Climate Forcings

Climate forcing refers to the notion that changes (by any mechanism) to the composition of the Earth's atmosphere will force imbalances in the long term radiation balance of the Earth which will then drive temperature changes (rising or falling). The industrial revolution has changed out composition as illustrated below where the extra forcing is measured in units of Watts per square meter.

Since about 170 watts per square meter on average is absorbed by the surface of the earth, a perturbation of 2 watts per square meter represents a 1% perturbation which is significant in the context of any energy balaced system

Formally, in the IPCC4 report there is a range in predicted temperatures (in the year 2100) of 1.5 to 5 degrees C.

5 degrees (= 9 degrees F) would be relatively catastrophic.

So, what drives this range and how can one estimate what is the most probable correct model?

Ice age calibration suggets 0.75 +/- 0.25 Degree C rise with every 1 watt per meter squared increase of forcing.

Summary of the relative forcings in the year 2000 compared to the year 1750:

1. CO₂ is 1.5; The canoncial value associated with a doubling of CO₂ (from 280 to 560 ppm) is 4.0 watts per square meter.

2. sum of all the other greenhouse gases shown is (2.54); however, since 2000 the CO₂ forcing term has risen faster than the others.

3. Black Carbon (basically particulate pollution suspended in the atmosphere or falling out at the poles) produces another 0.8 watts per square meter

4. Sun energy output of the sun has increased slightly. The energy reaching the top of our atmosphere from the sun is about 1300 watts per square meter and over the last 250 years the sun's output has increased buy about 1/4 of a percent or 0.3 watts pe square meter.

Summing all this up leads to a total positive forcing of 4.14 watts per square meter. Which implies 3 +/- 1 degree C increase

To counter that postive forcing

1. Aerosols combine to contribute -2.1 watts per square meter (about 1/2 of the positive forcing!)
   
   Oceanic dimethylsulfide (DMS) is the major natural source of sulfur to the atmosphere

2. Changing land use (greenspace converted to buildings/parking lots) slighly increases the overall reflectivity of the earth adding another -0.15 watts per square meter.

3. The net balance is then 1.95 watts per square meter.

So in round numbers 2 watts per square meter of net forcing implies 1.5 +/- 0.5 degree C net warming from 1750 to 2000.

The overall rise of the net forcing is illustrated below. The downward spikes seen since 1960 are the result of known, large scale volcanic events (El Chicon, Pinatubo) that temporarily greatly increase the sulfate aerosol content of the atmosphere.

But the most important thing to realize is that the potential positive forcings from greenhouse gas build up are pretty similar to the estimate negative forcings that occur during little ice ages (due to changing albedo/reflectivity of the planet, and net loss of greenhouse gases) ; thus Humans are potentially altering the climate (disturbing the energy balance) at the same amplitude as the alteration which causes little ice ages.