There are many questions around CO2 even basic ones. For example, the IPCC states ice cores show CO2 has been in the “range of 180 to 300 ppm over the glacial-interglacial cycles of the last 650 kyr”. A big unknown question noted by the IPCC is why does CO2 vary by this amount naturally? (AR4 Box 6.2 Page 446)

Another big area of question comes from the fact the IPCC does not have enough data on whether there has been an increase in rate CO2 entering the oceans. They cite some studies “suggesting the ocean uptake rate of total CO2 may have declined while others conclude that there is little evidence for a decline.” (AR5 Section 3.8.1 Page 293)

Other areas of question not fully understand include the full effects of CO2. For example, the IPCC explains they have “very high confidence CO2 has caused acidification of surface waters of between –0.0015 and –0.0024 pH units per year.” (AR5 Section 10.4.4 Page 906)

The IPCC goes on to explain CO2 is not uniformed around the global as there is more in the subtropics than at the poles because CO2 decreases towards them. As a result, the RF from CO2 is not uniformed around the globe. (AR5 Section 8.6.1 Page 702)

As a result, there is more CO2 RF or forcing in warm and dry regions than in wetter and higher altitude regions. (AR5 Section 8.6.1 Page 702)

Next, the IPCC address different types of forcing from CO2 like Radiative forcing (RF) and Effective Radiative forcing (ERF) where questions exist. RF sees temperature change from things like CO2 as well as other greenhouses gases more constant over time where ERF focuses more on the rapid change.

The IPCC goes on to tell us “ERF and RF value are significantly different for Anthropogenic aerosols owing to their influence on clouds and snow cover. These changes to clouds are rapid adjustments and occur on a time scale much faster than responses of the ocean.” (AR5 Chapter 8 Executive Summary Page 661)

The IPCC goes on to note they are “certain that the total anthropogenic ERF is positive.” They believe ERF has increased faster since 1970s. (AR5 Chapter 8 Executive Summary Page 661)

Aerosol Questions:

Another area of question is the effect of aerosols. For example, here the IPCC states, “Aerosols partially offset the forcing of the WMGHGs and dominate the uncertainty associated with the total anthropogenic driving the climate.” (AR5 Chapter 8 Executive Summary Page 661)

The IPCC admits they have changed their believed on the value range of aerosol forcing since AR4. They note they have lowered the effects of aerosol forcing to –0.35 (–0.85 to +0.15) W m–2. (AR5 Chapter 8 Executive Summary Page 662)