Grim Milestone

Carbon dioxide is a naturally occurring compound in the earth's troposphere. The quantity present has fluctuated over long stretches of what is called geologic time, but more recently has experienced significant changes over human timescales.

The concentration in the atmosphere changes by several parts per million in a predictable way each year, as seasonal changes in the photosynthesis experienced in mid-latitudes of the northern and southern hemispheres store and release carbon in the short term. Read more about the Keeling curve on my climate page.

On May 9 of this year -- our wedding anniversary -- the gas reached the historic milestone of 400 parts per million (0.04 percent). The word "historic" is important, in the sense that this is a level not reached in human history, though it was exceeded in pre-history. It will probably dip below 400 in coming months, but all indications is that it will continue to ratchet much higher until priorities change.

Visit the 400 page at 350.org for more information about the implications of this milestone, and follow Climate Justice to learn about the generational and geographic inequity of the problems run-away carbon loading is causing.

Colorado River Delta -- National Geographic

Climate change is not only cause -- nor perhaps in some cases even the main cause -- of the increasingly common failure of some rivers to meet the sea. But as National Geographic explains in its dry rivers photo essay, flood and drought contribute to the increasingly common and increasingly serious problem. Where rising seas meet retreating fresh water, the intrusion of salt into both surface water and ground water is an additional concern.