Climate Drivers Global Change Ecology Botany 275
External and Internal Drivers of Climate External Drivers -Sunspot Cycles -Orbital Variations Internal Drivers -Plate Tectonics -Volcanic Activity -Albedo -Greenhouse Effect
Climate forcing mechanisms Mechanism Time period 1. Solar Forcing Solar intensity (sunspots) (10 s to 100 s of years) Orbital Variations (Thousands of years) 2. Plate Tectonics (Millions of years) Mountain building, continent locations 3. Albedo (all time scales) 4. Aerosols (1-10 years) Volcanoes, pollution 5. Greenhouse Effect (all time scales) CO2, Methane, Water vapor 6. Land use (1 to 100 s of years)
Climate Drivers Incoming solar energy
Radiative Forcing of Climate Change Measured in units of watts per square meter (watts/m 2 ) Average solar radiation reaching the earth at the top of the atmosphere=1370 watts/m 2 This equates to 343 watts/m 2 when distributed uniformly over the earth s surface. For reference: A doubling of CO 2 from pre-industrial level of 280 ppmv to 560 ppmv results in radiative forcing of about 4 watts/m 2,
Estimated solar irradiance variations One way to measure solar intensity is from satellite observations, which are available only since the late 1970 s. These show solar variations of about 0.2 watts/m 2. This graph shows estimates of changes in solar output since 1750. 1750-2000
Figure 2.17
Figure 2.16
Sunspots
Sunspot Cycles Number of sunspots Maunder
Climate Drivers Orbital Variations or Milankovitch Cycles
Milutin Milankovitch 1879-1958 http://earthobservatory.nasa.gov/library/giants/milankovitch/
Current Eccentricity
Variation in Axial Obliquity, 40,000 year cycle Tilt of the axis http://earthobservatory.nasa.gov/library/giants/milankovitch/
Precession of the equinoxes, ~19,000 to 23,000 year Cycle: Direction of tilt Wobbling top http://earthobservatory.nasa.gov/library/giants/milankovitch/
Orbital Variations: Milankovitch Cycles Orbital Eccentricity. Shape of the Earth s orbit (cycles ~100,000 years) changes the distance between the Earth and Sun Axial tilt (cycles over 41,000 years) changes noon day Sun elevation and daylength Precession of the equinoxes (cycles over 19,000-23,000 years) changes when winter and summer occur on Earth.
Insolation at 65 degrees north latitude from the present to 1 million years ago Berger 1991
Vostok time series and insolation Atm Temp
Question: 1. How do Milankovitch cycles (e.g., changes in orbital parameters) lead to the onset and termination of ice ages? (e.g., warm summers & cold winters, cool summers and warm winters, etc.) 2. Propose a sequence of events or processes considering that the overall change in insolation is small (e.g., ~0.25 watts/m 2 ), but that the ice ages are global.
-35 per mille VSMOW -45 275 700 ppbv 400 ppmv 100 GISP2 (N. Atlantic SSTs) Methane CO2 Sea-Level (Ice Volume) 175 m 50 0 warm July (45 deg. N) W/m2-50 cold 25 20 15 10 5 Calendar ka Thousands of years ago Insolation January (45 deg. N) 0
-35 per mille VSMOW -45 275 700 ppbv 400 ppmv 100 Small ice sheets GISP2 (N. Atlantic SSTs) Methane CO2 Sea-Level (Ice Volume) 175 m 50 W/m2 0 Large ice sheets July (45 deg. N) Insolation -50 25 20 15 10 5 Calendar ka January (45 deg. N) Thousands of years ago 0
-35 per mille VSMOW -45 275 700 ppbv 400 ppmv 100 High CO2 GISP2 (N. Atlantic SSTs) Methane CO2 (from ice cores) Sea-Level (Ice Volume) 175 m Low CO2 50 0 July (45 deg. N) W/m2-50 25 20 15 10 5 Calendar ka Thousands of years ago Insolation January (45 deg. N) 0
per mille VSMOW -35-45 275 700 ppbv 400 ppmv 100 warm cold GISP2 (Oxygen isotopes- Measures air temperature) Methane CO2 (from ice cores) Sea-Level (Ice Volume) 175 m 50 0 July (45 deg. N) W/m2-50 25 20 15 10 5 Calendar ka Thousands of years ago Insolation January (45 deg. N) 0
Milankovitch Theory of Ice Ages The Milankovitch (1941) theory of the ice ages assumes that summer insolation anomalies at high latitudes in the Northern Hemisphere (NH) drive the ice ages: minimum summer insolation allows snow and ice accumulated in the cold season to survive, while maximum summer insolation tends to melt the ice sheets. Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Alternative Theory of Ice Ages Hansen et al. 2007 suggest that spring is the critical season for terminations, because the albedo feedback works via the large change in absorbed sunlight that begins once the ice/snow surface becomes wet, after which the surface albedo remains low until thick fresh snow accumulates. A spring maximum of insolation anomaly pushes the first melt earlier in the year, without comparable shortening of autumn melt, thus abetting ice sheet disintegration. And an increase of GHGs stretches the melt season both earlier and later, while also increasing midsummer melt. Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Figure 3. (a) Temperature, CO2, and sea level (SL), (b) late spring (April,May,June) insolation at 60 degrees N and (c) late spring (October, November, December) insolation at 75 degrees S. Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Could both theories contribute to glacial cycles? Perhaps the Milankovitch theory (summer/winter insolation) could lead to the initial formation of ice sheets, but the Hansen theory leads to the sudden termination.
Vostok Ice Core Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Question: 1. The increase in temperature in the Vostok ice core seems to preceed CO 2 rise by several hundred years. How do you account for this? What does this imply about the role of CO 2? 2. How do would you respond to a sceptic that uses this result to argue that we therefore should not be concerned about rising CO 2 levels?
Question: 1. How do would you respond to a sceptic that argues that solar output and sunspot activity control global temperatures (and therefore we should not be concerned about rising CO 2 levels)?
Solar forces have affected the climate system 1 Radiative forcing (W/m2) 0-1 -2-3 -4 1900 1950 2000
Figure SPM.2
Question: What is the role of external forcing likely to be in recent warming?
Question: 1. What seasonal distribution of insolation do you expect to lead to onset and end of ice ages? (e.g., warm summers & cold winters, cool summers and warm winters, etc.) 2. Why? Propose a sequence of events or processes that explain and support your answer to 1.
Variation in Orbital Eccentricity (~100,000 year cycle) perihelion aphelion http://earthobservatory.nasa.gov/library/giants/milankovitch/
SPM 3
External and Internal Drivers of Climate External -Sunspot Cycles (Decades) -Orbital Variations (Thousands of years) Internal -Plate Tectonics (Millions of years) -Volcanic Activity (1-3 years) -Albedo (All time scales) -Greenhouse Effect (All time scales)
What is climate? Climate is average weather and its variability for a particular region over a period of time Climate is what we expect, weather is what we get.
What is climate change? Climate change is a shift in climate relative to a given reference time period It is caused by: Natural factors -Solar variability -Volcanic dust levels -Internal variability -Geological change Human factors - Greenhouse gases - Aerosols -Ozone depletion -Land use change
Proxy data also indicate that the recent warming is likely unprecedented in at least the past millennium Source: IPCC(2001)
Review FAQ 6.1, Figure 1 What are the periodicities associated with each orbital variation?
Questions: 1. How do you expect orbital eccentricity and precision to interact to affect seasonal insolation? 2. How about obliquity and precision? 3. Under what conditions might we expect most seasonal variation in insolation?
Record of oxygen isotopes in ocean sediment over the last 800,000 years shows several glaciations (The last glaciation was 18,000 years ago) Warm (interglacial) Thousands of Years Ago Cold (glacial)