Just how much of the "Greenhouse Effect" is caused by human activity?
It is about 0.28%, if water vapor is taken into account-- about 5.53%, if not.
This point is so crucial to the debate over global warming that how water vapor is or isn't factored into an analysis of Earth's greenhouse gases makes the difference between describing a significant human contribution to the greenhouse effect, or a negligible one.
Water vapor constitutes Earth's most significant greenhouse gas, accounting for about 95% of Earth's greenhouse effect (4). Interestingly, many "facts and figures' regarding global warming completely ignore the powerful effects of water vapor in the greenhouse system, carelessly (perhaps, deliberately) overstating human impacts as much as 20-fold.
Water vapor is 99.999% of natural origin. Other atmospheric greenhouse gases, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and miscellaneous other gases (CFC's, etc.), are also mostly of natural origin (except for the latter, which is mostly anthropogenic).
Human activites contribute slightly to greenhouse gas concentrations through farming, manufacturing, power generation, and transportation. However, these emissions are so dwarfed in comparison to emissions from natural sources we can do nothing about, that even the most costly efforts to limit human emissions would have a very small-- perhaps undetectable-- effect on global climate.
Man-made and natural carbon dioxide (CO2) comprises 99.44% of all greenhouse gas concentrations (368,400 / 370,484 )--(ignoring water vapor).
Also, from Table 1 (but not shown on graph):
Anthropogenic (man-made) CO2 additions comprise (11,880 / 370,484) or 3.207% of all greenhouse gas concentrations, (ignoring water vapor).
Total combined anthropogenic greenhouse gases comprise (12,217 / 370,484) or 3.298% of all greenhouse gas concentrations, (ignoring water vapor).
The various greenhouse gases are not equal in their heat-retention properties though, so to remain statistically relevant % concentrations must be changed to % contribution relative to CO2. This is done in Table 2, below, through the use of GWP multipliers for each gas, derived by various researchers.
Converting greenhouse gas concentrations
to greenhouse effect contribution
(using global warming potential )
2. Using appropriate corrections for the Global Warming Potential of the respective gases provides the following more meaningful comparison of greenhouse gases, based on the conversion:
( concentration ) X ( the appropriate GWP multiplier (2) (3) of each gas relative to CO2 ) = greenhouse contribution.:
Compared to the concentration statistics in Table 1, the GWP comparison in Table 2 illustrates, among other things:
Total carbon dioxide (CO2) contributions are reduced to 72.37% of all greenhouse gases (368,400 / 509,056)-- (ignoring water vapor).
Also, from Table 2 (but not shown on graph):
Anthropogenic (man-made) CO2 contributions drop to (11,880 / 509,056) or 2.33% of total of all greenhouse gases, (ignoring water vapor).
Total combined anthropogenic greenhouse gases becomes (28,162 / 509,056) or 5.53% of all greenhouse gas contributions, (ignoring water vapor).
Relative to carbon dioxide the other greenhouse gases together comprise about 27.63% of the greenhouse effect (ignoring water vapor) but only about 0.56% of total greenhouse gas concentrations. Put another way, as a group methane, nitrous oxide (N2O), and CFC's and other miscellaneous gases are about 50 times more potent than CO2 as greenhouse gases.
To properly represent the total relative impacts of Earth's greenhouse gases Table 3 (below) factors in the effect of water vapor on the system.
Water vapor overwhelms
all other natural and man-made
3. Table 3, shows what happens when the effect of water vapor is factored in, and together with all other greenhouse gases expressed as a relative % of the total greenhouse effect.
Role of Atmospheric Greenhouse Gases
(man-made and natural) as a % of Relative
Contribution to the "Greenhouse Effect"
As illustrated in this chart of the data in Table 3, the combined greenhouse contributions of CO2, methane, N2O and misc. gases are small compared to water vapor!
Total atmospheric carbon dioxide (CO2) -- both man-made and natural-- is only about 3.62% of the overall greenhouse effect-- a big difference from the 72.37% figure in Table 2, which ignored water!
Water vapor, the most significant greenhouse gas, comes from natural sources and is responsible for roughly 95% of the greenhouse effect (4). Among climatologists this is common knowledge but among special interests, certain governmental groups, and news reporters this fact is under-emphasized or just ignored altogether.
Conceding that it might be "a little misleading" to leave water vapor out, they nonetheless defend the practice by stating that it is "customary" to do so!
Putting it all together:
total human greenhouse gas contributions
add up to about 0.28% of the greenhouse effect.