For someone so chronically late to meetings,I spend a lot of time thinking about time.I started my career in astrophysics,where the quick but finite speed of light mixes time with space.When we look up,we are seeing the ghosts of dead faraway galaxies as they were millions of years ago.There is an asymmetry there that feels somewhat unfair.We can see billions of years in the past,but nothing in the other direction.I wrote my thesis on time and the funny way it has of running in only one direction.Space allows you to reverse yourself,to admit mistakes and retrace your steps.Time is less forgiving.It carries you inexorably into the future without your consent.

There are ten billion trillion other planets,and no one I know lives on any of them.To my knowledge,this is the only good place in the Universe.I am sure there are other civilizations out there,people just like us sending messages out into the void.By the time we receive these communications,all of them will be dead.Time works that way.

A little more than nine billion years after the Universe banged into existence ("little," of course,being hundreds of millions of years longer than anything has ever lived) Hell came into existence.It was hell on Earth before it was Earth,a Hadean eon of boiling rock and an atmosphere choked with carbon dioxide.It was an inhospitable environment for life,so life appeared and thrived.Living things died,and their bodies left imprints in the rock as testaments to their short lives and the deep time in which they lived.Deep time on a planet is impossibly shallow as seen from the Universe.But it passed,pushing forward until just now,past transmuting into present.

I now spend my life studying the only good place.My concept of time is compressed in one direction and extended in the other.The satellite era encompasses roughly my own lifetime;I analyze space-based observations that reveal the climate changes through which I have lived and,in a small way,helped to cause.I use the laws of physics woven together with a patchwork of approximations to glimpse the future,the planet where I will eventually die.

My life,of course,is insignificant in the course of human history,a blip in the existence of the Earth,and meaningless in the span of the Universe My own Deep Time stretches back thirty-odd years,past happy Saturdays searching for fossils on the banks of the Olentangy River with my father.In the ugly park next to the doughnut shop and the Kroger there were hints of a past world,imprints of a time four hundred million years away from now.Rock time is tangible in ways that spacetime is not.When you look at a galaxy,you are seeing a dead thing in its shining prime.When you look at a fossil,you are seeing a dead thing.

I am lucky to be surrounded by colleagues who can listen to the past.Dead things carry information about their dead world,the planet that has been overturned and ploughed over and replaced multiple times by new planets with new life and new climates.They bear the imprints of plate tectonics and shifts in the Earth's orbit,of fluctuations in the chemistry of the atmosphere and collisions with rocks from space.Dead things,bubbles in ice cores,layer of sediment: together,these provide glimpses in what once was.They testify to the power of the atmosphere to change the Earth.

That atmosphere has changed many times over.We are changing it now.Perhaps the last time carbon dioxide concentrations were this high was three million years ago,when sea levels were twenty-five meters higher and the planet two or three degrees warmer.Perhaps we have already blown past that on our way to the Miocene (twenty or so million years ago) and then the Eocene (fifty million years ago),a time of palm trees and crocodiles in the Arctic.

I have found that the scientists who study the Earth's past are often the most alarmed about its future.They,after all,know what the planet is capable of doing.But in the end,there may be no real analogues to where we are going.The past is not the future: history repeats itself,but never exactly.Deep time is a long film whose frames are separated by hundreds of thousands,perhaps millions of years.Much can happen in the time between.We see the past as snapshots of settled equilibria,conditions that over time reconcile themselves with imposed changes.They are not faithful guides for the transient,disturbed state in which we now find ourselves.We may be headed to something far different than anything in the whole history of Earth.We may have the power to stop it.All we know is that this time,it will be different.