Atmospheric Greenhouse Gases

20,000 years
2000 years
The recent rate of change is dramatic and unprecedented; increases in CO2 never exceeded 30 ppm in 1,000 years – yet now CO2 has risen by 30 ppm in just the last 17 years.... IPCC 2007 FAQ 7.1
We refer to greenhouse emissions because recently black carbon(BC)/soot particles has been found to be second only to CO2 in global warming contribution. 

The Guardian (2011)published a thorough breakdown of GHG emissions by sources 

Industrial fossil fuel energy production and industrial food production (30-40%) are the two big sources of emissions. 

Emitting GHGs at all cause is catastrophically dangerous because of the extremely long atmospheric lifetime of carbon dioxide (and nitrous oxide) ​and chain reactions of interacting impacts and feedbacks​ ​


The fossil fuel industry is actually a source of all GH emissions,
​with the exception of halocarbons. ​​It is also the big source of
​health damaging air pollution. Coal combustion is a big source of
health damaging mercury and heavy metals. ​

​Atmospheric GHG concentration directly correlates with lower atmospheric radiative forcing. The increase in radiative forcing is the increase in the total heat in the climate system. 

This is a far more accurate indicator of warming because it includes future committed global average temperature increase as well as today's temperature increase. 

The main GHGs that naturally control ​​ice ages alternating with warm periods, and are emitted by us - are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). They are classified by the IPCC as 'long lasting' atmospheric GHGs, lasting from about a decade (CH4) to over a century (CO2 & N2O).   

Atmospheric CO2 has increased 40% since industrialization.
Methane has increased by 150%.
Nitrous oxide has increased by ​​​​20%.

GHG ice core data goes back 800,000 years. Atmospheric CO2 is nearly 30% higher than its maximum level over the past 800,000 years, from ice core data.​​
Research on ocean sediment indicates this is the highest in 15 million years.
​The rate of atmospheric CO2 increase in unprecedented.  

​​Methane is over double its 800,000 limit. 

These three main GHGs have different atmospheric lifetimes and different global warming ​​​capacities (global warming potential GWP). The table below is from the IPCC AR4 2007. 

Water vapour is the top GHG for warming contribution, because it is so abundant in the atmosphere. The other GHGs are classified as trace gases only existing in parts per million (CO2) or per billion (methane, nitrous oxide) of air. ​​It is classified by the IPCC as a global warming feedback, its most important capacity, rather than a GHG emission. Warmer air/atmosphere holds more water vapour. This +ve (amplifying) feedback about doubles the warming of a GHG alone. 
CO2. The IPCC gives no set metric on CO2 for atmospheric lifetime. The EPA assigns 100 years to CO2, but the big thing is that 20-40% of CO2 emissions last for 1000 years (IPCC AR5). In fact the lifetime is over 100,000 years. CO2 is also special for causing ocean acidification. The main sources are fossil fuel energy, deforestation and cement manufacturing (heating limestone). 

N20.​​ Nitrous oxide contributes the least of these 3 GHGs to global warming, but is increasing fastest. It has both an extreme long life time (114 years) and high GWP (298 over 100 years). The main sources are nitrogen chemical fertilizer (made from natural gas), livestock feed production ( same reason)​, livestock waste, and sewage.  
Nitrous oxide is doubly important now because it is the main cause of depletion affecting the stratospheric ozone UV protective layer.  

CH4. Methane lasts in the atmosphere 12 years which ​​that means methane emissions exert a warming 100 times that of CO2 over its lifetime. In addition methane disappears because it is oxidized to other GHGs - including water vapour, and  CO2 (that is not accounted in the methane GWP.  Main sources are livestock (ruminants exhale methane), natural gas industry (esp fracking), coal mining, natural wetlands, and rice paddies (wetlands).

A special feature of methane is the more of it there is in the atmosphere the longer methane will last in the atmosphere​

Methane feedback. The big thing about methane now is that we are into methane feedback. 
Atmospheric methane increase levelled off at 2000, but since 2007 has been on a sustained fast increase that the scientists say is due to methane feedback emissions from the warming planet. The sources are thought to be warming subarctic and tropical peatlands. 

The Health and Human Rights Approach to Climate Change

800,000 years ice core data
Tropospheric O3. Toxic ground level  ozone (distinct from stratospheric ozone that is the UV radiation shield) is a short lasting GHG. It is formed by a chemical reaction between oxides of nitrogen and ​volatile organic compounds (both from fossil fuels) which is catalyzed by solar warmth. It therefore increases with global warming. As well as being toxic to human health, it is toxic to green plants, so as its concentration increases with warming the capacity of green plants to fix carbon from CO2 will decrease, a +ve amplifying feedback.  
CO2 15 million year high
Recent Research

May 2015​​ R. Rhodes Paleoclimate. Enhanced tropical methane production in response to iceberg discharge in the North Atlantic.
Climate Emergency Institute
EPA May 2015 Atmospheric GHGs
800,000 yrs and ​1950-2015
2016 Atmospheric CO2 eq. was 489 ppm 2017 NOAA Annual Greenhouse Gas Index
CO2 equivalent, which is used by the IPCC, includes methane and nitrous oxide as well as CO2​.