IPCC Observations of Greenhouse Gas and Radiative Forcing Changes since 1750

IPCC Observations of Greenhouse Gas and Radiative Forcing Changes since 1750

Carbon Dioxide

The atmospheric concentration of carbon dioxide (CO2) has increased by 31% since 1750.
The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.
About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.

Methane

The atmospheric concentration of methane (CH4) has increased by 1060 ppb (151%) since 1750 and continues to increase. The present CH4 concentration has not been exceeded during the past 420,000 years.
Slightly more than half of current CH4 emissions are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and landfills). In addition, carbon monoxide (CO) emissions have recently been identified as a cause of increasing CH4 concentration.

Nitrous Oxide

The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750 and continues to increase. The present N2O concentration has not been exceeded during at least the past thousand years.
About a third of current N2O emissions are anthropogenic (e.g., agricultural soils, cattle feed lots and chemical industry).

Halocarbon Gases (CFCs, HCFCs, etc.)

Since 1995, the atmospheric concentrations of many of those halocarbon gases that are both ozone-depleting and greenhouse gases (e.g., CFCl3 and CF2Cl2), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments.
Their substitute compounds (e.g., CHF2Cl and CF3CH2F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs) and sulphur hexafluoride (SF6)) are also greenhouse gases, and their concentrations are currently increasing.

Radiative Forcing

The radiative forcing due to increases of the well-mixed greenhouse gases from 1750 to 2000 is estimated to be 2.66 Wm-2 (total):
1.66 Wm-2 from CO2;
0.48 Wm-2 from CH4;
0.34 Wm-2 from the halocarbons; and
0.18 Wm-2 from N2O.
The observed depletion of the stratospheric ozone (O3) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (ñ0.15 Wm-2).
The total amount of O3 in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O3 forming gases. This corresponds to a positive radiative forcing of 0.35 Wm-2.

Anthropogenic Aerosols

The major sources of anthropogenic aerosols are fossil fuel and biomass burning. These sources are also linked to degradation of air quality and acid deposition.
Since the SAR, significant progress has been achieved in better characterising the direct radiative roles of different types of aerosols.
Direct radiative forcing is estimated to be:
ñ0.4 Wm-2 for sulphate,
-0.2 Wm-2 for biomass burning aerosols,
-0.1 Wm-2 for fossil fuel organic carbon and
+0.2 Wm-2 for fossil fuel black carbon aerosols.
There is much less confidence in the ability to quantify the total aerosol direct effect, and its evolution over time, than that for the gases listed above. Aerosols also vary considerably by region and respond quickly to changes in emissions.