Download Climate Change Science and Engineering

CE 401
Climate Change Science and Engineering
orbital variations and climate, aerosols,
carbon cycle
25 January 2011
any questions from last time?
Earth radiation budget and GHG spectra
HW 3 is due today
HW 4 is posted on the web – due Thursday 1/27
HW 2 average was 3.5/10 :(. Distribution was bimodal with 6 10’s and 12 < 1/10
you need to: follow directions and answer the question, use a computer, think about your answer – does it make
sense?, write neatly, don’t let bad points drive your analysis (these “-99” points were clearly coded as bad), show
your work please! What happened to the use of MATLAB from the computer course?
solution is posted on the class website
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• astronomical forces drive global climate change – the Earth orbit
• seasons are driven by astronomical causes, as is the 24 h day/night cycle
ice ages:
Earth orbital changes that vary the solar input and cause the ice ages: the
Milankovich cycles – these cycles change the solar input to the Earth system
• not until the second half of 19th century was it accepted that there were indeed ice ages
• three main types of evidence that climate has radically changed in the past
• geological
• rock scouring
• moraines from glaciers
• valley cutting
• glacial erratics
• chemical
• isotope ratios in fossils and ice cores
• paleontological
• fossil distributions
• have been at least 5 major ice ages
• Earth has been in an interglacial period for
about 11000 yrs – very stable climate
• positive feedbacks
• ice  increased albedo  decreased T
• changes in ocean currents (e.g. Gulf stream to Europe)
• negative feedbacks
• causes of the ice ages are not understood in detail
• atmospheric composition
• changes in Earth orbit that are cyclic
• motion of tectonic plates  changed winds, ocean currents, etc.
• changes in solar irradiance
changes in Earth orbit – Milankovitch cycles
eccentricity changes
varies from nearly circular
to high eccentricity 0.058
with mean 0.028.
Caused by perturbations
from the other planets
e = 0.017 currently
shape of earth’s orbit changes during a cycle of about 100,000 years
axial tilt (obliquity) – increased obliquity  increased seasonal amplitude change
axis of rotation changes from about 21.5° to 24.5° --> seasonal variations over a period of
41,000 years. Tilt is the most significant cause of seasonal temp change. Modulates the
seasons, does not change climate overall. But ice sheets advance when summers are too
cool
axial precession – trend in direction of axis of rotation in inertial space – gyroscopic motion
the earth’s rotation axis precesses (wobbles) with a period of about 26,000 years
due to tidal forces exerted by sun and moon on solid Earth since Earth is not spherical
affects climate extremes
problems with the Milankovitch theory for ice ages:
• 100,000 yr problem: eccentricity variations should have a smaller impact that the other
mechanisms, but this is the strongest climate signal in the data record
• 400,000 yr problem: eccentricity variations also show a 400,000 yr cycle but that cycle is only
visible in climate records > 1My ago
• observations of climate changes show behavior much more intense than calculated
• the 23,000 yr cycle dominates, the opposite of what is observed
• the “reinforcement of causes” does not seem strong enough to initiate an ice age
• in the past 400k yrs, Milankovitch cycles match too well to ignore
• so the explanation is not 100% - there are still issues with the explanation
deg change
obliquity=axial tilt
long of perihelion
precession index
calc. insolation to TOA
Benthic and Vostok
ice cores
past and future daily average insolation at the top of the atmosphere at summer solstice, 65°N
next ice age: 10-100k yrs from now.
aerosols and their effects on climate
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big driver – HUGE driver, both human induced and natural RF
not well understood – hardly on the data sheets in 1990
not well measured – satellites and ground networks – short record of data
parameters that affect aerosols and their RF not understood
global effects not well understood – clouds (height and distribution)
increasing complexity of the modeling world
aerosols:
• solid particles or liquid particles suspended in the air
• size: few nanometers to microns in size: x100 - 1000 variation in size
• have major impacts on climate
• physical properties: shape, size, chemical composition
• particles at the small end play a large role in cloud physics
• condensation nuclei
• EPA regulates particles in the US PM10 and PM2.5 – CEE 341, 415
• key aerosol groups:
• sulfates
• organic carbon
• black carbon
• nitrates
• mineral dust
• sea salt
• aerosols of clump together to form complex mixtures
• 90% of aerosols (by mass) are natural in origin
• about 10% of global aerosols are generated by human activity
• problems in dealing with aerosol effect: diversity in size, composition and origin;
spatial and temporal variability; source; injection height
• aerosols removed primarily through cloud processing and wet and dry deposition