The Climate System
EESC 2100 Spring 2007
Lectures - Monday and Wednesday, 11:00 AM - 12:15 PM
Lab - Tuesday, 4:10 PM -7 PM
Climate changes: present, past, and future
Take away ideas and understandings
- What factors influence climate change?
- On what time scales do these factors operate and what controls the time
I. Review of planetary energy balance.
- Remember that in equilibrium, absorbed solar energy equals emitted heat.
- Absorbed solar energy depends on solar constant (intensity of Sun at Earth's
distance) and planetary albedo (fraction of incident sunlight reflected) (Fig
- Emitted heat depends on temperature at which Earth radiates to space; difference
between this temperature and surface temperature is an indicator of the greenhouse
effect, which depends on the concentration of greenhouse gases.
- Climate change occurs when either side of energy balance is perturbed.
Example 1: Increase greenhouse gases -> decrease IR
radiation to space -> absorbed solar exceeds emitted thermal -> temperature
must increase to restore balance.
Example 2: Increase planetary albedo -> decrease absorbed
solar -> emitted thermal exceeds absorbed solar.
- temperature must decrease to restore balance.
In general real climate changes involve changes of both sides of the energy
II. Past climate change.
- Historical (past 100+ years): Direct surface weather station measurements
of temperature indicate slowly rising global temperatures from late 19th
Century until about
1940, then weak cooling until 1965, then sharply rising temperatures up to
the present (Fig
This record contains information from a large number of land stations as
well as measurements of sea surface temperature. The latter include satellite
over the last two decades.
- Land measurements (Fig
- Global coverage between 1880 to 2000.
- Urban heat island effect.
- Two data sets one rural and the other containing data from
near or within cities. The two data sets show similar trends
and the differences between them are not statistically significant.
The warming in both far exceeds the uncertainties.
- Types of data and how measurements are weighted.
- More land in Northern hemisphere than southern.
- Oceans warm more slowly than land.
- Temperature measurements now cover 54% of land.
- Diurnal temperature range.
- Warming in recent decades has involved faster rise in daily
minimum than daily maximum temperatures (Fig
- Also a correlation between this trend and an increase in
- Sea Surface Temperature.
- Temperature within the upper few meters of the ocean.
- In the early days buckets of water were brought on deck and
the temperature measured.
- Since 1941 most measurements from ships have been engine intake
water temperature (Fig
- Land and sea surface temperatures show similar trends over the last
100yrs. Additional data does little to change the pattern (Fig
- Glacial advances and retreats in the Swiss Alps correlate with North
Atlantic sea surface temperatures.
- Past 1000 years: evidence from winter severity information, tree rings,
etc. suggests that there was a medieval warm period about 1000 years
ago, then a "Little Ice Age" from about 1400 to the late 19th
Century (See Fig 2 again).
- Proxy climatic indicators.
Proxy indicators of past climate are data derived from natural recorders
not man-made instruments. Such natural recorders include tree rings
recording annual tree growth, various properties of cores, including
some that are dependent on temperature, taken from the worlds great
ice caps, various measurements made on deep sea cores or cores removed
from tropical corals.
- During the last 1000 years tree rings have been used extensively
to estimate past climatic conditions prior to the instrumental record.
They are in fact the major contributor to the temperature curve for
the last 1000 years (Fig
- How are tree rings used as indicators of past climate?
Tree ring width's vary because of a number of local and regional
environmental factors. In general when conditions are "good" trees
grow more than when conditions are "adverse". What is good
or adverse depends on regional and local conditions. In arid regions
rainfall may be the most important control of tree ring width while
in high latitudes or high altitudes temperature may be most important.
A trees local position is also important to its growth response so
trees in the same region may show different growth histories depending
on their local setting. Replication of data from different local
trees is essential.
- How are climate chronologies developed from tree rings?
Chronologies are developed by counting rings in individual trees
and matching ring patterns between trees. For a particular region,
old wood preserved in swamps or buildings can be used to extend the
backward to before the age of living trees. In general continuous
chronologies can be developed for a region and they can extend back
hundreds of years. If a chronology is not continuous it can be dated
by carbon 14 dating.
- How is a climatic information extracted from tree rings?
Since tree rings respond to a variety of climatic factors the rings
record more than one climatic variable. The information about a single
variable, say temperature, is extracted in the following way. A tree
ring data set for a region is first subdivided in to subsets
of ring chronologies. One set is used to match against a known climatic
record such as the instrumental record of temperature for the last
80 years. This is done through what is called a regression analysis
in which the tree ring widths for the set of years chosen are correlated
with measured temperature for that same set of years. The result
of this correlation develops an equation or model of the relationship
between ring width and temperature. To verify or test the ability
of this model to predict temperature from tree ring width, the model
is applied to the tree ring chronologies that were not part of the
original calibration set, but grew when there are temperature measurements
with which to compare them.
- Climate change of the past several hundred thousand years.
- The record of ice cores.
- How does ice record climate change? (Fig
- How does ice record changes in atmospheric composition? (Fig
- Sedimentary archives
Sediments are laid down consecutively from oldest to youngest (this is called
the principle of superposition), and they contain material that provides
information on the time of their deposition and the environment in which
they were deposited (climate). Extracting the information from sediments
requires the application of proxies. Proxies are measurements or observations
that can be related to a climate parameter such as temperature, salinity,
aridity, wind speed, current speed, etc.
- Climate change over the history of the Earth.
- Important factors in long term climate change
- Changes in the astronomical configuration (Milankovitch)
- Changes in the carbon cycle
- Changes in the hydrological cycle
- Changes in the configuration of the continents (plate tectonics)
- Examples of past climate change.
- "Snowball Earth"
- Permo-Carboniferous glaciation
- Hothouse conditions in the Cretaceous
- Cenozoic build-up of glaciation
- Pleistocene glacial cycles
- Millenial scale variability of climate
III. External climate forcings (other than greenhouse gases).
- Solar luminosity variations.
- Sunspots are dark, decrease luminosity, but are surrounded by bright
faculae which cover a larger area; thus, Sun is brightest at peak of sunspot
cycle (Fig 10).
- Satellite observations since 1980 indicate that solar luminosity oscillates
slightly with the 11-year sunspot cycle (Fig
- Larger long-term variations may explain Little Ice Age (Maunder minimum),
but mechanism is not understood (related to sunspot number (Fig 12), cycle length (Fig
13)?); unknown potential contributor to future climate change.
- Other possible influences, e.g., charged particles from solar wind affecting
cloud droplet formation, but mostly speculation.
Anthropogenic (tropospheric) aerosols.
- Large ash and dust particles fall out of atmosphere quickly, do not
- Climate impact is favored by (a) material reaching stratosphere, above
altitude of scavenging by rain, (b) small particles, which fall out slowly;
under these conditions, volcanic aerosols can reflect sunlight (increase
albedo) for several years and cool climate (See
- Small particle formation: Injection of sulfur-bearing gases (e.g., SO2)
into stratosphere, photochemical reactions form small sulfuric acid (H2SO4)
- Not all volcanoes affect climate: Mt. St. Helens (1980) exploded sideways
14), sulfur-poor, thus no climate impact; Mt. Pinatubo (1991) exploded
vertically (Fig 15), sulfur-rich,
thus biggest climate impact of 20th Century. The eruption of Mount Pinatubo
presented modern climatologists with an opportunity to test the reliability
of numerical climate models to predict cooling from a volcanic eruption.
They could estimate the amount of aerosols propelled into the stratosphere
from this eruption, their spread around the world, and eventual removal
from the stratosphere. Using these estimates as input to the model they
asked the model to predict the effect of these aerosols on our planets
mean global temperature. These estimates are compared with instrumental
measurements of Earth's temperature over a period of several years after
the eruption (Fig 16). It is clear from
this figure that the model did a very good job. This does not mean that
the model would do as good a job with other factors forcing climate change
such as increasing CO2 input. About 10 other volcanoes have
probably affected climate in past century.
Biogenic regulation of climate (Gaia).
- SO2 emissions have probably more than doubled sulfate aerosol
concentration in 20th Century (Fig 17);
systematic upward trend in albedo (direct effect) may have offset part of greenhouse warming;
climate models agree better with observed temperature trend when aerosols
are included (Fig 18).
- other types of aerosols (desert dust, soot from combustion and biomass
burning) also important, but different (darker, absorb sunlight and
heat atmosphere as well), may reduce direct effect.
- aerosol forcing regional in nature (mostly Northern Hemisphere,
near and downwind of industrialized areas); some regions may cool
while others warm (Fig
- additional indirect effect: cloud droplets nucleate on aerosols;
more aerosols, more but smaller droplets that reflect sunlight better
and rain less; causes higher cloud albedo.
- Hypothetical example: Daisyworld (planet populated by white daisies
that like warm temperatures, black daisies that like cool temperatures);
if climate warms, white daisies thrive, albedo increases, limiting warming;
if climate cools, black daisies thrive, albedo decreases, limiting cooling.
- Real world possibility: dimethylsulfide (DMS) emissions by plankton,
leading to sulfate aerosol formation; primary nucleation source for oceanic
clouds; possible impact on climate change, but DMS dependence on temperature
not established (Fig 20).
IV. Natural variability.
- Short time scales (1-2 years): Random weather-related variations of turbulent,
- Interannual (2-8 years): Primarily ENSO; longer time scale due to interaction
of atmosphere with more massive ocean mixed layer and thermocline.
- Decadal-to-century scale: Due to changes of intermediate/ deep ocean circulation
and interaction with atmosphere; unknown magnitude and triggering mechanisms
leave open question of whether climate change is predictable.
- Modeling results of all important radiative forcings during the last 100+
(Figures in this section are from: Hansen, James E. Dangerous
Anthropogenic Interference: A Discussion of Humanity's Faustian Climate Bargain
Due (pdf - scroll to end of file). Presentation given at the Distinguished
Public Lecture Series at the Department of Physics and Astronomy, University
The question now is how much of the temperature change during the
last century (Fig. 21) can be explained by the known
- Summation of temperature forcings and their direction (1850 – 2000)
- Time series of individual radiative forcings
during the last 150 years (Fig.
of the model simulated global mean surface temperature change and the
instrumental record of that change since 1880 (Fig.
The answer to the above question is now clearer.
Most of the recorded global mean temperature change of the last century
can be explained
by known radiative
forcings. Greenhouse gasses are not the whole story but they
are an important and in fact dominant driving force.
Lecture text by Martin Stute, Julian Sachs, Stephanie Pfirman. Updated,
2004 by Jim Hays. Updated 2005, Sidney Hemming.