Short-Term Climate Variability (Ch.15) Volcanos and Climate Other Causes of Holocene Climate Change

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1 Short-Term Climate Variability (Ch.15) Volcanos and Climate Other Causes of Holocene Climate Change

2 Volcanos and Climate We learned in Chapter 12 that the volanos play an important role in Earth s climate on geologic time scale (an example?). Volcanic eruptions can also have a direct impact on climate through volcanic aerosols which reflect solar radiation back to space. Scientists once believed that the volcanic ash injected into the atmosphere during an eruption would result in an increase in global albedo, leading to a surface cooling. But volcanic ash, falling out of the atmosphere quickly has little impact on climate.

3 Volcanic eruptions Volcano puts out a mixture of ash, lava, and sulfur dioxide (SO2) Ash and lava fall out of atmosphere quickly (days to a few weeks at most) Oxidization of SO2 => forming H2SO4 (sulfuric acid) droplets (volcanic clouds) which spread globally in the stratosphere (if SO2 was injected to high latitude in the stratosphere) These particles can remain for several years, gradually dissipating in time Volcanic cloud reflects solar radiation, cooling the planet Also absorbs solar radiation and outgoing infrared, warming the stratosphere What important to climate is the gas (SO2) injected to the stratosphere!

4 The degree to which a volcanic eruption affects climate depends on the location, strength, and the way the atmospheric circulation distributes the aerosols. Volcanic eruptions at the tropics with the SO2 injected to the stratosphere have larger impact on climate. E.g., Pinatubo eruption lead to a reduction in the mean global surface temperature on the order of 0.5 o C in the late 1992.

5 Eruption of Mt. Pinatubo Philippine Islands

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8 Layer at ~22 km

9 Shuttle picture of Pinatubo aerosol layer

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15 Volcanic eruption effects on climate Cool surface and troposphere by reflecting solar radiation (typical effect is ~-0.1 to -0.5 o C) Warm stratosphere by absorbing solar radiation (about 1 o C) Effects last for a year or two

16 Why didn t Mount St. Helens eruption have a climate effect? Small eruption (didn t seem so around here!) Not much SO2 in eruption cloud Much of blast went sideways - didn t get to stratosphere Too far poleward

17 How do we know about earlier eruptions Ice layer has an anomalous high acidity during a volcanic eruption: volcanic eruption => acidic material => deposited on a glacier surface during snowfall From ice cores, record of the frequency and magnitude of past volcanic events can be constructed.

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19 Indonesian volcano Tambora Largest eruption in modern times (44 km altitude, km3 of ash, five times larger than the Pinatubo) Caused immense local destruction (92,000 people died) and the year without a summer in Europe and Asia (about 3 C decrease in average summer temperature)

20 Volcano summary For current (Holocene and Pleistocene) climate, volcanic eruptions are small perturbations on climate lasting a year or two Tropical eruptions have more climate significance In earlier eras, volcanic eruptions may have had more impact Large eruption events, sustained through time Significant alteration of atmospheric chemistry

21 Other Causes of Holocene Climate Change We used changes in ocean circulation (Younger Dryas) and volcanic eruptions to explain some of the more extreme changes in climate following the end of the last glacial period. Although a large number of theories have been put forward, the actual causes of Holocene climate change are not very well understood.

22 1. Orbital change and greenhouse gas variations In the Holocene, seasonality increased in the NH and decreased in the SH. CO2: 200 ppm (21,000 years ago) => 265 ppm (middle of Holocene). The broad pattern of change from the last glacial maximum through the Holocene climate optimum to the present can be explained based on orbital changes and greenhouse variation.

23 2. Solar variability We have already seen that the solar radiation has changed considerably over the long period of Earth s history. At the shorter time scale we tend to regard the solar output as constant. In fact, the solar output probably varies slightly all the time. The greatest changes in solar output are related to sunspot.

24 Sunspots Dark areas of lower-than-normal temperatures on the surface of the sun. Sunspot data since 1610 by Galileo (reliable data in the past 150 years). 11-year cycle in numbers and 22-year cycle of magnetic reversals. An increase in the number of sunspots leads to an increase in amount of solar radiation because the sunspots are surrounded by bright area of higher-than-normal temperature called plages. And the area of plages is larger than the area of spots.

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26 No explanation (no physical mechanism); Purely coincidental? The variation of incoming solar radiation between sunspot minimum and maximum is ~0.1% =>1.4 Wm -2 /4*(1-0.3) = 0.25 Wm -2. It is too small to have a direct climatic consequences (?).

27 14 C content of tree rings => sun spots: High sunspot activity => stronger magnetic field strength of the solar wind => greater deflection of cosmic rays away from Earth => less 14 C production in the atmosphere.. The direct observations and the proxy data indicate that Maunder Minimum ( ), Sporer Minimum ( ) and Wolf Minimum ( ), which might be partly responsible for the Little Ice Age, and great sun spot activity in , which might be partly responsible to Medieval warm period.

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29 In last 20,000 years Summary The broad trends: Orbital variation &CO2 Decadal-to-century: Solar activity and ocean circulation Shorter time scales: Volcanic activity The nature of the climate changes that occurred at given time may be a result of some combination of various factors (e.g., little ice age is due to both low solar activity and enhanced volcanic activity).

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31 Quiz #6 Biogenic Cloud-Albedo Feedbacks 1. Fill in the following feedback diagram assuming that global surface T has a negative coupling with marine algal productivity. Is this a negative or positive feedback on T? Biogenic Aerosol Cloud Albedo Marine Algal Productivity Global Surface Temperature 2. Why does lower sea surface temperature increase algal productivity? (ie: What happens to stratification, and mixing of nutrients?)

32 Quiz #6 Biogenic Cloud-Albedo Feedbacks 1. Fill in the following feedback diagram assuming that global surface T has a negative coupling with marine algal productivity. Is this a negative or positive feedback on T? Biogenic Aerosol Cloud Albedo (Positive!) Marine Algal Productivity Global Surface Temperature 2. Why does lower sea surface temperature increase algal productivity? (ie: What happens to stratification, and mixing of nutrients?) Lower sea surface temperature reduces stratification, allowing easier mixing of nutrients back to the surface, and therefore increasing productivity.