Sitting in my synagogue on Yom Kippur, it struck me that this is something I have done annually for almost sixty years and in the same house of worship for nearly thirty. This holiday, its annual predecessor Rosh Hashanah, and all holidays, both religious and secular, comprise a predictable cycle of my life. Similarly, my work life at the university where I have been teaching for thirty-five years has a repeated pattern of first day of classes, exams, papers, and final grades, broken only by the occasional sabbatical. Then there are the shorter cycles of the work week and weekend and of the rising from bed, having breakfast, going to work, etc., and finally going back to sleep.

But one cannot avoid a key message of Yom Kippur: we are born, we live, and we too soon die. The arrow of our lives has one direction and only one end. What matters is what we do before we get there.

The same dichotomy, between the arrows and cycles of time, is fundamental to much religious thought. As the late Stephen Jay Gould pointed out in his book Time’s Arrow, Time’s Cycle (whose title inspired that of this essay), the Biblical telling of the history of the world and of the Jewish people is a directional narrative. But we can also find in Ecclesiastes:

All the rivers

run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again.

As a geologist, I see this as a recognition of the water cycle.

Science is replete with cycles and arrows; of repeated phenomena and one-time events. Your body has a finite lifetime, but throughout its existence there are the repeated cycles of breathing; of blood flow; of cell creation and destruction. The Earth spins on its axis and moves around the Sun, producing day and night and the seasons. But one day the Sun will expand to encompass the orbit of the Earth, ending its history. And, of course, the universe as a whole began in an infinitesimal unit of time, expanded to its current day, and in some unimaginably distant time will die. But while it exists, our Sun will orbit the center of the galaxy every 225 million or so years.

As a geologist and paleontologist, my world view is full of beginnings and endings. We have a good idea of when the Earth was born; a less certain one of when life came into existence. The history of the Earth is full of directional changes in its land surface, climate, chemistry, and the life that inhabits it. At the same time, it is full of repetitions, even at very long time scales. The water cycle moves water from the oceans, to the atmosphere, to the land surface, and back again. The carbon cycle exchanges carbon between the animate and inanimate parts of the Earth system and within the biosphere, atmosphere, and oceans. Continents crash together and eventually move apart, only to collide again.

These cycles and arrows, even those that govern our own life, have always been things that we react to, rather than control. But this has changed. For two million years, the Earth has alternated between ice ages and intervening warm periods, primarily driven by cyclic changes in the planet’s orbit. We should be at the beginning of a long slow slide into another ice age. The actions of humans, however, have begun change this cycle into an arrow. We are putting more carbon dioxide into the atmosphere than has been seen for millions of years, inevitably warming the atmosphere. And even in the highly unlikely event we stopped the addition today, the impact of the added carbon dioxide will last for hundreds, perhaps thousands, of years. The alterations in the Earth’s atmosphere, the extinction of species, the modifications of the land surface, the acidification of the oceans, are all changes in directions whose endpoints we cannot see and whose impacts we cannot accurately predict.

During Yom Kippur we examine our past year and resolve to do better in the next. We also face our own mortality. Perhaps we need a Yom Kippur for humanity, to see what we have done and to face the serious consequences of our actions, including the end of society as we know it. Perhaps then we can then, acting as one, resolve to do better and protect our future.

My thanks to Rabbi Max Weiss, Oak Park Temple, for his discussion of arrows and cycles in Judaism. Oak Park Temple was a Scientists in Synagogues congregation in the 2016 cohort.

Glacier image by Daniel Foster.