The premise is inaccurate. While Southern Europe's Mediterranean climate (mild winters, hot dry summers) is a tremendous asset, it is not formally designated as its "most valuable resource."
Instead, the region's core assets are widely recognized as:
Climate enables many of these (agriculture, tourism, quality of life), but it's the cultural and economic utilization of that climate that creates value. Water scarcity and desertification (worsening with climate change) also make the climate a growing liability.