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