More than two miles above the Pacific surf, at the summit of the world's largest volcano, the evidence of human influence on global warming is in the air.

For a half century, sensors atop Mauna Loa on the island of Hawaii have captured the world-wide signature of increasing carbon dioxide in the atmosphere, due largely to burning coal, oil and natural gas. The carbon dioxide traps heat. For 50 years, these CO2 readings, known as the Keeling Curve, have been climbing steadily, setting and then breaking a new record every 12 months or so. (See the Keeling Curve)

Global concentrations of CO2 in 2006, not surprisingly then, reached the highest level since the record-keeping began in 1958, the World Meteorological Organization recently announced in its annual greenhouse-gas bulletin. Based on samples from 40 countries, the level of carbon dioxide in the air reached 381.2 parts per million, up fractionally from 2005 -- concentrations not seen in 650,000 years, scientists said.

This week, while diplomats from 180 countries argued over the cost of staving off predicted climate changes, the Mauna Loa readings started to approach even higher levels.

These annual measurements are the world's longest continuous record of CO2 concentrations and, plotted as data points in a rising arc, form one of the most important graphs in science.