Natural Causes of Climate Change

1. Natural Causes of Climate Change Geog410 Dr. Ye

2. Climate change theory needs to explain (1) on set of ice age through geologic time (long period of warmth interrupted by cooler period); and (2). account for warming and cooling period that occur within an ice age. Key is the flows of energy in and out of the system and the ways in which the energy is exchanged within the earth-ocean-atmosphere system

3. Basic Concepts: energy budget of the earth-atmosphere system

4. Albedo: percentage of solar radiation reflected by a surface

5. The Greenhouse Effect and Atmospheric Warming • Greenhouse effect: greenhouse gases (CO2, H2O, N2O, O3, CH4, CFCs) absorb energy of longwave radiation and re-radiate some energy back to the earth’s surface to keep it warm • Without greenhouse gases, the earth’s surface air temperature would be much colder • If the amount of greenhouse gases increase, the surface air temperature will increase (global warming)

6. Energy Budget by Latitude Figure 4.13

7. Short-term changes (within thousands of years) 1. Variation in solar irradiance • the sun’s surface temperature fell 11C in a single year of 1977-the normal temperature is 5438C); • the brightness of the sun faded in 1980s, so irradiance of the sun decline 0.07% from 1981-84. (computer simulation shows irradiance drop of 1-2% would bring about little ice age condition on the earth) 2. Sun spots: dark, circular areas over the sun’s outer surface or photosphere. In these areas, sun’s temperature is 1400C lower than surrounding areas. The number of them varies from 5-6 to 100 spots. An unusually high numbers of sun spots formed in a 200-year period around 1180A.D. No sunspots occurred between 1645-1715 (little ice age). It appears that sun spots also follows cycle of 11-year, 22-year, 33-year. This is believed to be related to sun’s surface magnetic field, which reverse its polarity every 22 years. 3. Variation in atmospheric dust Changes in transparency of the atmosphere result from changes in dust content, cloud cover, and ozone content of the upper atmosphere. A reduction in radiation absorbed by the earth by as little as 1% can produce a change in surface temperatures by as much as 1.2C to 1.5C. Volcanic eruption increases stratospheric temperatures decreases in earth’s surface temperatures (SO2). One hypothesis: during active times of earth-building forces, continued volcanic activity over long periods would have an extended effect. Once glaciations was initiated, the role of volcanic dust would be secondary to changing surface albedo.

8. Short-term climate change (continue) 4. Human induced changes in earth’s surface (a) Use of fire result in deforestation of large areas of the world (b) Farming. 50% of central Europe was converted from forest to farmland over the last 1000 years (c ) desertification (India; Africa, and South America). Misuse of marginal lands (d) Urbanization (e) raw material extraction; energy generation; and other processes that significantly altered the face of the earth

11. Long-term Climate Changes • Earth-sun relationships • the obliquity of the ecliptic: the angle of axis revolves around the sun (is 23.5°) varies 1.5° about the mean of 23.1° during a cycle of 41,000 years Obliquity of 0, equal length of day and night, lack of seasonal change; Obliquity of 54, great extremes in the length of summer and winter days and nights.

12. Earth-sun relationship (continue) (b) Earth’s orbital eccentricity Eccentricity (Le/α) (comparing the path to that of a true circle; smaller the number and more circular). Current is 0.017; it ranged from 0.001 to 0.054. It influences the amount of solar radiation intercepted by the earth and modifies the dates at which the solstice and equinoxes occur.

13. Continue (c ) precession of the equinoxes Days on which the earth reaches at perihelion and aphelion change over time. In 10,000 years, date of perihelion will pass to the Northern Hemisphere summer season. Milutin Milankovitch (Yugoslavian scientist) derived values going back thousands of years-Milankovitch cycles. Based on this cycle, the global cooling which started some 6000 years ago will continue.

14. Long-term Climate Change, Continue 2. Redistribution of continents Plat Tectonic theory suggests the present position of the world’s land masses are but a transitory location in the long-term evolution of the continents and oceans. The primary requirement for the formation of great ice caps is the polar location of continents; the role of mountain building and continental uplift increases the potential for ice caps;

15. 3. Variation in the oceans Changes in SSTs linked to changing circulation patterns and weather anomalies • Drops in sea level would increase heights of the continents and enlarge land masses in areas • Heat storage mechanisms; ocean is less variable in temperature; change in water temperature affect world climate. It also changes in salinity, evaporation rate and relative solar penetration • Mobile medium: ocean currents transport heat.

16. Basic concepts: differential heating of land and water

17. Land–Water Heating Differences  • Evaporation • Transparency • Specific heat: the amount of heat required to increase 1 gram of substance by 1°C. (water has the highest specific heat than anything else on the earth-5 times higher than earth’s rock materials). • Movement • Ocean currents and sea-surface temperatures • Marine vs. continental effects

18. 4. Extraterrestrial impacts Large objects from space have struck the earth, altered climate tremendously for short period of time to 1000 years Cretaceous-Tertiary geologic period (65 millions years ago), an event eliminated over half of all species of organisms living on the earth at the time. Firestorms generated from scattering of molten meteorite and gases (a) removed O2 and added CO2; (b) dust clouds blocked most the sun for month (huge cloud of soot mass of material blown into the atmosphere as the object struck was 100 times the mass of the meteorites)-halting or greatly reducing photosynthesis and had lethal effect on marine organisms; (c) earth’s surface temperature cools; (d) precipitation tuned into acid rain; (d) ozone in stratosphere disappeared for sometime; (e) largest animals were eliminated due to fire related shortage of food supply; (f) Temperature rebounded to levels higher than before the impact afterwards, much of the earth’s biota was killed or could not adjust to the changes; (g) changed seasonal variability of climate due to reduced seasonal CO2 variation; (h) the Milankovitch cycles were greatly strengthened by the event (took hundred thousands years to back to previous levels)

19. 5. Other theories Earth’s climate result from a spectrum of causal elements.