What was done
The authors describe and discuss what was revealed at a "Tropospheric Ozone Changes Workshop" held in Boulder, Colorado (USA) on 14-16 October 2009, where they say that "long-term ozone records from regionally representative surface and mountain sites, ozonesondes, and aircraft were reviewed by region."

What was learned
Logan et al. report that "in the Southern Hemisphere, surface measurements from South Africa and Tasmania and sonde data from New Zealand show a significant increase over the past 25 years." North of the equator, on the other hand, the story is somewhat different. In western Europe, for example, they write that "several time series of ~15-40 years ... show a rise in ozone into the middle to late 1990s and a leveling off, or in some cases declines, in the 2000s, in general agreement with precursor emission changes." Similarly, they state that "surface measurements within North America show a pattern of mostly unchanged or declining ozone over the past two decades that [also] seems broadly consistent with decreases in precursor emissions," while noting that "the Japanese sonde record suggests rising ozone into the 1980s and small changes thereafter."

What it means
The spatial and temporal distributions of these observations would seem to suggest that: whereas increasing industrialization originally tends to increase the emissions of precursor substances that lead to the creation of greater tropospheric ozone pollution, subsequent technological advances tend to ameliorate that phenomenon, as they appear to gradually lead to (1) a leveling off of the magnitude of precursor emissions and (2) an ultimately decreasing trend in tropospheric ozone pollution. And in light of these observations it can be appreciated that when atmospheric ozone and CO2 concentrations both rise together, the plant-growth-enhancing effect of atmospheric CO2 enrichment is significantly muted by the plant-growth-retarding effect of contemporaneous increases in ozone pollution, but that as the troposphere's ozone concentration gradually levels off and declines -- as it appears to be doing with the development of new and better anti-pollution technology in the planet's more economically advanced countries -- we should begin to see more-rapid-than-usual increases in earth's vegetative productivity, which should promote an acceleration of the greening of the earth phenomenon.