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

Avg. High

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

What can change the climate?

• 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

180 MYA Today

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/