ENV2073_f11_exam1_soln - 1. a. Planet #1 must have the...

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1. a. Planet #1 must have the dense, thick atmosphere of carbon dioxide. How do we know this? The actual temperature of the planet is much, much hotter than its predicted (effective) temperature. This enormous temperature difference is caused by a very, very strong greenhouse effect . The dense atmosphere of carbon dioxide allows incoming solar radiation (visible light) to enter, but then re-emitted infrared energy is very strongly absorbed and re-emitted back to the planet’s surface. This causes the planet to heat up by over 500 ºC! Such a strong greenhouse effect could only be caused by a thick atmosphere of greenhouse gases – in this case, carbon dioxide. Planet #2 shows a much smaller greenhouse effect – the planet warms by about 33 ºC over its predicted temperature. This means that planet #2 must have the moderately dense atmosphere with a small amount of carbon dioxide (and water vapor, though the problem didn’t say that). Finally, planet #3 exhibits almost no greenhouse effect whatsoever – only 5 ºC – which is consistent with the very thin, low-pressure atmosphere. b. Hopefully you recognized that planet #2 is Earth . The average temperature of our planet is about 15 ºC, thanks to our moderate greenhouse effect, and we have an atmosphere that consists predominantly of nitrogen and oxygen. Planet #1 is Venus . It is pretty amazing – even though Venus is closer than Earth to the sun, it “ought” to be slightly cooler than Earth because of its stronger albedo (reflectance). But Venus has a “runaway” greenhouse effect that has warmed the planet by over 500 ºC, which is more than 900 ºF! Some people worry that this could happen on Earth if we have significant positive feedbacks from greenhouse warming. So far, there is no sign of “runaway” warming on Earth, but many scientists worry that we could hit a “tipping point” if the atmospheric concentration of CO 2 gets too high. Planet #3 is Mars – there is virtually no atmosphere on Mars, and hence almost no greenhouse warming. Some scientists think that many millennia ago, all three of these planets had similar atmospheres, but the three atmospheres evolved differently over time for reasons that aren’t fully clear; hence, today, only one of the three planets has conditions suitable for life.
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2. I say yes, the effects of global warming are already being felt. There are multiple lines of evidence that support this statement. Foremost, we have temperature records from thermometers that show the planet has, on average, gotten warmer in the past 100 years, and especially in the past 40 years or so. Even if we account for the fact that thermometer readings are affected by urbanization and changes in land-use patterns, it still appears that the planet, on average, has warmed in recent decades. Now, you might debate whether that warming means that we should say the “effects” are
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ENV2073_f11_exam1_soln - 1. a. Planet #1 must have the...

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