What can we learn about climate from a new study on climate reconstructions?

[Andre]

A new study is published, "A surrogate ensemble study of climate reconstruction methods: Stochasticity and robustness."

From the abstract:

Reconstruction of the Earth's surface temperature from proxy data is an important task because of the need to compare recent changes with past variability. However, the statistical properties and robustness of climate reconstruction methods are not well known, which has led to a heated discussion about the quality of published reconstructions...

(...)
We find that all reconstruction methods contain a large element of stochasticity and it is not possible to compare the methods and draw conclusions from a single or a few realizations. This means that very different results can be obtained using the same reconstruction method on different surrogate fields. This might explain some of the recently published divergent results.

We also find that the amplitude of the low-frequency variability in general is underestimated. All methods systematically give large biases and underestimate both trends and the amplitude of the low-frequency variability. The underestimation is typically 20–50 %. The shape of the low-frequency variability, however, is in general well reconstructed...

http://web.dmi.dk/solar-terrestrial/staff/boc/reconstr.pdf of the full article

 

[Andre]

The first author elaborates about it http://www.dmi.dk/dmi/klimaets_hockeystav_er_braekket . Unfortunately it's in Danish but the google translater produces this of the first two paragraphs:

Climate hockey stick is broken

It has been shown in many contexts and has been the icon of where things have gone wrong with the climate since the pre-industrial times. This is known as the Mann curve or 'hockey stick' curve that shows the development of the Northern Hemisphere surface temperature over the last 600 years. A new Danish study breaking foundation of the curve.

"Hockey stick curve does not," says klimaforsker (climate researcher) Bo Christiansen from Denmark's Climate Center and add. "That does not mean that we cancel the anthropogenic greenhouse effect, but the foundation has become more nuanced."

 

[Xnn]

Too bad that it is so difficult to draw much of any conclusion from all of this.

Should be kept in mind that CO2 levels varied during this time with a corresponding influence on climate.

From 1520's to 1620, CO2 levels actually fell.
Then levels gradually rose until around 1740, after which they started climbing faster. Levels have continued to rise up to the present except during 1935-1945 period.


http://cdiac.ornl.gov/trends/co2/lawdome.html

and

http://cdiac.ornl.gov/ftp/trends/co2/lawdome.smoothed.yr75