How could humans cause climate change?


People have a lot of great questions about climate change.  Here are some comments and questions from people who are curious about climate change and our role in it.

There were huge changes in climate (ice age cycles) long before the industrial period.

It’s true, we can see evidence for bigger glaciers, lower sea levels, colder temperatures, and lower CO2 levels all over the planet, these records show evidence of ice age cycles.  These ice-age changes in CO2 were not caused by humans, but by natural cycles in the Earth’s orbit around the Sun and how the glaciers, oceans, biosphere, and atmosphere interact with one another.  Global sea level was about 120m lower than today during past ice ages (because in colder climates, snow that falls on land and does not melt can form a thick sheet of ice over land, removing the water from the oceans).


In this figure, ‘today’ is on the right, and you go back in time to 400,000 years ago on the left.  In the top panel, we can see global temperatures in red, then CO2 in green and sea level change in blue.  What do you notice?

  • The highs and lows seem bound (CO2 goes between ~180 and 280 ppm)
  • The warm times like today happened four times in the past 400,000 years, so about one ice age cycle happens every 100,000 years
  • The temperature seems to decrease slowly and gradually into an ice age and then they end ABRUPTLY into warmth
  • All three of these records are doing the same thing, so there is likely to be a link between temperature, CO2, and sea level changes

The climate has been stable since the dawn of human civilization, why would it change now?

The Earth’s climate has been relatively stable for the past 10,000 years, which has been great for human population growth, agriculture, and settlements.  During the last ice age, there was a thick sheet of ice that covered most of Canada and the northernmost United States (flowing as far south as Long Island), this was when CO2 was at ~180 ppm.  Pre-industrial CO2 was at about 280 ppm, which is the typical CO2 level for in between ice ages.   That means that 100 ppm of CO2 is the difference between our modern climate and an ice age.  Now you might wonder, how many ppms of CO2 are in the atmosphere today?  We are over 400 ppm.

The CO2 levels are now higher than they have been for millions of years because we are burning fossil fuels.  It may be possible that another ice age will not occur because the atmosphere will be too warm.

The Earth is so large and humans are so small!

Lots of small things can cause incredible damage.  A parasite, virus, or blot clot could kill a human.  The Earth is not infinitely large, and there is no way to truly dispose of our garbage. Our waste stays with us, in the air, water, and food.

Climate change is politically charged

I often wonder if an Inconvenient Truth had been done by a scientist instead of a politician if the topic of climate change would still be so politically polarized.  I wonder if the United States is the only country where climate change is actively denied by politicians and the general public.  I wonder who has the most to gain by denying climate change.  It’s easy to keep things the way they are, to not change our own personal habits, to blame corporations and politicians, but in the end, it’s the people who have the power to change the supply and demand of products and how they are distributed.


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