From 2002 to 2014, plants appear to have gone into overdrive, starting to pull more CO out of the air than they have done before. The result was that the rate at which CO accumulates in the atmosphere did not increase during this time period, although previously, it had grown considerably in concert with growing greenhouse gas emissions.
A 1993 review of scientific greenhouse studies found that a doubling of  concentration would stimulate the growth of 156 different plant species by an average of 37%. Response varied significantly by species, with some showing much greater gains and a few showing a loss. For example, a 1979 greenhouse study found that with doubled  concentration the dry weight of 40-day-old cotton plants doubled, but the dry weight of 30-day-old maize plants increased by only 20%.
In addition to greenhouse studies, field and satellite measurements attempt to understand the effect of increased  in more natural environments. In free-air carbon dioxide enrichment (FACE) experiments plants are grown in field plots and the  concentration of the surrounding air is artificially elevated. These experiments generally use lower  levels than the greenhouse studies.  They show lower gains in growth than greenhouse studies, with the gains depending heavily on the species under study. A 2005 review of 12 experiments at 475–600 ppm showed an average gain of 17% in crop yield, with legumes typically showing a greater response than other species and C4 plants generally showing less. The review also stated that the experiments have their own limitations. The studied  levels were lower, and most of the experiments were carried out in temperate regions. Satellite measurements found increasing leaf area index for 25% to 50% of Earth's vegetated area over the past 35 years (i.e., a greening of the planet), providing evidence for a positive CO fertilization effect.
Depending on environment, there are differential responses to elevated atmospheric CO between major functional types of plant, such as C3 plant| and C4 carbon fixation| plants, or more or less woody species; which has the potential among other things to alter competition between these groups. Increased CO can also lead to increased Carbon : Nitrogen ratios in the leaves of plants or in other aspects of leaf chemistry, possibly changing herbivore nutrition. Studies show that doubled concentrations of CO will show an increase in photosynthesis in C3 plants but not in C4 plants. However, it is also shown that  plants are able to persist in drought better than the  plants.