"Of the short-lived species, methane, tropospheric ozone and black carbon are key contributors to global warming, augmenting the radiative forcing of carbon dioxide by 65%. Others — such as sulphate, nitrate and organic aerosols — cause a negative radiative forcing, offsetting a fraction of the warming owing to carbon dioxide."

• climate models
• recent responses to large volcanic eruptions
• recent responses to solar cycles
• paleoclimate data
• data from the last Glacial Maximum
• and yes, data from the instrumental period

All of these different methods show strong agreement, overlapping in the IPCC climate sensitivity range of 2 to 4.5°C for a doubling of atmospheric CO₂ (2xCO₂).

Figure 2: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) and most likely value (3°C) are indicated by the vertical grey bar and black line, respectively (Knutti and Hegerl 2008)

Interestingly, Penner et al. find that whether the climate sensitivity parameter is on the low or high end, reducing anthropogenic emissions of the short-lived warming pollutants would achieve a significant reduction in global warming over the next 50-100 years.  In the red lines in the Figure 3, they employ a climate model with a sensitivity of 5°C for 2xCO₂, slightly outside the IPCC likely range.  The blue line is a climate model with a sensitivity of 2°C for 2xCO₂, on the lower end of the IPCC range.  Note that even with the lower climate sensitivity, the model shows the planet warming 3°C by 2100 in this emissions scenario (see the figure caption for further details).

Figure 3: Global mean temperature measurements (black) and projections based on an IPCC scenario with high emissions (A2) for a climate sensitivity parameter of 5°C (upper red) and 2°C (upper blue). Linearly decreasing the total anthropogenic radiative forcing owing to methane, tropospheric ozone and black carbon — starting in 2010 and achieving pre-industrial levels by 2050 — results in significant near-term climate mitigation (lower blue and red curves) (Penner 2010)

Unfortunately, reducing the short-lived cooling pollutants such as aerosols would cause a warming effect of similar magnitude, and so CO₂ remains the primary pollutant of concern.  Coincidentally, a group of scientists from NASA GISS just published a paper in Science entitled Atmospheric CO2: Principle Control Knob Governing Earth's Temperature.

Although it is important to reduce the remaining climate uncertainties, such as the magnitude of the impacts of short-lived pollutants, it does not change the fact that CO2 is very likely the driving force behind the current global warming, or that if we double the amount of CO2 in the atmosphere from pre-industrial levels, the planet will likely warm in the range of 2 to 4.5°C.