The Common Agricultural Policy is a system of state grants to farmers in the EU.
Why do farms warrant public funding in this way? Isn’t this, in effect a form of protectionism?
Farmers receive subsidies in many many countries around the world, including EU countries prior to the CAP, the US, or Switzerland. One justification that's commonly offered is that self-sufficiency is a strategic goal that requires state support. The original policy was also devised when Europe was just coming out of food rationing that lasted for a decade after WWII. Nowadays, this productivity objective has partly been replaced with policies designed to safeguard the landscape and ecosystems through specific agricultural practices. And of course other sectors of the economy also receive subsidies and support from states in various ways.
So what's specific to the CAP is not that farmers receive subsidies, it's that they may not receive subsidies from individual states, instead getting them solely through EU programmes. That's why the CAP was such a large part of the EU budget for many decades (less so now). EU federalists hoped that other sectors would follow but that never happened. In other domains (industry, defense, research, education, healthcare, etc.) individual EU member states fund specific policies or directly subsidizes businesses within the bounds set by EU rules (in particular the rules on “state aid”).
Not all of the so-called "CAP" has its primary goal of agricultural output. For example, in the UK there are large areas of land where the weather and soil conditions are only suitable for low-intensity sheep farming. However grazing sheep have been a part of the stable ecosystem of these areas for centuries, and removing them because they are "uneconomic" would cause the entire ecosystem to change dramatically, by destroying important habitats for wildlife, increasing soil erosion and hence changing the ecosystem of downstream river systems, increasing the risk of long-burning peat wildfires, etc.
In effect the farmers in these areas are being subsidized to maintain the environment, not to produce meat and wool.
In fact the policy of subsidies to these areas was changed to focus on the environmental protection issues, since the original payment rules led to environmental damage through attempts at unsustainable over-production to maximize the subsidy payments.
The same also applies in the high-intensity sector of the UK, where subsidies have been awarded to improve the environment in opposition to maximizing output - for example by extending the uncultivated borders of fields to provide wildlife habitats and corridors, maintaining hedgerows rather than replacing them with fences (or removing them completely in arable farming areas) etc.
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