As Dr. C wrote during his Surface Temperature Field Campaign, the weather in mid-December was cold in many parts of the United States. At our house here in Boulder, Colorado (Figure 1), this morning’s minimum temperature was -21 degrees Celsius. Just 20 kilometers east of here, the minimum temperatures was 27 degrees Celsius below zero, and about 50 km to the southeast of here, the minimum temperature reached -28 degrees Celsius. The weather reports were saying that those of us who live closer to the mountains weren’t having temperatures as cold as those to the east of us.

Figure 1. Map showing location of Boulder and CASES-99. The colors represent contours. The Rocky Mountains are yellow, orange, and red on this map. The colors denote elevation, with yellows, oranges and reds indicating higher terrain.

How does the air temperature relate to the surface temperatures that the students measured during Dr. C.’s field campaign? To answer this question, I looked at how the surface temperature related to the air temperature at our house.

The air temperature at our house was measured at 1-1.5 meters in our carport, and also on a thermometer I carried with me on our early-morning walk around the top of our mesa. That temperature, as noted above, was -21 degrees Celsius. To get the surface temperature, I put the thermometer I was carrying on the surface after I finished my walk. I am assuming that this temperature is close to the temperature that would be measured by a radiometer like the one used in GLOBE. I took the reading ten minutes later.

Just for fun, I also measured the temperature at the bottom of our snow (now 10 cm deep) and at the top of the last snow (about in the middle of the snow layer). At these two places, I put the snow back on top of the thermometer, waited ten minutes, and then uncovered the thermometer and read the temperature. The new snow was soft and fluffy, while the old snow was crusty; so it was easy to find the top of the old snow.

All of the measurements were taken close to sunrise, when the minimum temperature is normally reached, and the area where I took the measurements was in the shade.

Figure 2 shows the temperatures that I measured.

Figure 2. Temperature measurements at the snow surface, between the old and new snow, at the base of the snow layer, and at 1-1.5 meters above the surface at 7:30 in the morning, local time.

That is, the temperature was coolest right at the top of the snow. The temperature was warmer at the top of the old snow, and warmest at the base of the snow. As noted in earlier blogs, the snow keeps the ground warm.

The temperature at the top of the snow was also cooler than the air temperature. The surface temperature is often cooler than the air temperature in the morning, especially on cold, clear, snowy mornings like this one. However, on hot, clear, days in the summertime, the ground is warmer than the air.

Here are two sets of measurements taken in the Midwestern United States in October of 1999. Could you guess which measurements were taken at night, and which measurements were taken during the day even if the times weren’t on the labels? The first plot is from data taken after sunset, while the second plot was from data taken at noon.

Figure 3. Data from the 1999 Cooperative Atmosphere Exchange Study (CASES-99) program in the central United States, courtesy of J. Sun, NCAR.

According to the most recent report by the Intergovernmental Panel on Climate Change, land use change has a relatively minor impact on the recent rise in global average temperature.

Yet, as stressed in an earlier set of blogs on Iowa Dew Points and an apparent increase in stormy activity regionally, land use seems to be quite important at local and even regional scales.

Why the difference?

According to an article by Raddatz in a recent issue of Agricultural and Forest Meteorology, about 3.6% of Earth’s surface is covered in crops, and about 6.6% is in pasture. Figures 1 and 2 show what these percentages look like. Even if all the temperature trends associated with changes in land cover were all in the same direction, it would require large changes indeed to show up significantly in the average global temperature for a whole year.

Figure 1. Fraction of Earth’s surface covered with crops, rounded up to 4%.

Figure 2. Fraction of Earth’s surface covered with pasture, rounded up to 7%.

Further, crops grow actively only part of the year. So, for example, winter wheat or corn will lead to cooler maximum temperatures during their growing season. (Recall this is because more of the sun’s energy hitting the surface is going into evapotranspiration and less into heating). During the rest of the year, the stubble or plowed ground would have a different effect from surrounding green vegetation. In fact, the dormant fields could be warmer than grasslands if the grasses are green. Thus it is not surprising that converting natural land cover to crops or pasture does not always have the same effect on temperature change. In some areas, there is a cooling effect (e.g., if more moisture is being evaporated or transpired, or if more sunlight is reflected), while in other areas, there is a warming effect (e.g., more sunlight absorbed, less evaporation or transpiration). And finally, as pointed out in the Iowa Dew Points blogs, regional changes in land cover could have an indirect effect, like shifting the wind patterns. This can in some cases decrease the local influence on temperature. Figure 3 illustrates the “mixed” effect of crops.

Figure 3. Possible scenario showing effects of crops on the global average temperature, with the “red” representing a net warming effect over the whole year, and the “blue” representing a net cooling effect. This is only to illustrate that the net effect of crops will be mixed, rather than saying what the effect will be.

As noted in earlier blogs, cities also affect the temperatures, but they occupy a tiny fraction of Earth’s surface.

This does not mean that land use isn’t important. We live on land, which occupies only 30% of the globe. If we change our percentages of Earth’s surface to percentages of Earth’s land surface, they become bigger – 3.6% of Earth’s surface becomes 12% of Earth’s land surface, and 6.6% of Earth’s surface becomes 22% of Earth’s land surface! Furthermore, we humans aren’t evenly distributed: there are vast parts of Earth that are uninhabited. Human influence on land cover is where humans are. And, of course, those seasonal effects on temperature are important to us if they are happening where we live.

Figure 4. Fraction (12/100) of land surface on Earth covered with crops.

Figure 5. Fraction (22/100) of land surface on Earth covered with pasture.

Climate researchers know this. You have mainly heard about the predicted average annual temperature because it is the easiest “measure” that sums up all the details in one convenient number. This is particularly helpful in sending the message to governments, businesses, and individuals that Earth is getting warmer. Once you think about how this will affect how you live, one temperature is clearly not enough. You want to know how the temperature varies seasonally and where you live.

Also, climate models cover both a large area (the whole Earth) and a long time (decades), so they are expensive to run and require the biggest computers. And, they have to account for the various things that affect climate from the outside – the variability in the sun, the variation in greenhouse gases and particles, etc. By comparison, weather models are run for at most around 10 days or so. To make the runs doable, climate models work on points too far apart to really represent smaller-scale atmospheric motions (”weather”), smaller-scale regional effects (such as vegetation changes), and even terrain.

Some climate scientists have looked at regional climate changes by running weather-type (regional) models to describe what’s happening at the boundaries of a smaller area – such as the United States. (If the model only covers the United States, it still has to account for what the wind brings from the outside!) These models have taught us something. However, there is a worry that the climate models supplying the boundary information are themselves faulty because the effects of “weather” could affect the details of the local wind, temperature, etc.

So, in spite of the enormous costs, climate scientists are just now starting to run climate models on computers or clusters of computers working together that are powerful enough that global climate models can represent these regional changes better. The first such computer system was the Japanese Earth Simulator. It enabled model runs with grid points spaced at one-tenth the distance of most climate models. As more and more people start focusing on regional climate, and more computers with the capability of the Earth Simulator become available, the issues of our effect on Earth’s surface will be studied more intensely.

And our discussion didn’t even consider more long-term effects, such as the effect of land use on changes in greenhouse gas concentrations! Nor have we considered the effects of forests.

Reference: Raddatz, R.L., 2006. Evidence for the influence of agriculture on weather and climate through the transformation and management of vegetation: Illustrated by examples from the Canadian Prairies. Agricultural and Forest Meteorology, 142, 186-202.

[This blog reflects the help of many friends and colleagues. The story of how it developed shows how science research often works. The idea for this blog came from Professor Peter Blanken of the University of Colorado, who took advantage of a beautiful autumn day to take his biometeorology class outside so that the students could measure and compare the temperatures of yellow leaves and green leaves. I found out about Professor Blanken’s field trip from Joe Alfieri. Joe, a graduate student from Purdue University, was visiting me for a few weeks. Joe and I decided to have our own field trip, using the infrared thermometers we used to measure puddle temperatures. When Joe and I told a colleague here at NCAR, Jielun Sun, what we were doing, she suggested we borrow an infrared digital imager from Janice Coen, an NCAR scientist who uses it to look at forest fires. Sean Burns of NCAR and Jielun showed us how to use the camera. Joe processed the images and produced the figures. They are all gratefully acknowledged.]

What happens to leaves when they change color? Leaves are green because of chlorophyll, which is involved in photosynthesis. In photosynthesis, sunlight, carbon dioxide, and water are turned into glucose, which is used by the plant. In the autumn, as the days get shorter, photosynthesis stops and the chlorophyll disappears, leaving behind other materials that give the leaves their color.

As noted in a previous blog, shutting down of photosynthesis in the Northern Hemisphere autumn actually shows up as an increase in the carbon dioxide in our atmosphere (Remember – there is more land – and trees – in the Northern Hemisphere). GLOBE’s Seasons and Biomes Project and Carbon Cycle Project (found under the “Projects” drop-down menu at www.globe.gov) are both interested in the seasons and how they affect the earth system.

Green leaves also give off water vapor in a process called transpiration, which is a fancy name for evaporation from plants (mostly from leaves). When leaves open their “pores” (stomata) to allow carbon dioxide to enter for photosynthesis, water evaporates. Yellow leaves don’t transpire. Does this mean that the temperatures of green leaves and yellow leaves are different?

Blanken thought that the color of the leaves would affect their temperature. Joe Alfieri and I thought so too. But how much? We decided to go outside and measure leaf temperatures ourselves.

We still had the infrared temperature sensor from when we measured puddles. We used the sensors to measure leaf temperature. We found trees near the office with both yellow and green leaves, and measured the temperatures of individual leaves. We measured leaves in pairs – one yellow leaf and one green leaf for each tree. We measured leaves on “weeds” as well.

The GLOBE infrared thermometer wasn’t working, so we used another one. (More information on the GLOBE infrared temperature (”surface temperature“) protocol can be found at www.globe.gov in the “Teachers Guide”, in the drop-down menu for “Teachers”). We had compared it to the GLOBE instrument earlier and found the temperatures were off – but the temperature differences were the same for both instruments. So I will discuss temperature differences rather than actual temperatures. From the weather station at our building, the temperatures on all three days were between 20 and 25 degrees Celsius. We took data for red and brown leaves as well, but there were so few I am including only the yellow and green ones. Measurements were made during the last two weeks of September.

The first day, it was sunny. We knew that leaves in full sunlight would be warmer than leaves in shadow, so we tried to compare leaves that were either both in shade, or both in full sunlight. On this day, the yellow leaves were on average 1.6 Celsius degrees warmer than the green ones.

The second day was mostly cloudy with low clouds blocking the sun, making it easier to get leaves exposed to a similar amount of sunlight. On this day, the yellows were on average 1.2 Celsius degrees warmer than the green ones.

The third day was cool and windy. We found we had to hold the end of a leaf to measure it. Otherwise, the leaf would blow around and we couldn’t get a good reading. The yellow leaves were on average 1.3 Celsius degrees warmer than the green ones.

The measurements varied a lot for all three days. Differences varied from -0.2 Celsius degrees (green leaf warmer) to 7 Celsius degrees. Part of the reason for this variation is that some of the leaves were more shaded than others. Also, leaves directly facing the sun tend to be warmer. (If a leaf is oriented so its edge faces the sun, it will be cooler. I had a friend who really really liked to sunbathe. He found out that he could stay outside in cooler temperatures by tilting himself so that his body was directly facing the sun). Wind might make the temperature differences smaller. In spite of these factors, the yellow leaves were between 1 and 2 Celsius degrees warmer than the green ones.

The figures below show the same leaves photographed with an ordinary digital camera and an infrared camera (more properly, infrared imager) that scientists use to measure the temperatures of fires, trees, and surfaces. In the figures, the blue colors mean cooler temperatures than the yellow ones, which are cooler than the reds. Like our measurements, the infrared camera is “seeingâ€? yellow leaves as warmer as well. Notice that the stems are warmer, too, especially the thicker ones. We didn’t calibrate the camera exactly, but estimate the temperature difference between the yellow and green leaves to be about the same as we observed.

Figure 1. Poplar leaves photographed using a digital camera.

Figure 2. Same leaves, photographed using the infrared imager.

Why are the yellow leaves warmer? Remember that leaves lose water during transpiration. This means that the water turns from a liquid to a gas – it evaporates. Just as perspiration evaporating from our bodies keeps us cool, the water escaping from the leaf cools it off a little bit. It takes energy for molecules to escape a liquid to become part of a gas – and this energy loss is what cools the leaf. The same thing happens to you getting out of a swimming pool or shower – you are cooled off as the water on your skin evaporates.

POSTSCRIPT. If the leaves are cooler because of transpiration from open stomata, Sun hypothesized that yellow and green leaves should have the same temperature in the early morning, before the stomata open up. To test this, Blanken took leaf-temperature measurements at 7 a.m. Local Standard Time, 50 minutes after sunrise (6:10 a.m. Local Standard Time), on 15 October 2007, when the temperature was 4.5 Celsius degrees, relative humidity ~90+%. He found that the yellow leaves and the green leaves had about the same temperatures. This could be because the stomata are closed. However, the low temperature and high relative humidity that morning would reduce the evaporation rate, so even if the stomata were open, the leaf-temperature differences would still be small. In either case, the lack of temperature difference is related to little or no evaporation.

Recently, I posted the Hawaii record that showed that carbon dioxide has been increasing for the last several decades. To make the plot consistent with the global temperature plot, I showed only annual averages. Now, I show a copy of that same plot with seasonal information included.

Figure 1. Concentration of carbon dioxide at Mauna Loa, Hawaii (inset). NASA graph by Robert Simmon, based on data provided by the NOAA Climate Monitoring and Diagnostics Laboratory. Image from earthobservatory.nasa.gov/Newsroom/NewImages/…

This curve, which may be more familiar to many of you, has lots of wiggles. To look more closely at the wiggles, I obtained some data from the WLEF tower in Wisconsin, taken at 396 meters above the ground. The wiggles in Figure 2 show lots of variation from year to year, but there is a pattern. We can see the pattern easily if we average the data. During the winter, the carbon dioxide values are high. The values fall in the spring, and are smallest in July. By August, carbon dioxide values are increasing again.

What is happening? The WLEF tower is in a forest. During the spring and summer, the trees use up carbon dioxide in photosynthesis. As the trees leaf out, the carbon dioxide decreases. Once summer comes, photosynthesis starts slowing down, and so does carbon dioxide uptake. Like animals, both trees and the soils give off carbon dioxide in respiration. The curve shows the net effect of respiration and photosynthesis.

The carbon dioxide the tower measures does not just come from the forest – it can come from hundreds of kilometers away, and from grasses, shrubs, and crops as well as trees. Like the trees, these plants are also exchanging carbon dioxide with the atmosphere.

Figure 2. Monthly average flask values of CO2 from 396 meters above the surface. The inset shows the average for the ten years shown, to emphasize the change with seasons. Data collected by NOAA ESRL and The Pennsylvania State University and supplied by Ankur Desai (Dept of Atmospheric & Oceanic Sciences, University of Wisconsin-Madison).

Figure 2 is detailed enough to show lots of wiggles that don’t follow a smooth seasonal pattern. As the winds change, air with higher or lower values of carbon dioxide might be brought in. Where would carbon dioxide values be highest? Combustion produces carbon dioxide, so there will be higher values where there are lots of cars, factories, or fires. When trees are leafing out and growing, the carbon dioxide will be taken up. So it is possible that sharp peaks may be for times when the wind was bringing carbon dioxide from an area with lots of cities. Have you ever seen data on how much carbon dioxide is in the air near you?

Do you ever hear something you just can’t stop thinking about? About ten years ago, I heard a talk by Roger Pielke, Sr., where he compared the weather over northern Texas for two days, 100 years apart. The weather – high and low pressure areas, temperatures, and humidity – started out the exactly the same on two days in the world created by his version of a computer weather forecast model. But the land was different. It’s like someone took the weather from a 1991 TV weather map and put the highs, lows, and fronts on the United States of 1881, when there were only 44 states, and the land was prairie. And there was no television.

He picked a day when he knew there had been a strong thunderstorm – it had been observed and very carefully documented in 1991 by scientists during a field program. Since the storm already happened, he knew where the storm would form. And it’s a common place for storms to form – along a line where warm, moist air from the Gulf of Mexico meets dry hot air from the southwest United States. This line is called the “dry line.” It commonly forms in northern Texas. Storm forecasters watch the dry line closely to see whether new storms are forming.

Why the dry line? When air from two places flows together, something has to happen. The air can go sideways and “squirt” out of the way, or it can go up, or it can move sideways and up. This isn’t enough detail to know whether a storm will form, though. For a storm to form, the air has to go up over a small area, somehow. And it has to be moist enough to make a cloud.

That’s just the first step—the rising air cools. When the air gets cool enough, water starts condensing around tiny dust particles, and you get a cloud. If the air continues to rise – which it will if it is lighter (warmer) than the air at the same level outside the cloud – a very big cloud can form. And rain. Add to the mix a little wind change with height – we want the cloud to tilt over so the rain won’t fall into the updraft and kill the cloud. There is a very nice animation of a cloud forming on the GLOBE online Cloud Module.

We use computer models that describe how air behaves to predict where and how much air will rise. The model used by Pielke and his colleagues also describes how the soil and plants heat and moisten the air.

In the first model run, he put prairie into the model. In the second run, he put modern land use – including irrigation. After very carefully making sure that the beginning conditions are similar, they ran the model twice, once with the “old” land use and once with the “new.” The results were surprisingly different.

The “modern” computer run formed the strong thunderstorm that was observed. But the 1891 computer run formed much smaller clouds. Why?

The “modern” surface types heated at different rates. This leads to patches of warm air that start rising; and cool patches of air that move in to take their place. A “circulation” is set up with the updrafts over the warm region. This circulation helps get an updraft – and hence – a cloud, started in one place. This means the moist air near the surface can go up in one place, instead of many, concentrating the “energy” into one big storm; in this case, right on the dryline, where the air was already flowing together.

Next time – are there more storms than there used to be?

Thanks to Roger, Pielke, Sr. for supplying with journal articles on this topic.