Last week, my husband and I went hiking in Montana, looking for marine shelled fossils and birds along the way. We were in much the same place where we saw all the forest fires last year (see blog on Forest Fires).

This year, we found almost no fossils for several days. Yet, I took careful notes. Why?

• We thought there should be fossils there (our hypothesis).
• We didn’t find any (disproving our hypothesis).

So we learned something from the experience. From geologic maps, we can identify the type of rock (Bearpaw Shale). We knew that fossils form the nucleus of rocks called “concretions” in the Pierre Shale. There was Bearpaw Shale. There were concretions. But the concretions had no visible fossils.

From a personal point of view, this tells us not to expect to find too many fossils in this area.

From a geologic point of view, this poses the question – why are there so few? This fascinating question probably has a two-part answer. The first part involves where the creatures lived (Was the water too deep?). The second part involves whether the creatures were fossilized (Did the shells dissolve? Did they get crushed? Were the animals eaten?).

Null – or zero – results were also important in GLOBE’s past Contrail Count-a-Thons. If you see a contrail, and report it as “non-persistent”, “persistent,” or “persistent spreading,” that is valuable information. It tells us that there was enough water vapor for the contrail to exist. Indeed, the findings of former GLOBE PI Lin Chambers show – not surprisingly – that more humid air at jet altitude means longer contrails that can spread horizontally.

But a report of no contrails is interesting because it indicates very dry air – assuming that the observer is below where jets are known to be flying.

Another example of important “zero” observations is for precipitation. According to Nolan Doeskin of the Community Collaborative Rain, Hail, and Snow network, an observation of “no precipitation” is very important. “No rain” for a long period of time – or drought is significant information if you need water, which we all do.

Of course a “zero” observation doesn’t always mean that there is nothing there. As someone once said…

“The presence of absence doesn’t necessarily mean the absence of presence.”

One of the best examples of this involves life on other planets.

If I go to Mars and find no life, life could still exist there. Why? Because:

• I could have been in the wrong place (as in the fossil example).
• Or I could have not realized what kind of life I should be looking for.
• Or I could have been looking for it the wrong way. (maybe I needed a microscope, or some chemical test).

So if you are recording weather data for GLOBE, for CoCoRaHS, for some other organization, or for yourself – don’t forget to record the “boring” weather as well as the interesting weather.

(Note: We discuss carbon dioxide because it contributes to slightly over half of current greenhouse warming, but we must remember that methane, CFCs, ozone, and nitrous oxide together account with slightly less than half).

When I was a graduate student at the University of Washington, learning about weather and climate, I thought climate was boring, compared to tornadoes or thunderstorms. You averaged the temperature, rainfall, or wind, or – whatever – to get the climate of an area.

This changed around 1970, when I saw someone give a talk on the disturbing fact that the carbon dioxide in our atmosphere was increasing at a site on Mauna Loa in Hawaii. The speaker told us that he thought it was possible that this might make Earth’s climate warmer over time. (marked as “1″ on Figure 3). This was truly amazing! To me, the percentage of carbon dioxide was one of those numbers you memorized for class, like the conversion factor from Fahrenheit to Celsius. It was not supposed to change.

A few months later, another speaker said that the average global temperature, as far as he could see, would go down for a year or two after a volcanic eruption spewed dust into the stratosphere, and then warm up after the dust settled out. He didn’t think much else was happening.

Figure 3. Yearly average carbon dioxide concentration collected at Mauna Loa Observatory, Hawaii, USA. Data from C.D. Keeling and T.P. Whorf, and the Carbon Dioxide Research Group, Scripps Institute of Oceanography. Ppmv = parts per million by volume. For example 300 ppmv means that out of 1,000,000 molecules in the mixture of gases we call air, 300 are carbon dioxide.

In the meantime, the carbon dioxide kept increasing. In 1997, I worked for the first time with scientists who were measuring how much carbon dioxide was going between the surface and the air at a site near Wichita, Kansas, USA. To take this measurement, we needed a reference value for carbon dioxide, and we used “360 ppmv” (parts per million by volume). From this graph, we were clearly behind – the mean value had already gone up to 364 ppmv. In 2002, we took similar measurements in the same area, and I was surprised to see how much the value had changed in only five years.

The amount of carbon dioxide in Earth’s atmosphere has changed a lot over geologic time.

• At the end of the Permian period (about 250 million years ago) scientists have estimated that carbon dioxide in the atmosphere was a high as 10 times what it is today.
• During the mid-Cretaceous period, the dinosaurs also lived in a “greenhouse” world. Again, scientists estimate carbon dioxide could be as high as ~10 times what it is today.

Geologic evidence supports a warmer climate in both cases, especially in Polar Regions. This has a lot to do with the large changes that take place when snow disappears.

The gases in the air important to climate change are called “greenhouse gases.” To understand what a greenhouse gas does, you first have to understand a little about radiation. Not the science-fiction stuff that changes cockroaches into giant monsters that destroy cities, but the radiation we deal with every day - the sunlight that sustains life; the heat given off from a light bulb or a fire or even your friend. The radiation from the light bulb, a fire, or your friend, is called “infrared” radiation, or “long-wave” radiation. This is the radiation we measure with the infra-red instrument in the GLOBE Surface Temperature Protocol.

Most of the atmosphere is nitrogen and oxygen. These gases don’t interact significantly with infrared radiation. But the greenhouse gases do. A greenhouse gas molecule absorbs infrared radiation coming from one direction, gets energetic, and then re-radiates energy in all directions. Water vapor, which can account for up to 4% of the volume of air near the surface, is a greenhouse gas. There are many other important greenhouse gases, listed next to the pie chart in Figure 4. These gases all have molecules made up of three or more atoms. Nitrogen and oxygen are made up of only two atoms.

In the figure, we assume water vapor is 3% by volume. This is fairly humid even for surface observations. As you know from weather broadcasts, the water vapor content of air changes from day to day. And some parts of Earth are more humid than other parts. The 3%-by-volume value is reached near the surface in the Tropics, and in the mid-latitudes on very hot and humid days. In other areas, like the Sahara desert, the air is much drier. Cold air has less water vapor than warm air.

Figure 4. Gases in the Air (by volume) near Earth’s surface, for 3% water vapor content (corresponds to 68% relative humidity at 29°C or 19 gm water vapor per 1 kg dry air). CFCs are Chlorofluorocarbons, which have been shown to destroy ozone in the stratosphere.

How does a greenhouse gas make Earth warmer?

Imagine the sun heating Earth. The sunlight mostly goes right through the lower atmosphere and heats the ground. The ground warms up, and heats the air near the ground, mostly by warm air currents rising from the ground. These air currents cause distant objects appear to waver on hot summer days.

This warm ground and air give off infrared radiation. Greenhouse gas molecules overhead intercept this energy before it escapes to space, and re-radiate it in all directions. This means that some radiation still goes up – but some goes down as well. The net effect is less radiation – and therefore less energy – escaping to space. Too see an animation of how a greenhouse gas works, see LEARN: Atmospheric Science Explorers and look for the greenhouse effect.

The major greenhouse gases in Earth’s atmosphere are carbon dioxide and water vapor, and both occur naturally. If it weren’t for these gases, the Earth would have been much cooler. The amount of water vapor in Earth’s atmosphere probably hasn’t changed much for a long time. Water cycles through the air quickly, largely because it so easily condenses or freezes and turns into rain or snow. But gases like carbon dioxide cycle through the atmosphere slowly. This means the fraction of air that is carbon dioxide also changes slowly.

The problem today is that human activity is increasing the carbon dioxide content in the air. There is also more methane and nitrous oxide than there used to be.