The eastern and central U.S. has experienced a general cooling trend in the past century under intensifying global warming. Focusing on the summer daily maximum surface air temperature in the second half of the 20th century, we analyzed observed surface air temperature and found that the cooling consists of two different regions in separate periods, corresponding to the two leading modes of temperature variation. The cooling in the north-central U.S. occurred in the last quarter of the century during a strong global warming epoch, while the cooling in the south-eastern U.S. appeared in the late 1950s through early 1970s when the earth experienced slight cooling. To separate local (land use) from remote (SST) forcing mechanisms responsible for these cooling trends, we also carried out 50-year GCM simulations where either land use characteristics or SST was kept constant. The half century simulations indicate that the U.S. cooling trends are positively (negatively) correlated with north (equatorial) Pacific SST, suggesting that the concurrent occurrences of the positive phase of the Pacific Decadal Oscillation (PDO) with more frequent El Nino events during the last quarter of the 20th century may have contributed to the cooling. The gradual evolution of agricultural practice in increasing cropping intensity has also partly contributed to the cooling. Another explanation for the north-central U.S. cooling is the increased cold air advection associated with the net ridging in the western U.S. that is itself partly supported by increased warming in the western U.S. during the peak warming period.