Was the twentieth century warming mostly due to human emissions?

There has been no statistically significant warming for at least 15 years. Yet some people, like commentator “Michael the Realist”, who is currently trolling Joanne Nova’s blog, are claiming otherwise. His full claims are as follows

Again look at the following graph.

Now let me explain it to the nth degree.
# The long term trend over the whole period is obviously up.
# The long term trend has pauses and dips due to natural variations but the trend is unchanged.
# The current period is at the top of the trend.
# 2001 to 2010 is the hottest decade on the record despite a preponderance of natural cooling trends. (globally, ocean, land and both hemispheres)
# Hotter than the previous decade of 1991 to 2000 with its preponderance of natural warming events.
# Every decade bar one has been hotter than the previous decade since 1901.

Please explain why the above is true if not AGW with proof.

The claims against the warming standstill I will deal with in a later posting. Here I will look at whether the argument proves, beyond reasonable doubt, that AGW exists and is significant.

There might be a temperature series, but there is no data on greenhouse gases. There is data on the outcome, but there is no presentation of data on the alleged cause. It is like a prosecution conducting a murder trial with a dead body, with the cause of death not established, and no evidence presented linking the accused to the death. I will have to fill this bit in. The alleged cause of most of the twentieth century global warming is human greenhouse gas emissions. The primary greenhouse gas emission is CO2. First I will compare estimated global CO2 emissions with the warming trend. Second, I then show evidence that the twentieth century warming is nothing exceptional.

The relationship of CO2 emissions to average temperature is weak

Some time ago I downloaded estimates of national CO2 emissions data from what is now the CDIAC website, then in filled my own estimates for all major countries where there were data gaps, using the patterns of other countries and my knowledge of economic history. This shows steady growth up to 1945 (with dips in WW1, the Great Depression and at the end of WW2) The post war economic boom, the 1973 oil crisis, the recession of 1980-81 and the credit crunch of 2008 are clearly visible. It therefore seems reasonable and not too dissimilar from the increase in atmospheric CO2 levels.


I have charted the growth in human CO2 emissions against the HADCRUT3 data, putting them on a comparative scale. The 5 year moving average temperature increased by around 0.5oC between 1910 and 1944 and 0.6oC between 1977 and 2004. In the former period, estimated CO2 emissions increased from 0.8 to 1.4 giga tonnes. In the latter period, estimated CO2 emissions increased from 4.9 to 7.4 giga tonnes. The period in between the 5 year moving average temperature decreased very slightly and CO2 emissions increased from 1.4 to 4.9 giga tonnes. 1945 and the late 1998 have two things in common – the start of a stall in average surface temperature increases and an acceleration in the CO2 emission rate of increase. On the face of it, in so far as there is a relationship between CO2 emissions and temperature, it seems to be a pretty weak one.

The longer view

The case for claiming human emissions affect temperature is even weaker if you take a longer perspective. Human CO2 emissions were negligable before the industrial revolution, yet there is plenty of evidence that temperatures have shown larger fluctuations in last couple of millennia. Four example are Law Dome, Esper et al 2012, Gergis et al 2012 and the CO2 Science website.

This Law Dome ice cores are the highest quality ice cores in Antarctica.


There seems to be no warming there at all. With 75% of the global ice packs in Antarctica it is fortunate that there is nothing exceptional about Antarctica warming. But maybe the Arctic is different.

Esper et al 2012, published in Nature, has the following Summer temperature reconstruction for Northern Scandinavia over two millennia.


There is a twentieth century uptick, but only in the context of a long term cooling trend.

Focussing on the last 130 years shows something at odds with the global position.


The highest temperatures were in the 1930s, just like the record temperatures in the USA. The warming trend from the mid-1970s is still far greater than the global averages, but less than the warming trends in the early twentieth century. It corroborates data that shows recent warming trends are higher in the Arctic than the global average, but also shows claims that there is nothing significant in these trends.

I find the most convincing evidence is from the withdrawn Gergis 2012 temperature reconstruction for the combined land and oceanic region of Australasia (0°S-50°S, 110°E-180°E). This is because it set out with the aim of showing the opposite – that the recent warming was much more significant than anything in the last millennium. Despite breaking their own selection rules for proxies, they managed to only demonstrate that the last decade of the last millennium the warmest by the narrowest of margins. See below.


There are many reasons to reject the paper (see here), but one significant point can be illustrated. There were only three reconstructions had any data prior to 1430. There were two tree ring studies from New Zealand, and coral study from Palmyra Atoll. Plotting the decadal averages shows that the erratic Palmyra data suppresses the medieval period and exaggerates the late twentieth century warming. Further, Palmyra Atoll is over 2000 km outside the study area.


Finally, CO2Science.org specialises in accumulating evidence of the impacts of CO2. It also has a database of studies on the medieval warm period. There is a graph that summarizes the quantitative studies


Figure Description: The distribution, in 0.5°C increments, of Level 1 Studies that allow one

to identify the degree by which peak Medieval Warm Period temperatures either exceeded

(positive values, red) or fell short of (negative values, blue) peak Current Warm Period

temperatures.

In conclusion, on the face of it, there is very weak support for human emissions being the cause of most of the warming in the last century by the fact that changes in human emissions do not appear to move in line with changes in temperature. The case is further weakened by evidence that at times in the last 2000 years were warmer than in the current period. It does not discount the possibility that human emissions are responsible for some of the warming. But demonstrating that empirically would mean understanding and accurately measuring the full extent of the natural processes, then demonstrating that these were not operating as strongly as in previous epochs. By definition, the evidence will be more circumstantial than if there was a direct correlation. Furthermore, the larger the actual human impact the more circumstantial will be the evidence.

Tung and Zhou claim of constant decadal anthropogenic warming rates in last 100 years

Bishop Hill reports on

A new paper in PNAS entitled ‘Using data to attribute episodes of warming and cooling in instrumental records’ looks important. Ka-Kit Tung and Jiansong Zhou of the University of Washington report that anthropogenic global warming has been overcooked. A lot.

My comment was:-

My prediction is that this paper will turn out to have exaggerated the anthropogenic influence, rather than have under-estimated it.

The relevant quote:-

The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07–0.08 °C/decade

Greenhouse gas emissions have not been increasing at a steady rate. The most important is CO2. A couple of years ago I tried to estimate from country data (filling in important gaps) how global CO2 emissions had increased. The increases per quarter century were

1900-1925 85%

1925-1950 60%

1950-1975 185%

1975-2000 45%

That meant global CO2 emissions increased more than 12 times (1100%) in 100 years. The conversion rate to retained CO2 seems to be roughly constant – 4Gt of carbon equivalent to increase CO2 levels by 1ppm. Furthermore, the C20th warming was nearly all in two phases. 1910-1945 and 1975-1998. Rather than temperature rise being related to CO2 emissions, it seems out of step. That would imply a combination of two things for the anthropogenic warming rate to be constant at 0.07–0.08 °C/decade. First is that CO2 has massively diminishing returns. Second is that CO2 emissions alone have a much smaller impact on the global average temperature changes (as reported in HADCRUT4), than this paper concludes.

Supplementary Information

This source of the emissions data is

Boden, T.A., G. Marland, and R.J. Andres. 2010. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001_V2010

The CO2 levels are for Mauna Loa back to 1959, and estimated backwards from there to 1780.


The above chart shows by estimated CO2 emissions (expressed in units of 10Gt of carbon equivalents) shown as against the HADCRUT3 data set. This shows a slow rate of increase in CO2 emissions in the first half of the twentieth century, with falls in emissions during the Great Depression (1929-1933) and at the end of the Second World War (1945). From 1950 to 1973 there was a huge upsurge in emissions with the post-war economic boom, then stalls in 1973 (The OPEC oil embargo) and 1980-83 (global recession). After 2000 there was another surge in emissions, mostly due to rapid growth in China.

The temperature increases followed a different pattern. There were two periods of increasing temperatures in the twentieth century – From 1910-1945 and 1975-1998. The decadal changes graph below shows clearly the change in emissions. The temperature changes by decade exaggerate the falls in temperature in the Edwardian decade and the 1940s.


What is clearly illustrated is why I believe the anthropogenic influence on temperature was not similar in every decade from 1910, as Ka-Kit Tung and Jiansong Zhou claim.

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