ATMS 321: Natural Climate Variability Chapter 11 Solar Variability: Total solar irradiance variability is relatively small about a tenth of a percent. Ultraviolet variability is larger, and so could affect surface climate through the ozone layer, probably. Volcanic Eruptions: Volcanoes can have an influence on climate by putting sulfur dioxide (SO2) into the stratosphere, which oxidizes twice to form sulfuric acid (H2SO4), which condenses to form tiny droplet aerosols that float around for a year or so in the stratosphere and reflect solar radiation, cooling the surface. For this to be effective you need an explosive eruption with a lot of sulfur and it helps it if is in the tropics, where the resulting aerosols will last longer and reflect more solar radiation. The most recent example is the Pinatubo eruption in the Philippines in June of 1991, before that el Chichon in Mexico.
Pinatubo Eruption: Effect on Earth s Energy Balance Soden, et al., Science, 296,727 About six months after the eruption the absorbed shortwave is down by about 4 Wm - 2 and the outgoing longwave is decreased by about 2.5 Wm - 2. The absorbed shortwave reduction is felt at the surface, the longwave is felt more in the atmosphere. It takes about 6 months for the SO2 to be fully converted into aerosols that will reflect solar radiation.
The temperature responds by cooling about half a degree Celsius. This change is modeled pretty well by the Global Climate Model (GCM). The model also captures the observed reduction in atmospheric water vapor that occurs with the cooling. Water vapor s saturation vapor pressure depends on temperature, so as the surface and atmosphere cool we expect the water vapor mixing ratio to decline also. Since water vapor is the primary greenhouse gas, this constitutes a positive feedback. Warmer climate > more water vapor > more greenhouse effect > warmer climate
Milankovitch: The Orbital Parameter Theory of Pleistocene Ice Ages Eccentricity: A measure of by how much the orbit deviates from a perfect circle and is thereby more elliptical Earth s orbit about the Sun has an eccentricity that varies from zero to about 0.06, and varies with periods of about 100,000 and 400,000 years. It is currently about 0.015. Since the distance from the sun varies, the insolation will be less at aphelion and more at perihelion, so the insolation goes through one max and one min per year. The annual mean variation depends on eccentricity squared, and so for Earth s modest (e<0.06) eccentricities, eccentricity mostly affects the annual cycle of insolation. Obliquity: The tilt of the Earth s axis of rotation relative to the plane of the Earth s orbit. Currently 23.45, and varies from 22-24.5 with a period of about 41,000 years. Obliquity defines the strength of the seasonal shift of insolation from hemisphere to hemisphere. As the obliquity increases, the poles get more annual mean insolation by getting more in summer. For zero obliquity, the poles never get any direct sunlight. Longitude of Perihelion: Measures the phase of the seasons relative to perihelion. A standard definition is to measure the angle between the vernal equinox and perihelion!. It varies with periods of about 19,000 and 23,000 years.
Eccentricity: Varies with periods of 100,000 and 400,000 years.
Distribution of Insolation with orbital parameters: TSI fixed Insolation = S o 4 s(e,!,", x,v) = S o 4!s(", x,v) d 2 d(v) 2 e = eccentricity! = longitude of perihelion! = obliquity x = sine of latitude v = angle defining time of year d = Earth- Sun distance, which is a function of eccentricity and longitude of perihelion. If we are interested primarily in insolation at Northern Summer Soltice (since we think the insolation over the NH continents is critical for ice ages, we can write approximately, that. (see book) Insolation = S o 4 s(e,!,", x,v) # S o 4!s(", x,v) ( 1+ 2esin! ) The function!s(!, x,v) shows the distribution with latitude and season that depends on obliquity, and the Precession Parameter = 2esin! is a critical measure of how much insolation we get during summer
Insolation Distribution and Obliquity. The top panel on the left shows a distribution function s! (!, x, v) for insolation for eccentricity zero and obliquity of 23.5. You would multiply this by S/4 to get the actual insolation as a function of latitude and season. The global mean of this distribution function is 1.0. The bottom panel shows the sensitivity of the top function to an increase in obliquity of 2. You can see that the summer value in high latitudes increases by more than 0.12, or close to 10%. Since the wintertime value does not change, in the annual mean you get more insolation in high latitudes with increased obliquity and correspondingly less in lower latitudes. One might therefore expect an increase in obliquity to result in less glacier ice in high latitudes. Right?
The past and future history of Obliquity and the precession parameter. The obliquity (solid line, scale on right) varies fairly regularly with about a 41,000 year period. The precession parameter 2esin! has cycles of 19,000 and 23,000 years, so it beats a bit, and it also depends on the eccentricity. When the eccentricity is small, it does not matter so much. Can you use this plot to predict the ice age 20,000 years ago and the transition to an interglacial after that? What about the glacial transition to the interglacial 125,000 years ago?
Insolation anomalies as a function of latitude and time calculated from orbital parameters.
Testing the Milankovitch theory with Paleo- Data 1. As we have seen, you can argue that the last couple of glacial interglacial cycles are synched up temporally with decreased high latitude Northern Hemisphere insolation for glacial maxima and increased high latitude Northern Hemisphere insolation for interglacials. So we can argue that orbital parameters are setting the timing of these changes. 2. The 0 18 isotope global ice volume record has some similar periodicities to those of the orbital parameters. One perplexing thing is that the paleo record has a huge 100,000 year peak in the recent record, even though the annual mean insolation associated with eccentricity is small. The seasonal cycle must be very important in explaining this, as the precession parameter depends on eccentricity and affects the summertime insolation. 3. The climate in the two hemispheres is simultaneous, with ice ages in the NH and SH at the same time. This must be due to the carbon cycle response to glacial ages, perhaps through the oceans. Since the atmospheric CO2 concentration goes up and down and explains about half of the temperature change, and CO2 is well mixed, the CO2 concentration can synchronize climate change in the SH with land ice volume in the NH.
EDML is a compound acronym meaning EPICA Dronning Maud Land in Antarctica (75S, 0E, 2,892 meters above sea level) EDC and Byrd Station are also in Antarctica NGRIP is from Greenland
What you are supposed to be able to discern from this is that when Northern Greenland is cold during the MIS3 period (beige shading and black line), the region near the EDML site in Antarctica is warm (purple line), and vice versa. The idea is that the ocean heat flux in the Atlantic has a see- saw in time. When it is really cold in the North Atlantic and deep water formation there is suppressed, the Antarctic Atlantic Ocean warms up. This makes sense in terms of the global circulation of deep water in the Atlantic. Note that stadial means a period of relative cold within a glacial age. These cold spells are called Heinrich events in this case (H1, H2, H3, H4 and H5 in the NGRIP record (black line). These events tend to be warm in the EDML curve from Antarctica. EPICA, Nature, 444, 195- So during a Heinrich event the North Atlantic area is relatively cold during an ice age and a big surge of ice flow occurs. The cold is inferred from the ice core isotope record and the surge of glacier ice is inferred from ice- rafted debris in ocean cores from the North Atlantic Ocean. When this occurs the meridional overturning circulation of the Atlantic Ocean is reduced or suppressed entirely. When this happens the South Atlantic near Antarctica is relatively warm, as measured by isotopes in ice near that region (EDML core). It is also shown that the longer the stadial event (cold even) in the North Atlantic, the greater warming is observed in the South Atlantic. The physical connection is the ocean circulation. When more deep cold water is formed in the N. Atlantic, more deep cold water is brought up around Antarctica in the South Atlantic.