International law is notoriously weak on enforcement. It is often said that international law cannot be enforced since there is no international police force to enforce decisions of international tribunals  and to punish states for the transgression of rules of international law.  In international law, the makers of the law, that is states, are those that have to enforce the law.  States are usually reluctant to pursue enforcement action against other states unless the transgression of international rules affects their vital interests.  Furthermore, the purpose of international law is the preservation of peace. If states started to enforce international law to the point of executing acts of aggression against other states, international law would start to look incoherent.

International law is an incentive-based instrument. States often follow international law because doing otherwise would affect their reputation. States also follow international law because they know that their failure to follow the rules will be an invitation for other states to demonstrate non-compliance, something that is not in their interests.  States tend to demonstrate high vigilance for the monitoring and enforcement of international treaties on the proliferation of weapons of mass destruction but rarely do they demonstrate the same vigilance for the enforcement of environmental treaties.  In a world in which regional conflicts, terrorism, and various diseases are still prevalent the protection of environment does not always get the priority it deserves.

Having said that, states are increasingly undertaking obligations that could make a difference in the protection of environment.  Many international treaties contain provisions that provide that states have to notify and consult with other states for activities that they undertake in their territory that are likely to have adverse consequences for other states.  States are to execute Environmental Impact Assessments and Strategic Impact Assessments for plans and projects likely to have adverse effects on the environment of other states.

Certain international treaties provide a mixture of sticks and carrots to ensure state compliance.  In the ozone regime, for instance, a multilateral fund is established  that provides assistance to developing countries to comply with the regime.  However, if the enticement provided by the fund does not suffice states are entitled to impose trade sanctions on states that have not complied.  A number of other instruments are increasingly adopting compliance procedures the purpose of which is to identify the causes of noncompliance and respond accordingly either with financial assistance or with sanctions.