Transcript lecture 13 for 351 - Department of Atmospheric Science

A Changing Climate: Past,
Present and Future
AT 351
Lecture 13
Dec 7, 2009
Outline
• Current Climate
• What can change climate?
• Observations
– Proxy records
• Predictions
• Facts and misconceptions
• The ozone hole
What is Climate?
• The slowly varying aspects of the
atmosphere–hydrosphere–land surface
system
• Climate is often considered to be an
“envelope of possibilities” within which the
weather can bounce around
• “Weather tells you what to wear today …
climate tells you what clothes to buy!”
Current Climate
Current Climate
Fort Collins Climate
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
41°
45°
51°
61°
68°
78°
85°
84°
74°
64°
50°
42°
Avg. Low 14°
18°
25°
34°
44°
51°
57°
55°
45°
35°
24°
16°
Mean
28°
32°
38°
48°
56°
66°
72°
68°
60°
50°
38°
28°
Avg.
Precip.
0.4
in
0.4
in
1.4
in
1.8
in
2.7
in
1.9
in
1.8
in
1.3
in
1.3
in
1.0
in
0.7
in
0.5
in
Avg.
High
What can change the climate?
180 MYA
Today
• Plate tectonics helps to explain climate shifts on the order of millions
of years
• The positioning of the continents helps to dictate ocean currents and
how energy can be distributed on the planet
• Also, the movement of continents creates land features such as
mountains and volcanoes that have a large impact on local climate
What can change the climate?
• Changes in the earth’s orbit will affect the
amount of incoming solar radiation – known as
the Milankovitch cycles
• Eccentricity of the earth’s orbit happens on a
cycle of about 100,000 years
• Precession of earth’s axis has a 23,000 year
cycle
• Changes in the tilt (obliquity) of the earth’s axis
happen on a 41,000 year cycle
What can change the climate?
• Solar output can vary slightly depending on its
magnetic field
• The sun’s magnetic field has a 22 year cycle – it
reverses itself every 11 years
• The reversal of the sun’s magnetic field is
associated with a maximum in sunspot activity
• The sun emits slightly more energy during
periods of maximum sunspot activity
What can change the climate?
• Volcanic eruptions release ash, dust, and sulfur
gases into the atmosphere
• Sulfur combines with water vapor in the
atmosphere to produce a thick haze of sulfuric
acid particles
• This thick haze can block a portion of the sun’s
incoming energy
• A large eruption can cool the surface
temperature by a small amount for 1-3 years
after the original explosion
What can change the climate?
• Changes in the composition of the earth’s atmosphere
will affect the radiation budget of the earth
• Sulfate aerosols (like those brought about by volcanic
eruptions) act to cool the planet by blocking shortwave
radiation from the sun
• Some other aerosols, along with gases such as water
vapor and carbon dioxide act to warm the planet by
blocking long wave radiation from escaping the earth
(remember the greenhouse effect?)
• A distinct minimum in carbon dioxide has been found to
coincide with the past ice ages
• Aerosols that act as CCN can also change the thickness
and lifetime of clouds, which can also affect the climate
What can change the climate?
Past Climate Record
• Much of the data about the past climate comes
from ice cores extracted from Greenland and
Antarctica ice sheets
• Concentration of oxygen isotopes is used to
back out the temperature at the time the ice was
formed
• Ice cores also contain bubbles of air and actual
aerosol particles that can be analyzed to find the
composition of the atmosphere at that time
Past Climate Change
Current CO2 levels are 100ppm
above background levels!
CO2 Levels (ppm):
• Last Glacial
Maximum – 180
• Preindustrial – 280
• Currently – 380+
The Temperature Response
• Climate models can do a
pretty good job of
simulating long term
temperature patterns…
• IF anthropogenic forcing
is included
• Natural forcing
(Milankovitch cycles,
solar forcing, volcanoes)
alone cannot explain the
temperature change that
has occurred
Geographic Differences
Models also do well at the regional scale.
Arctic Sea Ice
Arctic Sea Ice
Feedback Mechanisms
• Water vapor feedback (positive)
– A warmer surface will evaporate more water
– Water vapor adds to the atmospheric greenhouse effect
• Snow-albedo feedback (positive)
– Melting of bright snow and ice leaves behind dark water and
reduces the albedo of the earth
– A lower albedo means more of the sun’s incoming radiation is
absorbed rather than reflected
• Clouds (unknown)
– High clouds tend to warm the atmosphere by releasing less
infrared radiation to space
– Low clouds can cool the atmosphere by reflecting large amounts
of solar radiation
– The net effect of clouds is still uncertain
Surface Temperature Projections
Predicted Precipitation Changes
Emission Scenarios
Facts
• Climate change is real – the climate has changed in the
past, it is changing now, and it will change even more in
the future
• It is extremely likely that the current warming trend has
been brought about by human activity
• The effects of the warming will not be felt uniformly
across the globe – some areas will be affected more
than others, and some places might even cool
• The sea level is currently rising due to thermal expansion
and the melting of land ice
• The exact response of the climate system to this
warming is uncertain, but it is expected that precipitation
extremes will increase in many areas
Misconceptions
•
•
•
•
Climate vs. Weather
What is “real science”?
Solar variability
The global cooling scare of the 70’s
Misconceptions
• Climate vs. Weather
– Isolated weather events do not necessarily mean that
the climate is changing
– Hurricane Katrina was not necessarily a result of
climate change
– Climate change also does not mean that temperature
is increasing everywhere
• some places are more sensitive than others
Misconceptions
• Solar variability
– Some people will claim that since our atmosphere is
so massive the only thing that can affect our climate
system is the sun
• Although we know we can affect things like the ozone layer
– Solar variability has been researched and is well
documented, as of yet it cannot be shown that
variability of solar output explains all of the changes
we are seeing
Misconceptions
• What is “real science”?
– Misinformation is rampant in the
media and public opinion
– Real science is peer-reviewed
such as articles found in
scientific journals
Misconceptions
• Scientists thought the planet would cool in the 70’s
– The fact is that there was never any consensus in the
community about a possible episode of global cooling
– In the late 60’s through the 70’s:
• 7 articles predicted global cooling
• 44 predicted global warming
• 20 were neutral
– A summary from USA today can be found at:
http://www.usatoday.com/weather/climate/globalwarming/2008-02-20-global-cooling_N.htm
The Ozone Hole
• The Ozone Hole forms over Antarctica around
the time of SH Spring
• CFC’s are responsible for this loss, but not
directly
• Ozone depletion due to CFC’s happen around
the globe and is governed by the so-called
“Chapman Chemistry” discovered in 1930’s
– This only considers the gas phase
– This does not explain the amount of Ozone loss in
Anarctica, partly because this only happens higher up
in the stratosphere than where the hole occurs
The Ozone Hole
• The type of ozone depletion in anarctica is driven by
Polar Stratospheric Clouds (PSC)
– These only form when the stratosphere is significantly cold (i.e.
polar night conditions, 185-190K)
– These allow chemical reactions on their surface that activate
chlorine in the stratosphere which leads to ozone depletion
The Ozone Hole
• This heterogeneous chemistry allows chlorine to be
transferred into “active” reservoirs
• The cold temperatures that are found in the Antarctic
stratosphere during winter along with the increasing
sunlight in the SH spring leads a massive depletion of
ozone
• The Polar vortex plays a significant role by not allowing
the ozone rich air from lower latitudes to mix with the
ozone depleted air over the pole
• Videos of this can be found at:
http://ozonewatch.gsfc.nasa.gov/