If global temperature increases as much as 4 C by 2100, what global effects might you anticipate as ...

(Answer to question 1 )  Some possible answers:
 Growing seasons for agricultural products will become longer, possibly increasing yields.
 Evaporation will increase, causing more droughts and the drying up of lakes and reservoirs. Note: Fewer students recognize that increased temperatures invigorate all aspects of the hydrologic cycle, so that increased evaporation will also mean increased precipitation. However, since the evaporation and precipitation processes do not necessarily happen equally at the same locations, some places can expect drought whereas other can anticipate increased rainfall.
 Less snowfall in mountains during the winter, which will impact recreational winter sports and tourism and also diminish the amount of snowmelt runoff in springtime, which is critical for crop irrigation in many regions.
 Geographic ranges of plants and animals will change as temperatures change, allowing warm-climate plants and animals to expand to higher latitudes and higher elevations. Cold-climate adapted land plants and animals may go extinct, however, as the necessary conditions for their survival disappears at the polar edges of continents.
 Tropical diseases and parasites will expand north and south from the equator to afflict larger regions, including places where populations do not have immunities.
 Hurricanes will become more numerous or more powerful, or both, because hurricane formation and power rely on warm sea-surface temperatures.

(Answer to question 2 )  The rise of agricultural societies may have had two impacts that could cause climate change. One is changes in land use by replacing forests with crop land. The crop-land surfaces would probably have higher albedo, which could cause warming. However, there may also be less photosynthesis compared to the previous forest, so that would lead to an increase in atmospheric carbon dioxide. Some agricultural practices, like growing rice, increase the atmospheric concentration of methane, an important greenhouse gas.

(Answer to question 3 )  Water vapor concentrations are not thought to change dramatically from year to year because the hydrologic cycle efficiently moves water molecules from vapor to liquid and back again. So, although water vapor is abundant in the atmosphere, it probably doesn't change very much, so it is insignificant for explaining changing temperature. In contrast, measurements of air samples and air bubbles trapped in glacial ice show that carbon dioxide and methane concentrations have increased dramatically in the last century.

(Answer to question 4 )  Volcanoes emit gases to the atmosphere. Emitted carbon dioxide contributes to warming. Emitted sulfurous aerosols contribute to cooling. On historic time frames, the focus is on the larger volume of sulfurous aerosols compared to carbon dioxide. During historic times, therefore, highly explosive volcanic eruptions have typically been followed by unusually cool temperatures. On geologic time scales of millions or tens of millions of year, however, long periods of greater volcanic activity may also contribute significant amounts of carbon dioxide. During times when there are longer mid-ocean ridges or when sea-floor spreading rates are unusually high, there should also be more carbon dioxide released to the atmosphere; this may account for both warmer temperatures and higher carbon dioxide levels at times in the geologic past.

(Answer to question 5 )  Satellite measurements show that solar radiation varies with the 11-year sunspot cycle. In addition, there are longer trends of increasing or decreasing radiation, on the scale of many decades to centuries, which are interpreted from theoretical calculations and consistent with the limited period of satellite data collection. On the longer time frames, the Milankovitch cycles that describe variations in the tilt and wobble of Earth's rotation axis and variations in the shape of Earth's orbit around the Sun, also cause changes in the amount of solar radiation reaching Earth.

(Answer to question 6 )  a) Overall temperature has cooled over the last 180 million years, with many fluctuations.
b) Overall temperature has cooled during the Cenozoic, especially since about 40 million years ago.
c) Temperatures have fluctuated strongly during the Quaternary between very cold (ice age) and very warm times.
d) During the last 21,000 years, the overall temperature has warmed, because the peak of the last ice age was at 21,000 years ago.
e) There have been some ups and downs in temperature over the last 1000 years but the overall trend has been toward warmer conditions.
f) The overall trend over the last century has been warming, with some short-lived cooling spells (e.g., 19301940).
g) Temperatures have increased over the last decade.

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