( CNN ) -- Most of us can appreciate that the world is an ancient place and that a lot has changed in the almost 4.6 billion years since it took its shape . It 's not easy to have a feel for the amount of time that has passed , but grappling with deep time helps you understand why an atmospheric carbon dioxide concentration ( CO2 ) of 400 parts per million ( ppm ) is meaningful . Deep time is geologic time and the scale needed to fathom the evolution of life , mountains , oceans , and Earth 's climate . Climate , one must note , is not weather . Weather is experienced day to day . Climate occurs on longer scales : the tropics are hot and wet ; Antarctica is freezing and miserable . If you wanted to consider the climate of the whole planet you would need to consider its temperature . Without greenhouse gases the world would be a much colder ( ~30˚C or 65˚F colder ) and lonelier place . All of us have felt the greenhouse effect because water vapor is a greenhouse gas and when summer humidity is high you just ca n't get any relief . Even at night when the sun is gone , water vapor keeps radiating the heat . Dry desert nights are just the opposite , with temperatures falling fast when the lights go out . CO2 works the same way as water vapor . More CO2 radiates more heat , increasing the average temperature and turning more surface water into water vapor , which radiates more heat . This is an example of a positive feedback and positive feedbacks push temperatures higher than CO2 alone . Global climate models tell us that doubling CO2 will lead to a global temperature increase between 2 and 4.5˚C ( that 's 3.6 to 8˚F ) , reflecting the climate sensitivity to CO2 . Since 400 ppm marks a 43 % man-made CO2 rise ( from 280 to 400 ppm in ~200 years ) , we should soon expect 1˚C ( 1.8˚F ) of warming if low-end estimates of climate sensitivity are correct . Well , a 1˚C ( 1.8˚F ) global temperature increase is close to what we 've already measured , but full warming is not expressed overnight or even over decades because the world is mostly a cold ocean that helps ameliorate immediate warming by taking up heat . Also , man-made atmospheric particles could be masking up to 0.5˚C ( 0.9˚F ) of the potential temperature rise . With this in mind , we expect temperatures to further increase even if CO2 stopped rising , and so it 's not alarming to assume that climate sensitivity might be higher than the belief of an optimist . So , how much CO2 is too much ? This is where deep time helps frame our expectations of things to come . First , as far as we know , the rate of our CO2 rise is unprecedented in Earth history . That 's saying something , is n't it ? During the well-known glacial-interglacial cycles that occurred in the last one million years , CO2 fell to a cold 180 ppm and back up to 280 ppm . Those changes took thousands to tens-of-thousand of years , so , 400 ppm not only breaks that record , but it breaks it in record time . When climate scientists say that they are worried about 2 to 5˚C ( 3.6 -- 9˚F ) of global warming , keep in mind that global temperature was , at most , 4 to 5˚C ( 7.2 -- 9˚F ) colder during the last ice age when miles of ice piled up on land and pushed enough dirt to make Long Island . Small numbers , like 4˚C ( 7.2˚F ) of cooling can mean big things , but that 's not where we 're headed any more . Now we 're headed deeper into the past . At best , CO2 levels are similar to 4 million years ago when global temperatures were 3 to 4˚C ( 5.4 -- 7.2˚F ) higher , and there was little-to-no ice on Greenland , and sea levels were at least ~~20 m ( 65 feet ) higher . Even if we were to resolve to live in the warmth of the relatively recent past , the nagging problem is that atmospheric CO2 levels will continue to rise and stay around for a very , very long time without intentional intervention and/or a surprising technological innovation . CO2 is increasing at about 3 ppm per year and we will be arriving at a minimum CO2 concentration of 700 ppm by year 2100 if nothing is done . In terms of our history , that places us somewhere beyond 35 million years ago when there was no permanent ice on the poles and sea level was over 200 feet higher than today . Add more CO2 to the mix and we step back towards peak temperatures 50 million years ago when CO2 is estimated to have been about 1000 ppm and sub-tropical plants and crocodiles inhabited the poles . There 's lots of uncertainty when trying to reconstruct ancient climates and the factors that produced them , but having some sense of the past informs us in the same way as knowing the history of our grandparents and the potential inheritance of future ailments . Deep time tells us that we are traveling forward to a world that is an extraordinary warm and different place , something Earth scientists refer to as a greenhouse world . Hopefully we will resist the urge to take the trip , but if we do n't , I know a few people who would be happy to study how it all went down . The opinions expressed in this commentary are solely those of Mark Pagani .
Mark Pagani : CO2 levels in atmosphere undergo major changes . He said research on the past shows the dangers of high levels of greenhouse gases . Today 's CO2 levels comparable to time when Greenland had little land , which was several feet higher . Others : With more CO2 , we hark back to when crocodiles and sub - tropical plants inhabited the poles .