What was the IPCC AR4 Most Certain About?

With attention focused on the release later this week of the Working Group I report of Fifth Assessment of the IPCC, I thought that it would be worthwhile to present the findings of Working Group I from the Fourth Assessment (AR4) which were expressed with the greatest certainty, that is with 95% confidence or greater.

In a 2011 paper published in Climatic Change (here in PDF), Rachel Jonassen and I looked at all of the 2,744 findings presented by the AR4 accompanied by associated likelihood terminology (shown above). Of those total findings, 573 came from Working Group I. Of those 573, a total of 17 non-unique findings were presented with a confidence level of greater than 95%. Note that the Summary for Policy Makers introduced a likelihood term not included in the guidance -- "unequivocal" as follows:

• Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level (see Figure SPM.3). {3.2, 4.2, 5.5}

Including the "unequivocal" finding there appears to be about 10 unique findings that were expressed with a 95% or greater likelihood.

Here is the full list:

From the Summary for Policy Makers

• It is virtually certain that over most land areas, there will be warmer and fewer cold days and nights, warmer and more frequent hot days and nights. [Tables 3.7, 3.8, 9.4; Sections 3.8, 5.5, 9.7, 11.2–11.9][WG1]
• The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to known natural causes alone. [4.8, 5.2, 9.4, 9.5, 9.7][WG1]

From the Technical Summary

• It is virtually certain that over most land areas, there will be warmer and fewer cold days and nights, warmer and more frequent hot days and nights. [TS/3.8, 5.5, 9.7, 11.2–11.9; Tables 3.7, 3.8, 9.4][WG1]
• It is virtually certain that anthropogenic aerosols produce a net negative radiative forcing (cooling influence) with a greater magnitude in the NH than in the SH. [TS/2.9, 9.2][WG1]
• From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols and land surface changes, it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750. [TS/2.9][WG1]
• It is extremely unlikely (less than 5%) that the global pattern of warming during the past half century can be explained without external forcing, and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling. [TS/2.9, 3.2, 5.2, 9.4, 9.5, 9.7][WG1]
  

From Chapter 2

• The combined anthropogenic RF is estimated to be +1.6 [–1.0, +0.8]2 W m–2, indicating that, since 1750, it is extremely likely that humans have exerted a substantial warming influence on climate. This RF estimate is likely to be at least five times greater than that due to solar irradiance changes. For the period 1950 to 2005, it is exceptionally unlikely that the combined natural RF (solar irradiance plus volcanic aerosol) has had a warming influence comparable to that of the combined anthropogenic RF. It is extremely likely that the total anthropogenic RF is larger than +0.6 W m–2. it remains extremely likely that the combined anthropogenic RF is both positive and substantial (best estimate: +1.6 W m–2). [2.ES, 2.9.2, 9.2.1.1-9.2.1.2][WG1]
• From the current knowledge of individual forcing mechanisms presented here it remains extremely likely that the combined anthropogenic RF is both positive and substantial (best estimate: +1.6 W m–2). [2.9.2][WG1]
• Over particularly the 1950 to 2005 period, the combined natural forcing has been either negative or slightly positive (less than approximately 0.2 W m–2), reaffirming and extending the conclusions in the TAR. Therefore, it is exceptionally unlikely that natural RFs could have contributed a positive RF of comparable magnitude to the combined anthropogenic RF term over the period 1950 to 2005 [Figure 2.23][WG1]

From Chapter 3

• The global land warming trend discussed is very unlikely to be influenced significantly by increasing urbanisation [3.2.2.2][WG1]

From Chapter 6

• It is virtually certain that millennial-scale changes in atmospheric CO2 associated with individual antarctic warm events were less than 25 ppm during the last glacial period. This suggests that the associated changes in North Atlantic Deep Water formation and in the large-scale deposition of wind-borne iron in the Southern Ocean had limited impact on CO2. [6.ES][WG1]

From Chapter 9

• The combined anthropogenic RF is estimated to be +1.6 [–1.0, +0.8]2 W m–2, indicating that, since 1750, it is extremely likely that humans have exerted a substantial warming influence on climate. This RF estimate is likely to be at least five times greater than that due to solar irradiance changes. For the period 1950 to 2005, it is exceptionally unlikely that the combined natural RF (solar irradiance plus volcanic aerosol) has had a warming influence comparable to that of the combined anthropogenic RF. It is extremely likely that the total anthropogenic RF is larger than +0.6 W m–2. it remains extremely likely that the combined anthropogenic RF is both positive and substantial (best estimate: +1.6 W m–2). [2.ES, 2.9.2, 9.2.1.1-9.2.1.2][WG1]
• Extremely likely: Warming during the past half century cannot be explained without external radiative forcing. Anthropogenic change has been detected in surface temperature with very high significance levels (less than 1% error probability). This conclusion is strengthened by detection of anthropogenic change in the upper ocean with high significance level [9.4.1.2, 9.4.1.4, 9.5.1.1, 9.3.3.2, 9.7][WG1]
• It is extremely unlikely (less than 5%) that recent global warming is due to internal variability alone such as might arise from El Niño (Section 9.4.1). The widespread nature of the warming (Figures 3.9 and 9.6) reduces the possibility that the warming could have resulted from internal variability. No known mode of internal variability leads to such widespread, near universal warming as has been observed in the past few decades. [9.7][WG1]
• For the period 1950 to 2005, it is exceptionally unlikely that the combined natural RF (solar irradiance plus volcanic aerosol) has had a warming influence comparable to that of the combined anthropogenic RF. It is extremely likely that the total anthropogenic RF is larger than +0.6 W m–2. [2.ES, 2.9.2, 9.2.1.1-9.2.1.2][WG1]
• Many observed changes in surface and free atmospheric temperature, ocean temperature and sea ice extent, and some large-scale changes in the atmospheric circulation over the 20th century are distinct from internal variability and consistent with the expected response to anthropogenic forcing. The simultaneous increase in energy content of all the major components of the climate system as well as the magnitude and pattern of warming within and across the different components supports the conclusion that the cause of the warming is extremely unlikely (less than 5%).

Details on our paper:

Jonassen, R. and R. Pielke, Jr., 2011. Improving conveyance of uncertainties in the findings of the IPCC, Climatic Change, 108:745-753, http://dx.doi.org/10.1007/s10584-011-0185-7.

Abstract: Authors of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) received guidance on reporting understanding, certainty and/or confidence in findings using a common language, to better communicate with decision makers. However, a review of the IPCC conducted by the InterAcademy Council (2010) found that “the guidance was not consistently followed in AR4, leading to unnecessary errors . . . the guidance was often applied to statements that are so vague they cannot be falsified. In these cases the impression was often left, quite incorrectly, that a substantive finding was being presented.” Our comprehensive and quantitative analysis of findings and associated uncertainty in the AR4 supports the IAC findings and suggests opportunities for improvement in future assessments.