[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.

During the last several blogs, I’ve written about how humans affect climate locally. Today, I am writing about young people noticing things locally, but many of these changes are related to global changes.

I was privileged to be the moderator for the International Polar Year Pole-to-Pole Videoconference, which was coordinated by GLOBE for the Seasons and Biomes Project. For a transcript of the event, see the transcript page. The Seasons and Biomes Project, which is based in the University of Alaska at Fairbanks, is teaching students how to notice changes in seasons in their biomes –- and how the seasonal markers are changing; e.g. budburst, green-up, green-down, freeze-up and break-up. The videoconference brought together scientists studying both the Arctic and the Antarctic, with students from both the Arctic and Antarctic.

Where? The Antarctic scientists and students were in Ushuaia, Tierra del Fuego, Argentina, on the extreme southern tip of South America. The Arctic scientists and students were from Fairbanks, in the middle of Alaska, and Healy, about 200 kilometers southwest of Fairbanks. Fairbanks is at 64.84°N, and Healy is at 63.97°N, and close to Denali National Park, which is named after the highest mountain in North America. Ushuaia at 54.8°S, is the southernmost city on Earth, only 1000 km from Antarctica, and it has a ski area with such good snow that many Olympic teams go to practice there.

Why now? The videoconference honors the beginning of the International Polar Year (IPY, http://www.ipy.org), which runs from 1 March 2007 to March 2009. IPY is dedicated to science related to the Earth’s Polar Regions. There have been three earlier International Polar Years, with the last one in 1957-1958. But scientists see this International Polar Year as especially urgent, because there are big changes at the poles. The average temperature at the Earth’s surface has been rising over the last century. And, while not all parts of Earth have been warming (some parts are actually cooler!), the poles are warming more than anywhere else on Earth. This warming has meant big changes in polar regions -– the permafrost is thawing out, damaging houses and roads due to erosion, the ice sheets are becoming smaller and thinner, threatening the polar bear’s habitat, and affecting the lives of many people. Melting of ice on land has increased sea level, which is starting to affect people in coastal areas around the world. And, the changes in the ice and the land surface can affect both ocean currents and weather and climate patterns.

During the web chat, we heard from four scientists in Alaska (Dr. Elena Sparrow, Dr. Dave Verbyla, Dr. Javier Fochesatto, and Dr. Derek Mueller), two scientists from Ushuaia (Dr. Gustavo Lovrich and Sr. Daniel Leguizamon), one scientist from the U.S. National Science Foundation (Dr. Martin Jeffries), and one scientist from the U.S. National Snow and Ice Data Center (Dr. Walt Maier). These scientists answered questions from the students, who were from three schools in Fairbanks (Pearl Creek, Moosewood Farm, and Effie Kokrine), the Healy school, and the school in Ushuaia. Then, the students asked each other questions about the weather and climate in their areas, whether the climate seemed to be changing, and things that students were doing to reduce their impact on climate change.

Some questions were simple but fundamentally important -– “What is it like in the Antarctic in December?” It’s easy to read in a book that the Southern Hemisphere has summer while the Northern Hemisphere has winter, but it’s a fact many forget when thinking about what causes seasons -– especially those of us who haven’t had the chance to feel winter while talking to someone on the opposite side of the Earth feeling summer!

Many questions were about how the environment was changing -– we heard about the number of polar bears declining, but that coyotes and magpies (a bird) were coming farther north than in the past. At the same time, krill and the animals that eat krill, like the whales, are declining in numbers around the Antarctic. One student pointed out that the ice hockey field in Healy was thawing out in February, when the temperature reached 55°F (13°C, unusually warm for that time of year in Healy).

Are all these changes related to warming in the Polar Regions? This question wasn’t answered for everything, but the decline in whales around Antarctica seems related to warming. Dr. Lovrich, who studies a 5 cm long shrimp-like animal called krill, described a strong link to warming. During the winter, when the sun is low in the sky and the days are short, krill feed on algae that grow underneath the sea ice. There is less sea ice compared to previous years, hence less algae available during winter for krill. This translates to less food for animals that eat krill e.g. penguins, seals, and whales. Populations of these animals would be adversely affected.

And, students talked about other causes of changes near the poles -– such as what high values of ultraviolet radiation, resulting from the ozone hole over Antarctica, might do. (The Alaska scientists noted ozone was affected in the high northern latitudes, but much less.) Also, the students in Ushuaia noticed changes in where beaver and foxes live, but this could be related to more houses where forest used to be.

There was much talk about the relationship between ocean currents and climate. Both salt content and cooler temperature make water denser. The ocean current associated with the Gulf Stream (east of the US) sinks at high latitudes as it cools. If the water doesn’t sink; the current cannot continue, and the water “backs up” and goes nowhere, just like water in a sink with a clogged drain. No northward current and the areas whose climate is warmed by the Gulf Stream cool off -– especially Western Europe.

An additional surprising effect: The oceans take up carbon dioxide, meaning lower amounts in the air to warm our climate. When you have ocean currents sinking downward; the carbon dioxide is carried down with them. And somewhere else, water rises that is low in carbon dioxide, since it hasn’t been at the surface for a long time, so more carbon dioxide can be absorbed. If there is no more fresh water arriving at the surface to absorb carbon dioxide, the ocean absorbs less carbon dioxide. That is -– stopping the Gulf Stream means a faster rise in carbon dioxide in the air.

In the Antarctic, a big cooling in temperature happened 13 million years ago. This is the same time the Antarctic Continent broke off from South America. This enabled ocean currents –- and air currents -– that isolated the continent and made it the coldest place on earth.

Finally, students talked of things they could do to slow the increase of carbon dioxide that is warming our planet. They noted simple things that we can do every day, like recycling, walking places, and carpooling.

How do seasons affect the environment where you live? Have you noticed how seasons change from year to year? Have you talked to your parents about how things change? Think about those changes and what might be causing them. Keeping track of such changes is one of the main goals of GLOBE.

By the calendar, Spring in the Northern Hemisphere began on 21 March. At our house, we heard the first robins singing on 27 February. Snow is still possible in Colorado for a few more months, but the air temperatures are, on average, getting warmer. The trees are leafing out.

On the GLOBE web site, you can see some reports of budburst. These reports should creep northward in the Spring in the Northern Hemisphere. Similarly, you might also look for how far north the Rubythroated Hummingbirds have gone (also see www.rubythroat.org). Most importantly, look for other signs of Spring’s arrival (or Autumn’s arrival) at your home or school. From my time working in the tropics, the biggest changes seem to have to do with rainfall.

Want to see the seasons from space? A recent issue of EOS, which is published by the American Geophysical Union, points to a web site — bluemarble.nasa.gov, where you can do just that.

I found it most interesting to compare the winter image to the summer image. Looking at where I live, in the west-central United States, the ground appears brownish when there is no snow. This fits with what I saw outside. The grasses were dormant, and the trees had lost their leaves. In the summer, the same area is green, because the satellite, like us, “sees” the leafed-out trees and green grasses. How do things change where you live? How does it compare to what the satellite image shows?

These images are from the Terra MODIS (MODerate-resolution Imaging Spectroradiometer), and they have a resolution of 500 meters. This is big enough to see a big lake, but not a football field. Before the images could be produced, the effects of clouds, dust, haze, and even the air between the satellite and the surface had to be removed.

Happy Earth Day!