Data from IPCC Third Assessment Report (TAR)[i]

This paper is not intended to be a comprehensive discussion of the scientific basis for climate change. Rather, the discussion here is intended to provide a context for the action program we describe. The following is a summary of the findings of the Intergovernmental Panel on Climate Change (IPCC) that shape our program of recommendations.[ii]

• The global average surface temperature has increased over the 20th century by about 0.6°C.
• Temperatures have risen during the past four decades in the lowest 8 kilometers of the atmosphere.
• Snow cover and ice extent have decreased.
• Global average sea level has risen and ocean heat content has increased.
• Changes have also occurred in other important aspects of climate.
  • Increase in precipitation
  • Increase in heavy precipitation events
  • Increase in cloud cover
  • Decrease in extreme low temperatures, increase in extreme high temperatures

 The IPCC has concluded that the warming described above is due to human activity. It states:

“Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.”[iii]

 Among the most striking findings presented :

• 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.[iv] (Emphasis added)
• 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 ^v (Emphasis added).

The IPCC concludes:

“In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations[v].” (Emphasis added)

Atmospheric CO₂ Concentration and Stabilization

The IPCC has several scenarios that it uses to project possible stabilization levels for atmospheric CO₂. These scenarios, referred to as A1, A2, B1 and B2 use differing assumptions regarding population growth and human environmental impact. They are “business as usual” scenarios that do not take into account GHG emissions mitigation activities. The projected emissions amounts and corresponding atmospheric CO₂ concentrations are shown in the charts below (taken from IPCC TAR). These charts show that even under the most optimistic scenario, the atmospheric CO₂ stabilizes at a level almost 50% higher than today.

As these charts demonstrate, actions must be taken to stabilize the atmospheric CO₂ concentration in order to prevent unprecedented and unknown effects on the climate.

From these projections, IPCC concludes:

“…to maintain a constant future CO₂ concentration, anthropogenic CO₂ emissions would ultimately have to be reduced to the level of persistent natural sinks.”

 This statement is sometimes known as “the scientific (ecological) imperative.” The following chart shows the atmospheric CO₂ concentrations that would result under different emissions scenarios. Note that even the most optimistic scenario (B1) results in atmospheric CO₂ that is stabilized at a level (475 ppm-575 ppm); far higher than today.