Recently on an email list, Hiazar Hiazarus stated this: “It was hotter in the 30s and much hotter during the medieval warming period. Long term it’s been cooling for the last 13 thousand years after a sharp warming that ended the last ice age. We are in an interglacial period that probably will end on the next thousand years. If carbon goes below 200 parts/million then agriculture ends because plants cant grow fast enough. There’s been a warming since the 1750s when the little ice age ended.”
This is a number of separate claims. Let’s examine each.
This graph of global temperature records since 1850 is from Phil Jones of the Climatic Research Unit in 2013, who got the data from Morice et al., (2012). He is one of the authors of this paper.
It shows that no, the 1930s were NOT hotter than now. The graph sets 1940 as 0. Compared to that, global temps are as much higher as that period was compared to the lowest during the time period covered: 1910. That is, the rise between 1910 and 2012 doubled compared to the rise between 1910 and 1940.
There was indeed a slight cooling between 1940 and 1985, but it can be seen that it wasn’t a big deal. The reason is explained in an essay by Anne-Marie Blackburn in Skeptical Science. In summary, this was due to sulphate aeorosols in the atmosphere. The source was in part a number of large volcanic eruptions, in part industrial pollution. That is, it’s like walking under the shade of a tree on a hot day. While you are shielded from sunshine, you feel a little cooler, but the day hasn’t cooled.
Medieval warm period
This is a “brief climatic interval that is hypothesized to have occurred from approximately 900 CE to 1300.” Experts don’t even agree that there was global warming during this time. Things certainly were warmer in Europe, Asia and Greenland, but there is argument regarding whether that applied elsewhere on Earth. [All my quotes here are from the Encyclopaedia Britannica.]
Naturally, the technology of temperature measurement was not yet available then, even for those regions. “The climatic conditions of older periods, however, have to be ‘reconstructed’ with the help of historical documents (which contain figures on food production, the length of growing seasons, and the duration of ice on bodies of water) and other indirect measures of climate (including ‘climate proxies’ such as tree rings, ice cores, and sediment cores)… There is little evidence that such conditions prevailed on a global scale, however. Indeed, some climate proxies point to several periods of extended drought during the MWP in some locations, such as the Sierra Nevada mountains of North America, parts of Australia, and the Asian steppe, whereas other areas, such as northern China, experienced a mix of heavy rainfall and drought.”
Even assuming that the effect was global, this graph from Morberg et al. (2005) shows that at its highest, it was about that toward the end of the 20th Century. Since then, every year but one has exceeded global temps of that century’s highest, 1998.
Also, the Medieval Warm Period was due to dissimilar mechanisms from the present one. That was a period with very low volcanic activity, while, as I’ve stated, recently we’ve had a lot of sulphates in the stratosphere, some from volcanoes. Industrial air pollution has a complex effect. Some stuff, like tiny black carbon particles, substantially add to warming. However, overall, the haze over cities and industrial regions actually causes a cooling. If we could have air as clear as it was in medieval times, our temps would be MUCH higher.
And we are currently in a period when sun activity should lead to global cooling instead of the frightening rate of warming.
Temperature pattern over last 13,000 years
I could find no reference specifically discussing 13,000 years before the present, but denialist Lord Monckton [PDF document] claims that the past 10,000 years were warmer than the present. As usual for this man, he is cherrypicking.
The claim is based on Easterbrook’s work on Greenland ice cores, critically examined here. These ice cores looked at temperature indicators up to 1855. Um… current human-driven global warming was just starting to take off then. Also, one location can only be a component of the total examination of a global pattern.
Easterbrook is quoted as claiming, “There has been no significant warming from 1987 to the present, so the top of the core is representative of the present day climate in Greenland.” I don’t know what planet he is on, but all the records I’ve looked at show that global temps have been climbing for the sky since.
We are in an interglacial period
True. So what?
If there were no human interference in climate, that would end in about 1000 years. It is ending now, with extreme rapidity, by going into a full greenhouse world, like the one that ended the Permian period 250 million years ago.
If goes below 200 parts/million…
It is now officially above 400 ppm the year around. I don’t think this is a worry.
There has been a warming since the 1750s
This is back to the Medieval Warm Period. It ended. That means, that areas where it was observed got cooler again. Then, things swung up once more.
Have another look at the Morberg graph above. It shows the pattern.
I truly wish I could reverse my conclusion that humans are cooking this planet, but the evidence supports my view, not Hiazar’s.
Moberg, A., Sonechkin, D. M., Holmgren, K., Datsenko, N. M. & Wibjörn, K. (2005). Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature, 433 (7026), pp 613-617.
Morice, C. P., Kennedy, J. J., Rayner, N. A. & Jones, P. D., (2012). Quantifying uncertainties in global and regional temperature change using an ensemble of observational estimates: the HadCRUT4 dataset. Journal of Geophysical Research, 117, D08101, doi:10.1029/2011JD017187