Natural causes of climate change
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Natural causes of climate change

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The Earth’s climate is dynamic and characterised by trends, aberrations and quasi-periodic oscillations varying over a broad range of time-scales [1], which are governed by external ...

The Earth’s climate is dynamic and characterised by trends, aberrations and quasi-periodic oscillations varying over a broad range of time-scales [1], which are governed by external (extraterrestrial systems) and/or internal(ocean, atmosphere and land system). Trends are largely controlled by plate tectonics, and thus to change gradually on million year time scale. Aberrations occur when the certain thresholds are passed and are manifested in the geological record as the unusual rapid (less than a few thousands of years) or extreme change in climate. The quasi-periodic oscillations are mostly astronomically paced; they are driven by astronomical perturbations that affect the earth’s orbit around the sun and the orientation of earth’s rotation axis with respect to its orbital plane. These perturbations are described by the three main astronomical cycles: eccentricity, precession and obliquity, which together determine the spatial and seasonal pattern of insolation received by the earth [2], eventually resulting in climatic oscillations of ten to hundreds of thousands of year [3].Sun being the main source of energy for the earth system controls the climate of it. Variation in solar activity and cosmic ray intensity has direct influence over climatic features such as cloudiness, temperature and rainfall [4]. Volcanic eruptions also force all elements of the climatic systems up to a varying degree but producing long term climatic signals in the ocean. The cumulative volcanic cooling effect at present offsets about one third of anthropogenic warming [5].Other than these causes paleoclimatologists also relates the past climate changes with movement of solar system[6], interplanetary dusts and influence of asteroids[7].However the recent variability in climate what earth is experiencing is unlikely due to any of the individual above factors rather it is due to the compound effect of complex interactions of all the natural as well as anthropogenic forcings.

References:
1. J. C. Zachos, M. Pagani, L. Sloan, E. Thomas, K. Billups, Science 292 (2001) 686-693.
2. G. Kukla, Nature (London) 253, 600 (1975).
3. J. D. Hays, J. Imbrie, N. J. Shackleton, Science 194 (1876) 1121-1132.
4. N. Marsh, H. Swensmark, Space Sci. Rev. 94 (2000) 215-230.
5. T. L. Delworth, V. Ramaswamy, G. L. Stenchikov, Geophys. Res. Lett. 32 (2005) L24709.
6. K. Fuhrer, E. W. Wolf, S. J. Johnsen, J. Geophys. Res. 104(D24) (1999) 31043-31052
7. P. Hut, W. Alvarez, W. P. Elder, T. Hansen, E. G. Kauffman, G. Keller, E. M. Shoemaker & P. R. Weissman, Nature Vol. 329, 10 September, 1987

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Natural causes of climate change Natural causes of climate change Presentation Transcript

  • Natural causes of climate change Debasish Chakraborty Roll No - 4843 Division of Agricultural Physics
  • Climate Variability Inherent characteristics of earth’s climate which manifests itself in changes of climate with time Climate Change It refers to the statistically significant variation in either the mean state of the climate or in its variability persisting to an extended period It is mainly influenced by Natural factors and nowadays it is speeded up by the Anthropogenic factors
  • Solar Output Earth-Sun Geometry Interstellar Dust Earth’s Internal Volcanic Eruption Mountain Building Continental Drift Atmosphere-Ocean Heat Exchange Atmospheric Chemistry Natural Factors Influencing Earth’s Climate Extraterrestrial
  • Earth-Sun Geometry Motion Return Period Effect Rotation 24 Hours Day - Night Revolution 365.25 Days Seasons Axial Tilt 41,000 Years Season, Shifting of tropical & polar belts Eccentricity 100,000 Years Seasonal Extremities Precession 23,000 Years Shifting of Solstices & Equinoxes Wandering of Poles Chandler’s Motion
  • The Astronomical causes The Milankovitch Theory (1924): Johannes Kepler (1571 – 1630): Earth has elliptical orbit path around the Sun Joseph Alphonse Adhemer (1842): Precession cycles , alternate hemisphere James Croll Curve of eccentricity, Winter temperature main for Glaciations and Positive Feedback mechanism Discovered and computed relationship between Earth’s orbital parameters (Precession, obliquity & eccentricity) and solar radiation Summer insolation in the polar latitudes is the main cause of glaciation
  • Eccentricity  Controls the shape of the Earth’s orbit  Eccentricity(e)= (a2 – b2 )½ /a2  The difference in between the perihelion & aphelion is 3% and responsible for 7% variation in Solar radiation on the earth  It varies from about 0.0669 to 0.0001  Earth is now experiencing a lower eccentricity of about 0.0167.  The total cycle takes about 100,000 years
  • Precession Cyclical variation of the earth’s rotation around it’s polar axis By this the timing of equinoxes and solstices changes, so sometimes it is called ‘Precession of Equinoxes’ ‘Precession of Equinoxes’ has a cycle of approximately 23,000 years Is our pole or North star constant? Now our pole star is ‘POLARIS’ But around 2500 BC in the time of Pyramids it was ‘THUBAN’ or ‘ALPHA DRACONIS’.
  • Precession • Now in the time of perihelion Southern Hemisphere is closer to the Sun • Southern Hemisphere summer receives 4% more solar radiation than Northern Hemisphere. • After 11,000 years Northern Hemisphere will be closer to the Sun in the time of perihelion
  • Obliquity or Axial Tilt  It is the cyclical variation of the Earth’s tilt of polar axis  This tilt can deviate from 22.10 to 24.50  At present the tilt is 23.440 and it is decreasing  The Obliquity or Axial Tilt cycle takes about 41,000 years
  •  When the Tilt is small there is less climatic variation between the summer and winter seasons in the middle and high latitudes.  Warmer winters allow for more snow to fall in the high latitude regions  Cooler summers cause snow and ice to accumulate on the Earth's surface because less of this frozen water is melted  So the net effect of a Smaller Tilt would be more extensive formation of glaciers in the polar latitudes. Obliquity or Axial Tilt
  • Milankovitch Cycle and Glaciations Core Selection  Contains continuous climatic record and fast accumulation rates  Located between Africa, Australia and Antarctica to avoid the influence by variation of erosion from the continents Geological Data The Oxygen isotopic composition (δ 18 O) Estimate of summer sea surface temperature at the core site(Ts) Percentage of Cycladophora davisiana
  • Orbital Data  The orbital and insolation changes was calculated using numerical procedure Chronological Models  For testing the orbital theory ,age models must be developed to express each geological variable as a function of time SIMPLEX- Age is estimated as an exact linear function of depth ELBOW- More chronological information and is not expressed as a simple function of depth PATCH- Time series where the records of two cores at the 8 – 7 stage boundary was joined to provide longer and statistically more useful records
  • Fig; Variations in obliquity, precession, and the corresponding frequency components of climate over the past 500,000 years
  • Fig; Variation in eccentricity and climate over the past 500,000years Conclusions: Climate variances is concentrated in three discrete peaks at periods of 23K,42K and 100K years These peaks correspond to the dominant period of the earth’s orbit & contain 10,25 and 50 percent of the climatic variances The changes in the Earth’s orbital geometry are the ‘Fundamental causes of the succession of ice ages’
  • Continental Drift-Theory of plate Tectonics Motion in earth's asthenosphere which causes global scale dynamics of the rigid lithospheric plates Different Mechanisms: Plates moves at an average rate of 3 cm per year
  • Evolution of present continental conformation
  • Altering the Distribution of Land Masses Changing Continental Elevation Variability in Atmospheric Concentration of CO2 Major Factors ofMajor Effects of Plate Tectonics
  •  Thermal Inertia: Oceans have a higher thermal heat capacity than land.  High Latitude Land Area: More glaciers over land, higher albedo, cooler temperature  Surface Albedo: Amount of ocean versus land surface at low latitudes: great affect of absorbed solar energy or reflection Altering the Distribution of Land Masses:
  •  Restrictions to Ocean Currents: Oceanic circulation is primary mechanism by which heat is redistributed from equatorial to polar latitudes. Continental barrier blocks the oceanic circulation & thus creates problems to redistribution of heat throughout the globe. Cretaceous warming T.H. van Andel, "New Views on an Old Planet, a history of global change", Cambridge (1994)4
  • Changing Continental Elevation  Change in Elevation or Mountain Building: Atmospheric temperatures decrease with increase in height (6.5°C per km). High elevation leads to cold temperatures which ultimately produces snow coverage & high albedo situation. Even today the Himalayan range is rising by about 1mm/year as Indian plate slowly moves towards the Eurasian plate.  Atmospheric circulation: Wind belts of the earth are influenced by pressure differences across mountain chains. A change in topography may alter the distribution of air masses.  Regional Climate: Different climate in different sides of the mountain
  • Variability in Atmospheric Concentration of CO2 Atmospheric carbon dioxide is released in the process of weathering of minerals . Weathering of minerals is accelerated by Higher temperature & Collision of Continents Carbon dioxide is released through Drifting of the Continents(Mountain Building) Volcanic Activity
  • Volcanoes VEI 0 1 2 3 4 5 6 7 8 Eject Volume <10,00 0 m3 >10,0 00 m3 >1,000, 000 m3 >10,000, 000 m3 >0.1 km3 >1 Km3 >10 Km3 >100 Km3 >1000 Km3 Plume <100 m 1 Km 5 Km 15 Km 25 Km >25 Km >25 Km >25 Km >25 Km Example Mauna Loa Strom boli Galeras (1993) Cordon Caulle (1921 Eyjafjallaj okull (2010) Mt. St. Helne s (1980 ) Mt. Pinat ubo 1991 Tambo ra (1815) Taup o (26,5 00 BP) Frequency many Many 3477 868 421 166 51 5 0
  • Volcanic Eruption: Influence of Volcanoes Very Short Term Volcanic ash Short Term Mid to Long Term Aerosol Atmospheric Circulation Hydrological Cycle Ocean Scientists belief that volcanoes are having profound effect on climate. But these effects are time dependent.
  •  These ashes lasts for few months  Blocks the penetration of sunlight into the atmosphere  Tropospheric temperature drops down Volcanic ash
  • Net Radiative Balance = -3 W/m2 Decrease in Global Surface Air Temp.= 0.5 K Global Visible Optical Depth Maximizes to 0.15 Mount Pinatubo,1991
  • Hydrological Cycle Precipitation is more sensitive to variation in solar SW radiation Precipitation drops over land in 1st year due to rapid radiative cooling But the precipitation over ocean are delayed & maximum reduced after 3-4 years of eruption Geographically the precipitation anomalies are located in low latitude monsoon region
  • Atmospheric Circulation  In the 2nd year following eruptions, the NH winter tropospheric circulation has typically been observed to display features of an anomalously positive AO situation  The Zonal mean expression is the creation of Low pressure at the high latitudes & High pressure at the Mid latitudes i. e. North Atlantic and Mediterranean sectors (North Atlantic Oscillation)  Pole ward shift of Atlantic storm track & an increased flow of warm air to Northern Europe and Asia where anomalously higher winter temperature is observed  Only low latitude volcanic eruptions could effect the AO/NAO phase Stenchikov, Hamilton,Ramaswamy, 2003
  • Ocean  Ocean Comprises almost the entire thermal capacity of the climate system. Their thermal inertia delays full scale response of the earth’s surface temperature to greenhouse warming  Ocean warming cause expansion of water and therefore effects the sea level, called thermosteric height  The maximum Heat content and Sea level decrease after Pinatubo eruption is 5 * 10 22 J & 9 mm respectively Stenchikov et. al. (2007)  The characteristics e-folding time for ocean heat content or steric height is about 40 – 50 years and complete relaxation requires 2-3 relaxation times
  • Oceanic Circulation  SW cooling from volcanic aerosols results in a cold surface temperature anomaly  Volcanically induced cooling leads to reduced precipitation and river run-off in High latitudes of Northern Hemisphere  So, upper ocean condition in the higher latitudes of the northern hemisphere becomes saline (denser)  Colder ocean temperature & enhanced salinity destabilizes the water column , making them more prone to ocean convection which in turn enhances the MOC (Meridional Overturning Circulation)  The maximum increase in MOC is 1.8 sverdrups or 9%
  • Solar Activity and Climate  Solar activity and cosmic ray is negatively correlated  There is a strong correlation between the previous cold and warm climatic periods & high and low levels of cosmic ray intensity
  • Cosmic Rays and Cloud Cover  Clouds effect the irradiative properties of the atmosphere by both cooling through reflection of incoming SW solar radiation and heating through trapping of outgoing LW radiation  High optically thin cloud tend to heat and low optically thick cloud tend to cool the atmosphere
  • Increasing Cosmic Ray intensity leads to increase in the low level cloud coverage which ultimately reduces the surface temperature. > 6.5 Km < 3.2 Km 6.5 – 3.2 Km
  • Cosmic Ray & Rainfall  The Cosmic Ray intensity is directly proportional to the rate of ion generation over the mid latitudes  The Rate of ion generation is highly correlated to the total surface of cloud cover with a correlation coefficient of 0.91 .  Stozhkov analyzed 70 events of Forbush decrease of CR intensity observed in 1956-1993 & compared this event with the rainfall data over USSR. He showed that the daily rainfall levels decreases by 17%.
  • Cosmic Ray & The Earth’s Temperature  Swensmark showed that the increase of air temperature by 0.30 c corresponds to a decrease of CR intensity of 3.5%.  This Decrease in CR intensity reduces the global cloudiness by 3%, equivalent to an increase of solar irradiance by 1.5 W/m2  This change in irradiance is 5 times bigger than the solar cycle irradiance change (0.3 W/m2 )
  • Forecasting of Climate Change by Cosmic RayRays  CR intensity predicted on the basis of monthly Sunspot numbers from model of Convection- Diffusion & Drift mechanism.  The correlation coefficient is 0.97.
  • Earth’s Climate and movement of solar system  Paleoenvironmental records suggests that in the last 520 M years Earth has gone through four alternating warming and cooling periods with temperature change more than 50 c  During this periods the Solar system crossed galaxy arms four times  In this time inflow of comets from Oort’s cloud occurs, which results in an increase in concentration of interplanetary dusts in Zodiac cloud and cools earth’s climate Fuhrer, Wolff, Johnsen, 1999
  • Earth’s Climate and Comet  Comet in past have struck the Earth with huge force to make major changes in the Earth’s climate (the famous Dinosaurs killing mass extinction at the end of the cretaceous)  The study published in ‘New Scientists’ journal’ tells us a step forward that the comet struck the earth near India & is the main cause for extinction of Dinosaur
  • IPCC WG1 AR4 Observed Changes and Future Prediction  Natural factors leading to a cooler climate  Anthropogenic factors are more dominant
  • Conclusions:  The changes in the past climate is mostly correlated with the change of Earth-Sun geometry i. e. Milankovitch Cycles  The changes due to Volcanoes & solar activity is the causes of increasing of the abruptness of the change.  Though with the advancement of science many methods are developed (proxy data) to know about the past, but the accuracy is questionable  As the climate change is the net effect of complex interaction of all the factors ,more research is needed to gather the knowledge about interactions to make our predictability more accurate
  • What is our future? Thank You