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Lecture 11

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Department
Environmental Science
Course
EESA09H3
Professor
Tanzina Mohsin
Semester
Summer

Description
Lecture 11: Future Winds and Global Climatic Change Outline Part I: Climate Change Primer What does climate mean? What factors influence climate? Historical climate change Global warming The greenhouse effect Global warming Mitigation Reality Check Part II: Research on Torontos climate Part 1: Climatic Change Primer Climate versus Weather In discussions of climatic change an important distinction should first be made between the concepts of climate and weather. Weather can be defined as a description of the current conditions of the atmosphere: temperature, pressure, humidity, winds, etc. recall the quantities and instruments for observation from Lecture 1. A prediction of the weather in the future consists of a set of estimates for the future values of these quantities. The sensitivity of the weather to very small changes places a very short limit (5-10 days) on our ability to make accurate predictions. Climate, in contrast, is a description of typical atmospheric conditions typical weather. This can take the form of statistical functions such as averages, variabilities, or extreme values of the quantities listed above. Because of the large amount of variability in weather conditions, at least 30 years of observational data should be used to describe climate, and ideally much more. Knowledge of climate allows us to form long-term expectations of what the weather will be like, although it doesnt provide specific predictions for example, our knowledge of climate in Toronto tells us that a typical July day will be hot and sunny, but we know that some days will be cool and wet. Predictions of climatic change, as we will see, are predictions of how typical weather conditions will change over a long period of time. What factors influence climate? Latitude The latitude of a location affects the angle of incidence of sunlight. The part of the Earths surface that is perpendicular to sunlight receives the greatest amount of radiation per unit of surface area in spring and autumn this is near the equator. Because of the Earths axial tilt, the angle of incidence at any location changes with the seasons; in the Northern Hemisphere we receive the most concentrated sunlight in June, when the sun is highest in the sky (we also get more hours of sunlight per day at this time). Global circulation We have seen in earlier lectures that prevailing winds have persistent effects on local weather conditions. Jet streams high overhead can affect climate too, by controlling the creation and movement and thus frequency of storms. Ocean currents also affect local climates by transporting warm water from the equator to higher latitudes, and thus redistributing heat to different parts of the globe. Air masses Air masses pushed around by prevailing winds and topography provide transportation of warm or cold, moist or dry air. Geography Geographical features such as oceans or mountains strongly affect local climates see for example the land/sea breeze. Land cover Urban heat islands are caused by changes in surface albedo, evapotranspiration, heat capacity and topography. Other land surface types such as forests, deserts or fields of crops also have a strong influence on their own climate. Historical climatic change Volcanic eruptions e.g. Mt. Pinatubo in 1991 Particularly powerful volcanic eruptions are capable of launching dust and sulfuric acid particles into the stratosphere. These aerosols (tiny particles suspended in the air) can remain in the stratosphere for several years, as there is no rain to wash them out as happens in the troposphere (sometimes in the form of acid rain). They are too small to fall very quickly, so it takes a long time for them to settle back into the lower atmosphere. While they remain in the atmosphere, these particles have the property of reflecting sunlight, reducing the amount that reaches the Earths surface (i.e. they increase the albedo of the Earths system). This causes noticeably cooler conditions around the world for a short period of time.One particularly alarming example of this occurred in 1816, known colloquially as the Year Without a Summer or Eighteen Hundred and Froze To Death. Massive eruptions occurred in the preceding few years, combining to load the stratosphere with reflective aerosols. In 1816 there were snowstorms in June in North America and Europe and snow cover lasting into July. Some victims froze to death, and even more starved as a result of massive crop failures. Solar variation Changes in the strength of the sun alter climate on Earth for obvious reasons. In Lecture 1 we talked about the suns gradual increase in strength on a billion year time scale when we discussed the Gaia Hypothesis. Cycles in solar features that have a period of decades or longer have been observed, and may indicate chanths in solar strength that could cause climatic change on Earth. For example, in the 17 century the sunspot cycle came to a halt, and no sunspots were observed for several decades an event called the Maunder Minimum. This roughly coincided with a climatic period called the Little Ice Age, where anecdotal reports of temperatures in Europe indicate a temperature drop of between 0.5 and 1C. However, there is significant dispute over whether this change in temperature was a local or global phenomenon, and in any case it is unproven that the low temperatures were caused by an actual decrease in solar strength. Direct observations of solar strength have only been available since the development of human-built satellites in the 1970s. Through these observations we have observed a slight change (0.01%) in solar s
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