Are greenhouse gas emission levels on the rise in the United States and elsewhere?

Recent world greenhouse gas (GHG) emissions rates and more recent GHG trends in the United States are examined.

There is little doubt that global climate change is a trend and that the cause is an increase in greenhouse gas emissions (GHG).  That’s not to say that this recent winter was necessarily the result of climate change. While many see a definite relationship between human activity and the warming trend some are more skeptical.  We’ll leave any clear answer to that alone for now.  Instead, let’s look at the world GHG emission levels over recent history from a variety of anthropogenic (human-made) sources.  Then we’ll consider recent U.S. levels and look at trends.  Remember that GHG includes the most prevalent one, carbon dioxide, plus methane, nitrous oxide, much less prevalent Hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs or CxFx ).

The United Nations Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report:  Climate Change 2007 holds some interesting information.  IPCC is a United Nations-sponsored scientific body that is open to all UN members.

Chart on right: Figure 1.1b Global anthropogenic greenhouse gas emissions in 2004.  Vertical scale is in Gt CO2eq/yr = Teragrams (or million metric tons) of carbon dioxide equivalent per year. Source: Adapted from Olivier et al., 2005, 2006 as quoted in www.ipcc.ch. See link above for footnotes and more complete description of figure.

Even with agreed upon methodologies, in providing the following summarizing table, the Fourth Assessment Report concludes, “These values should be regarded as indicative only as some uncertainty remains, particularly with regards to methane (CH4) and nitrous oxide  (N2O) emissions, for which the error margin is estimated to be  on the order of 30–50%, and CO2 emissions from agriculture, which have an even larger error margin.”

Understandably, it cannot be easy to gather concise data on what are mostly colorless, odorless emissions from many diverse activities in all countries worldwide.  The Fifth Assessment Report is due out in 2014 and the process of putting together is starting now.  More than 800 authors in three working groups will be involved in writing the assessment, and months of review will be involved. 

The trends in the Fourth Assessment report show generally increasing GHG output. But that was before the recession in Europe and the U.S. during 2008 and ’09.  Currently, China has replaced the U.S. as the world’s largest GHG emitter. 

The global warming potentials (GWP) of various GHG emissions are shown below. Carbon dioxide is the reference gas, with a value of one. The U.S. Environmental Protection Agency (EPA) has published The draft 1990-2010 Greenhouse Gas Emissions Inventory   in the Federal Register for a public comment period.  The Executive Summary shows the U.S. trends in table form.  This is shown in abbreviated form for the major categories of GHG emissions below.  

Gas

Global Warming Potential (GWP) 

 CO2

 1

 CH4*

 21

 N2O

 310

 HFCs

 Nine listed, from 140 to 11,700

 CxFx

 Four listed, from 6,500 to 9,200

 SF6

 23,900

Summarized from IPCC, 1996 as shown in Draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010, page ES-3 

*CH4 GWP includes direct effects and indirect effects due to the production of tropospheric ozone and stratospheric water vapor.  The indirect effect due to the production of CO2 is not included. (CH4Converts to CO2 over a period of years.)  The definition of a GWP for a particular greenhouse gas is the ratio of heat trapped by one unit mass of the greenhouse gas to that of one unit mass of CO2over a specified time period. 

Figures in the chart below are in CO2 Equivalent as Teragrams (Gt).  A Gt equals one million metric tons.

Gas/source

1990

2005

2006

2007

2008

2009

2010

CO2

5,100.5

6,114.2

6,026.2

6,127.5

5,928.6

5,503.4

5,718.8

CH4

668.3

633.7

646.4

656.4

663.2

671.8

666.2

N2O

327.7

346.2

352.0

352.2

334.5

322.6

325.7

HFCs

36.9

120.2

123.5

129.5

129.4

125.7

135.4

PFCs

20.6

6.2

6.0

7.5

6.6

5.6

5.6

SF6

32.6

17.8

16.8

15.6

15.0

13.9

13.8

Total

6,186.6

7,238.3

7,170.9

7,288.8

7,077.4

6,643.0

6,865.5

Net Emissions (sources and                      sinks effects)

5,376.9

6,169.5

6,052.7

6,212.6

6,021.9

5,612.3

5,823.0

Decreasing emissions with concomitant periods of lower economic output or high fuel can be seen here. Also, some Ozone Depleting Substances (ODS) were phased out by signatories to the Montreal Protocol and 1990 Clean Air Act Amendments and have been replaced by some HFCs and PFCs.  PFC emissions from aluminum production have dropped due to industry efforts and lower industrial activity levels. The report points out that while HFCs and PFCs, along with Sulfur hexafluoride (SF6) are acceptable substitutes for ODS, they are also potent greenhouse gases. PFCs and SF6 are essentially not broken down in the atmosphere and keep accumulating.  Sulfur hexafluoride has the strongest GHG effect of any chemical evaluated thus far.

“Sinks” as used here, are forests, urban trees, and landscape and food material in waste dumps.

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