"demonstrably necessary" is a correct and usable phrase in written English.
You can use it to describe a situation when something is necessary beyond any doubt. For example, "This measure is demonstrably necessary to ensure the safety of the public."
So: preëmption, yes — when demonstrably necessary, and as a last resort.
Instead of playing sleight of hand with history, while deferring for a year the comprehensive spending review which is now demonstrably necessary, Mr Brown should come clean.
In such cases, exceptions to these policies regarding publication, classification, and access by foreign students and scholars may be made, but only in those very rare instances where the area of work is crucially important to MIT's educational mission and the exception is demonstrably necessary for the national good.
Two genes are demonstrably necessary for GAF and further characterization of their proteins is important for developing natural products as control measure against grassy weeds.
"At each stage, such activities must be demonstrably lawful, necessary and proportionate.
It is also demonstrably unnecessary.
It makes sense when a government demonstrably wants to respect rights but lacks the necessary resources or technical know-how.
America thinks in terms of shaping an international environment so that if force becomes necessary it can be justified because diplomacy has been demonstrably exhausted and Iran, by stubbornly refusing to alter its nuclear program, will appear to have essentially brought war on itself.
But such squabbles wouldn't be necessary if we weren't using an authentication method that is demonstrably broken.
Individuals who come from an educational environment such as that found in certain rural or inner-city environments that has demonstrably and directly inhibited the individual from obtaining the knowledge, skills, and abilities necessary to develop and participate in a research career.
Both the traditional and molecular activity assessments indicated that free ammonia had a limited inhibitory effect on Nitrobacter spp. In fact, the pH changes necessary to induce high free ammonia concentrations (>10 mg-N/L) had a demonstrably more important inhibiting effect on nitrite oxidation than free ammonia.
