Climate Change Part 3. The gases in air

Posted on 08/07/2007 by peggy

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.

Gases in the air by volume, near Earth's surface

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.

Posted in Atmosphere, Carbon, Climate Change, Earth System Science | 5 Comments

What affects Earth’s climate?

Posted on 07/16/2007 by peggy

When I arrived at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, in the 1970s, I started hearing the debate about whether the global climate was getting warmer or cooler. From the “warming” graph in the previous blog, there wasn’t much of a trend at that time.

On our bulletin board, we posted two articles next to each other. One was about an NCAR scientist who said that Earth’s climate was getting warmer because of more carbon dioxide in the atmosphere. The second was about a scientist who said it was getting cooler because of changes in Earth’s orbit and tilt. The changes, he argued, could mean the beginning of a new Ice Age in a few hundreds of years or so. Beneath the two articles, we posted Robert Frost’s Poem “Fire and Ice,” because it starts as

“Some say the world will end in fire,
Some say in ice.”

Surrounded by talk about climate, I retreated into the library and found a wonderful book that was published in the early 1900s, and I realized I hadn’t heard much new about the factors that affect Earth’s climate – the ingredients were:

  • How much sunlight is available to heat Earth
  • How much heat escapes to space

Which are related to:

  • What gases are in the atmosphere (especially carbon dioxide and water vapor)
  • How much dust there is in the atmosphere (including that resulting from volcanoes)
  • How much ice there is on Earth’s surface (since ice reflects light back to space)
  • Where the continents are

And, as I mentioned earlier under “regional climate”

  • Properties of the rest of Earth’s surface (other than ice cover)

And of course stuff going on in the atmosphere:

  • Amount and height of clouds
  • Winds, etc.

And

  • What the ocean is doing

Finally, Walter Orr Roberts, the head of NCAR at the time, was interested in the effects of:

  • The changing sun

And, as noted in the article about the scientist who thought an ice age was coming:

  • Changes in Earth’s orbit and tilt
Posted in Climate Change, Earth System Science | 2 Comments

It is getting warmer!

Posted on 07/05/2007 by peggy

Figure 1 shows how Earth’s average temperature has changed over time from two research groups – one the National Climate Data Center in the United States, and the second from the Climate Research Unit at the University of East Anglia in the United Kingdom.

Global average temperature as a function of time

Figure 1. Thermometer-based global average temperature as a function of time, plotted relative to the average between 1960 and 1990. This includes land and ocean. From the NOAA Web page.

In both graphs, temperatures are compared to the 1960-1990 averages. Any temperature below the red “zero” line is cooler than that period, and any temperature above the red “zero” line is warmer. The temperature records are carefully chosen to avoid too much influence of the warming of cities (see previous blogs on the urban heat island), changes in thermometers at a given site, changes of measurement locations, and other changes that influence local temperature. And, because oceans cover 70% of Earth’s surface, extra care is taken in analyzing sea surface temperature data from ships. You can find more information about how temperature data are treated in a 1990 Scientific American article by Philip D. Jones and Tom M.L. Wigley (pp 84-91). A graph of surface air temperature relative to the 1951-1980 mean, from the Goddard Institute for Space Studies (GISS), shows similar trends (Figure 2). In this case, satellite-based sea-surface temperatures are used from 1982 on.

The graphs are for Earth’s yearly average temperature at about 1.5 m above the surface (the height temperatures are measured by weather services, and the height at which GLOBE students take their temperature observations, see Instrument Construction from the Teacher’s Guide, p 14). That includes the land, which covers 30% of the surface, and the ocean, which covers 70% of the surface.

The GISS graph in Figure 2 includes the temperature departures from the 1951-1980 averages. You can see that some places – like the Arctic, are much warmer than that average. And some paces, like parts of the high latitudes in the Southern Hemisphere, are actually cooler.

Temperature departure from 1951-1980 mean

Figure 2. (Top) temperature departure from 1951-1980 mean as a function of time. (Bottom) map of temperature departure from 1951-1980 mean for 2005. Source: Goddard Institute for Space Studies. Numbers are degrees Celsius.

We tend not to remember averages, though. We remember hot days, or cold days, or storms, so we (even scientists!) are tempted to think that a hot day means “global warming” and a record low means that “global warming” isn’t happening. That is why looking at graphs of average temperature data is helpful. The graph enables us to study the patterns in climate data which represent average weather over time.

What is the difference between weather and climate? In the words of a middle-school student, “Weather helps you decide what to wear, and climate helps you decide what clothes to buy.”

Next: What affects Earth’s climate?

Posted in Air Temperature, Atmosphere, Climate Change, Earth System Science | 11 Comments

Puddles

Posted on 05/29/2007 by peggy

I like puddles, and I have become more interested in them lately. Why?

On 29 May 2002, we took observations of the heating and moistening of the lower atmosphere using an aircraft and surface sites observations in the Oklahoma Panhandle (The Western Track in Figure 1). Two days before we took our data, a heavy rain brought 80 mm of rain to the point labeled 1, with the points labeled 2, 3, and 10 getting 30 mm or less.

Map showing the location of aircraft flight tracks

Figure 1. Map showing the location of aircraft flight tracks (white lines) and sites where we took special measurements (numbered 1-10). The observations I write about are along the Western Track, on the left side of the picture. The long white lines outline part of the state of Oklahoma.

We have been trying to see how well a land surface model would do in predicting the observed heating and moistening, given the weather conditions – temperature, solar radiation, wind, rainfall, and so on as input. And the model didn’t work very well near Site 1. No matter what we did.

We have an idea why: Puddles.

As you can see from Figure 2, there were puddles near the southern end of the flight track. In fact, one road was blocked by water. And the land surface model didn’t account for evaporation from puddles. We think this could explain why the measurements showed more moistening (and less heating) of the air than the model did.

Photograph of puddles near the southern end

Figure 2. On 29 May 2002, photograph of puddles near the southern end of the Western Track, shown in Figure 1. The spots are on the aircraft windshield.

I decided that I had better learn more about puddles. So the first day there were puddles outside my office, I went outside and took puddle temperatures with a GLOBE infrared sensor (see the GLOBE Surface Temperature Protocol).

I was surprised – the puddles were warm compared the ground around them. This is not what I expected. Puddles like those in Figure 2 were cooler than the surrounding ground on 29 May. So I became even more excited about puddles. There is nothing more fun – and sometimes more awful! – than taking measurements you don’t understand.

I’m starting this project by just trying to figure out how fast the puddle disappears. On an asphalt surface, this tells me how fast the puddle evaporates. I’m also measuring the temperatures of the surface around the puddles.

Figure 3 is a picture of the puddle that I measured. The chalk rings are drawn around the puddle so that I can see how fast it is drying out.

Puddle with outlines of water's edge

Figure 3. Puddle with outlines of water’s edge. The lines alternate between light yellow and light pink. Yellow or pink dashed lines are where the chalk is too light to see easily. By the time this picture was taken, the puddle was almost gone, with shallow water in a few places in the small left circle. Times are when the lines were drawn. UTC = MDT + 6 hours.

What did I learn? Figure 4 showed results more like what I had expected. As the sun got higher in the sky, the asphalt surrounding the puddle warmed more than the puddle itself. And the difference between the puddle temperature and the asphalt temperature got bigger, until around 11 a.m., when it started to get cloudy. After that, the temperature difference became smaller.

Graph of temperature of the puddle

Figure 4. Temperature of the puddle in Figure 3 as a function of time. The skies were mostly clear until about 11 a.m. MDT. After that, the sky got cloudier with time. It was overcast by 11:50. Local solar noon (when the Sun is highest in the sky) is around 13 MDT.

Does this offer a clue to why the first puddle I looked earlier at was warmer than the surrounding surface? I think it does. That day, it was also cloudy in the afternoon.

If this puddle had lasted longer, AND if this puddle cooled more slowly than the dry asphalt once the skies were cloudy, then this puddle might have ended up warmer than the asphalt. And maybe it has something to do with the fact that water stores heat well.

So I need to look at more puddles. And, while doing this simple experiment, I noticed that I could have done some things better:

  • I didn’t want to use the oven mitt on the radiation thermometer, as recommended for the GLOBE Surface Temperature Protocol, so I kept the radiation thermometer outside so that its temperature was the same as the air temperature. But I soon discovered that the air temperature where I kept the radiation thermometer was different enough from the puddle site that the measured surface temperature changed rather rapidly for about five minutes (the differences weren’t that bad). So I’m not too sure about the temperatures before 8 a.m. After 8 a.m., I still left the instrument outside but oven mitt on – and the measurements were more consistent.
  • Toward the end of the observations, I realized that I had made a bad assumption: that all the asphalt outside the puddle was the same. It wasn’t. The puddle was in a place that had been repaired. You can see the difference in Figure 5. The area to the north of the puddle was up to 3 degrees cooler than the area to the south of the puddle! ). So I only use the temperatures on the south side in Figure 4.
  • I had carefully drawn chalk rings around the puddle, so that I could see how fast it evaporated. But I forgot to take an important observation – how deep the puddle was! So – even though I could tell that the puddle was evaporating, I couldn’t tell how much water was evaporating – and I wanted to know that.
  • If clouds are important, as they would be if the puddle stays warm after the skies cloud over, but the surface around it cools off – I need to be more careful about writing down when there were clouds.
  • Fourth, once the puddle got quite shallow, it was basically wet asphalt. I should have taken the temperature of the wet asphalt as well.

The puddle and its environment

Figure 5. The puddle and its environment. Note the puddle lies on the south (right) end of an asphalt square that was slightly warmer than the darker asphalt to the right. (Although dark things are usually warmer than white things, the warm temperature could have something to do with different materials being used, or the thickness of the asphalt layer.)

I have some of the other data below. You’ll notice I may have made a few mistakes! (It’s important to keep track of them, so you can learn from them). The times are important because I might want to check other weather data I can get from the Web or from the automatic weather station on top of our building.

Next time I will be more thorough. I’ll let you know what happens. In the meantime, think about how you can use measurements or simple observations to describe some things that are happening around your home or school.

Table: Puddle measurements on 6 May 2007.

Time (LDT) Cloud Comments
0743 Clear Photos 7:50 of puddle.
0845 Clear First Ring
0945 - Second Ring; took photo.
1045 0.4 Cu Drew third ring; took photo
1115 Broken Cu Drew 3 yellow circles where water still is. Got cloudy after second reading. Photo.
1125 - Puddle almost gone. I take temperature measurements just to show how much they vary around and in where the puddle was.
- - Note temperatures on patch of asphalt to the north are cooler than temperature on the original asphalt to the south.
1150 Cloudy Used average temperatures just for south for time series since just started taking measurements to the north (don’t have complete record).
1220 Cloudy I recorded a temperature north of the puddle that was warmer than the temperature to the south of the puddle. I wonder if the readings are just reversed?
1225 Cloudy Puddle basically gone
Posted in Atmosphere, Backyard Science, Data included, Earth System Science, Hydrology, Watersheds | 6 Comments

Are there more storms than there used to be?

Posted on 05/11/2007 by peggy

The work of Roger Pielke, Sr., discussed in the last blog, suggests that thunderstorms might be more common than they were 100 years ago. Are they?

My first job in science was as a college student. Ten hours a week, I worked on putting together a ‘tornado climatology’ for Professor Grant Darkow at the University of Missouri. To do this, I had to find out when and where tornadoes occurred in my home state of Missouri. I used records from the U.S. Weather Service (Monthly Weather Review, for more recent years, Storm Data), and weekly newspapers from small towns around the state.

Why? We wanted to see if we could learn more about how tornadoes form by knowing better about:

  • When tornadoes form
  • Where tornadoes form
  • How big they are
  • How long they stayed on the ground
  • What time of day they happen
  • What direction they come from

This information usually came from eyewitness reports of a funnel cloud and damage reports that lined up along a narrow track that lined up with the funnel cloud. Ted Fujita, who is known for the Enhanced Fujita Scale of tornado intensity, developed methods for associating damage patterns with tornadoes.

What did we find out? We found out that there were more tornadoes where there were more people. If there are more people, the chances go up of someone seeing a tornado. And tornado damage is more obvious (at least when you aren’t looking for it) where there are houses than in an open field. Also, if there are enough people, there is often a small newspaper to report the tornado.

The area around St. Louis, Missouri, had the most tornadoes per unit area. This was not only because there were a lot of people in and around St. Louis, but there was a storm chaser and scientist named Ed Brooks who found and reported tornadoes that no one would have known about otherwise.

Looking at the map in Figure 1, there also seems to be more tornadoes on the west side of the state than the eastern side, although Kansas City (where the Missouri River reaches the west side of the state) probably accounts for some of the high numbers there.

Number of tornadoes in Missouri by County

Figure 1. Number of tornadoes in Missouri by County between 1916 and 1969. St. Louis is on the east side of the state, in the county that has 33 tornadoes. Kansas City is at the north end of the straight-north-south part of the state’s west border. Figure Courtesy of Grant. L. Darkow, University of Missouri-Columbia.

We also found out that the number of tornadoes went up with time, with a rapid increase in the 1950s. Some of this is related an increase in the number of people and newspapers with time.

Also, the Weather Service (then the Weather Bureau) started tornado forecasting in the early 1950s. So not only were people reminded to look for them, but weather forecasters would look for evidence of tornadoes to check to see how good their forecasts were. About the same time, the Weather Service started to publish summaries of tornadoes and other severe weather in Storm Data, making the information much easier to find.

Apparently my hard work between about 1965 and 1968 didn’t increase the number of tornadoes counted – aside from a big peak in 1967, the number of tornadoes was about the same as in the late 1950s.

Note that the tornado deaths stayed about the same in spite of the increase in population. If the number of tornadoes really had increased, a steady death rate might mean better tornado warnings. But we felt that the small change in the number of deaths meant that the number of destructive tornadoes hadn’t changed that much (and thus probably the total number didn’t either).

Tornadoes and tornado deaths by year

Figure 2. For the state of Missouri, tornadoes and tornado deaths by year. Note the big jump in the mid-1950s, which corresponds to when the Weather Service formally started recording storm occurrence in Storm Data. Figure courtesy of Grant L. Darkow. University of Missouri-Columbia.

Tornadoes occur with strong thunderstorms, which usually produce lots of rain. An easier question to answer is whether there are more heavy rainstorms.

Why is this? First, there are lots and lots of rain gauges. But they are not everywhere, so scientists lump together several gauges in an area. Obviously, the time changes will be more accurate if the area is large enough to have lots of rain gauges with records for a long time.

But the areas you look at should be small enough so that you can see how things vary from place to place. By very carefully combining rain gauge records and measuring how accurate the resulting trends are, Pavel Groisman and his colleagues at the National Climatic Data Center found out that they can detect an increase in very heavy rainfall in the east-central United States. In other places, the trend is too small to say for sure using their methods.

Since thunderstorms clouds are cumulonimbus clouds, you could count observations of cumulonimbus clouds to see whether there are more thunderstorms. (To see what a cumulonimbus cloud looks like, see the cloud chart.) Between the early 1950s and the early 1990s, when human observers were replaced by automated weather stations, there was a good continuous record of cloud-type observations by U.S. Weather Service human observers. (Now, students like you take observations for GLOBE-related projects like the CloudSat mission for “ground truth.”) During this time, the number of cumulonimbus cloud observations increased during the spring and fall, but the reports didn’t change much during the summer. The authors suggest that the spring and fall increase is related to more warm days as winter gets shorter.

So the small number of clues here suggests that there are more heavy rain events in some parts of the country, and there are more thunderstorms clouds – in the spring and fall. But we really can’t see whether there are more – or fewer – tornadoes.

And the much harder question is to answer is, “why?”

My sincere thanks to Grant Darkow for checking my facts and allowing me to reproduce his figures. And I wish to dedicate this blog to the people of a beautiful town I visited and enjoyed very much about 10 years ago – Greensburg, Kansas, which had a devastating tornado last week. Best wishes for a full recovery – and no tornadoes for another 100 years – at least.

Posted in Atmosphere, Climate Change, Earth System Science | Leave a comment