Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties alter the energy balance of the climate system. These changes are expressed in terms of radiative forcing, which is used to compare how a range of human and natural factors drive warming or cooling influences on global climate. Since the Third Assessment Report (TAR), new observations and related modelling of greenhouse gases, solar activity, land surface properties and some aspects of aerosols have led to improvements in the quantitative estimates of radiative forcing.

Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and landuse change, while those of methane and nitrous oxide are primarily due to agriculture.


o Carbon dioxide is the most important anthropogenic greenhouse gas. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. The annual carbon dioxide concentration growth-rate was larger during the last 10 years (1995 - 2005 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements (1960 - 2005 average: 1.4 ppm per year) although there is year-to-year variability in growth rates.

o The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land use change providing another significant but smaller contribution. Annual fossil carbon dioxide emissions increased from an average of 6.4 .

o The global atmospheric concentration of methane has increased from a pre- industrial value of about 715 ppb to 1732 ppb in the early 1990s, and is 1774 ppb in 2005. The atmospheric concentration of methane in 2005 exceeds by far the natural range of the last 650,000 years (320 to 790 ppb) as determined from ice cores.

Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. It is very likely that the observed increase in methane concentration is due to anthropogenic activities, predominantly agriculture and fossil fuel use, but relative contributions from different source types are not well determined.

o The global atmospheric nitrous oxide concentration increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005. The growth rate has been approximately constant since 1980. More than a third of all nitrous oxide emissions are anthropogenic and are primarily due to agriculture.
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Source: INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE / Climate Change 2007: The Physical Science Basis

Terms explained:
ppm (parts per million) or ppb = (parts per billion, 1 billion = 1,000 million) is the ratio of the number of greenhouse gas molecules to the total number of molecules of dry air. For example: 300 ppm means 300 molecules of a greenhouse gas per million molecules of dry air.
Radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it. In this report radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre.
Fossil carbon dioxide emissions include those from the production, distribution and consumption of fossil fuels and as a by-product from cement production.


Annotations:
abundance = Fülle, Überfluß
aerosol = Spraydose, Schwebstoff
nitrous oxide = Stickoxyd
ice core = Eisbohrkern
fossil fuel = fossiler Brennstoff
anthropogenic = vom Menschen verursacht


Assignments:
1. Which factors are responsible for the energy balance of the climate system?
2. What are the effects of positive and negative radiative forcing and how is radiative forcing measured?
3. Which chemical substances have dramatically increased in the past 250 years?
4. Who/what is mainly responsible for the increase of methane and nitrous oxide?
5. Which 'man-made' greenhouse gas is the one that mostly affects our atmosphere?
6. How do scientists determine the increase of greenhouse gases produced over sevearl thousand years?
7. From what you know about the greenhouse effect, what consequence does it have for us?