Increase in temperature extremes is one of the main expected impacts of climate change, as well as one of
the first signs of its occurrence. Nevertheless, results emerging from General Circulation Models, while
sufficient for large scales, are not enough for forecasting local trends and, hence, the IPCC has called
for local studies based on on-site data. Indeed, it is expected that climate extremes will be detected much
earlier than changes in climate averages. Heat waves are among the most important and least studied
climate extremes, however its occurrence has been only barely studied and even its very definition
remains controversial. This paper discusses the observed changes in temperature trends and heat waves
in Northwestern Mexico, one of the most vulnerable regions of the country. The climate records in two
locations of the region are analyzed, including one of the cities with extreme climate in Mexico,
Mexicali City in the state of Baja California and the Yaqui River basin at Sonora State using three different
methodologies. Results showed clear trends on temperature increase and occurrence of heat waves in
both of the study zones using the three methodologies proposed. As result, some policy making suggestion
are included in order to increase the adaptability of the studied regions to climate change, particularly
related with heat wave occurrence.