Posted by Co2sceptic on Mar 8th 2013
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This is a paper that was bound to cause lurid headlines along the lines that the Earth is warming faster than at anytime during the past 11,000 years; that the temperature rise and rate of rise of the past 100 years is like nothing we have seen since the last Ice Age, and that global temperatures have been rising abnormally since the start of the industrial revolution. The obvious conclusion drawn is that it is all down to mankind’s emission of greenhouse gasses. The paper in question is in the journal Science and is produced by a team of researchers from Oregon State University and the department of Earth and Planetary Sciences at Harvard University. It describes a reconstruction of regional and global temperature for the past 11,300 years – the most recent climatic epoch termed the Holocene.

Such reconstructions are important because they allow today’s global temperature to be placed into a historical context, possibly enabling some conclusions to be reached about distinguishing anthropogenic influence on climate from natural variability.

The researchers use 71 globally distributed temperature records – obtained from marine and terrestrial proxies like corals or the shells of certain marine organisms – with time resolutions ranging from 20 to 500 years meaning that what they obtain is a broad overview of the temperature variations of the Holocene. Their median temporal resolution is 120 years.

And that is the key point to bear in mind. This is a broad brush look at the past 11,000 years with very poor sensitivity to temperature events that take place on the scale of centuries – like what has happened recently. Hence it is not possible to look at the past 150 years – about which we have high temporal resolution data – and compare it to this Holocene temperature reconstruction.

I also disagree, for the same reasons, with the authors’ statement made early in the paper that proxy-based temperature reconstructions of the past 1500 years suggest that the warming of the past few decades is unusual relative to pre-anthropogenic variations. There are many studies that suggest that the Medieval Warm period of about 1000 years ago was comparable to today’s temperatures. Yet it is not well represented in this rough reconstruction.

Their conclusions: After the end of the last glaciation starting around 11,300 BP (Before Present) temperatures rose by about 0.6 deg C until they reached a plateau for 4,000 years between 9,500 BP and 5500 BC. Then a long-term decline started resulting in about 0.7 deg C of cooling to about 200 years BP. (above)

One interesting thing about the data is that most of this cooling comes from northern hemisphere extra-tropical sites. The researchers say that high-latitude cooling and low latitude warming is the pattern expected by changing solar insolation due to the Earth’s decreasing orbital obliquity since 9,000 years BP.

Half Empty Or Half Full

The authors conclude that the decade 2000 – 2009 has not yet exceeded the warmest temperature of the Holocene. In fact 2000 – 2009 was warmer than 72% of the Holocene and that given the rate of warming seen throughout the 20th century, if it continued, temperatures would not exceed the maximum seen in the Holocene until 2100.

Of course, another way to put this is that current temperatures are colder than 28% of the Holocene. According to this research the temperatures seen in the 20th century were about average for the Holocene.

In the paper, and in the many press reports of it, it said that the climate has been warming since the industrial revolution kicked into gear, and that the two are related via greenhouse gas emissions. But prior to 1950 anthropogenic greenhouse gas emissions were to weak to have an a significant effect on the atmosphere. It is implausible to suggest that mankind’s greenhouse gas emissions in the 19th century were changing the climate causing global temperature rise.

To my mind it should have said climate has been warming fairly steadily since the end of the Little Ice Age in the 17th century, which was an event with no anthropogenic connection. There is also the question of from which end to look at the data. Many reports emphasise that current temperatures are greater than 90% of the Holocene. They could have said that current temperatures were cooler than 10% of the Holocene. (The more reasonable figure of 28% I use comes from deeper in the research paper and was reached after its discussion of possible errors.)

At least this paper casts doubt on the statements often made that it is currently warmer than it has been for thousands of years. It’s not, and it won’t be unless the global temperature rises at the same rate as it did between 1950 – 2009 for another 90 years. Oh for a crystal ball.