Significant Figures by Peter Gleick

[An early version of this essay was originally published on my Forbes blog in 2012. It has been edited and updated.]

Geologic time scales are long – far too long for the human mind to easily comprehend. Over millions, and tens of millions, and hundreds of millions of years, the Earth has changed from something unrecognizable to the planet we see on maps, plastic globes, and photos from space. The Atlantic Ocean didn’t exist eons ago and it will literally disappear in the future as the continental plates continue to shift inch by inch. A visitor from outer space millions of years ago would have looked down upon land masses and land forms unrecognizable today. As John McPhee notes in his book, Assembling California, “For an extremely large percentage of the history of the world, there was no California.” Or North America, China, Australia, Hawai’i, Mt. Everest, Grand Canyon, or any of the other landforms and natural symbols we think of as immutable.

Humans cannot relate to these changes. Our perception of time is short — measured in days, months, years, or decades, not millennia or eons. And our perception of the world around us is similarly driven by events with human time scales. Again, John McPhee:

The two time scales – the one human and emotional, the other geologic – are so disparate. But a sense of geologic time is the most important thing to get across to the non-geologist: the slow rate of geologic processes – centimeters per year—with huge effects if continued for enough years. A million years is a small number on the geologic time scale, while human experience is truly fleeting – all human experience, from its beginning, not just one lifetime. Only occasionally do the two time scales coincide. When they do, the effects can be as lasting as they are pronounced.

Nowhere is this collision of time scales more pronounced than in the current climate change debate. There are a variety of reasons why a few people still find the reality of human-caused climate change to be inconceivable. Leaving aside those who are unfamiliar with or ignorant of the science, those who simply shill for the fossil-fuel industry, and those who for political reasons must toe an ideological line that contradicts scientific conclusions, there remain some whose world view prevents them from accepting that humans can influence something so vast and global as the climate. Coupled with the fact that the Earth’s climate fluctuates naturally, this group has never been able to accept the reality of human-caused climate change. For regular readers of the blogs of climate contrarians (or their comments on this and other essays on climate change), this sentiment will be familiar. Here are a few (of the more polite) examples from comments I’ve received:

I don’t deny that the climate changes, it’s been changing since there has been an atmosphere to change. And it’s common knowledge that the earth goes through cycles of climate, what is not known is the exact causes of these changes or cycles.

Observed climate changes since 1850 are linked to cyclical, predictable, naturally occurring events in Earth’s solar system with little or no help from us.

Global Warming, Global Cooling and Global Climate Change have been happening for millions of years – long before any possible human influence – Climate Change is natural and nothing new.

This is a manifestation of the collision that McPhee describes, the conflict of human and geologic time scales.

Climate does change naturally, for reasons well understood by scientists. But it does so over thousands or tens of thousands of years – time scales so slow as to be imperceptible to humans. The causes of these natural climate changes are the cumulative result of tiny but cosmic changes, including incremental shifts in the orbit of our planet around our star, the tiny but real wobble of the Earth’s axis, and variations in the output of energy from the Sun. These natural factors — the Milankovitch cycles — lead to changes in the Earth’s climate. They cause the ice ages, and they cause the warm interglacial periods. But they happen slowly – in geologic time unseen, unperceived, and unfelt by humans. The peak of the last ice age was 20,000 years ago, long before any recognizable form of human civilization existed. The next ice age isn’t expected to start for thousands of years and may not peak for tens of thousands of years, and who knows what kind of civilization will exist then.

Graphical representation of Milankovitch cycles. From

Graphical representation of Milankovitch cycles. From

Human-caused climate changes are different. As the planet’s population has grown past 7 billion people, and as we have learned how to mobilize and burn vast quantities of carbon-based fossil fuels (ironically, created over geologic time scales) to satisfy our short-term energy demands, humans are now powerful enough to overwhelm slow geological cycles. We are, for the first time in the 4+ billion year history of the Earth capable of altering the largest geophysical system on the planet – the climate – and we are doing it on a human time scale of years and decades, with consequences we are only just beginning to comprehend.

Atmospheric CO2 over the past 800,000 years with the dramatic increase in the past century (shown on right edge). Data from Vostok ice cores. CO2 concentrations today are at 407 ppm and climbing rapidly.

Ironically, our effect on the climate, while fast in geological terms, is still slow enough for policy makers, climate contrarians and skeptics, and those simply not paying attention to either actively deny it or to just look the other way, committing the planet to more and more change.

Some will never be able to accept that humans can affect the global climate, no matter the evidence. They will continue to conflate geologic and human time scales and assume that what is occurring today must be what has always occurred in the past — natural. But their inability to comprehend the planetary influence of humans isn’t based on reviewing and rejecting the scientific evidence, which is clear to 97-98% of climate scientists publishing in the field. It is based on ignoring or disbelieving that evidence, just as some dogmatically refused to abandon their belief in a geocentric universe for reasons that had nothing to do with science. Alas, these modern-day dogmatists are unlikely to change their minds, at least not on a human time scale. And we don’t have time to waste.

Peter H. Gleick



  1. #1 Mentifex (Arthur T. Murray)
    Seattle WA USA
    March 28, 2017

    As a newbie here to — which I have embedded in my local websurfing page — I was shocked to see the name “Peter Gleick” this morning, because over the last two decades I have seen that name only on the hardcopy pages of my daily New York Times. So now I have read the above article on “Climate Change and the Collision between Human and Geologic Time” and please let me tell y’all that yesterday at I uploaded a potentially immortal artificial intelligence that could take a very “Long Now” view upon what human beings are doing to this little blue planet.

  2. #2 SteveP
    March 28, 2017

    “Ironically, our effect on the climate, while fast in geological terms, is still slow enough for policy makers, climate contrarians and skeptics, and those simply not paying attention to either actively deny it or to just look the other way, committing the planet to more and more change.”

    Nicely stated.

    As I watch the Trump administration gleefully dismantle years of environmental work, I try not to despair. I still have a lot of hope. But looking at Tillerson as secretary of state, I suspect the following: the massive wealth accumulated by the North American fossil fuel industry has been used to form an alliance with common Russian fossil fuel interests in an act of… self preservation….. or worse. We can all create conspiracy theories until the cows come home but it is difficult for the non-elites to pierce the veil of secrecy that surrounds what is going on , so we can only guess.

    We of the environmental tribe have, I feel, been somewhat naive in not recognizing the powerful forces of what I call automobile culture and leisure culture, which the fossil fuelists have used to shape and control our current world.

    I think that we have, thus far, done the right thing in bringing attention to this problem and continuing to exert pressure to fix it. The current dynamics are forcing things in an unfortunate direction that is currently out of our control. Are we planning for the day when the forces of nature make headlines so astounding that the nature of the problem becomes clear to even the most hardened ideologue ? That would be a key time to overturn this unfortunate current dynamic, IMO.

  3. #3 Peter Gleick
    March 28, 2017

    Steve, thank you for your thoughtful comments.

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