Paleoclimatic reconstruction using noble gas concentrations in the Carrizo aquifer of southwest Texas and water ages determined through simulation of groundwater age reveals abrupt late Holocene temperature increases previously unidentified through 14C dating. Of particular interest is a temperature increase of up to 3.4¿C in the first half of the last millennium following a cold period between ~3.7 and 0.9 Kyrs BP. Wet, cool periods in the region are associated with El-Nino dominated conditions, while warm, arid events are linked to multi-decade La-Nina dominant events. The data shows a slow decrease in temperature between ~1,200 and 200 Kyrs BP, a decrease that accelerated in the late Pleistocene and early Holocene. This decrease was followed by warming in the last millennium, that seems to be continuing today.