CH09 Lecture 1 The Atmosphere and Climate Change


Hi Students, In this lecture video we will be learning about the basic science behind global climate change. This starts with an understanding of the earth’s atmosphere, and how it affects temperatures in the air, the land and the water. The learning objectives for this first lesson on climate change include understanding how the troposphere and the stratosphere differ, being able to explain the greenhouse effect, and how it relates to earth’s climate system, and last, to be able to understand and identify what causes natural climate variability. In 2010 the IEA wrote that “The next decade is critical. If emissions do not peak by around 2020, . . . the needed 50% reduction by 2050 will become much more costly. In fact, the opportunity may be lost completely.”We have made some significant progress towards this goal, but there remain a lot of work to do to limit climate change. Some researchers now believe that the earth’s climate system is already committed to a 2 degree Celsius increase in temperature since the beginning of the industrial revolution. For cattle ranchers in Texas, 2011 was a terrible year—the hottest and driest year observed since record keeping began in 1895. In 2011, Texas saw more than 90 days with temperatures over 100ºF (38ºC)—roughly double the normal number.

Lakes, streams, and stock-watering ponds dried up and disappeared along with the grass on baking pastures leaving cattle thirsty and starving. Drought also meant no harvest of hay—critical for winter feed. By the end of the year, Texas ranchers were forced to sell or slaughter 1.4 million cattle, or roughly 11 percent of the state total, possibly the biggest sell-off in American history. Because Texas is the nation’s largest producer of beef cattle, impacts were felt in beef prices across the country. This was good news for ranchers in other regions, where the value of herds rose sharply, but it wasn’t easy on meat packers and consumers. With no soil moisture and with spring rains at half their normal level, field crops suffered, too. Just over half of the state’s 20 million acres of cropland were harvested. More than half the cotton crop failed entirely. Many farmers didn’t even bother planting wheat on their bone-dry fields. The federal government stepped in to ease the pain, however; U.

S. taxpayers covered more than $5 billion in crop insurance, disaster payments, and other supports to farmers who lost their cotton, sorghum, wheat, and other crops. Total economic losses associated with that one year of drought are estimated at more than $10 billion. This drought was extreme even for a region accustomed to heat and drought, but the state climatologist has warned that these dry conditions could last for a decade or longer. Much of the South and the West have seen similar extremes. Is all this just weird weather? Or is it part of a larger pattern? This is the question policy makers, scientists, and the public are all asking. In a complex and rapidly shifting system like weather, most of us tend to see data selectively, focusing on pieces of evidence that confirm our expectations. For some observers, several bad years of drought are confirming proof of human-caused climate change predicted by climatologists, ecologists, marine scientists, and others.

For others, a string of bad years is simply more variation in the complex system we call weather. How do we know what’s really going on? Climate scientists—like all scientists—seek their answers in data—that is, in collected observations. The longer the data record, the more years and the more sources of evidence or places considered, the better. Climatologists conclude that the droughts seen across much of the southern and central United States, and many other regions of the world, represent a long-term shift toward increased heat and energy storage in the atmosphere. They warn that if we don’t act soon—in the next few years at the most—those changes will shift us to an entirely new climate regime. The evidence indicates that extreme summer heat, with drying soil and crops, is the new normal for much of the southern and central United States. Texas is expected to look like the Arizona desert does today. Illinois and Iowa, the center of our farm economy, will look like Texas today.

Global average temperatures have closely tracked CO2 concentrations in our atmosphere. CO2 has increased from 285 parts per million (ppm) in 1880 to over 400 parts per million, with the fastest change in the past 50 years. Today’s average temperature is about 0.7 degrees warmer than a century ago. AS of February 2016, we have seen a 4 ppm increase in CO2 (to an average of 404 PPM) over a single one year period; the most in the earth’s recent history. Clean, dry air is 78 percent nitrogen and almost 21 percent oxygen, with the remaining 1 percent composed of argon, carbon dioxide (CO2), and a variety of other gases. Water vapor (H2O in gas form) varies from near 0 to 4 percent, depending on air temperature and available moisture. Minute particles and liquid droplets—collectively called aerosols— also are suspended in the air. Atmospheric aerosols and water vapor play important roles in the earth’s energy budget and in rain production.

The atmosphere has four distinct zones of contrasting temperature due to differences in absorption of solar energy (fig. 9.3). The layer immediately adjacent to the earth’s surface is called the troposphere (tropein means “to turn or change,” in Greek). Within the troposphere, air circulates in great vertical and horizontal convection currents, constantly redistributing heat and moisture around the globe (fig. 9.4). The troposphere ranges in depth from about 18 km (11 mi) over the equator to about km (5 mi) over the poles, where air is cold and dense. Because gravity holds most air molecules close to the earth’s surface, the troposphere is much denser than the other layers: it contains about 75 percent of the total mass of the atmosphere. Air temperature drops rapidly with increasing altitude in this layer, reaching about –60°C (–76°F) at the top of the troposphere. A sudden reversal of this temperature gradient creates a boundary called the tropopause.

This temperature boundary occurs because ozone (O3) molecules in the stratosphere absorb solar energy. In particular, ozone absorbs ultraviolet (UV) radiation (wavelengths of 290–330 nm; see fig. 2.13). This absorbed energy makes the stratosphere warmer than the upper troposphere. The stratosphere extends about 50 km (31 mi) out from the tropopause. It is far more dilute than the troposphere, but it has a similar composition except that it has almost no water vapor and nearly 1,000 times more ozone. Unlike the troposphere, the stratosphere is relatively calm. There is so little mixing in the stratosphere that volcanic ash and human caused contaminants can remain in suspension there for many years. Convection cells circulate air, moisture, and heat around the globe. Jet streams develop where cells meet, and surface winds result from convection.

Convection cells expand and shift seasonally. The sun supplies the earth with abundant energy, especially near the equator. Of the solar energy that reaches the outer atmosphere, about one-quarter is reflected by clouds and atmospheric gases, and another quarter is absorbed by carbon dioxide, water vapor, ozone, methane, and a few other gases (fig. 9.5). This absorbed energy warms the atmosphere slightly. About half of incoming solar radiation (insolation) reaches the earth’s surface. Most of this energy is in the form of light or infrared (heat) energy. Some incoming solar energy is reflected by bright surfaces, such as snow, ice, and sand. The rest is absorbed by the earth’s surface and by water. Surfaces that reflect energy have a high albedo (reflectivity). Fresh snow and dense clouds, for instance, can reflect as much as 85 to 90 percent of the light falling on them.

Surfaces that absorb energy have a low albedo and generally appear dark. Black soil, asphalt pavement, and water, for example, have low albedo, with reflectivity as low as 3 to 5 percent. The atmosphere absorbs or reflects about half of the solar energy reaching the earth. Most of the energy reemitted from the earth’s surface is long-wave, infrared energy. Gases and aerosols in the atmosphere absorb and re-radiate most of this energy, keeping the surface much warmer than it would otherwise be. This absorption is known as the greenhouse effect. In this figure we see that visible light enters the atmosphere easily, as the earth’s atmosphere is virtually transparent to visible light. However, when this light reaches the earth, it warms dark surfaces with low albedo values, and this heat is emitted by the earth and renters the atmosphere as infrared energy. Think of this energy like a heat lam you sometimes see in bathrooms.

As you can see from this figure however, infrared wavelengths are highly absorbed (blocked) by gasses in the atmosphere, including CO2. This is why heat is trapped in the atmosphere by CO2 and forms the theoretical basis of the green hose effect. This table shows the Albedo values for some common earth surfaces. Hundred can see fresh snow has a very high albedo number and reflects a high percentage of visible light back into the atmosphere. Dark soil reflects very little visible light back into the atmosphere, and thus is heated by the light in this heat is emitted into the atmosphere where it is absorbed by carbon dioxide and other greenhouse gases. If our atmosphere didn’t capture this reemitted energy, the earth’s average surface temperature would be about –6°C (21°F), rather than the current 14°C (57°F) average. Thus, energy capture is necessary for liquid water on earth, and for life as we know it. The greenhouse effect is a common term to describe the capture of energy by gases in the atmosphere. Something like the glass of a greenhouse, the atmosphere transmits sunlight but traps some heat inside.

Also like a greenhouse, the atmosphere lets that energy dissipate gradually to space (see Key Concepts, p. 218). The balance of the rate of incoming energy and outgoing energy determines the temperature inside the greenhouse. The policy issue that faces us is that we are slowing the rate of heat loss, thus increasing heat storage in our “greenhouse.” We are doing this by adding CO2, CH4, and N2O to the atmosphere at levels the earth has not seen since before the appearance of humans as a species. The question is whether we will be able to agree on changing this trend. A great deal of the the incoming visible light is used to evaporate water. Imagine the sun shining on the Gulf of Mexico in the winter. Warm sunshine and plenty of water allow continuous evaporation that converts an immense amount of solar (light) energy into latent heat stored in evaporated water. Now imagine a wind blowing the humid air north from the Gulf toward Canada. The air cools as it moves north (especially if it encounters cold air moving south). Cooling causes the water vapor to condense.

Rain (or snow) falls as a consequence. Note that it is not only water that has moved from the Gulf to the Midwest: 580 calories of heat have also moved with every gram of moisture. The heat and water have moved from a place with strong incoming solar energy to a place with much less solar energy and much less water. This redistribution of heat and water around the globe is essential to life on earth. The oceans absorb heat created by incoming solar radiation. Ocean currents act as a global conveyor system, redistributing warm and cold water around the globe. These currents moderate our climate: for example, the Gulf Stream keeps northern Europe much warmer than northern Canada. Variations in ocean salinity and density, low (blue) to high (yellow), help drive ocean circulation. These deep ocean currents are critical to life, and to maintaining the earth’s current climate. These currents, also known as the “ocean conveyor” can take hundreds of years to migrate around the planet. We will learn in later lessons that the increase in heat from the human-caused (anthropogenic) additions to the greenhouse effect not only increase air temperatures, but sea temperatures as well.

This, along with melting of glaciers, may have a deleterious effect on on thermo-haline circulation, click here. The extra CO2 also causes addition of carbonic acid to the ocean increasing ocean acidification..