Carbon Dioxide - How much is human activity adding to the atmosphere ?

Carbon Dioxide or CO2 is frequently mentioned as the primary catalyst for the observed warming of the planet over the past several decades, and mankind's CO2 emissions are pointed to as the primary factor in the increasing volume of CO2 in the atmosphere. However, how much have CO2 levels really increased and how much is human activity contributing to this increase ? Are there other sources ?

Measured CO2 emissions by human activity vs net atmospheric CO2

Comments on this graph :

• Total CO2 emissions per Dept of Energy : http://www.eia.doe.gov/aer/txt/ptb1119.html
• Historic CO2 emissions calculated by tabulating coal, brown coal, peat, and crude oil production by nation and year, going back to 1751.  Data prior to this point assumed to be level (close to unmeasurably low) - spike in 18th C begun by coal and oil energy use.
• Atmospheric CO2 levels are measured by hundreds of monitoring stations across the globe. Independent measurements are also conducted by airplanes and satellites. data for periods prior to are determined from air bubbles trapped in polar ice cores. In pre-industrial times over the last 10,000 years, CO2 was relatively stable at around 275 to 285 parts per million. Over the last 250 years, atmospheric CO2 levels have increased by about 100 parts per million. Currently, the amount of CO2 in the atmosphere is increasing by around 15 gigatonnes every year.

[ sources : ice core air bubble CO2 levels : http://www.ncdc.noaa.gov/paleo/metadata/noaa-icecore-2419.html ; modern day CO2 atmospheric levels measured in Hawaii - http://www.esrl.noaa.gov/gmd/ccgg/trends/ ; cumulative CO2 emissions data by human activity - http://cdiac.ornl.gov/trends/emis/overview_2006.html ]

The Earth absorbs a large volume of this increase in CO2 pollution into natural absorption sinks (plant absorption and sea absorption), however increased absorption into the oceans in particular will lead to a change in the acidity in sea water, significantly harming sea life including plankton, a key supplier of oxygen in the atmosphere.

In addition to direct pollution from the burning of fossil fuels, deforrestation also reduces the planet's ability to naturally absorb CO2 - as such, deforrestation is another human activity which causes increased CO2 volume in the atmosphere.

What do we know about CO2 that makes it a greenhouse gas, a gas that by its very nature, traps heat ?  According to radiative physics and decades of laboratory measurements, increased CO2 in the atmosphere is expected to absorb more infrared radiation as it escapes back out to space.  In 1970, NASA launched the IRIS satellite measuring infrared spectra.  In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations.  Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period (Harries 2001).  What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as CO2 and methane (CH4) absorb energy.  The change in outgoing radiation was consistent with theoretical expectations.  Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect". This result has been confirmed by subsequent papers using data from later satellites (Griggs 2004, Chen 2007).

Outgoing radiation is the reflected heat from the Sun, reflecting off of the Earth's surface back out into space.  As CO2 and related global warming gases absorb this radiated heat, preventing it from escaping, the atmosphere warms.  Warmer air warms the surface, melting ice and causing thermal expansion of water molecules in the Oceans.

This chart details the change between 1970 and 1998 (28 yrs) in absorption of normally reflected sunlight back into space.  As you can see there is less reflected energy since 1970 at exactly the wavelengths at which Carbon Dioxide and Methane absorb radated heat.  This is directly observed evidence of the increasing trapping of reflected sun light being caused by higher concentrations of CO2 and CH4 in the atmosphere.

[ source : http://www.nature.com/nature/journal/v410/n6826/abs/410355a0.html ]

When greenhouse gases such as CO2 and CH4 absorb infrared radiation, the energy heats the atmosphere which in turn re-radiates infrared radiation in all directions. Some makes its way back to the earth's surface. Hence we expect to find more infrared radiation heading downwards from the upper atmosphere, back to the ground. Surface measurements from 1973 to 2008 find an increasing trend of infrared radiation returning to earth (Wang 2009).

This graph represents the amount of radiated heat energy at the surface, note the correlation between the peaks in radiated heat and the wavelengths at which carbon dioxide and methane generate radiated heat.  This is direct, measured, imperical evidence that CO2 and CH4 significantly add to radiative heat, warming the atmosphere and the surface of the Earth.

[ source : http://ams.confex.com/ams/Annual2006/techprogram/paper_100737.htm ]

With more energy coming in than escaping back out to space, our planet accumulates heat. The Earth's total heat build up can be derived by adding up the heat content from the ocean, atmosphere, land and ice (Murphy 2009). Ocean heat content has been measured down to 3000 metres deep. Atmospheric heat content is calculated from the surface temperature record and heat capacity of the troposphere. Land and ice heat content (eg - the energy required to melt ice) are also included.

[ sources : http://www.agu.org/pubs/crossref/2009/2009JD012105.shtml ; http://www.nature.com/nature/journal/v453/n7198/abs/nature07080.html ]

From 1970 to 2003, Earth has been accumulating heat at a rate of 190,260 GigaWatts with the vast majority warming the oceans. Considering a typical nuclear power plant has an output of 1 GigaWatt, imagine 190,000 nuclear power plants pouring their energy output directly into our oceans. What about after 2003? A map of of ocean heat from 2003 to 2008 was constructed from ocean heat measurements down to 2000 metres deep (von Schuckmann 2009). Globally, the oceans have continued to accumulate heat to the end of 2008 at a rate of 0.77 ± 0.11 Wm?2, consistent with other determinations of the planet's energy imbalance (Hansen 2005, Trenberth 2009). The planet continues to accumulate heat.

Warmer oceans will lead to higher precipition and more storms as more and more water vapor enters the atmosphere.   Water vapor is also a significant heat trapping agent.

[ source : http://journals.ametsoc.org/doi/abs/10.1175/2008BAMS2634.1 ]

So, we have established CO2 emissions by human activity are up significantly.  We have determined there is significantly more CO2 entering the atmosphere and the Earth is absorbing CO2 at similar rates to the past.  We have determined that CO2 and CH4 interfere with the reflection of sunlight back into space, trapping the heat and radiating it in all directions.   We have measurements of higher radiated heat at the wavelengths of CO2 and CH4, we have measurements of rising temperatures across the board, with the highest in the oceans.    We anticipate more water vapor to enter the atmosphere, a feedback loop which will accelerate warming and also cause more storms such as cyclons and hurricanes, localized flooding and changes in participation leading to droughts in some areas and high precipition in other areas.

So we see a direct line of evidence that we're causing global warming. Human CO2 emissions far outstrip the rise in CO2 levels. The enhanced greenhouse effect is confirmed by satellite and surface measurements. The planet's energy imbalance is confirmed by summations of the planet's total heat content and ocean heat measurements.