Carl Sagan testifying before Congress in 1985 on climate change

right our uh next witness is dr carl sagan of cornell university a man who needs no introduction he comes to us today from the center for radio physics and space sciences we welcome you and we're very pleased that you would take the time out of your schedule to come to a place like washington where everything seems to be living in today and not in tomorrow to share with us your particular view of how our past and our present uh may well affect our future carl thank you very much thanks very much senator dernberger senator gore

senator burdick i'm glad to be here as i understand my uh my function it is uh to give some sense of what the greenhouse effect is to try to say something about uh greenhouse effect on uh on other planets to again underscore that this is uh is a real phenomenon and then perhaps i can take liberty to say a few remarks about what to do about it the power of human beings to affect and control and change the environment is growing as our technology grows and at present time we clearly have reached the stage where

we are capable both intentionally and inadvertently to make significant changes in the global climate and in the global ecosystem and we've probably been doing on a smaller scale things like that uh for a very long period of time uh for example uh slash and burn agriculture uh which has been uh with us for tens of thousands of years probably changes the climate to some extent by changing the albedo the reflectivity of the earth that massive changes have occurred is clear from the historical record for example egypt was once the basket of the roman empire it

may be the same role as the american midwest plays today that is certainly no longer the case it's not a greenhouse effect issue it may be an overgrazing issue but is an example of how humans are perfectly capable of making these unexpected and inadvertent changes because the effects occupy more than a human generation there is a tendency to say that they are not our problem of course then they are nobody's problem not on my tour of duty not on my term of office it's something for the next century let the next century worry about it

but the problem is that there are effects and the greenhouse effect is one of them which have long time constants if you don't worry about it now it's too late later on and so in this issue as in so many other issues we are passing on extremely grave problems for our children uh when the time to solve the problems if they can be solved at all is now if you um if you ask what determines the earth's climate clearly the main the main thing that determines it is uh sunlight sunlight is what heats the earth

uh not all the light that uh arrives at the earth from the sun goes to eating the earth some of it is reflected back it's just the the part that is absorbed uh and what happens is there's a certain rate at which sunlight is absorbed by the earth's surface and there's a certain rate at which the earth's surface radiates to space what comes from the sun is in the ordinary visible part of the spectrum that our eyes is sensitive to what the earth radiates into space is in the infrared part of the spectrum longer waves

than red that our eyes are not sensitive to but it's as legitimate excuse me a form of light as the kind that we're that we're used to now if you calculate what the temperature of the earth ought to be from how much sunlight is being absorbed uh equalling how much infrared radiation would be radiated to space you find that the earth's temperature by this simple calculation is too low it's about 30 centigrade degrees too low and why is it too low it's too low because something was left out of the calculation what was left out

of the calculation the greenhouse effect the air between us is transparent except in los angeles in places of that sort uh in the ordinary visible part of the spectrum we can see each other but if our eyes were sensitive at say 15 microns in the infrared we could not see each other the air would be black between us and that's because in this case carbon dioxide carbon dioxide is very strongly absorbing at 15 microns and other wavelengths in the infrared likewise there are parts of the infrared spectrum where water vapor absorbs where we could not

see each other if if we were only as far apart as we are in this room if you add these infrared absorbing gases to a planetary uh to a planet then what happens is the sunlight comes in as before but when the surface tries to radiate the space in the infrared it is blocked it is impeded by the absorbing gases and so the surface temperature has to rise so that there is an equilibrium between what comes in and what goes out so this is the greenhouse effect it is a misnomer for more reasons than one

it's a misnomer in particular because that's not how the florist greenhouse works but that's a very minor point there are other gases which absorb in the infrared uh all of many of which have been mentioned already nitrous oxide methane the halocarbons and these are products uh partly of agriculture it's fertilizers refrigeration aerosol spray cans and so on all products of our technology we don't generate much water into the atmosphere but we certainly do generate a great deal of carbon dioxide through the burning of wood and fossil fuels and apparently benign [Music] activity who could object

to humans burning oil and coal gas and wood i'd like to stress that the greenhouse effect makes life on earth possible if there were not a greenhouse effect the temperature would as i say be 30 centigrade degrees or so colder and that's well below the freezing point of water everywhere on the planet uh the oceans would be solid after a while a little greenhouse effect is a good thing but there is a delicate balance of these invisible gases and uh too much or too little greenhouse effect can mean too high or too low a temperature

and here we are pouring enormous quantities of co2 and these other gases into the atmosphere every year with hardly any concern about its long-term and global consequences now certainly not all aspects of how increased co2 and other gases into the atmosphere affect the climate are known there are still many uncertainties although the overall picture is i think quite clear and quite widely understood and accepted but there are questions about aerosols about clouds you heat up the earth how much increase or decrease in cloudiness is there how does that change the albedo or reflectivity of the

earth there's questions about the ocean and its response time to an increase in co2 there are feedback effects and therefore it is certainly worthwhile to spend some additional money on uh further research on the subject another point is that the significant temperature changes on the earth between uh ice ages and out of ice ages glacial and interglacial time periods seems to be connected with quite small changes in the amount of sunlight that reaches the earth due to changes in the earth's orbital properties and that is a suggestion that uh the earth's climate system may be

uh uh very delicately dependent on the sorts of factors that we're talking about here and that's why it makes sense to study past climatic change uh on the earth as an attempt to obtain some calibration another source of calibration is the other planets every planet with an atmosphere has some degree of a greenhouse effect the most spectacular case by far is the greenhouse effect of venus it's the nearest planet it's a planet about the same mass radius density as the earth but it uh is spectacularly different in several respects one of which is that the

surface temperature is about 470 degrees centigrade 900 fahrenheit and that enormous temperature um is not due to it's being closer to the sun because venus is surrounded with bright clouds and in fact because it reflects so much light back to space if that's all that was happening it would be cooler not warmer than the earth the reason for this uh absurdly high temperature on the surface of venus which is uh well understood i mean soviet spacecraft have landed on venus and in effect stuck out a thermometer there's no doubt that the surface temperature is very

high um and and later u.s spacecraft have as well the reason is a massive greenhouse effect in which carbon dioxide plays the major role now the amount of co2 in the venous atmosphere is much larger than here the atmosphere is almost entirely carbon dioxide and there's 90 times more of it uh there than here but it is an indication of what can happen in an extreme case you look at mars or jupiter or titan the big moon of saturn and you have additional examples of greenhouse effects different gases different amounts of sunlight reaching the surface

different planetary albedos and cloudiness and in all those cases there is also a greenhouse effect in addition it has been possible to calculate those greenhouse effects fairly accurately so that the kind of theoretical uh armamentarium which is used to calculate the greenhouse effect greenhouse effect changes on the earth is also used for other planets and therefore can be calibrated to some extent against those other planets if we keep coming out with the right answer in all those different cases then probably we understand fairly well how greenhouse effects work it would however be worthwhile in along

the lines that senator gore was talking about to have an increased program through nasa to understand the greenhouse effects on other planets this might be a very uh practical uh application of planetary exploration as you've heard the the best estimates they certainly have some uncertainty attached to them are that uh at the present rate of burning of fossil fuels the present rate of increase of minor infrared absorbing gases in the earth's atmosphere that there will be a several centigrade degree temperature increase on the earth global average by the middle to the end of the next

century and that has a variety of consequences including redistribution of local climates and through the melting of glaciers an increase in global sea level there is concern on a somewhat longer time scale about the collapse of the west antarctic ice sheet and a general rise of many many meters in in sea level so we we have a kind of handwriting on the wall certainly there's more research to be done but as i say there is a consensus what can be done about it the idea that we should immediately stop burning fossil fuel has such severe

economic consequences that no one of course will take it seriously but there are many other things that can be done one has to do with subsidies for fossil fuels more efficient use could be encouraged by fewer government subsidies secondly there are alternative energy sources some of which are useful at least locally solar power is certainly one that might be of more general use safe fission power plants which are in principle possible and then on a longer time scale the prospect of fusion power fission infusion power plants in principle vent no infrared active gases and therefore

whatever other problems they may provide they do not provide a greenhouse problem i'd like to close by just saying a few words on the the kind of perspective that this problem as related problems pose to us here is a problem which transcends our particular generation it is an intergenerational problem if we don't do the right thing now there are very serious problems that our children and grandchildren will have to face it is also a global problem it is no good if uh just one or two major industrial nations take major steps to prevent uh a

major increase still further in co2 and other greenhouse gases because other nations may uh through their industrial development um caused the problem by themselves and not to say that this is inevitable but just to give an example uh the largest coal reserves on the planet are the united states soviet union and china china is undergoing a very major uh industrial development um and the burning of coal is certainly uh something that must be very attractive for the chinese looking into the future i would say that there is no way to solve this problem even if

the united states and the soviet union were to come to a perfectly good accord on this issue without involving china and many other nations that will be developing rapidly in the time period we're talking about so here is a a sense in which the nations to deal with this problem uh have to make a change from their traditional concern about themselves and not about the planet and the species a change from the traditional short-term objectives to longer-term objectives and we have to bear in mind that in problems like this the initial stages of global temperature

increase one region of the planet might benefit while another region of the planet suffers and there has to be a kind of trading off of of benefits and uh and suffering and that requires a degree of international amity which certainly doesn't exist today i think that what is essential for this problem is a global consciousness a view that transcends our exclusive identifications with the generational and political groupings into which by accident we have been born the solution to these problems requires a perspective that embraces the planet and the future because we are all in this

greenhouse together thank you mr chairman oh thank you very much just a