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