Lecture 16

• Observations of climate change• Feedback mechanisms• Air pollution• The stratospheric ozone hole• Changing land surfaces• Greenhouse gases and global warming

– Global warming and H2O– Climate modeling

• Climate change assessments

Global average surface temperature

+ive/-ive feedback mechanisms

• Feedbacks occur when one change leads to some other change that can act to reinforce or inhibit the original change

• Reinforcing, +ive feedback, e.g. water vapor feedback, ice/albedo feedback

• Inhibiting, –ive feedback. Example CO2 and photosynthesis – end result is equilibrium

Ice albedo temperature feedback

Air pollution (aerosols and gases that are harmful to life)

• Natural sources e.g., from volcanoes, soils• Anthropogenic sources

– Carbon monoxide– Lead– Sulfur dioxide, sulfur trioxide (+H2O)– Sulfuric acid– Nitric oxide, nitrogen dioxide– hydrocarbons, (volatile organic compounds)– ozone

The stratospheric ozone hole

• Ozone produced photochemically in the stratosphere

• Ozone plays a fundamental role in the radiation budget and dynamics of life

• Minimum amount of ozone is observed in spring in SH (October). Why?

• Discussed ozone depletion as a result of release of CFCs.

• The winter atmosphere above Antarctica is cold, -90C.

• Strong temperature gradient from pole to mid-latitudes. From thermal wind arguments, this results in strong westerly winds encircling the South Pole, so called polar vortex

• Results in isolation of SP air, keeping it very cold and chemically isolated

• Polar stratospheric clouds can form• Add sunlight and the result is rapid

destruction of ozone

Total column ozone over Halley Bay station

Notice change after 1975

Oct monthly mean ozone concentration

’87, ’89, ’90, ‘91

Why not a NH ozone hole?

• The NH polar vortex is continually being bombarded with Rossby waves that have propagated from the troposphere in the NH. This disturbs the vortex and does not allow the same chemical isolation (nor as strong a vortex) as occurs in the SH.

Stratospheric polar vortex

WMO Scientific assessments of ozone depletion: 2002, 1998, ’94, ‘91, ‘89, ‘85

Life cycle of CFCs: Cl and Br compounds in the atmosphere cause the ozone depletion

over Antarctica

Changing land surfaces

• Desertification: spreading of a desert region because of a combination of climate change and human impacts on the land. Examples: overgrazing, deforestation without reforestation, diversion of water away from a formerly fertile region, farming on land with unsuitable terrain or soil

• The Sahel or sub-Sahara (14-18N)

• Partly due to natural variability – depends on the northernmost location of the ITCZ for rain. Shifts by 100 km from year to year

• Enhanced by biogeophysical +ive feedback mechanism

The incredible shrinking Aral Sea

1973

1987

2000

Urban heat island effect

Notice the warmth of Pittsburgh compared to the surrounding rural area

Greenhouse gases and global warming

• H2O, CO2, CH4, CFCs are all gh gases

• Humans have changed the amount of gh gases in the atmosphere over the past century by 25%

• Based on the radiation concepts of the greenhouse effect, temperature should have increased a lot too. Hard to quantify

• Complex system

Global warming and atmospheric water

• Human activities add little H2O to the atmosphere directly

• Saturation water pressure increases very rapidly with temperature

• Since H2O is a strong gh gas this will lead to still warmer temperatures, still more water vapor etc

• On the other hand cloudiness may also increase, which would be a cooling effect

Global warming and atmospheric H2O, continued

• More aerosols can mean more CCN, more droplets can form in clouds and the clouds may reflect more

• Indirect aerosol effect on climate

• Contrails are airplane-induced clouds

Stratus clouds off California, ship tracks can be seen in the clouds since their reflectance

is enhanced by increased aerosols

Climate modeling

• GCMs, global climate models consist of an atmospheric model, coupled to an ocean model, coupled to a sea ice m., land surface model

• Give statistical estimates of future conditions

• Sensitivity studies to understand processes

Schematic of atmospheric processes included in a GCM