meteo 1020 – lecture 4 the natural variability of the...
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Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
• We have discussed evidence that suggests the earth is warming due to human outgassing of carbon dioxide
• Could we be wrong? Consider the major criticisms of the global warming hypothesis:
• Instrumental error• Urban heat island• Others
• A potential cause of the present warming may be due to natural variability in the climate system.
• We will examine some of these issues in the present chapter
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Distinguishing between internal or external forcing…
A force as we use it here is energy imparted to a system that causes it to change in some way….
External Forcing:
Internal Forcing:
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Distinguishing between internal or external forcing…
A force as we use it here is energy imparted to a system that causes it to change in some way….
External Forcing: A force that cannot itself be modified by changes to the system.
Internal Forcing: A force that itself can be changed by changes to the system – i.e. feedbacks.
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
• Natural variability can be classified by its forcing type (internal or external to the climate system) and whether it is periodic/cyclic or episodic.
Modes of Natural Variability that we know of:timescale form forcing
1. Diurnal 24 hours cyclic external1. Seasonal 90 days cyclic external2. El nino ~3-5 yrs cyclic internal3. North Atlantic Oscillation ~interannual cyclic internal?4. Pacific Decadal Ocillation ~decadal cyclic Internal?5. Volcanism geologic episodic external6. Ice ages ~100,000 yrs cyclic Both
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
The climate effects of volcanic eruptions:
What makes an eruption climatically significant?
• nature of the eruption –lava vs. ash (ash is more significant)• composition – need high sulfur dioxide gas content• location – Tropical eruption spread globally
Mt Pinatubo Eruption – June 15, 1991
In most eruptions, the ejected solids are locally significant but have a minor global effect. If the sulfur dioxide gas can reach the stratosphere, it condenses into small sulfuric acid droplets that have long residence times in the stable stratosphere. It is this cloud of sulphur particles that can spread and influence climate over several annual cycles.
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Pinatubo Sulfuric Acid layer
Tropical Thunderstorms
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Where a volcano happens is critical to the climate impact….
Arrows show the Brewer-Dobson Circulation in the stratosphere.
The color contours show ozone concentration.
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Where a volcano happens is also important….
Note that the energy balance showed net negative anomaly due to the Pinatubo aerosol in the stratosphere.
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Absorbed Solar Radiation Anomaly
Emitted Infrared Radiation Anomaly
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
The slight negative net radiation anomaly resulted in a measureable dip in global temperatures.
Krakatoa Volcanoe(s)
Eruptions in 535 AD, 1680, 1883…
Yellowstone SuperVolcano
Yellowstone SuperVolcano
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
TOA Solar Emitted IRClimate System
Something that modifies the input
Absorbed Solar
Feedbacks in the Climate System
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Feedbacks in the Climate System
Feedbacks can be positive or negative:
Positive feedback drives the system to instability – runaway changeNegative feedback drives the system to stability – stasis
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
The Gaia Hyopthesis – Credited to James Lovelock – claims that the bisophere responds to maintain the earth’s temperature in some sort of equilibrium – i.e. a negative feedback.
Solar Irradiance Increasing
Low Albedo High AlbedoThe Daisy World Model:
Feedbacks Associated with Marine Low Clouds
Feedbacks Associated with Marine Low Clouds
Albedo of marine low clouds depends on Droplet number and size that in turn depends on aerosol – anthropogenic and natural
In the presence of warming: Decrease droplet number and increase droplet size – lower albedo –more sunlight to ocean – more warming. A positive feedback.Or…Increase droplet number and decrease droplet size – higher albedo – less sunlight to ocean – cooling. A negative feedback
Which is it?
We don’t know! We don’t if this feedback is even important.
Ocean Acidifies
Feedbacks Associated with Marine Low Clouds
Coverage of marine low clouds depends on warming
In the presence of warming: Cloud coverage of marine low clouds decreases due to mixing of warmer air into marine boundary layer resulting in more warming and a positive feedback.
Warmer air results in optically thicker clouds due to the saturation vapor pressure increase with temperature. A negative feedback
Which is it?
We don’t know. But it does appear that this feedback is important in the real climate system.
An actual biology-cloud cycle in the Southern Ocean…
Aerosol varies in tandem with the ocean biology and and clouds vary with the aerosol
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Feedbacks in the Climate System
The main negative feedback in the climate system that is best understood….
The Planck Feedback: As the surface warms, it emits more infrared energy as the 4th power of the temperature.
Recall that this 4th power depdendence causes the Earth to attain a new equilibrium radiating level to keep the effective temperature at 255 K.
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
The Set of Known Feedbacks in the Climate System
1. Albedo Feedback – warming(cooling) melts(freezes) ice which lowers(increases) albedo causing more warming(cooling) –drives the further and further change – a positive feedback. (Reasonably certain – land surface issues and sea ice challenges)
2. Water Vapor Feedback – warming(cooling) causes more(less) water vapor to be in the air leading to increasing(decreasing) greenhouse effect resulting in more(less) water vapor – a positive feedback. (Very certain – physics pretty well understood)
3. Ocean Heat Uptake – For several centuries, the oceans will continue to absorb excess heat from the atmosphere and store that energy. This is a negative feedback but BAD for ocean ecosystems. (Reasonably certain)
3. Low Cloud Feedback – warming(cooling) results in more(less) low cloud cover causing the albedo to increase(decrease) causing the cooling(warming) of the system – drives the system to equilibrium – a negative feedback. (Most uncertain)
4. High Cloud Feedback – Tropical cirrus drive this quantity. Cold ice clouds near the top of the troposphere act to trap IR radiation. Because, tropical cirrus get colder as the climate warms – a positive feedback. (Reasonably Certain – Jury out)
Meteo 1020 – Lecture 4The Natural Variability of the Earth-Atmosphere System
Feedbacks in the Climate System
Taken together, clouds exert a positive feedback with a very uncertain magnitude
From Dufresne and Bony (2008) – Journal of Climate
Putting some numbers with the feedbacks…. (W/m2/K)