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Roger Andrews: The Sea Level House of Cards: Part 1 – The Science
Guest post by Roger Andrews on the IPCC’s sea level rise attribution:
THE SEA LEVEL RISE HOUSE OF CARDS
PART ONE: THE SCIENCE
by Roger Andrews : Dec 2012
According to the recently-released draft of the IPCC AR5 global sea levels rose by about 0.17m in the 20th century and are projected to rise by 0.29 to 0.96m more in the 21st. This projection is based on the assumption that the warming generated by man-made greenhouse gases causes ice to melt and sea water to expand, thereby raising sea levels. Here we will review the supporting data the IPCC presents in the AR5 draft to see whether this assumption stands up to scientific scrutiny.
If the assumption is correct then there has to be a time-dependent relationship between sea level rise and AGW, so the first question to be addressed is; when did man-made greenhouse gases begin to have a significant impact, or in other words, when did AGW start? According to the following statements in the AR5 draft Summary for Policymakers it didn’t start with the Industrial Revolution. In fact it didn’t start until quite recently (the boldface is mine):
It is very likely that more than half of the ocean warming observed since the 1970s is caused by external forcing, mainly due to a combination of both anthropogenic forcing and volcanic eruptions (see Figure SPM.4). It is extremely likely that this warming has resulted in global mean sea level rise due to thermal expansion during this period.
It is extremely likely that human activities have caused more than half of the observed increase in global average surface temperature since the 1950s. There is high confidence that this has caused large-scale changes in … sea level in the second half of the 20th century.
Climate model simulations that includeonly natural forcings (volcanic eruptions and solar variations) can explain a substantial part of the preindustrial inter-decadal temperature variability since 1400 but fail to explain more recent warming since 1950 (see Figure SPM.4).
Two of these statements cite Figure SPM.4, so let’s see what this Figure shows. The upper graph in Figure 1 below reproduces the comparison of observed and modeled global land surface temperatures shown in Figure SPM.4 and the lower graph plots the difference between temperatures modeled using natural plus anthropogenic forcings and natural forcings only, revealing the anthropogenic contribution. According to the lower graph roughly 80% of the total anthropogenic land surface warming since 1860 occurred after 1960.
FIGURE 1 (From AR5 Figure SPM.4)
CLIMATE MODEL SIMULATIONS, LAND AIR TEMPERATURE
Figure 2 reproduces the Figure SPM.4 model-observed comparison for ocean heat content. It doesn’t go back far enough to provide a complete picture, but the data that are available indicate that all of the anthropogenic increase in ocean heat content occurred after 1970.
FIGURE 2 (From AR5 Figure SPM.4)
CLIMATE MODEL SIMULATIONS, OCEAN HEAT CONTENT
Based on these results we can reasonably pick 1970 as the year in which man-made emissions first began to have a significant warming impact, or in other words the year in which AGW began.
Now that we’ve established this we can look at some long-term observational data. First glacier ice loss, which is one of the two major contributors to sea level rise and which is assumed to be controlled by air temperature. AR5 Figure 4.12 plots various estimates of cumulative global glacier mass loss since 1800, and Figure 3 is a cleaned-up version of it:
FIGURE 3 (From AR5 Figure 4.12)
CUMULATIVE GLACIER MASS LOSS
If AGW is the only thing causing glaciers to retreat we should see a hockey stick shape that breaks upwards around 1970, and if an AGW component is superimposed on a natural component we should see an increase in the rate of glacier mass loss after 1970. But we see neither. The data show glacier retreat beginning after the Little Ice Age, which is generally agreed to have ended around 1850, picking up speed in the mid-1900s and if anything tending to slow down rather than speed up after the onset of AGW. There’s nothing here to suggest that the current phase of glacier retreat is anything other than a natural event that started well over a hundred years before the onset of AGW and which continued unaffected by man-made greenhouse gas emissions after AGW began. And in Section 13.3.2.1 of the AR5 draft the IPCC admits that at least the first 50 years – and depending how we interpret the wording maybe the first 120 years – of glacier retreat indeed probably was a natural event:
Overall, the combined records suggest that the net decline of global glacier volume began in the 19th century before significant anthropogenic radiative forcing had started, and was probably the result of natural climatic variability
The other major contributor to sea level rise is thermal expansion of sea water, which is governed by ocean heat content. Unfortunately there is OHC record before 1954 (and arguably no reliable OHC record before the ARGO data became available in 2003) so we will move on directly to sea levels.
Figure 4 shows four reconstructions of mean global sea levels based on tide gauge records, two of which (Jevrejeva et al. and Church & White) are presented in the AR5 draft. Like the ice mass loss reconstructions they show sea levels rising at a roughly constant rate since the end of the Little Ice Age and no visible acceleration in the rate of rise after the onset of AGW in 1970:
FIGURE 4
MEAN GLOBAL SEA LEVEL FROM TIDE GAUGE RECORDS
The level of correlation between sea level rise and AGW can also be evaluated statistically by comparing annual changes in sea level against anthropogenic forcings, and Figure 4 is an XY plot of annual sea level change since 1880 using the Jeverejeva data) against the GISS annual anthropogenic forcing estimates, which are substantially the same as those used in the IPCC’s climate models. The correlation coefficient (R2 = 0.000) is about as low as it’s possible to get.
FIGURE 5
ANNUAL CHANGES IN SEA LEVEL VS. GISS ANTHROPOGENIC FORCING
So how does the IPCC conclude that man-made greenhouse gases are causing sea levels to rise? It cites two basic lines of evidence. First is the postulated acceleration in the rate of rise during the 20th century – presumably related to AGW, although the IPCC doesn’t explicitly say so – cited in AR5 Section 13.2.2:
(T)he rate of sea level rise has increased since the late 19th century and has continued during the 20th century …. the estimated acceleration in GMSL since 1900 ranges from0.000 mm yr–2 in the Ray and Douglas (2011) record, 0.013 mm yr–2 in the Jevrejeva et al. (2008) record, and 0.012 mm yr–2 in the Church and White (2011) record.
Two positive estimates and one zero estimate hardly make a compelling case for the existence of an acceleration component, and if the acceleration began in 1900 it predated AGW by 70 years and can’t have been caused by it anyway. There are also doubts as to whether the 20th century sea level record even contains an acceleration component. The sea level data on which Church and White based their acceleration estimates, which are presented in Figure 5 of their 2011 paper (here) and reproduced in Figure 6 below, show no change in the rate of sea level rise since 1930:
FIGURE 6: CHURCH & WHITE (2011) SEA LEVEL RECONSTRUCTIONS
The second line of evidence comes from “semi-empirical” climate models which “do not explicitly simulate the underlying processes, (but) assume that sea level rise is caused primarily by changes in global ice volume and global ocean heat content in response to changes in global temperature or radiative forcing” (AR5 Section 13.1.5). Translated this means that the models use observational data as well as forcings to simulate the ice melt and ocean heat content contributions to sea level rise, which should make it a lot easier to match observations. But as shown in Figure 7 (a cleaned-up version of AR5 Figure 13.4 e) they still can’t hindcast 20th century sea levels:
FIGURE 7: OBSERVED VS. MODELED MEAN GLOBAL SEA LEVEL
But the fact that the models can’t hindcast 20th century sea levels doesn’t stop the IPCC using them to project 21st century sea levels, and Figure 7 shows the results:
FIGURE 8: SEA LEVEL OBSERVATIONS VS. IPCC PROJECTIONS
With that our review of the IPCC’s basic data ends because there are no more basic data to review. What’s the conclusion? That there is no evidence to support the assumption that man-made greenhouse gas emissions are causing sea levels to rise. Zero, zip, zilch, nada. The science behind human-induced sea level rise isn’t just flawed, it’s nonexistent.
And now it gets worse, because the science used to develop the 20th century sea level rise estimates is flawed too. Details in Part II.
2012-12-29 08:47:29
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