Muir Glacier 1941

Muir Glacier 2004

Causes of the recent warming

What is the cause of the recent warming?

• We have had about 0.7C of warming over the last 100 years

• Clearly there are more greenhouse gases in the atmosphere…..

• But what is the context?• Other long-term changes

– Tectonic: The northern movement of the continents produces a slow cooling ~ 5-10C over the past 100 Myrs

– Insolation has been dropping. This amounts to about 1C over the last 6000 years

– Millennial-Scale oscillations?

CO2

• We have very good data on the natural range of CO2 during the past glaciations from the ice bubbles.– An aside on natural sources of

CO2

• We passed the maximum natural range about 1900….

• At that point fossil fuels were only a fraction of total human carbon input

200 Million Tons All volcanic inputs

CO2

• About half stays in the atmosphere

• 15-20% goes into the biosphere– There are places with

substantial “reforestation” (Eastern US for example)

– Faster plant growth

• 25-30% into the shallow ocean– CO2 is more easily dissolved in

cold water

Methane

• Even more extreme story with Methane.

• We were outside the natural range well before the industrial revolution

• Human sources are about double all natural sources

• Methane accounts for about 16% of the total greenhouse-gas effect.

Let’s Pretend…..• The short-term

climate is driven by:– The Sun Spot

Cycle– Dust from volcanic

explosions– ENSO (El Nino

Southern Oscillation)

– “Anthropogenic Influence” Atmospheric CO2 buildup

Let’s Pretend…..

• The yellow line is the sum of JUST these 4 factors

• The black line is the instrumental surface temperature record (i.e. reality)

• Works pretty good…hunh? • Make some predictions……?

Sulfate Aerosols

• Like many things, the role of SO2 is complex• Like water….as a vapor SO2 it is a greenhouse

gas, as a particle it has a cooling effect• BUT, it reacts with water vapor to form sulfate

particles (and sulfuric acid…acid rain)

Brown Clouds

• These are essentially smog due to human activity– Stoves

– Cooking fires

• Two effects– Absorb solar radiation and

heat lower atm (2-3 km altitude)

– Blocks Sunlight producing ground cooling

• May effect the monsoons

Global Dimming• Because of aerosols

pollution, the brightness of the Earth's surface has dropped about 7%

• This probably represents a net cooling due to more sunlight being reflected by the increased particle load in the atmosphere

Radiative Forcing

• Remember, the Earth has a natural greenhouse effect

• The greenhouse gases we have added has enhanced the back radiation by about 2.7 W/m2

But….the warming is actually pretty small…

• We have increased CO2 by 35%, equivalent CO2 by 60%

• Why ONLY a 0.7C temp increase?– Ocean Thermal Inertia

– Aerosols

– Insolation

• The net effect of the various sources of radiative forcing is still a bit tough to sort out.

Greenhouse Gas Production