Somehow the branches and leaves are sturdy enough to hold you. Your feet will feel a bit warm¾ leaf surface temperatures on a sunny day might reach in the low 100s, but all-in-all a pleasant experience.
Now do the same thing, between stop lights across, say, 10th Street in downtown DC. Guaranteed that your feet will be burning by the time you reach the other side, as pavement temperatures can brush 150° F.
It gets worse. Wherever there isnt a street in downtown Washington, theres likely to be a building. In order to protect the impressiveness of the federal government, they are, by law, limited in height, with about ten stories being a typical maximum. Even though the outside temperature is approaching 100° , its a cool 72° inside, thanks (or no thanks) to the invention that made legislating a year-round task in Washington: air conditioning.
All of the heat energy required to cool those buildings, and then some, is now cascading down the street corridors, raising things a few more degrees.
The picture is obvious: replacing a forest with pavement drastically alters the local climate, right in those locations where there happen to be plenty of weather stations. More subtle changes occur in smaller towns or when forests are replaced by farms¾ as was common during the development of intensive agriculture in Tidewater Virginia. Now the farms are being replaced by homes and concrete, etc.
One of the currents stirring through the climate change business is that we may not have paid enough attention to changes in land use as an influent on historical temperature records. We hope this little discussion demonstrates why it may be very important.
Dewpoints and Don'tpoints
Its an undisputed fact that simply changing atmospheric carbon dioxide will not produce very much global warming. Most computer calculations give a bit more than 1°C of warming for the equivalent effect of doubled carbon dioxide. We are about one-half way towards this point right now, when all the human greenhouse gasescarbon dioxide, methane, chlorofluorocarbons, etc.are taken into account.
The reason the computer spits out much larger warmings has to do with the so-called "positive water vapor feedback". Carbon dioxide provides a bit of warming. The surface of the ocean warms up and evaporates more water. Water is a potent greenhouse gas, so it gets warmer.
As the surface heats up, more and more water is evaporateduntil theres so much water vapor in the atmosphere that the suns energy becomes more directed towards evaporation than direct heating. This is one reason why the temperature range on a humid day is generally less than it is on a dry day, or why theres such a huge temperature swing between night and day in the desert. There are other reasons, too.
The bottom line is that warming does increase evaporation, and the amount of water in the air determines something called the dewpoint, which is the temperature at which water will condense. You can note the dewpoint on most any summer evening in Virginia by sticking a thermometer on a grassy surface that has begun to accumulate dew.
The dewpoint is actually determined by the relative density of water vapor molecules in the air. Where there are a lot, the dewpoint is high. Because water content increases with temperature, raising the mean value of the earths temperature will raise the dewpoint. And people near an ocean might especially notice the phenomenon.
The bottom line is that regional dewpoints should be going up. Our plot doesnt extend as far back as the ones for temperature because dewpoint records are only regularly taken at airfields and National Weather Service offices. Readers may also note this is not a statewide record, but rather is from Norfolk.
Annual averaged daily dewpoint temperatures for Norfolk, VA for the period 1948-1997.
Surprise! The only significant trend is in winter, and it is down. Contrast this to the finding of Livezey & Co., detailed in our sidebox, showing that U.S. fall and winter temperatures have been going up. In general, the drier it is in the winter, the colder it is.
Just to keep track, lets call the score on this one point: Greenhouse 0, Visitors 1.
Dewpoint is a very potent integrator of how much moisture is in the air¾ but it has other, more practical applications, such as telling people when to chill out.
There are a passel of heat stress measurements that all attempt to quantify the combined effects of temperature and atmospheric moisture on people. Some go even further, adding in the effects of wind.
They go by the names of Humiture, the Steadman Index, the Temperature-Humidity Index etc...(Weve been waiting for some time for "Tempidity"), but in Virginia maybe this can be simplified a bit by simply looking at dewpoints over a range of observed temperatures.
The fact of the matter is that there are very few cool days in a Virginia summer, and the ones that are (which means a high of less than 80° F) are either cloudy and rainy (which cools the temperature to near the dewpoint), or clear, breezy and cool, as a result of a welcome, but unfortunately temporary, visit of a high-pressure system of Canadian origin. Obviously any stressful physical activity is appropriate under these conditions, although this does not mean that we recommend planting trees in thunderstorms.
Also, we dont have a lot of days in which the temperature exceeds 95° F, especially in rural areas. We trust that people have the good sense not to go for a run under these conditions.
That leaves us with the toasty middle, between 80° F and 95° F. Try these simple rules:
Less than 60° FWhatever you want
60-65° FMake sure well watered
65-70° FLimited to moderate exercise
70-75° FCaution: Stop with any discomfort
Greater than 75° FDont!
Now, if youve just gotta have that dewpoint because youre about to go for some exercise, you can get it on our website: http://www.people.virginia.edu/~climate, and click on "Current Conditions".
Predictions of altered precipitation in a greenhouse world are all over the map. Some computer models show increases in storminess (i.e. a stronger jet stream) across the North Atlantic. Logic suggests that the jet stream will actually power up fewer and/or weaker surface storms because it maintains a more "summer-like" orientation during warm periods. The big, cyclonic storms that derive much of their power from the jet stream largely disappear in the summer.
At the same time, surface heating is supposed to be more (even though it isnt, here). Everything else being equal (a rare bird), warmer surface temperatures will lead to increasing buoyancy of the moist air near the ground and sea, resulting in an increased frequency of showers and thunderstorms.
Which, if any, of these two processes is winning out? At least in the United States and Virginia, the surface just doesnt seem to have warmed very much at all. Temperatures rose about two degrees between 1915 and 1930, fell by about 1.5° from 1930 to the late 1960s, and then rose back to where they were 70 years ago.
Given our little discussion above, precipitation, if anything, should be decreasing slightly. But unnoticed for the first 95 years of this century is that rainfall has been slowly, but noisily (meaning that there is a lot of variation from year-to-year) creeping up.
Annual total precipitation for the U.S. for the period 1895-1998.
The country averaged about 28 inches per year at the beginning of this century, and will finish the century averaging around 31. Thats a 10% increase.
One of the big fears about greenhouse effect-related changes is that any increases in rainfall will be overcompensated for by increases in evaporation from higher temperatures. The net result would be less moisture available for crops and forests.
But thats not at all what has happened in the U.S. Instead, precipitation has increased by about 10% while temperatures have barely budged. The bottom linewhich may come as a shock to someis that the nations overall moisture status has actually improved.
In Virginia things arent as rosy. Theres no trend at all in annual precipitation, although there has been a tendency for dry summers in the last two decades. While our overall moisture status doesnt seem to show any trends, there has been some propensity for agricultural distress, largely a summer phenomenon, in this period.
Annual total precipitation for Virginia for the period 1895-1998.
Summer total precipitation for Virginia for the period 1895-1998.
Winter total precipitation for Virginia for the period 1896-1999.
Its nice to see that the Virginia and National precipitation figures have some internal consistency. While Virginia total precipitation hasnt changed this century, the fact that the U.S. total has would imply that there has been an increase elsewhere. Inspection of national records shows that indeed there has been an increase in the Southwest.
It is chic these days to say "well, this variable or that may not be changing in a fashion predicted by greenhouse changes, but the big picture reveals a suspicious fingerprint." That statement simply does not apply here in Virginia. Overall, theres just no evidence here pointing in that direction.
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