The IPCC Says, “The Observed Patterns of Warming…, And Their Changes Over Time, Are Only Simulated By Models That Include Anthropogenic Forcing”

SUMMARY

This post is the second in a series of posts that could be called the “casual observer posts”. The first was “Satellite-Era SST Anomalies: Models Vs Observations Using Time-Series Graphs And 17-Year Trends.”Like it, this is a post about perceptions, about what the casual observer would perceive if he or she were to investigate the differences between the observed Sea Surface Temperature data and the multi-model mean of the coupled climate models used by the IPCC in their AR4.

This post compares the linear trends of the observed Sea Surface Temperature, on Global and Hemispheric bases, from 1900 to 2005, and the corresponding linear trends of the multi-model mean output of the coupled climate models used by the IPCC for AR4. The data are broken down into three periods: the two warming and one flat (or cooling) periods that are acknowledged by the IPCC. The data is presented on an annual basis to allow for the easier identification of the start and end years of those periods. And the observational Sea Surface Temperature data in this post uses the average of four long-term Sea Surface Temperature datasets: the NCDC’s ERSST.v3b, and the Hadley Centre’s HADISST, HADSST2, and HADSST3.

INTRODUCTION

The quoted portion of the title of this post is an edited version of a sentence from the IPCC AR4 Working Group 1 Summary for Policymakers. It’s from the fourth bullet-point paragraph under the heading of “Understanding And Attributing Climate Change” (page 10). The paragraph reads in full [my bold face]:

It is likely that there has been significant anthropogenic warming over the past 50 years averaged over each continent except Antarctica (see Figure SPM.4). The observed patterns of warming, including greater warming over land than over the ocean, and their changes over time, are only simulated by models that include anthropogenic forcing. The ability of coupled climate models to simulate the observed temperature evolution on each of six continents provides stronger evidence of human influence on climate than was available in the TAR. {3.2, 9.4}

The highlighted sentence assumes the coupled climate models used by the IPCC have skill in recreating the observed warming of global surface temperatures. And it implies that the rise in global surface temperatures could only be caused by Anthropogenic Forcings. Why? Because the skillful climate models tell us so. But suppose the models aren’t so skillful.

On the subject of the rise in global surface temperatures since 1900, the IPCC was kind enough to suggest further reading with the bracketed chapter and section numbers of 3.2 and 9.4, so that quote from the Summary for Policymakers leads to the following two quotes from Chapters 3 and 9 of the IPCC AR4:

First, in Chapter 3 Observations: Surface and Atmospheric Climate Change, under the heading of “3.2.2.5 Consistency between Land and Ocean Surface Temperature Changes”, the IPCC states with respect to the surface temperature variations over the period of 1901 to 2005 (page 235):

“Clearly, the changes are not linear and can also be characterized as level prior to about 1915, a warming to about 1945, leveling out or even a slight decrease until the 1970s, and a fairly linear upward trend since then (Figure 3.6 and FAQ 3.1).”

And second, in Chapter 9 Understanding and Attributing Climate Change, under the heading of “9.4.1.2 Simulations of the 20th Century”, the final paragraph (page 686) begins:

“Modelling studies are also in moderately good agreement with observations during the first half of the 20th century when both anthropogenic and natural forcings are considered, although assessments of which forcings are important differ…”

From these two quotes, we can note that the IPCC acknowledges two warming periods during the 20^th Century, separated by a period where temperatures were relatively level, or decreasing slightly, and we can note that the early 20^th Century model outputs are “in moderately good agreement with observations.” The phrase “moderately good agreement” agreement is obviously vague and subjective, but the intent of the sentence is to reinforce how well the models can reproduce the rise in surface temperature over the early part of the 20^thCentury.

THE TIME PERIODS SELECTED

So let’s first take a look at the time-series graphs of annual Global and Hemispheric Sea Surface Temperature anomalies, as represented by the mean of the four long-term Sea Surface Temperature datasets, ERSST.v3b, HADISST, HADSST2, and HADSST3, to determine what years to use for the warming and level (cooling) temperatures periods. The graphs aren’t annotated and can serve as a reference.

Figure 1 shows that global Sea Surface Temperature anomalies dropped quite rapidly from 1900 to 1910, then rose from 1910 to 1944. Sea Surface Temperature anomalies appear to be relatively flat from 1944 to 1975, and then they rose again from 1975 to 2005.

Figure 1

The Northern Hemisphere periods are slightly different, Figure 2, with the change from the cooling period to the second warming period occurring in 1976, instead of 1975.

Figure 2

The Southern Hemisphere Sea Surface Temperatures anomalies show significantly different periods of warming and cooling. Refer to Figure 3. The initial warming period runs from 1911 to 1941. There appear to be two choices for the end year of the cooling period: 1955 and 1964. We’ll use 1964, since 1964 would reflect the drop in temperature associated with the 1963/64 eruption of Mount Agung in Indonesia. (Feel free to rerun the trend analysis using 1955 if you’d prefer, but it’s not going to significantly alter the conclusions of this post.)

Figure 3

GLOBAL TREND COMPARISONS

Figure 4 compares the linear trend of the Global Observed Mean and Multi-Model Mean Sea Surface Temperature anomalies from 1910 to 1944. During this period, the initial warming period, the combination of natural and anthropogenic forcings caused the model mean Global Sea Surface Temperatures to rise at a rate of only 0.035 deg C per decade, but the observed rate was about 3.8 times higher at 0.134 deg C per decade. One might conclude that Global Sea Surface Temperatures can rise quite strongly for multidecadal periods without the assistance of Anthropogenic and Natural Forcings.

Figure 4

For the period of 1944 to 1975, Figure 5, the combined effects of natural and anthropogenic forcings caused Global Sea Surface Temperatures of the model mean to rise at a rate of 0.025 deg C per decade, which is slightly less than the trend of 0.035 deg C per decade for the early warming period. But the observations show that Global Sea Surface Temperature anomalies during this period were relatively flat, rising at a rate of only 0.003 deg C per decade, which is a significant decrease from the rate of the early warming period of 0.134 deg C per decade. So the rate at which Global Sea Surface Temperatures rise can also drop significantly more than can be explained by changes in Natural and Anthropogenic Forcings. This appears to contradict the often-used explanation that the leveling off of global temperatures during this period was caused by an increase in anthropogenic aerosols.

Figure 5

The period of 1975 to 2005, Figure 6, is the only period when the Anthropogenic and Natural Forcings-driven multi-model mean Sea Surface Temperatures align with the trend of the observations. The observed linear trend of 0.141 deg C per decade is slightly higher than the 0.124 deg C per decade trend of the model mean. The observed trends for the early warming period (0.134 deg C per decade) and the late warming period (0.141 deg C per decade) are quite similar. But the model mean trend for the Global Sea Surface Temperature anomalies during the early warming period (0.035 deg C per decade) is only 28% of the trend of the modeled late warming period (0.124 deg C per decade).

Figure 6

NORTHERN HEMISPHERE COMPARISONS

In the Northern Hemisphere, the dissimilarities persist between observed and model mean Sea Surface trends for the early warming period and mid-Century cooling period. Refer to Figures 7, 8, and 9. Here, though, during the cooling period, Northern Hemisphere Sea Surface Temperature anomalies drop at a rate that of -0.044 deg C per decade, while the model mean rise at a rate of 0.013 deg C per decade. There are no similarities between the observations mean and the forcings-driven model-mean during the first 75 years of the Northern Hemisphere Sea Surface Temperature anomaly data. No similarities whatsoever.

Figure 7

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Figure 8

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Figure 9

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SOUTHERN HEMISPHERE COMPARISONS

And the differences between the observations-mean and the forcings-driven multi-model mean do not get any better in the Southern Hemisphere (Figures 10, 11, and 12) during the early warming and cooling epochs. The plus for the Southern Hemisphere: the late warming period started about a decade earlier, so the model and observed Sea Surface Temperature trends in the Southern Hemisphere align for a little longer.

Figure 10

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Figure 11

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Figure 12

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CLOSING

Figure 13 compares the linear trends of the observations mean and the multi-model mean during the two warming epochs of the Global Sea Surface Temperature that occurred between 1900 and 2005. The trends are extended for 30 years and are zeroed at year zero. There are significant differences between the trends of the natural and anthropogenic forcings-driven model mean for the two periods. Clearly, during those two epochs, the increase in anthropogenic forcings results in the difference in the trends of the models. But the observed trends in sea surface temperature show little difference, indicating that the increased anthropogenic forcings had little impact.

Figure 13

As shown in this post, the natural and anthropogenic forcings-driven sea surface temperatures of the models do not capture the mid-century flattening of Sea Surface Temperatures. And they do not explain the early 20^th Century rise in observed Sea Surface Temperatures, which rose at a rate that was comparable to the observed rise in the latter part of the 20^th Century. So, on what basis are we to believe that rise in sea surface temperature in the latter part of the 20^th Century was caused by Anthropogenic Forcings? The fact that multi-model mean sea surface temperature output aligns with observed sea surface temperatures for a few decades at the end of the 20^thCentury? That agreement between models and observations that occurs for a period that is less than 30% of the term of the data appears to be nothing more than happenstance.

The natural multidecadal variability of the observed Sea Surface Temperatures are the basis for the differences between the trend of the mid-20^thCentury flattening of Sea Surface Temperatures and the trends of the two warming periods, but natural multidecadal variability cannot be used to explain why the increased amount of anthropogenic greenhouse gases had virtually no impact on the trend of the latter warming period observations when compared to the observations of the first warming epoch.

So, let’s drop back to the IPCC quote included in the title of this post. That full sentence reads:

“The observed patterns of warming, including greater warming over land than over the ocean, and their changes over time, are only simulated by models that include anthropogenic forcing.”

One might conclude that that sentence actually points to one of the many failings of the climate models used by the IPCC in their AR4.

SOURCE

Al of the Sea Surface Temperature data used in the Observations-mean data and the IPCC AR4 multi-model mean data used in this post are available through the KNMI Climate Explorer. The SST data is found at the Monthly observations webpage, and the model mean data is found at the Monthly CMIP3+ scenario runswebpage.

ABOUT: Bob Tisdale – Climate Observations