Dr. Roger Pielke Sr. has written a blog post addressing the disagreement between himself and Skeptical Science (SkS) regarding the contribution of CO2 to the net positive anthropogenic radiative forcing. Initially Dr. Pielke cited a presentation he gave in 2006 which said (on slide 12):

"The CO2 contribution to the radiative warming decreases to 26.5% using the IPCC framework given in Slide 9"

This "radiative warming" refers to the human plus natural positive radiative forcings ('natural' being solar). As Dr. Pielke's presentation was given in 2006, before publication of the IPCC Fourth Assessment Report (AR4), his reference to the IPCC is to the Third Assessment Report (TAR) published in 2001. In his new post, Dr. Pielke also references a previous post on his blog on the same subject, which concludes (emphasis added):

"For all of the human-caused warming radiative forcings, which includes the 0.5 Watts per meter squared value for the shortwave albedo change, and estimating tropospheric ozone as 0.3 Watts per meter squared, the aerosol black carbon direct effect as 0.2 Watts per meter squared, the black carbon on snow and ice as 0.3 Watts per meter squared, the semidirect indirect effect as 0.1 Watt per meter squared, and the glaciation indirect effect as 0.1 Watt per meter squared (with the latter two forcings using a nominal value, since these forcings are very poorly known),

the contribution due to CO2 will fall to about 28%."

In this case Dr. Pielke refers to only the human positive radiative forcings, excluding the contribution of solar irradiance.

In short, Dr. Pielke has argued that CO2 contribution to the total positive radiative forcing (since pre-industrial times) is between 26% and 28% (depending on whether solar effects are included), whereas in our previous post, SkS concurred with the AR4 radiative forcing estimates, which put CO2 at approximately 50% of the total positive radiative forcing (nearly twice Dr. Pielke's estimate).

Below we discuss some problems SkS has identified in Dr. Pielke's estimate, and provide a detailed up-to-date estimate of these values. The main underlying problem is that Dr. Pielke is relying on an estimate he made in 2006, failing to account for advances in climate research over the past 5 years, and thus his sources are at least 5 years out of date. Additionally, he appears to have made some mathematical errors in his calculations.

Dr. Pielke estimates the radiative forcing from methane at 0.8 Watts per square meter (W/m2), which is significantly larger than the IPCC estimate (both TAR and AR4) of 0.48 W/m2. To support this value, in his 2006 presentation Dr. Pielke references research by "Drew Shindell and colleagues; Keppler et al." (slide 11), and on his blog posts, references Keppler et al. (2006). Keppler et al. do not estimate the methane radiative forcing in their paper - the 0.8 W/m2 figure is Dr. Pielke's estimate based on Keppler et al.'s results.

However, as we noted in our previous post, both the atmospheric methane concentration and radiative forcing are well-known quantities. The IPCC TAR and AR4 best estimates of the methane radiative forcing are 0.48 W/m2, 0.49 W/m2 according to Skeie et al. (2011), and 0.504 W/m2 in 2010 according to the NOAA Annual Greenhouse Gas Index (AGGI). Thus Dr. Pielke's methane forcing estimate appears to be 60% too high.

Additionally, Dr. Pielke appears to have double-counted the methane forcing in his calculations:

"By summing the 0.8 Watts per meter squared for methane and using the total of 2.4 Watts per meter squared of the well-mixed greenhouse gases from the IPCC Report..."

The 0.48 W/m2 methane forcing is included in the 2.43 W/m2 best estimate forcing for well-mixed greenhouse gases in the IPCC TAR (the best estimate is 2.64 W/m2 in the AR4). Thus, summing Pielke's estimated methane forcing (0.8 W/m2) and the IPCC TAR greenhouse gas forcing (2.4 W/m2) double counts the methane forcing.

Dr. Pielke also estimates "*0.5 Watts per meter squared value for the shortwave albedo change*," which is a forcing not included in the TAR or AR4. In his presentation (slide 11), Dr. Pielke claims:

"For the period 2000-2004, a CERES Science Team assessment of the shortwave albedo found a decrease by 0.0015 which corresponds to an extra 0.5 W m−2 of radiative imbalance according to their assessment."

However, there are a number of problems with this estimate. Most importantly, the data in question only cover a period of 4 years. Changes in the Earth's albedo (reflectivity) over a 4-year period tell us little or nothing about changes in albedo over the past century. It's apples and oranges; one is short-term, the other is long-term.

Four years is also simply far too short of a timeframe to ascertain a meaningful trend. From Loeb et al. (2007):

"Commonly used statistical tools applied to the CERES Terra data reveal that in order to detect a statistically significant trend of magnitude 0.3 W m

^{−2}decade^{−1}in global SW TOA flux, approximately 10 to 15 yr of data are needed. "

Additionally, there is significant uncertainty regarding this short-term albedo change (i.e. see Wielicki et al. 2005 and many other papers on the subject). While the CERES data Dr. Pielke references estimated a decrease in the Earth's albedo from 2000 to 2004, albedo change estimates over the exact same timeframe using Project Earthshine data found an even larger ** **** increase** in albedo from 2000 to 2004 than the CERES-estimated decrease.

Loeb et al. (2007) also used a revised version of the CERES data to show that no statistically significant changes in the Earth’s albedo occurred between 2000 and 2005. More recently, Palle et al. (2009) conclude:

"Earthshine and FD [International Satellite Cloud Climatology Project flux data] analyses show contemporaneous and climatologically significant increases in the Earth's reflectance from the outset of our earthshine measurements beginning in late 1998 roughly until mid-2000. After that and to date, all three show a roughly constant terrestrial albedo, except for the FD data in the most recent years"

We should also note that an albedo increase/decrease due to increasing cloud cover would also be accompanied by an increased/decreased greenhouse effect, making the net effect on the climate even more uncertain.

But the bottom line is that in order to incorporate an albedo forcing into these estimates, we must use an estimated albedo change from pre-industrial to Present. We should also investigate the cause of any albedo change to determine if it should be treated as a forcing or as a feedback. If it's a forcing, then it's not anthropogenic, and Dr. Pielke was incorrect to include it in the anthropogenic forcings. If it's a feedback, then it should not be included in the calculation of total forcings at all.

Ultimately, for this calculation, Dr. Pielke's 0.5 W/m2 albedo forcing estimate is unjustified and not supported by more recent observations and scientific literature.

Dr. Pielke cites Hansen and Nazarenko (2004) in estimating the albedo effect of soot on snow and ice at 0.3 W/m2, and the net black carbon forcing at 0.5 W/m2. However, as the IPCC AR4 noted three years later, the magnitude of the black carbon radiative forcing remains uncertain. The best estimate of Skeie et al. (2011) of 0.45 W/m2 for the black carbon forcing is in rough agreement with Dr. Pielke's estimate. Ramanathan and Carmichael (2008) give a best estimate for the black carbon forcing at 0.9 W/m2.

In short, the black carbon forcing remains highly uncertain, but Dr. Pielke's estimate is reasonable.

Dr. Pielke's tropospheric ozone forcing estimate is somewhat unclear. He states that the associated forcing is 0.3 W/m2, but the IPCC TAR estimate is 0.35 W/m2, and Dr. Pielke appears to believe the value should be higher:

"Ozone was responsible for one-third to one-half of the observed warming trend in the Arctic during winter and spring [Drew Shindell]"

This release from NASA GISS appears to be the source, from which, if we are interpreting his presentation correctly, Dr. Pielke estimates an additional 0.3 W/m2 on top of the IPCC 0.35 W/m2 tropospheric ozone radiative forcing.

However, there are more recent estimates of this forcing, in addition to the IPCC's 0.35 W/m2 (both TAR and AR4). The best estimate from Skeie et al. (2011) was 0.44 W/m2, and the best estimate from Cionni et al, 2011 (submitted), on which Shindell is a co-author, is 0.23 W/m2. Thus Dr. Pielke's estimate of 0.65 W/m2 appears to be much too high.

Dr. Pielke also identifies a "glaciation effect" as causing a 0.1 W/m2 forcing, which, in a recent talk, he clarifies as "*An increase in ice nuclei increases the precipitation efficiency*." Lohmann et al. (2007) is a very good paper on this subject, and explains the effect, described as the aerosol indirect effect:

"Global climate model studies suggest that if, in addition to mineral dust, hydrophilic black carbon aerosols are assumed to act as ice nuclei at temperatures between 0 and –35°C, then increases in aerosol concentration from pre-industrial to present times may cause a glaciation indirect effect (Lohmann, 2002a). The glaciation effect refers to an increase in ice nuclei that results in a more frequent glaciation of supercooled stratiform clouds and increases the amount of precipitation via the ice phase. This decreases the global mean cloud cover and allows more solar radiation to be absorbed in the atmosphere. Whether or not the glaciation effect can partly offset the warm indirect aerosol effect depends on the competition between the ice nucleating abilities of the natural and anthropogenic freezing nuclei (Lohmann and Diehl, 2006)."

Lohmann et al. (2007) note that the aerosol indirect glaciation effect is negligible. However, Perlwitz and Mlller (2010) conclude:

"Despite the high complexity and nonlinearity of the microphysical interaction between aerosols and clouds, modeling studies generally indicate that the net effect of this interaction is to reflect more radiation back to outer space [Forster et al., 2007], although recent results show that aerosols acting as ice nuclei could counteract the cooling effect significantly [Storelvmo et al., 2008]. A few observational studies seem to confirm a relation between soil dust aerosols and cloud cover."

In short, the aerosol indirect glaciation effect remains far from clear. Dr. Pielke also identifies the aerosol semi-direct effect, which involves tropospheric aerosols absorbing shortwave radiation, as causing a 0.1 W/m2 forcing. However, the IPCC has not included this as a positive forcing becase

"the semi-direct effect is not strictly considered an RF because of modifications to the hydrological cycle"

Additionally, Lohmann et al. identify the semi-direct effect as most likely causing cooling:

"The semi-direct effect refers to temperature changes due to absorbing aerosols that can cause evaporation of cloud droplets, as was shown in a large eddy model simulation study that used black carbon concentrations measured during the Indian Ocean Experiment (Ackerman et al., 2000). It ranges from 0.1 to –0.5 Wm-2 in global simulations"

The IPCC AR4 also lists the semi-direct effect as "positive or negative" and "small" potential magnitude, and the indirect effect as "positive" and "medium" potential magnitude, where as Dr. Pielke lists both as positive and equal in magnitude (0.1 W/m2). In short, the magnitude and roles of the aerosol semi-direct and indirect glaciation effects in terms of radiative forcings remain far from clear.

Dr. Pielke's estimate for the CO2 radiative forcing (1.4 W/m2) is both outdated and not consistent with the value in the IPCC TAR (1.46 W/m2); it appears that he either rounded the value down or eyeballed the IPCC TAR radiative forcing graphic rather than looking up the precise value. However, 1.46 W/m2 was the estimated value in 2001, when the TAR was published. In 2007, when the AR4 was published, the CO2 forcing had already increased to 1.66 W/m2. More recently, the NOAA AGGI estimated the CO2 forcing at 1.79 W/m2 in 2010, and Skeie et al. at 1.82 W/m2.

In other words, the CO2 radiative forcing has increased 25% over the past decade. Some of the other forcing estimates (like tropospheric ozone and black carbon) have changed mainly as a result of new research, but the CO2 forcing has changed as a result of rapidly increasing CO2 emissions and atmospheric concentrations.

As Isaac Held noted,

"I think it is very generally recognized that, for the same global mean forcing, aerosols perturb the mean precipitation field more than do the well-mixed greenhouse gases (WMGGs). So if, up to the present, anthropogenic aerosols and WMGGs have had comparable effects on regional precipitation, say, the WMGG effect will undoubtedly grow and will be essentially irreversible on the time scale of several centuries, in the absence of geoengineering, while the aerosol effect will likely be bounded by its current magnitude, and the WMGGs will dominate."

Below we summarize various estimates of the CO2 contribution to the net positive radiative forcing. We believe Dr. Pielke has committed two types of errors: mathematical (double-counting and rounding), and using outdated sources.

We believe the first column is a replication of Dr. Pielke's estimates. The second column corrects Dr. Pielke's math errors by eliminating the double counting of methane, and correcting rounding errors for the CO2 and solar forcings. The third column provides the IPCC TAR estimates which were the basis of Dr. Pielke's estimates, but which, for the most part, we believe are more accurate than Dr. Pielke's suggested values.

The fourth and fifth columns correct for the out-of-date references by using the IPCC AR4 and Skeie et al. (2011) estimates. Bear in mind we have not included the uncertainty ranges - these are all just best estimates of the respective positive radiative forcings (in W/m2).

Forcings | Pielke 2006 | Pielke Math Corrected | IPCC TAR | IPCC AR4 | Skeie 2011 |
---|---|---|---|---|---|

CO2 | 1.40 | 1.46 | 1.46 | 1.66 | 1.82 |

CH4 | 0.80 | 0.80 | 0.48 | 0.48 | 0.49 |

other LLGHGs | 1.00 | 0.49 | 0.49 | 0.50 | 0.51 |

tropospheric ozone | 0.65 | 0.65 | 0.35 | 0.35 | 0.44 |

black carbon | 0.50 | 0.50 | 0.20 | 0.10 | 0.45 |

albedo | 0.50 | 0.50 | 0 | 0 | 0 |

aerosols (semi-direct+indirect) | 0.20 | 0.20 | 0 | 0 | 0 |

stratospheric water vapor | 0 | 0 | 0 | 0.07 | 0.07 |

contrails | 0.02 | 0.02 | 0.02 | 0.01 | 0 |

solar | 0.25 | 0.30 | 0.30 | 0.12 | 0.12 (AR4) |

Total Positive Forcing | 5.32 | 4.92 | 3.30 | 3.29 | 3.90 |

Total Anthropogenic Forcing | 5.07 | 4.62 | 3.00 | 3.17 | 3.78 |

CO2 contribution to Total | 26.3% |
29.7% |
44.2% |
50.5% |
46.7% |

CO2 contribution to Anthropogenic | 27.6% |
31.6% |
48.7% |
52.4% |
48.2% |

If we correct for Pielke's double counting and rounding errors, the CO2 contribution to the total net positive forcing increases to approximately 30%. When we use up-to-date research for all forcings, the CO2 contribution increases to close to 50%, as we originally argued. We again note that this fraction will continue to increase along with continually increasing human CO2 emissions.

We are still interested in Dr. Pielke's answer our original question on this subject:

"Approximately what percentage of the global warming (increase in surface, atmosphere, ocean temperatures, etc.) over the past 100 years would you estimate is due to human greenhouse gas emissions and other anthropogenic effects?"

We suggest a back-of-the-envelope answer to this question by applying the probabilistic estimate of transient climate sensitivity by Padilla (2011):

"we find a most-likely present-day estimate of the transient climate sensitivity to be 1.6 K with 90% confidence the response will fall between 1.3–2.6 K"

We can use this range of transient climate sensitivity (alpha = 0.35 to 0.70 K/Wm-2) and scale the transient climate response (we're currently 49% of the way to the radiative forcing associated with CO2 doubling [~1.8 out of 3.7 W/m2]) to estimate the amount of CO2-caused surface warming:

Where F is the radiative forcing. Using the Skeie et al. (2011) CO2 forcing best estimate of 1.82 W/m2 for 2010 and the Padilla (2011) range of transient climate sensitivity parameters, this corresponds to **a CO2 contribution of 0.64 to 1.28°C, with a best estimate of 0.79°C warming of average global surface temperature**.

We can also consider the expected warming for the net anthropogenic forcing, which Skeie et al. estimated at 1.4 W/m2 and the IPCC AR4 estimated it at 1.6 W/m2. Using these two estimates and the Padilla transient sensitivity range yields **a net anthropogenic warming of 0.49 to 1.12°C with a central estimate of 0.65°C warming of average global surface temperature**.

Dr. Pielke, would you concur with these estimated ranges of CO2 and anthropogenic warming?

The main points here are that CO2 is responsible for approximately 50% of the net positive radiative forcing since pre-industrial times (a percentage which will only continue to increase in the future). In the absence of negative forcings, CO2 would have contributed 0.79°C of the 0.8°C observed global surface temperature rise, and hence we would expect the total observed rise to be double that. This tells us that the negative forcings (primarily from human aerosol emissions) have offset approximately 50% of the net positive forcings.

We also found that the net anthropogenic radiative forcing (sum of all positive and negative forcings) accounts for approximately 80% of the observed average surface warming over the past century (~0.65 out of 0.8°C). The other ~20% is a combination of natural forcings (primarily solar), and perhaps a bit of natural variability.

Another key point is that aerosols have a short atmospheric lifetime, unlike long-lived greenhouse gases. Thus their large offsetting of close to 50% of the net positive radiative forcing is only temporary, and will decline rapidly if we reduce aerosol emissions. This is why, as Isaac Held noted in the quote above, we fully expect CO2 and other greenhouse gases to continue as the dominant cause of global warming, and why although we need to address other issues like land use change, CO2 emissions are rightfully the primary target in mitigating climate change.

Posted by dana1981 on Tuesday, 11 October, 2011

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