ES 1111 Chapter 8: The Causes of Climatic Change (with some chapter 2)
Earth-Sun Relationships • Energy received from the Sun drives weather and climate, so it is obvious to start with changes associated with the Sun and our orbit around the Sun • The regular changes of the Earth’s orbit around the Sun, and changes in the Earth’s rotation about its tilted axis, control the diurnal and seasonal cycle and dominate the climatology of the Earth
Changes in Eccentricity • Eccentricity: Measure of how circular or elliptical our orbit is around the Sun. • Changes in 100,000 and 410,000 year cycles • Current value is 1.7% and decreasing. • Fluctuates between 0.1% and 5.4% • Low values of eccentricity correlate to no change in insolation during the year (whereas now there is a difference between aphelion and perihelion)
Precession of the Equinoxes • Precession: The change in the direction that the North Pole points (slow top-like rotation). • Changes in 19,000 to 23,000 (22,000) year cycle • In 12,000 years, we will experience summer in December and winter in June. • This can combine with eccentricity to have a seasonal effect. For example, if the summer solstice occurs during perihelion, summers would be warmer and winters colder than today.
Obliquity • Obliquity: The inclination (tilt) of the Earth’s axis to the plane of the Earth’s orbit. • Changes in a 41,000 year cycle • The current value is 23.4 degrees and decreasing. • Fluctuates between 22 degrees and 24.5 degrees. • Changes in obliquity will have a big effect on seasons. • If the obliquity reduces to zero, the seasonal cycle would vanish and pole-to-equator contrasts would sharpen. This would lead to colder periods at high latitudes.
Earth’s Orbital Cycles Figure 2.5, Page 21
Earth’s Orbital Cycles Figure 2.6, Page 22
Climate and Earth’s Orbital Cycles • Astronomer Milankovitch hypothesized that synchronization of the three cycles could have a major impact on the Earth’s climate • The key to explaining ice ages are changes that would cause insolation at high latitudes to decrease during the summer (thereby allowing winter snowfall to remain largely intact)
Changes in Solar Output • The Sun is a very stable star, but there are fluctuations in output that are detectable using satellites. • Sunspot – dark features seen on the sun that correspond to cool areas compared to the surrounding photosphere. • A sunspot has two parts: • Umbra: the central dark area • Penumbra: lighter area surrounding umbra • The number of sunspots varies in an 11-year cycle. • When the number of sunspots is large, solar output is slightly higher (0.1%) • Why higher? Aren’t sunspots cold (and therefore radiate less according to Wien’s law)?! • Surrounding sunspots are bright areas on the sun called faculae. • The increase in radiation from the faculae compensate for the cold sunspots.
Sunspots and Faculae Figure from other text, showing sunspots and faculae
Sunspot Cycles Figure 2.10, Page 28
Sunspots and Solar Constant Figure 2.11, Page 29
Sunspots and Climate Change • “Maunder Minimum” – time during late-17th century with an almost complete lack of sunspots. • The Maunder Minimum coincides with the “Little Ice Age” • Observed difference of 0.1% is too small to have a direct effect on temperature. So how can sunspot activity (or lack thereof) cause climate shifts? • During increased solar activity, 20% of the change in output takes place in the ultraviolet part of the spectrum since the faculae are very hot and emit ultraviolet, X-rays, and gamma rays. • Possible linkage between stratosphere and surface temperatures (changes in stratospheric ozone due to more UV radiation could lead to changes in insolation in the lower atmosphere). • Computer models suggest increased UV radiation could lead to the expansion of the Hadley cell and cause a poleward shift of storm tracks. • More UV radiation could initiate more cloud condensation nuclei (free radicals) and make it cloudier.
Solar Causes of Climate Change • Sunspots also have a 22-year magnetic field cycle (magnetic field of sunspots reverse after each 11-year max/min cycle). Changes could result in changes in the interaction of the solar wind with the Earth’s magnetosphere. • There could also be a linkage between a change in gamma rays emitted by the sun and the electrical field of the Earth…which in turn could cause changes in thunderstorm activity. • Gleissberg cycle – longer sunspot cycle of 90-100 years that causes variation in the peaks of the 11-year sunspot cycle. • Study by Lean: Changes in UV forcing can account for half of the warming between 1860 and 1970, and account for 1/3 of the rapid increase seen between 1970 and 1990. • Longer-term changes: The sun is slowly increasing its output of radiation. • 4 billion years ago, the solar constant was only 80% that of today.
Tidal Forces and Climate Change • Gravitational attraction causing the atmosphere and oceans to fluctuate, and can also cause changes in Earth’s orbit • Tides - combined pull of Moon and Sun or from pull of other planets (smaller than first source) • Gravitational forces can impact weather and climate by • changing location of perihelion and perigee • planetary tides changing solar output? (coincidence that 11 year sunspot cycle matches closely with Jupiter’s 11.86 year orbital period?) • Earth speeding up or slowing down in orbit (spend more time on winter/summer side of orbit)
The Biosphere and Climate Change • The biosphere is the totality of living matter on the land and in the oceans. • The biosphere can impact climate by: • Acting as a temporary source and sink of greenhouse gases • Photosynthesis: carbon dioxide removed • Decay: methane (greenhouse gas) released • Productivity of biosphere increases with increasing CO2 (Negative feedback to global warming). • Producing particulates (volatile organic substances): phytoplankton producing dimethylsulphide converts to sulphate particulates, and sulphate particles are excellent CCN. Increased clouds – negative feedback to warming • Changing albedo of the surface • Desertification – deserts have high albedo • Deforestation – forest and snow has a lower albedo than just snow.
Volcanoes and Climate Change • Benjamin Franklin first identified the potential of volcanoes to alter the climate. • Vast amounts of dust thrown into the stratosphere, as well as vast amounts of sulphur dioxide (CCN), results in more sunlight scattered. • Little vertical motion in stratosphere – minute particles can remain suspended for several years and be spread globally. • Short-lived change – effects last for 2-3 years • Substantial summer cooling, some winter warming (greater circulation) has been observed with eruptions of Pinatubo and El Chichon). • May trigger lasting change when combined with other perturbations
Changes in Volcanic Activity • Can changes in sea level create stresses on the Earth’s crust such that volcanic activity increases? • Evidence exists that volcanoes in Mediterranean erupt more commonly during rapid sea level changes. • Can changes in atmospheric circulation also lead to changes in crustal stresses? • Increase in global atmospheric circulation should cause Earth to rotate more slowly due to the Conservation of Angular Momentum • El Nino of 1997-98 caused Earth to slow 0.4 ms. • Changes in rotation rate could cause changes in stress that could also unleash volcanoes.
Plate Tectonics and Climate Change • Arrangement of continents – Movement of continents in Northern Hemisphere poleward • More land and less water to be up in high latitudes • Results in cooling trend in high latitudes. • Mountain uplift • Alter global circulation by producing Rossby waves in the atmosphere
Other Causes of Climate Change • Asteroid/Comet Impact – Massive global cooling would result in mass extinctions (more on this topic in the last unit of this course) • Man-Made Climate Change • Consumption of Fossil Fuels (release of carbon dioxide into the atmosphere) • Nuclear Winter – global cooling after massive nuclear exchange (more on this topic later too)