Isoprene is a simple organic molecule ( C₅H₈ , or, more accurately CH₂=C(CH₃)−CH=CH₂ ) which is emitted into the atmosphere by (some) plants - notably oaks, poplars, eucalyptus and pines. The first report of isoprene emissions from plants was published in 1957.

It's now known to be the second most prominent hydrocarbon in the atmosphere (after methane) - with global emissions estimated at 600 million metric tons per year.

As well as being a potential 'Greenhouse Gas' itself * isoprene levels have been found to affect the ozone and particulate content of the atmosphere.

The complex feedback mechanisms for isoprene production from plants are poorly understood. For example, the levels of isoprene tend to drop as CO₂ levels rise - but the reasons are not clear.

In addition, the biological reasons for the evolution of isoprene emissions are unknown. As are the reasons why some plants emit the compound and other don't.

It's also unclear how climate change will affect future levels of isoprene (and vice versa)

[…] estimates of how isoprene emission will change in the future are not approaching consensus. Even the direction of change, whether increasing or decreasing with climate change, is under debate.

Source : Plant, Cell & Environment, Volume 37, Issue 8 p. 1727-1740

The lack of knowledge about the bi-directional feedback effects of isoprene in the atmosphere make the construction of accurate computer models of climate change more problematic.

Incorporating a mechanistic treatment of isoprene emissions within land-surface schemes has recently become a focus for the modelling community, the aim being to quantify the potential magnitude of associated climate feedbacks. However, these efforts are hampered by major uncertainties about why plants emit isoprene and the relative importance of different environmental controls on isoprene emission.

Source : Atmospheric Environment, Volume 43, Issue 39, December 2009, Pages 6121-6135

Further reading Isoprene research – 60 years later, the biology is still enigmatic Plant, Cell & Environment, Volume 40, Issue 9 p. 1671-1678

* Editor's Note : There seems to be limited research into the effects of isoprene itself as a simple 'Greenhouse Gas'. References needed.