Climate Change: Causes, Consequences, and Current Trends, Exams of Meteorology

Download Climate Change: Causes, Consequences, and Current Trends and more Exams Meteorology in PDF only on Docsity! Jon Ahlquist 12/10/2006 MET1010 Intro to the Atmosphere 1 Chapter 16: Climate Change The Earth’s Changing Climate Possible Causes of Climate Change Global Warming The Recent Warming Future Warming Climate and averaging Climate: average weather National Weather Service climatological “normals”: average over most recent 3 decades; currently 1971-2000. See http://lwf.ncdc.noaa.gov/oa/climate/normals/usnormals.html Averaging period is arbitrary, though. For various studies, averaging period can be as short as a few weeks or as long as hundreds of years No matter what the averaging period, climate is always changing with time. Scales of Changing Climate Not only does climate change on all time scales. Climate changes can be due to very local effects such as street light in fig. 16.1 and urbanization in general, to continental scale (continental drift), and planetary (orbital fluctuations). More on each of these later. Fig. 16.1, p. 432 Determining past climate Recent climate: direct measurements (over 100 years for major cities in US, nearly 200 years for Europe). For historical era: written reports of freeze dates, floods, etc. Chinese records go back several thousand years. Supplemented by: tree rings, identification of plants from seeds and pollen found in various layers Oxygen isotope ratios in coral, etc. Geological record (dry lake beds, fossils, etc.) Climate simulation programs can check consistency of observations and estimate climate in places where observations are sparse. Ice Ages (pp. 432-433, 435) Ice age: glaciers cover significant area of Earth. We are currently in a minor ice age, with glaciers covering about 10% of Earth’s surface. If all glaciers were to melt, sea level would rise by about 200 feet. Warming a few degrees C: sea levels up by ~1/2 m Sea level rising in some places (ice melting) and dropping in others (land rebounding from weight of past ice now gone that compressed it) Most recent major ice age: ~18,000 years ago, when sea level may have been ~400 feet lower, opening Bering land bridge between Asia and North America Precise no. of earlier ice ages is hard to say. Advance of new glaciers “erases” evidence of earlier glaciers. Climate During Past 1000 years (p. 437) Moderate during 1000- 1400 AD Cooler between ~1400 and 1900 AD (Viking colony in Greenland perished) Definite warming since 1900 Fig. 16.6, p. 437: N Hem temp Jon Ahlquist 12/10/2006 MET1010 Intro to the Atmosphere 2 Causes of Climate Change: Plate Tectonics (p. 439-441) Continental drift proposed by Alfred Wegener, who also proposed ice crystal process (p. 171) Fig. 16.8 (p. 440) shows estimated land distribution 180 million years ago and now Orbital Fluctuations Serbian geophysicist Milutin Milankovitch (1879-1958) developed earlier ideas from self- educated Scotsman James Croll (1821-1890) that variations in the Earth’s orbit could explain climate fluctuations. Orbital variations include: Orbital eccentricity (how round the orbit is) Wobbling of the Earth’s axis Changes in tilt of the Earth’s axis These explain many fluctuations on time scales of 10s to 100s of thousands of years. Particles in the Air (pp. 443-446) Particles from wind-blown dust, fires, chemical reactions, etc. get in tropospheric air Dust from volcanic eruptions & asteroid impact can enter stratosphere These particles affect Earth’s radiation budget (how much visible light is reflected or absorbed & how much infrared from surface is absorbed) Volcanoes & asteroid impact are the 2 leading contenders for what killed dinosaurs (& 70% of all species on Earth at the same time) Particles in the Air (continued) Volcanic dust from major 19th & 20th century eruptions has been tied to cooling of up to ½°C for 1-3 years after eruption. 1816, “Year without a summer,” followed huge eruption of Mt. Tambora in Indonesia in 1815. June 1991 eruption of Mt. Pinatubo in Philip- pines cooled global temperature by 1°C in 1 yr Large nuclear war would probably put great amount of dust into the air, causing “nuclear winter,” killing millions or more (p. 444). Variation in Solar Output (p. 446) Satellite measurement of solar output only since 1980’s, but sun spots counted since Galileo observed them in 1612. Number of sunspots goes up and down with an 11-year cycle. Sunspot cycle does not repeat exactly. Numbers vary over time. E.g., few sunspots during 1650-1700: Maunder minimum Variation in Solar Output (continued) At time of sunspot maximum, sun emits about 0.1% more energy, which is so small that it was undetectable before satellite measurements. Variation in solar output may account for some climate fluctuations on time scales of decades to centuries. See http://en.wikipedia.org/wiki/Solar_variation which is the source for the sunspot graph on the preceding slide.