They have samples from England and Wales listed in the sample origination table too, the plot thickens lol.
Here is a link to the full publication with sources cited for review:
Reading through it they mention similar trends to America and listed the source they used to compare with. So naturally i read through it and found more information to share:
"A total of 38,681 samples of cannabis preparations were received and analyzed between January 1, 1995 and December 31, 2014
. The data showed that, while the number of marijuana samples seized over the last four years has declined, the number of sinsemilla samples has increased. Overall, the potency of illicit cannabis plant material has consistently risen over time since 1995 from approximately 4% in 1995 to approximately 12% in 2014
. On the other hand, the CBD content has fallen on average from approximately 0.28% in 2001 to <0.15% in 2014, resulting in a change in the ratio of THC to CBD from 14 times in 1995 to approximately 80 times in 2014.
It is concluded that there is a shift in the production of illicit cannabis plant material from regular marijuana to sinsemilla.
Link to Changes in Cannabis Potency over the Last Two Decades (1995-2014) - Analysis of Current Data in the United States
It would seem the main attribute to potency increase is the increase in seedless marijuana production.
I cant speak from experience on the 60s 70s 80s 90s, seedless medical marijuana availability has always been high for me. Ive only been smoking since 2011 and growing since the end of 2012. Lived in medical marijuana tolerant areas since starting like here in Tacoma, WA and San Diego, CA as well. The reason these types of reports always seem strange to me is because there are 20 year old clones that hit these thc % numbers and higher like Mr.brunch mentioned. Many people Ive met/know that have been smoking most of their life speak highly of strains from the 70s - 90s that they search for and enjoy still today.
Arent there landraces that test higher than 10%? Is it more to do with poor growing/storage and transportation methods?