Scientists are well aware of the potential implications that climate has on these trees, what they aren’t aware of is the affect that the reduction in forest will have on the world’s ecosystems.   Trees act like giant lungs, taking in carbon dioxide and releasing oxygen.  Studies have shown that trees take in more than 50% of human-generated carbon dioxide and store it.   Therefore, if these big trees continue to die, there’s more carbon dioxide left in the atmosphere, which can lead to additional atmospheric warming.  Furthermore, if the trees are dead, they cannot provide the key nutrients, such as nitrogen or seeding, to the surrounding soil to allow the forest to re-establish itself after fire or windstorm.

Forest die-off can also affect things like surface moisture and climate classification.  Heat and drought affect each tree species differently, which can result in a long-term shift in the dominant species found in a location.  For example, a forest may become grassland.  This will also affect soil moisture, as there will be no tree canopy to intercept rainfall or prevent the exposure to harsh sun and wind.

But it goes further than that.  Trees provide homes to many different types of animal life, from mammals to birds and reptiles.  As the trees die, these animals are forced to look for a new habitat.   It is feared that as trees die, so will different species that rely on these old trees.

The GLOBE Program has protocols that can aide in the examination of how these forests are changing. Looking at land cover classification while taking air temperature and precipitation measurements can start the foundation for an exploration between climate change and land cover change.  The month of January features a repeat of the Climate and Land Cover Intensive Observing Period (IOP).  With that IOP, teachers and students are encouraged to classify their land cover as well as take photographs.  By keeping these records over the years, GLOBE schools can contribute to studies following forest mortality.

Suggested activity: Participate in the January Climate and Land Cover IOP by establishing or visiting your land cover site.  Submit your photographs and land cover classification to the GLOBE website.

-Jessica Mackaro

Cloud Forest located in Mount Kinabalu, Borneo Photo Credit: Nep Grower

Scientists became interested in how cloud forests work after they started studying some of the amphibians and migratory birds that live in cloud forests.  For a long time, a lot was known about the animals, but not about the vegetation that provided homes for all these animals.  This inspired a group of researchers from the University of California at Berkeley to research the cloud forests of Monteverde, Costa Rica. The cloud forests in Monteverde receive precipitation about 9 months out of the year.  During the other three months, Monteverde receives very little precipitation, but it does get fog.  Some parts of the forests will have fog for an average of 13 hours per day.  This fog forms when moist air from the Caribbean Sea condenses under the forest’s canopy.

A quetzal - a bird that lives in cloud forest trees Photo Credit: Drew Fulton (Canopy in the Clouds)

In order to study where the water in the trees comes from, scientists heated a spot on their branches and then tracked how the warmed water under the spot moved.  If the water moved towards the leaves, it came from the roots.  If the water moved towards the trunk, it came from the leaves.  After studying trees both in and out of cloud forests, the scientists found that the trees in the cloud forests could store 20% more water for growth via foliar uptake than the trees outside of the cloud forests.   Scientists had long suspected that the ecology of cloud forests was tied to the fog and low-level clouds, but not until this research was conducted were they able to say that cloud forests do obtain water via the clouds.

For more information, here is a research group in Costa Rica that studies cloud forests.

Some of our GLOBE schools are near cloud forests.  We would love if you could share your pictures and experiences via email to science@globe.gov or by leaving a comment.  Also, for all our GLOBE schools – we want to wish you a Happy New Year and remind you to always keep investigating!  You might find something amazing.

-Julie Malmberg

The current mountain pine beetle outbreak spanning Colorado and southern Wyoming (previously reported on in this blog) has spread to 4 million acres since 1996 making it the largest recorded epidemic in these areas. Tree canopy loss within stands of affected pine trees is a symptom of this epidemic. Part of the loss occurs during the “red phase,” wherein the trees still retain red needles, and the remaining loss occurs during the “gray phase,” wherein the trees have shed all of their needles and some small branches. While these visually dramatic effects of the beetle epidemic are evident, how it influences specific environmental factors, such as hydrology and transmittance of solar radiation, is less understood.

Healthy pine forest near Niwot Ridge, Colorado (Photo taken by Steve Miller/CIRES)

Example of a beetle-affected pine forest (Photo from the Denver Post)

Research is underway to comprehend any potential effects the pine beetle epidemic has on the environments in which it is prevalent.  One such project, which I worked on during my summer internship with GLOBE, focused on studying solar radiation in affected and unaffected tree stands. A change in solar radiation transmittance through the canopy can have effects on the snowpack accumulation (or ablation), surface sensible and latent heat fluxes, and other energy and water balance parameters. To quantify and compare the levels of solar radiation transmitted through healthy pine canopies with transmittance through trees affected by pine beetles, we used data from pyranometers (instruments that measure solar radiation) to analyze healthy sites and a red phase site.

Our study found that, on average, the ratio of the solar radiation recorded by sensors that were placed directly under the canopy (“under”) to those placed in nearby open tree clearings (“open”) was closer to unity at the red phase site.  This indicates that nearly the same amount of solar radiation was transmitted through the trees as was transmitted through the relatively treeless clearings.  On the other hand, this ratio was reduced at most healthy sites, indicating that less solar radiation was being transmitted through the healthy tree canopies.   Similar results were found by comparing the amount of solar radiation received at the top of the tree canopies (using pyranometer data from an instrumented tower) compared to that received below the tree canopies (the “under” sensors).

A pyranometer sensor under the tree canopy (Courtesy: Dave Gochis/NCAR)

Thus, our results imply that more solar radiation was transmitted through the red phase canopy, perhaps due to its loss of needles.  However, these results do not prove that pine needle loss caused more solar radiation to be transmitted.  There are several other factors that could have led to these results that need to be considered, such as forest tree density, instrument placement, and other site specific details (i.e. altitude, slope angle, etc).  Furthermore, additional sites need to be analyzed in order to more firmly establish these results and understand the variability within the various tree stands.  Nonetheless, if the tree stands affected by pine beetles do allow more solar radiation to reach the ground below their canopies, that could lead to more snow melting and ultimately have effects on water storage and the hydrological and energy cycles in the region.

When beetles kill the trees, they impact the local land cover.  Are there changes happening to your local land cover and if so, what effects of land cover change have you noticed in your community?  Send us an email at science@globe.gov or post a comment to let us know!

The Sahel zone in Africa (image courtesy of kidsmaps.com)

The Scientists found that the average temperature in the Sahel warmed by 0.8 degrees Celsius and rainfall decreased by as much as 48% between 1954 and 2002.  Due to the hotter, drier climate the trees started dying – one out of every six trees present in 1954 died by 2002.  And, not only did specific trees die, but whole species actually disappeared from this region; as much as one in five tree species were no longer present by 2002.  Fruit and timber trees, which require a lot of moisture and also are very important to local populations, were impacted the most.  In general, the scientists found that climate change is causing vegetation zones to shift, moving toward areas with more moisture.

A dead Ironwood tree in Senegal (photo by Patrick Gonzalez)

One of the Intensive Observing Periods (IOPs) of the Student Climate Research Campaign (SCRC) is the Climate and Land Cover project. During this campaign, students take photographs throughout the year and classify land cover areas (check out the land cover protocol) near their schools and upload these data to the GLOBE database.  These kinds of photographs are exactly the kind of data scientists need to study how land cover changes over time, including to monitor the presence of trees in various regions around the world.  GLOBE students can contribute to the database by participating in these IOPs, and help study trends in their local landscape and vegetation over time.

Have you heard stories (anecdotes) about land cover changing due to climate change near your school or home?  If so, send us an email or add a comment to let us know!  And, don’t forget to take part in the Climate and Land Cover IOPs!

-JSM

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.

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.

Figure 8. Me enjoying the relatively cool temperatures in a hole. The shade helped, too! Photo by Lorrie McWhinny.

Figure 9 shows how the temperature varied beneath a winter wheat field in south-central Kansas during the late spring-early summer of 2002. The temperature 7.5 centimeters below the surface (blue curve) reaches a maximum in the early afternoon, with the peaks slightly later as you go to lower levels. Note that the daytime temperature at 7.5 centimeters below the surface is warmer than that at 15 centimeters, and so on, with the coolest temperatures at 80 centimeters below the surface.

Figure 9. Soil temperature as a function of day of year for a winter-wheat site in south-central Kansas. The distances in cm (centimeters) indicate how far below the ground surface the measurement is being taken. Day 138 is 18 May, Day 150 is 30 May, Day 180 is 29 June. All data for 2002. Date collected and processed by Professor Richard Cuenca, Oregon State University. The maxima in the blue curve occur in the early afternoon.

These data, which are fairly typical, are consistent with our impression that the soil is usually cooler than the surface for most of the day during summertime. (The cooler surface temperatures on some days appear to be related to rainfall.)

The surface temperature for the same site appears in figure 10. Notice how the surface temperature peaks during the day about five degrees higher than at 7.5 centimeters during the first part of the data record, and then 10-15 degrees higher than the temperature at 7.5 centimeters late in the record. The change is related to cooling of the winter wheat (the sensor is measuring the temperature of the winter wheat) due to evapotranspiration during the first part of the record. Once the winter wheat stops growing and becomes golden, transpiration is no longer happening and the dry wheat and then the wheat stubble and ground surface are strongly heated by the sun.

Figure 10. For same site as Figure 4, except for surface temperature. Note that the wheat becomes golden (senescent, stops growing, is almost ready to harvest) around Day 150 (30 May).

The same happens to the bare ground at other sites – the surface is much warmer than the temperature 7.5 cm below the surface.
Have you been in a cave in the summertime? Caves, being farther from the surface, are even cooler. At the Devil’s Icebox, a cave not far from where I am writing this blog in Columbia, Missouri, the temperature stays at about 56°F (13°C) all year, even though the average summertime high temperature in Columbia is in the upper eighties (around 30° Celsius) and the average wintertime low temperature here is in the mid-teens (around -8° C).

So in the summer, the ground gets cooler as you dig down, — at least through the upper few meters. In the winter, the ground gets warmer farther down. And in caves, the temperature doesn’t change much at all. In fact, I once read that a cave temperature is a good first guess of the average above-ground air temperature at the cave’s location.

Similarly, people in many countries take advantage of the cool below-ground temperatures too store food during the hot summer. Also, some people take advantage of the temperature several feet below the surface to heat their homes in the winter and cool their homes in the summer.

If you look at a stream, it is surrounded by higher ground. If you include all the ground that is feeding that stream, this area is the stream’s watershed. Let’s think a bit about what we know regarding watersheds:

1. If the amount of precipitation is the same, more water flows out of bigger watersheds. Like bigger roofs shed more water.
2. In big watersheds, it takes time for the water to flow out. Unlike our roof example, we cannot assume that the flow will be fastest when there is heavy rain upstream. In fact, it will take awhile for the rain to reach the outlet or mouth of the watershed.
3. If the rain is light and the ground is dry, the ground might “soak up” the rain – and you might not see a change in the river or stream flowing out of its watershed. (This works in our roof example only if you are unfortunate enough to have a very leaky roof!).

Figure 4 shows an example of a watershed that I have studied. This is the Walnut River watershed, southeast of Wichita, Kansas. All the water falling on the area outlined flows into the Walnut River, which empties at the bottom of the picture.

Figure 4. The Walnut River watershed, which lies east of Wichita, Kansas, USA. Gray lines are contours; a red solid line outlines the watershed, and blue solid lines show the Walnut River and its tributaries.

This watershed measures 100 km from north to south, and about 60 km from east to west – a lot larger than the roofs.

A few years ago, scientists at Northern Illinois University and Argonne National Laboratory estimated the water budget of this watershed.

What does this mean? In simple terms:

All the water coming into the watershed = All the water going out of the watershed

It would be easy to do this if all you had to worry about was:

Rain falling in the watershed = Rain flowing out of the watershed

But it’s much more complicated, since

1. The water might evaporate before it leaves the watershed, and
2. Water will soak into the soil, and
3. Water might flow deeper underground – that is, the watershed might “leak.”

In fact, all of these things happened in the Walnut River watershed. And these things can be complicated.For example, the amount of water evaporating or soaking into the soil changes with the surface. In Figure 5, grasslands are shown in green. Much of the area not colored is covered with crops (mostly winter wheat).

What would happen to rainfall in the late summer when the grass covers the ground but the winter wheat has been harvested? Do you think more water will run off the fields of harvested wheat? What about plowed fields?

Figure 5. Contour map of the Walnut River watershed (outlined) with grasslands shaded in green.

Now think about the cities and towns. Figure 6 shows some of the larger towns in the same area.

Figure 6. Map showing the upper (top) and lower (bottom) Walnut River watershed. Each small square is a mile (1.6 km) on a side.

Everywhere there is yellow, there is a city. The big yellow spot to the left (west) is Wichita. Much of Wichita is covered with concrete. Now think about what happens with a heavy rain? How much water would soak into the ground in Wichita, compared to the grassy areas farther to the east?

Now, think about the future. How much water would soak in if there were more cities, and the cities were bigger?

Clearly, what we do can affect the amount of water leaving the watershed.

Think about the watershed where you live. And watch for more about watersheds in the future: Visit the GLOBE Watershed Dynamics Earth System Science Project (ESSP) to find out more about this project and announcements for upcoming workshops. This project will be examining the flow of water through a watershed and how humans are impacting runoff and stream flow.

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.

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?