This volume contains the proceedings of the Third Conference on the Climatic Impact Assessment Program (CIAP), held at the DOT Transportation Systems Center from February 26 to March 1, 1972. It includes 45 invited papers, 20 unscheduled presentations, and edited question-and-answer sessions following some of the papers. The conference was essentially a progress report on CIAP. Therefore, some of the papers contain new data not yet published elsewhere, and others describe experimental equipment. Reports on work relevant to CIAP, though not sponsored by it, are also included. The subjects covered include aircraft-engine emissions, the nature of the 'undisturbed' stratosphere of 1974, the nature of the 'perturbed' stratosphere of 1990-2025, the possible resulting tropospheric perturbations, and the biological and economic effects of such perturbations.