Dec 21, 2015
But the Earth has an atmosphere!
• Dominated by gases such as nitrogen and oxygen
• Also includes greenhouse gases.
What is a Greenhouse Gas?• Greenhouse gas is a gas that that is relatively
transparent to solar radiation, but absorbers and emits in the infrared…the type of radiation the earth and clouds emits.
• Some examples:– Water vapor– Carbon dioxide– Nitrous oxide– Methane– Chlorofluoromethanes
But it is worse than that…
• There are a number of natural “amplifiers” of mankind’s emission of greenhouse gases.
• The warming due to increased carbon dioxide, methane, and other greenhouse gases will cause more water to be evaporated from the earth’s oceans.
Why?• The amount of evaporation increases with
temperature.
• Water vapor is the most potent greenhouse gas and thus causes even MORE warming.
• This is caused a positive feedback.
But it gets even worse…
• Warming temperatures melt snow and ice.• Snow and ice help cool the planet because they
reflect much of the sun’s radiation….that is why you need sun glasses while skiing.
• As the snow, melts less radiation is reflected to space and more is absorbed.
• Thus, the earth gets warmer, which melts more snow.
• Another positive feedback!
The Technology of Climate Prediction
• Atmospheric scientists use complex climate prediction models…called General Circulation Models…to predict the future climate.
• These models are similar to weather forecast models, but allow the gases in the atmosphere to change.
• They also simulate the evolution of the oceans.• Have to assume the future emission of greenhouse
gases by mankind…a major uncertainty.• These models are not perfect and cannot exactly
replicate the current climate….but they are close and getting better each year.
Details on Current Study: GCM
IPCC Report, 2001IPCC Report, 2001
• Which Scenario Will Mankind Follow?
How good are are climate models?
• The technology is constantly being tested and improved in our weather prediction models.
• But even more important, how well can we duplicate the climate of the past hundred years?
Projected warmingin 21st century expected to begreatest over land and at most high northern latitudes
Projections of Future Changes in Climate
What about the Northwest
• Several groups are using very high resolution simulations forced by the global climate model predictions
• Gives a view of the local implications of global warming.
• Could our mountains and other local features make it worse or better?
Bottom Line
• Global warming and its local implications are serious.
• There are significant uncertainties, but they are NOT in whether global warming will occur, but rather the magnitude and its local effects.
Fig. 14-11, p. 395
Axis now
Axis in approximately 11,000 years
January July
(b) Conditions now23 1/2°
(a)
July January
(c) Conditions in about 11,000 years
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