week 3 Flashcards
(25 cards)
What is radiative forcing (RF)?
defined as the instantaneous change in Earth’s top-of-atmosphere (TOA) energy balance due to a change in external factors
i.e how much the Earth’s energy budget is “disturbed” before any climate response occurs
-for example, a doubling of CO₂ would lead to an instantaneous RF change of approximately +3.7 W/m²
What is effective radiative forcing (ERF)? Why would ERF be used over RF?
defined as the change in Earth’s energy balance after very fast atmospheric adjustments (occurring within days to months), but before the surface temperature has fully responded
- ERF is preferred over RF because it is a more accurate representation of radiative forcing
-includes these short-term feedbacks and the atmosphere’s way of adjusting to them
-IPCC now uses ERF instead of RF
How has estimated global average temperature changed over the last million years?
the temperature is extremely cyclic. There are peaks of extreme warming followed by troughs of extreme cooling, with natural cycling between conditions happening approximately every 100,000 years
show large dips that correspond to periods when the planet entered into ice age
-most recent ice age, about 20,000 years ago, was on average about 5°C cooler
State the four things that drive natural variability in climate?
The four key things that drive natural variability in climate are:
◦
Orbital Variations: Changes in Earth’s orbit and axial tilt over long timescales, known as Milankovitch cycles.
◦
Solar Variability: Changes in the sun’s energy output.
◦
Volcanic Activity: Large volcanic eruptions.
◦
Ocean Circulation: Variations in ocean currents and circulation patterns.
What are the two types of variations in solar activity? What do they lead to?
Alterations to the Sun-Earth system:
-changes lead to seasonal variations, prolonged periods of cooler or warmer climates, and variations in daylight hours
Alterations in the solar output:
-changes can lead to changes in atmospheric chemistry (particularly in the stratosphere), decadal heating cycles, and can also form prolonged periods of cooler or warmer climates
What is meant by the Milankovitch theory? State what the 3 types are
Milankovitch theory was developed by Serbian astrophysicist Milutin Milankovic in the 1920s
-studied how solar radiation changed to explain the irregularity of ice ages
-showing that it was due to changes in the shape of the Earth’s orbit around the Sun
1. Eccentricity: The shape of the Earth’s orbit around the Sun.
◦
2.Precession: The wobbling of the Earth’s rotational axis.
◦
3. Obliquity: Changes in the tilt of the Earth’s axis
Describe what eccentricity is and how it affects climate? include equations
Eccentricity refers to the shape of Earth’s orbit around the Sun, specifically how elongated or circular it is (Earth’s orbit is more elliptical rather than perfectly circular)
-orbits of high eccentricity happen every 100,000 years on average
eccentricity is high (more elliptical orbit), the difference in distance between Earth and the Sun at perihelion (closest approach) and aphelion (farthest distance) becomes more significant= changes total amount of radiation absorbed by the planet
-temps are higher at low eccentricity
-eccentricity of an ellipse is defined by the ratio e = c/a, where a is the semi-major axis and c is the distance from the center to the focal point where the Sun is located. The relationship between the semi-major axis, semi-minor axis (b), and c is given by a² = b² + c²
How much of an effect does eccentricity play?
Earth’s current eccentricity= 0.0167
-enough to cause approximately a 3.3% difference in the distance between the Earth and the Sun
-causes about a 6.8% difference in incoming solar radiation between June and December
During orbits of high eccentricity the Sun-Earth distance can be about 10% greater at one point compared to another, and incident solar radiation can be up to 21% less at one point compared to another within the orbit
Describe what obliquity is and how it affects climate?
Obliquity refers to the tilt of Earth’s axis relative to its orbital plane around the Sun
lt is not constant but varies between approximately 22.1 and 24.5 degrees over a cycle lasting about 41,000 years
-currently 23.5 degrees
affect the distribution of solar radiation across Earth’s surface
Examples:
1. low (e.g., 22 degrees), polar regions receive less solar radiation during summer and more during winter, reducing the temperature contrast between seasons (milder winters, cooler summers)= ice sheets grow
2. high (e.g., 24.5 degrees), polar regions receive more sunlight in summer and less in winter, increasing seasonal temperature contrasts (warmer summers, cooler winters), which can cause ice sheets to melt
Describe what precession is and how it affects climate?
Precession refers to the gradual wobble or “precession” of Earth’s rotational axis over time
-changes the orientation of Earth’s axis with respect to its orbital path around the Sun
-operates over a period of about 26,000 years
-caused by the gravitational attraction between the Earth and other bodies in the solar system, particularly Jupiter
-influence the timing of Earth’s seasons and the distribution of solar radiation across different latitudes
-does NOT change the yearly globally averaged incident solar radiation received by the Earth, rather changes the quantity of incident radiation at each latitude during a season
Example:
currently, the Northern Hemisphere is further from the Sun in summer than in winter, but in about 11,000 years, the Northern Hemisphere will be closer to the Sun in summer than in winter
How has the Milankovitch cycles been verified?
verified in the 1970s following the examination of deep-sea sediment cores and subsequently ice cores
-results showed that the ice ages observed in these records occurred when the Earth was going through the different stages of orbital variation predicted by Milankovitch’s theory
What are solar output changes? What are the possible climatic effects?
Solar output changes are alterations in the actual energy output of the Sun
Climatic effects:
-More shortwave radiation incident on the Earth, leading to warming
-increases in absorption of radiation at particular wavelengths, leading to warming
-increases in ozone produced by extra UV radiation in the stratosphere, which warms the stratosphere
What are sun spots and how do they cause variability?
Sunspots are areas that appear as temporary dark spots on the Sun’s surface
-appear dark as cooler than surrounding areas
-linked to Sun’s magnetic activity
-number of sunspots increase/decrease over a regular 11 year cycle
-more sunspots=generally more solar energy (more shortwave radiation coming to the Earth, which can lead to warming)
-sun’s total brightness at solar maximum is about 0.1% higher than solar minimum
-sunspots themselves are dark and cooler, the surrounding brighter regions called faculae (or torches) emit more at shorter wavelengths
What is the Maunder Minimum?
an example of a “Grand Solar Minimum”
-sunspots virtually disappear for decades
-particularly well observed from 1645 to 1715
What occurred during the Little Ice Age?
-cool climatic period in Europe and North America
- 1300 to 1870
-Europe and North America experienced much colder winters
-Maunder Minimum (1645-1715) coincided with, or overlapped with it
-divided into two phases:
1. the first starting about 1300
2. continuing until the late 1400s, and one from 1650 to 1870
still considerable uncertainty regarding the direct link between the solar cycle and these climatic effects
Is the sun causing global warming?
Sun is not causing the current global warming trend
-actually observed is warming at the surface and cooling in the stratosphere
-satellite measurements since 1978 show no upward trend in the amount of solar energy reaching our planet
-Since 1750, the warming driven by greenhouse gases from human burning of fossil fuels is over 270 times greater than the slight extra warming from the Sun over the same period
-IPCC estimate solar contribution to be 1% across industrial era
What is the volcanic climate influence?
-inject huge amounts of material into the atmosphere, including sulfur dioxide, which turns into sulfuric acid droplets and forms reflective aerosols, and microparticles of dust that absorb and scatter solar radiation
-increase the reflection of radiation from the Sun back into space, leading to a cooling of the Earth’s lower atmosphere (troposphere)
-cooling effects are typically short-term, lasting maybe one to three years on average
Example:
Mount Pinatubo eruption in 1991
What have the results shown when modelling anthropogenic vs natural impacts?
When global climate models are run using only natural forcings (like orbital variations, solar variability, and volcanic activity), they can reproduce historical temperature changes going back a million years reasonably well
-however diverge significantly from actual observed temperature anomalies (e.g., the black line) from the 1960s and 1970s onwards, particularly in the 1980s
-models are rerun to include anthropogenic forcings results tend to agree extremely well with the actual observations
How does ocean circulation influence climate?
ocean currents play a crucial role in redistributing the absorbed radiation (heat) around the Earth
significant impacts on regional weather patterns and global climate
What is ENSO?
ENSO stands for the El Niño-Southern Oscillation
-coherent, large-scale fluctuation of ocean temperatures, rainfall, atmospheric circulation, vertical motion of the air, and air pressure across the tropical Pacific
-coupled ocean-atmosphere phenomenon
-considered one of the most important climate phenomenon
What is the Walker circulation (WC)?
The Walker Circulation (WC) is a pattern of atmospheric circulation across the tropical Pacific
-normally equatorial Pacific Ocean has a pool of relatively warm water in the upper ocean in the west and a shallower layer of relatively cool water in the east, maintained by easterly surface winds.
-leads to strong convection over the warm western Pacific where air rises, forms clouds and precipitation, spreads out at the top of the troposphere, cools as it moves eastward, descends over the cooler eastern Pacific, and then returns along the surface as easterly winds, completing the circulation cycle
What changes occur to the WC during El Nino?
-warm phase of ENSO
-changes occur to the Walker Circulation
-thermocline (the layer separating warm upper water from cold deeper water) becomes deeper in the eastern Pacific and roughly the same depth across the entire Pacific
-warmer water extends further east across the Pacific
-Convection and precipitation occur mostly across the entire Pacific.
-the air spreading out at the tropopause collides, leading to reduced upper-level wind speeds and a weakened Walker Circulation.
-this also results in reduced surface wind speeds.
-there is a reduced or zero thermocline gradient across the Pacific.
-sea surface height is higher in the eastern Pacific
What changes occur to WC during la Nina?
-opposite phase of ENSO
-changes occur to the Walker Circulation:
-heating due to a very deep equatorial layer in the western ocean tends to confine convection and precipitation to that narrow region.
-this leads to increased upper-level wind speeds and an enhanced Walker Circulation.
-there are increased surface wind speeds.
-the thermocline is much shallower in the eastern Pacific.
-there is an enhanced thermocline gradient across the Pacific.
-sea surface height is lower in the eastern Pacific.
-nutrient-rich water is closer to the surface off the coast of South America.
What is Southern Oscillation Index (SOI)?
-measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific; it indicates the state of the Southern Oscillation during El Niño and La Niña episodes
-calculated based on the difference in air pressure anomaly between Tahiti and Darwin, Australia
-Prolonged periods of negative SOI values coincide with abnormally warm ocean waters in the eastern tropical Pacific=El Niño
-Prolonged periods of positive SOI values coincide with abnormally cold ocean waters in the eastern tropical Pacific= La Niña
