We examine constraints obtained from SNIa surveys on a two parameter model of dark energy in which the equation of state w ( z ) = P ( z ) / \rho ( z ) undergoes a transition over a period significantly shorter than the Hubble time .
We find that a transition between w \sim - 0.2 and w \sim - 1 ( the first value being somewhat arbitrary ) is allowed at redshifts as low as 0.1 , despite the fact that data extend beyond z \sim 1 .
Surveys with the precision anticipated for space experiments should allow only slight improvement on this constraint , as a transition occurring at a redshift as low as \sim 0.17 could still remain undistinguishable from a standard cosmological constant .
The addition of a prior on the matter density \Omega _ { m } = 0.3 only modestly improves the constraints .
Even deep space experiments would still fail to identify a rapid transition at a redshift above 0.5 .
These results illustrate that a Hubble diagram of distant SNIa alone will not reveal the actual nature of dark energy at a redshift above 0.2 and that only the local dynamics of the quintessence field can be infered from a SNIa Hubble diagram .
Combinations , however , seem to be very efficient : we found that the combination of present day CMB data and SNIa already excludes a transition at redshifts below 0.8 .