The financial strains created by crises in Japan and Europe highlight a growing problem: The rich world is getting close to the point where it won't be able to bear the costs of another disaster.

Japan and Europe face very different crises—one brought on by nature, the other man-made. But from a financial perspective, they're strikingly similar. In both cases, the mounting costs of mitigating disaster are stretching governments' already overburdened finances. In Japan, the...