Lecture 1: Natural climatic variation Flashcards
Key uk info on biomass energy generators and recycling
-we are burning Canadian biomass – shipped across for use
-50% of ‘recycling’ in the UK is burnt – this is thought to be as polluting as burning coal
Topics covered in this and the following lecture
what are past natural causes?
why is it fossil fuels now?
what are the future projections?
- A brief summary of CO2 and temperature trends; the physics of
greenhouse gases
- Natural sources of climate variation:
- Milankovitch cycles (eccentricity, precession, obliquity)
- Thermohaline Circulation
- Solar cycles
- Oscillations (El Niño)
- Volcanoes
- Past human effects on climate
- The IPCC (intergov panel on climate change )
-climate modelling: singling out fossil-fuel derived greenhouse gases
Current climate change
Global average temperatures have increased rapidly over the last few decades
Atmospheric CO2 concentration has reached record levels
see: http://www.climatecentral.org/news/world-passes-400-ppm-threshold-permanently-20738
^ See site for animated flux over time
2024: we are now at 410-420 ppm atmospheric CO2– never dipping below 400 at any point in the year
How can we be certain climate change is due to increased greenhouse gas concentrations?
combined world averages ‘kick period’ from 1960 onwards in CO2 and greenhouse gas
– but notably sea level + land and sea surface temp. rising from start of 20th century
Radiative forcing is the difference between solar irradiation (short-wave radiation energy) absorbed by the Earth and energy radiated back to space (as long-wave radiation energy)
A positive forcing is a factor that causes more energy to be retained by the Earth than lost
-The physics of greenhouse gases (CO2, methane) as + radiative forcings is irrefutable
- Increases in GHGs -> increases in temperature.
Greenhouse effect is a natural phenomenom that keeps the Earth 23 degrees C warmer for example Durham is 10 degrees toady and would be –13 without this effect. It is necessary to maintain a liveable temperature on Earth – However we are causing an increase.
Molecules absorb energy (heat) radiated from Earth’s surface, and radiate back towards Earth
-But what about natural sources of variation?
- What are they and how do we rule them out?
Cycles related to the Earth’s orbit:
Milankovitch cycles
Milankovitch cycles
*Calculated effects of orbital variations on insolation received at the Earth’s surface
*Play a key role in driving global climatic changes on time scales of 104 – 106 years
-e.g. the glacial–interglacial cycles of the Quaternary ice age
*Principally result from the gravitational effects of the Sun, Moon and larger planets impacting Earth’s orbit
*Variations in three aspects of the Earth’s orbit are important in relation to global climate:
-eccentricity, precession (of the equinoxes) and obliquity
see: https://www.youtube.com/watch?v=PFfwIOzVlh8
Eccentricity in the Earth’s orbit - not a perfect circle due to the influence of other plan
Earth’s orbit around the Sun varies from nearly circular to more elliptical
-Cyclical, every 100-400 k years
This affects seasonality of climate
-Greater difference in radiation when orbit is more elliptical
See: From NOAA Paleoclimatology slide set ‘The Ice Ages’:
While variations in orbital eccentricy have a small impact on the total amount of radiation received at the top of Earth’s atmosphere (ca. 0.1 percent), the primary importance of the eccentricity cycles is to modulate the amplitude of the precession cycle.
When eccentricity is high (more elliptical), the effect of precession on the seasonal cycle is strong. When eccentricity is low (more circular), the position along the orbit at which the equinoxes occur is irrelevant since all points on the orbit become, in effect, perihelia.
The orbit of the Earth changes from nearly circular (eccentricity equal to 0.00) to more elliptical (eccentricity equal to 0.06).
These changes occur in two broad frequency bands: one at periods of around 100,000 years and one at periods near 400,000 years.
Aphelion: when the Earth is furthest from the sun in its orbit (Now: N hemisphere summer)
Perihelion: when the Earth is nearest to the sun in its orbit (Now: N hemisphere winter)
eccentricity, obliquity and precession
See for eccentricity, obliquity and precession: https://science.nasa.gov/science-research/earth-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/
All three processes are occurring at the same time
Obliquity (tilt), axial precession, and eccentricity combine to effect oscillations in daily insolation
See: By Incredio : https://commons.wikimedia.org/wiki/File:MilankovitchCyclesOrbitandCores.png, CC BY-SA 3.0
, https://commons.wikimedia.org/w/index.php?curid=45036243
Precession of the Earth’s axis
From NOAA Paleoclimatology slide set ‘The Ice Ages’
Over time seasonality has changed e.g. 11,000 years ago amplitude in seasonality was much greater
*The Earth’s axis of rotation ‘wobbles’
*The equinoxes and solstices slowly shift in relation to when the Earth is closest to the sun (perihelion)
*When the North Pole is pointing toward the Sun at perihelion, the contrast of winter and summer is greater
*This axial rotation (‘precession’) makes a complete cycle every 22,000-26,000 yrs
Obliquity
Earth’s axial tilt varies from 24.5 degrees to 22.1 degrees at periods of close to 41,000 years.
Axial tilt affects the distribution of solar radiation on Earth’s surface. When the tilt is decreased,
polar regions receive less sunlight; when it is increased, polar regions receive more sunlight.
*‘Tilt’ of the Earth’s axis relative to perpendicular of orbital plane varies between 22.1 and 24.5
-Cyclic, every ~41,000 years
*Again, affects seasonality- stronger when ‘tilt’ is greater, and vice versa
oscillations in eccentricity, obliquity and precession are captured in ocean life forms and ice as well as sediments
http://www.ucmp.berkeley.edu/fosrec/Wetmore.html
^ oscillations are captured in 18O (and temperature) variation in benthic foraminifera (calcium skeleton sea creatures) : driving glacial-interglacial cycles
- In colder climates, more 16O-water is locked up in ice sheets (oceans are richer in 18O). In warmer climates, oceans are globally poorer in 18O as more 18O-water enters the water cycle, and there is less 16O locked in ice.
- Orbital cycles related to greenhouse gases : Methane and CO2 levels at different times in history can be measured by quantifying the gas trapped in ice cores. Ruddiman (2005) Scientific American March 2005, 42-53; Ruddiman (2003) Climatic Change 61: 261-293.
Miocene (Tertiary) sediments at Ptolemais, Greece – an example of Milankovitch cycle
This is an open gas coalmine layering of coal (black – plant matter) and Marl (white, inorganic material.) Marl is washed in during wet intervals when plant matter was less whereas coal is laid down in drier periods
-Black layers: Lignite (low grade coal), laid down in drier periods, swamp vegetation.
-White layers: Marl (inorganic, sedimentary rock), laid down during wetter intervals.
Bottom line: Orbital cycles produce predictable changes in radiative forcing, and therefore in global climate
Effects of orbital cycles
*Principally associated with the ~100 k year glacial-interglacial cycles during the Quarternary
*Precession also associated with cycles in monsoon strength
-Rainfall over continents in lower-middle latitudes
Bottom line: Orbital cycles produce predictable changes in radiative forcing, and therefore in global climate
Ocean thermohaline circulation
*Driven by gradients in ocean temperature and salinity, affecting seawater density
-North Atlantic deep water (NADW) formation
*Thought (at least in part) responsible for past abrupt climate changes
-Bipolar ‘see-saw’
see: https://www.youtube.com/watch?v=vGBaL3udkHI
Rapid addition of large volumes of freshwater to the North Atlantic, e.g. during deglaciation, can cause reduction or even cessation of NADW formation
This results in much reduced heat transfer to higher northern latitudes and rapid cooling, especially in the northern hemisphere. Heat energy carried elsewhere (higher Southern latitudes)
Summary
*As well as the many natural forcings acting on climate, there is evidence that humans have affected greenhouse gas concentrations in the past, leading to a climate that is different from that expected with natural forcings
*Using known natural forcings, we can pinpoint the most likely effects of current and future greenhouse gas concentrations on climate
*There is now overwhelming evidence that recent and ongoing climate change is being driven by anthropogenic forcings (CO2 and methane emissions)
