Temperature trends at the Mauna Loa observatory, Hawaii

Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthropogenic CO<sub>2</sub> in the atmosphere. For the same reasons that this site provides excellent globally averaged CO<sub>2</sub> data, it may provide temperature data with global significance. Here, we examine hourly temperature records, averaged annually for 1977–2006, to determine linear trends as a function of time of day. For night-time data (22:00 to 06:00 LST (local standard time)) there is a near-uniform warming of 0.040 °C yr<sup>−1</sup>. During the day, the linear trend shows a slight cooling of −0.014 °C yr<sup>−1</sup> at 12:00 LST (noon). Overall, at Mauna Loa Observatory, there is a mean warming trend of 0.021 °C yr<sup>−1</sup>. The dominance of night-time warming results in a relatively large annual decrease in the diurnal temperature range (DTR) of −0.050 °C yr<sup>−1</sup> over the period 1977–2006. These trends are consistent with the observed increases in the concentrations of CO<sub>2</sub> and its role as a greenhouse gas (demonstrated here by first-order radiative forcing calculations), and indicate the possible relevance of the Mauna Loa temperature measurements to global warming