Notes: Fri 27 Sept

Temperatures, continued

overhead: some temperature plots versus time for two cities at the same latitude. The average temperature of the city at higher elevation was lower than the city at low elevation for every month of the year. This once again emphasizes that latitude is not the only factor that determines temperature.

Land and Ocean Climates

Water moderates temperature for several reasons.
  • First, some incoming energy goes into evaporating water (this is an example of latent heat). So less energy is left over to warm the air. This helps to make hot areas cooler.
  • Water can penetrate down past the surface of the water. This allows a whole volume of water to absorb energy, again leaving less energy to warm the air. The surface of the Earth, on the other hand, can only absorb energy right at its surface.
  • The specific heat of water is high, meaning it takes a lot of energy to increase its temperature. On the flip side, it releases a lot of energy when it cools. This means that water in hot areas can absorb a lot of energy and still not change temperature very much, and when this water moves to colder regions, it gives off energy and warms the air.
  • The last reason was just mentioned above: that water moves around and distributes energy. In contrast, a hot place on a continent can not move around and share its energy with surrounding environments.

    Some concrete examples of the moderating effect of water:

  • the interior of Canada has more extreme seasonal climate variations and a bigger range of temperatures than Vancouver, which is on the coast.
  • Kansas, in the interior of the U.S., has hot summers and cold winters. Compare that to San Francisco, which is about 60 degrees year round.

    Representing temperature on a map

    To show the temperature patterns in a region, we draw isolines of temperature, or isotherms. These are lines of constant temperature, just as topo maps have lines of constant elevation. On an overhead, we saw that the isotherms on a world map were more squiggly toward the North than in the center. This is because there is more land in the Northern hemisphere (remember the question from lab 1?), and there are more temperature extremes on land than there are over the ocean.

    definition: thermal equator is line around the globe that goes through the hottest points. It will differ from the real equator (0 degrees latitude) and typically will go over landmasses (since water has a moderating effect on climate and land does not).

    Most climate-action will occur over continental landmasses (Siberia is a good example of extreme temperatures). Remember that the elevations at the poles are important causes of the difference in their temperatures. The North Pole is mostly water and relatively flat. The moderating effect of water and the low elevation contribute to its relative warmth compared to the South Pole. The South Pole has an elevation of 9300 ft and has a few mountain ranges. In addition, it has a high albedo and is covered in ice and snow. All these factors contribute to the South Pole being quite cold.

    The Greenhouse Effect

    When we look at global climates over the past 65 million years, we see that temperature has been dropping, and the trend has been toward colder and drier climates. If we look on a smaller timescale, we see that temperatures have increased since the "Little Ice Age" (mentioned earlier in the term while discussing sunspots). In the 1970's, temperatures actually decreased. So the question we face when trying to analyze temperature changes is what scale do we examine them on? We need to consider a global timescale, not a geologic one. The data then shows that the general trend is toward increasing temperatures. The cause of this has been attributed to the greenhouse effect.

    What is the greenhouse effect? It is the natural trapping and recycling of energy that originally came from the sun but has been re-radiated from the Earth back to the atmosphere and from the atmosphere back to the Earth. This effect makes our planet livable and makes the temperature 30 to 35 degrees C warmer than it would be without it.

    So what is the big deal about carbon dioxide causing the greenhouse effect? Greenhouse gases act to accelerate the natural greenhouse effect and increase the recycling of energy back down into the ground. This increases temperatures.
    Actually, many gases are greenhouse gases, but humans are putting a lot of CO2 into the air, especially the lower atmosphere. Furthermore, CO2 has a long residence time, meaning that once it is put in the air, it stays there for a long time. There is also a correlation: the % CO2 in the atmosphere is increasing measurably, and at the same time, average temperatures are warmer no matter where you go. Even though the percentage of CO2 emissions from North America has dropped, the CO2 emissions have increased from 1.6 x 106 tons in 1950 to 5.2 x 106 tons in 1980, and show no signs of getting better.

    What will the effect be of ever-increasing CO2 levels in the atmosphere? You need models to try to predict what will happen. The early models made incorrect predictions, which overestimated the rise in temperature that would occur. They were off partly because the CO2 cycle is not completely known. A lot of CO2 is tied up in the atmosphere and oceans. More subtle effects are photosynthesis and seasonality of plant life. Then you have to take into account that adding more CO2 may cause a natural buffer to try to balance the CO2 excess (one possible example: leaves on trees might get larger overall to increase rates of photosynthesis and thereby help lower CO2 levels).

    Not only were the first models too simplistic, as we just discussed, but the effect of aerosols and of burning fuels was not taken into account. The effect of aerosols is opposite of the greenhouse effect.

    1. aerosol particles themselves reflect energy back to space, which makes for cooler temperatures.
    2. the particles also serve as nucleii for condensation. The result is sulfate haze, which again increases albedo (reflectivity). Sulfate haze is not just man-made, but occurs naturally due to plankton and marine animals, though in much smaller quantities.

    So why don't we just reduce our CO2 output? Besides economic reasons, the proof is not convincing enough to those in power. For instance, very hot summers have been followed by cool summers, which is not what you'd expect. This makes people think that global warming is nonsense. However, you can not just compare one summer to the next summer. You have to look at a trend over a longer period of time.

    That's it for Friday's notes. Monday's notes have a lot of Greenhouse Effect information, if you missed any. I'd recommend looking at your notes for that 11:15 PM Thurs. 3 Oct.

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