Earth's Future Climate (1986)

[Music] [Music] hello and welcome to this episode of our series NASA biology on Earth and in space in our last show Earth's atmosphere was discussed in today's program we will hear how carbon dioxide in the Earth's atmosphere may become a major economic and environmental problem research scientist Dr James casting from the NASA am Research Center tells us about the carbon dioxide cycle and how it influences life on our planet well I'd have to say that the carbon dioxide cycle is generally misunderstood at most most levels and including uh some of the highest research levels because there is so much that goes on into controlling the levels of carbon dioxide in the atmosphere most of the current interest in CO2 is focused on the changes in CO2 that are occurring right at this uh time in our history and that is uh due to the burning of fossil fuels or at least largely due to the burning of fossil fuels and you can read about it in the papers there's a lot of interest because the current increase in Co 2 uh is thought to be contributing to uh or will be contributing to an enhanced Greenhouse Effect and therefore the the global temperatures on the Earth are likely to rise over the next 50 to 100 years CO2 happens to be a very interesting gas because the control mechanisms are rather complex and they occur on a variety of different time scales uh so that on very long time scales there may be one mechanism which controls CO2 level whereas on shorter time scales the control is exerted by a totally different type of mechanism so what I hope to do in this presentation is to go over uh the natural controls on co2 starting off with the very longest time scales the the billions of year time scales during which the Earth has evolved and then I'm going to work my way up uh through the 100 million year time scale of going back to the Cretaceous Period when the dinosaurs were run roaming around the earth and then up to a relatively recent time scale of the plene Ice Ages which has have been occurring with regularity over the past couple of million years and then finally say just a couple of words about the fossil fuel problem which hopefully will be placed in somewhat of a better perspective by going over the the history of CO2 on these other time scales so what I'd like to do is start out with the with the very longest time periods uh the earth was formed four and a half billion years ago and as I've sh in my first slide there there have been a number of changes in the Earth's environment that have been important on that time scale one of the most important of which is the fact that the sun's output has changed over the past 4 and a half billion years this is not something that's unique for the sun this happens to all main sequence Stars almost all Astro uh theories the sun's Evolution agree that the sun has to increase in in in output as it ages so the implications of this for climate were first pointed out back in 1972 in a paper by Carl San and George Mullen where they uh showed that all other things being equal uh that is if the composition of the Earth's atmosphere had not changed during the past 2 4.5 billion years the Earth's surface should have been completely frozen over prior to about 2.3 billion years ago that is the oceans would have been frozen and complete glaciation so this is at at odds with what we know from the geologic record there's very good geologic evidence for the existence of running water back to the earliest uh known Rock record 3.8 billion years ago and in fact there's no evidence that there was any glaciation whatsoever until two and a half billion years ago the the so-called huronian glaciation now the question is could you could the Earth really have had CO2 levels that were a thousand times higher than today the answer to that question is yes uh the Earth has lots of CO2 in fact the amount in the atmosphere is really a trivial fraction of the Earth's total carbon inventory there is currently about 60 times the amount of CO2 that's in the atmosphere which is dissolved in the ocean as carbonate and bicarbonate ions uh and even that is only a small fraction of the carbon most of the carbon actually exists as carbonate rocks on uh in sediments U and if you took the total uh amount of carbon in carbonate rocks and put it all into the atmosphere as CO2 you'd end up with an atmosphere which would look very much like the atmosphere of Venus which has about 90 bars of carbon dioxide in its atmosphere Earth has one bar of uh carbon dioxide a bar is the total surface pressure on the present Earth so that would be an at atmosphere 90 times as thick as the Earth's present atmosphere but composed entirely of CO2 if you put even a small fraction to get a thousand times the present CO2 level we're talking about a few tth of a bar of CO2 clearly a small fraction of the Earth's total carbon and the question is how do you decide how much carbon was tied up in the carbonate rocks back in the past and how much was in the atmosphere and in the ocean well the first thing to notice is that the atmosphere and the ocean are linked in the sense that CO2 can dissolve out of the ocean or can redissolve in the ocean and on long time scales the atmosphere in the ocean tend to stay in equilibrium by long time scales uh you're really controlled by how fast the ocean can turn over and expose new surface water to the atmosphere and it turns out that it takes about a thousand years for the ocean to do that so that on any time scale longer than a thousand years the atmosphere and ocean stay in equilibrium and what you're really interested in is the total amount of carbon dioxide in the atmosphere ocean system versus the total amount of carbon dioxide in the the carbonate Rock Reservoir well to determine the relationship between those two you have to look at at the so-called rock cycle which is is also referred to as the carbonate silicate geochemical cycle well this sounds complicated uh but in fact it's it's not all that uh not all that complex uh and I have here a slide that illustrates this thing schematically with just two simple chemical reactions uh the first is a is What's called the silicate weathering reaction in which you start with calcium carbonate or calcium silicate cao3 combine that with atmospheric CO2 uh and end up with calcium carbonate plus silica sio2 uh this actually happens in liquid water solution you need rainfall and precipitation and you you weather or that is erode and chemically react uh rocks on on the continents release these minerals silicate minerals into the rain water and then that water containing dissolved minerals and carbon dioxide which once it ol olves is in the form of bicarbonate ions that water washes down into into the oceans filling up the oceans with calcium ions and with bicarbonate ions then in the oceans the calcium and bicarbonate get together precipitate out as calcium carbonate and go down and form sediments on the uh floor of the ocean this is accomplished today in the present Oceans by the activities of biological organisms plankton in particular who form their shells out of calcium carbonate on the early Earth that is probably was probably not the case but in fact you can form calcium carbonate inorganically as well as if you just allow the calcium or bicarbonate concentrations to achieve higher levels well that is the loss mechanism for CO2 from the atmosphere ocean system because you you then you're in this process you're taking CO2 out of the atmosphere and you're putting it into the sediments as calcium carbonate what happens to these sediments though is that the the carbon is not lost irrevocably in fact these sediments uh are on the ocean floor but as we all know the ocean floor is not a static uh platform because of plate tectonics the ocean floor is constantly moving in fact ocean floor is constantly being created at the mid ocean ridges the centers of the Ocean Oceans and then it spreads out gradually towards the Continental margins and at the Continental Mar margins it get subducted back down well when that happens the carbonate sediments that are on the ocean floor get brought back down to higher temperatures and pressures within the within the Earth's crust and going down into the to the mantle and at at higher temperatures and pressures just the reverse of the weathering reaction occurs and this is the second reaction shown on the slide and it's referred to as silicate reconstitution or carbonate metamorphism and and what happens here is that calcium carbonate from the sediments recombines with silica which which also is contained in the sediments to reform calcium silicate plus CO2 and the CO2 bubbles back into the atmosphere through volcanos and through uh other cracks in the Earth's surface so that in the very long term this is the restoring mechanism by which CO2 re-enters the atmosphere ocean system if you didn't have this mechanism occurring then the uh atmosphere would become depleted in CO2 in a very short time scale and the the greenhouse effect would go away and in fact the Earth would get very cold well understanding this cycle is the key to understanding how the Earth's atmosphere could have had very high CO2 levels in the past but at any rate the best explanation for why carbon dioxide levels could have been higher on the early Earth has to do with the fact that if the Earth had been much colder than today then the silicate weathering process would have gone more slowly and therefore the loss process for CO2 would have decreased and CO2 levels would have naturally built up in the atmosphere so that uh probably the Earth never did freeze over almost certainly the Earth never did freeze over entirely uh but in fact there was a natural feedback mechanism that provided uh climatic stability by enhancing the amount of CO2 in the early atmosphere well this same rock cycle that controls the carbon uh carbon dioxide concentration of the atmosphere on very long billions of year time scales uh can also operate on shorter time scales uh in particular there's some evidence now that uh CO2 levels may have been significantly enhanced during the Cretaceous Period which is the time about roughly a 100 million years ago when the dinosaurs were roaming the Earth uh we we know from the climatic record that in fact the Cretaceous Period was very warm period in Earth history and that the globally average surface temperature during the Cretaceous was probably about 10° Centigrade warmer than the present Global temperature well there are several possible reasons why it could have been warm during the Cretaceous one of which is that the continents were in different positions in particular they were more clustered towards the equator there was less Continental area near the poles on which uh ice caps could accumulate so in fact this this is a a very likely explan explanation for part of the temperature increase during the Cretaceous and the best climate models that have been run indicate that you can account for roughly half of that 10 degree global warming by just simply moving the continents around to different positions but in fact that still leaves about five degrees to be accounted for and there the interesting thing that has come out just in the last couple of years is that uh it's very likely that that other five degrees of warming was accounted for by higher CO2 levels in the Cretaceous atmosphere the reason for this is that the rock cycle uh was probably operating faster essentially during the Cretaceous than it is now the evidence for this being the fact that the seaf Flor spreading rate the rate at which the seafloor is created at the mid ocean ridges and moves towards the Continental margins uh can be measured by uh looking at magnetic anomalies in the ocean floor magnetic anomalies are merely the signature which is left when the Earth's magnetic field reverses as it does every few thousand years on sort of a random basis Anyway by looking at these magnetic anomalies we can estimate how fast the ocean floor was spreading uh and uh the estimates disagree U to some to numerically as to what the change was but they do all agree that spreading weights rates were significantly higher back around 100 million years ago so if you think of that in terms of the carbonate uh cycle uh the carbonate sediments that are on the ocean floor would have been brought uh to the to the Continental margins and subducted and metamorphosed more quickly at that time than they are today so that that should give you a a significantly enhanced CO2 production rate during the Cretaceous and in fact people have now tried to make ra rather elaborate geochemical models of the CO2 cycle uh to see what effect that would have on atmospheric CO2 it is now possible to measure CO2 uh uh in ice cores that are taken from Antarctica and from Greenland and these ice cores can be dated back to 30 or 40,000 years by just simply looking at the annual layers of ice and counting Counting layers within these ice ice cores are little air bubbles that were trapped during the formation of the ice and which have maintained the composition of the atmosphere in which they were uh in which from which they formed and now measurements by a number of different groups have shown that in fact during the last ice age that is prior to about 11,000 years ago the CO2 concentration of the atmosphere was about 200 parts per million and then it suddenly jumped up to about 250 parts per million uh at right at the close of the last ice age well this is very interesting because of course climatologically if you get a CO2 increase uh that should give you a global warming and uh the question now is could this CO2 increase uh explain the could that possibly be the reason that we that we popped out of the last ice age and second of all what in what could possibly be causing CO2 to change uh dramatically uh in such a short time scale well the answer uh is probably uh has to do with the biological productivity in the surface ocean and this is something that is not normally thought about but uh if you make a simple two boox model of the ocean that is divide the ocean up into a a surface layer which contains most of the uh biology the species which are uh performing photosynthesis and then a deep ocean which contains the rest of the uh water in in the in the oceans you find out that the uh concentration of and observations show that this is true the concentration of dissolved CO2 in surface water is significantly different than it is in deep water the reason being that organisms are constantly taking CO2 out of surf surface water and incorporate incorporating it into their bodies as organic matter and as carbonate uh into their shells as carbonate uh then as these organisms die they fall down into the deep ocean uh most of the organic matter is reoxidized and much of the carbonate redissolves into the ocean so the net effect is that you have a biological pump which pumps carbon from the surface ocean to the deep ocean we now have been measuring monitoring CO2 levels very accurately of at the monoa observatory in Hawaii ever since 1958 and in fact CO2 has been increasing constantly since that time uh although there is a there's a slow increase it's gone up from about 315 parts per million in 58 to about 340 parts per million today superimposed on this slow increase is a five or six part per million wiggle which is due to the photosynthetic cycle uh on the continents in particular in the northern hemisphere but nevertheless this ex this increase in CO2 is thought to be due primarily to burning of fossil fuels and if one takes all of the fossil fuels which have been burned since the the late 1800s uh if about 50% of that carbon is going into the atmosphere then that is enough to explain the observed CO2 increase uh that is the current theory and uh it's likely that we're going to continue burning carbon dioxide that uh carbon dioxide levels will continue to increase and in fact the best estimates are that uh CO2 levels will approximately triple over the next 200 years uh if CO2 levels triple then uh the the Earth's surface should warm up by four or five degrees Centigrade during that time which will make the Earth warmer than it has been at any time since the late Cretaceous uh this has now been predicted in in reports by both the uh National Academy of Sciences and the Environmental Protection Agency and in fact uh it's something that we should probably look forward to in our future the greenhouse effect is basically due to the fact uh that CO2 and water vapor for that matter uh both are fairly transparent to to visible radiation that most of the sun's energy comes us comes to the Earth in the in the form of visible radiation just light that we see U on the other hand the Earth radiates back most of this this energy as as infrared energy or heat and both CO2 and water uh absorb infrared energy very well therefore they let the sunlight in but trap the outgoing infrared radiation the reason see is it kind of counterintuitive but you don't hear much about the greenhouse effect of water water is actually a much better greenhouse gas than is CO2 it absorbs throughout much of the infrared Spectrum whereas CO2 uh really the the absorption is more concentrated in specific areas however water is controlled water can condense out of the atmosphere and therefore it's controlled very much by its saturation Vapor uh pressure so what what actually happens in terms of the climate is that CO2 can be thought of as a driver mechanism if you increase CO2 you increase uh surface temperature by increasing the greenhouse effect that evaporates more water and the water amplifies the effect of CO2 so that the greenhouse effect is due to both gases but CO2 is the one that you're more worried about from a climatic standpoint one of the most uh productive ways of looking at the carbon cycle I think is to compare Venus and Earth and Mars and try to see why they each evolve so differently Venus has a very in fact both Venus and Mars have predominantly CO2 atmospheres there's a big difference in pressure though Venus has a surface pressure of about 93 bars uh that is 93 times the surface pressure on the earth most of that about uh 97% of that is CO2 Mars on the other hand uh is again about 96 or 97% CO2 but the surface pressure is only 6 mbars or 6 1,000 of the surface pressure on the earth so that here we've got vast differences in the total amount of carbon as carbon dioxide on these uh in these atmospheres now you ask why why do we see such great discrepancies between the planets let's take uh Mars case of Mars first Mars in the first place we don't know for sure if Mars was in was ever endowed with as much carbon dioxide as the Earth was but uh even if it was uh G given a large CO2 atmosphere and I should say that there is some evidence for it in that uh a lot there are there is evidence for running water in the past on Mars there are channels that were very obviously formed by by running water and which probably uh could only have been formed by precipitation from a very uh relatively warm dense atmosphere the most likely explanation for the for primitive Mars just as it is for primitive Earth is that you had a dense carbon dioxide atmosphere prod producing a big Greenhouse Effect and that that allowed liquid water to exist and to form those channels well now Mars had a problem though and that was that Mars is significantly smaller than the earth and small objects tend to lose their heat they cool off much more rapidly than big objects do uh and it's it's the internal heat of the earth that keeps plate tectonics going and that keeps and it's plate tectonics that keeps carbon cycling through the CO2 geochemical cycle well in Mars uh it was too small very early in its history uh plate tectonics ceased if indeed it ever operated in the first place and Mars was unable to recycle car carbon dioxide the way that the Earth does so if you had liquid water there which uh which there is evidence uh for it occurring uh you should have formed carbonates uh and uh subtracted CO2 from the atmosphere but you weren't adding it as you were in the case of the earth so the process was essentially irreversible and Mars the atmosphere became thinner and thinner and and the climate became cooler and cooler now Venus uh and finally uh it would be very interesting to consider what would be the case if Mars was the same distance that it is now from the Sun but if it was the SI the same size as the Earth well then the same process and and had the same carbon dioxide endowment then the same process which we think kept the early Earth warm ought to be keeping Mars warm today you would in fact expect for instance if you took the Earth and put it at Mars's orbit you would in fact predict or or at least I would predict that uh the climate should be much warmer than the present Mars in fact you should have an average surface surface temperature which is warm enough to allow liquid water to exist and that you should have a dense CO2 atmosphere which is maintained by the carbon cycle because like you know the analogy to the Earth is that you have to have liquid water in order to keep the carbon cycle in Balance so Mars's problem is not that it's too far out basically but that it's too small now if you go to Venus the problem is different uh Venus is the same size as the Earth so from the tectonic standpoint one would expect that it should be possible to recycle carbonates and in fact Venus has the opposite problem from Mars it's got a huge dense CO2 atmosphere instead of a relatively thin uh atmosphere what happened to Venus uh the answer is is the obvious uh again which is that Venus was too close to the Sun I think that the key issue 50 to 100 years in the future is going to be carbon dioxide I think that we're just seeing the the beginning of this uh CO2 as a climatic problem it already gets a lot of press but in fact uh even though CO2 levels have been observed to go up uh it we have never seen the climatic effect of CO2 during that say next 100 years or so the sea level May begin to rise by a meter or two now that again doesn't sound like that much but in fact that would flood significant areas of coastal real estate and economically that that would uh have very severe consequences and I think when when we begin to notice a problem an environmental problem is when it really begin begins to hit us in the pocketbooks and I I think that all the scientific evidence indicates that ce2 is going to be a big economic uh environmental problem uh and much bigger 50 years from now than it is today each year we learn more about the planet in which we live as tenants of this planet we have the responsibility to use this knowledge to provide for a better world for the generations to come through its various research activities NASA will strive to achieve this goal thank you for joining us this is Lyn bondan saying goodbye from the NASA Lewis Research Center Cleveland la [Music]