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Climate Change Climate Change hp://www.independent.co.uk What determines the temperature of the earth? The earth’s interior is very hot, but the mantle is a good insulator. The amount of heat seeping out of the interior is very small compared to the energy received from the sun. How do we measure heat flow? The temperature gradient in the upper part of the crust is determined by directly measuring temperatures at different elevaons in boreholes. Heat flow at the surface of the earth. Note how the areas of highest heat flow follow the mid-ocean ridges. The largest areas of measurement uncertainty are along the very crests of the ridges and under the Greenland and AntarcCc ice caps. Davies and Davies, Solid Earth, vol 1, p 5-24, 2010. What determines the temperature of the earth? The earth’s interior is very hot, but the mantle is a good insulator. The amount of heat seeping out of the interior is very small compared to the energy received from the sun. The principal factor controlling the temperature of the earth is the fracon of light it absorbs from the sun. We have good pictures of the earth taken from space, so we know how much light the earth reflects. So, we know how much power the sun provides to the earth. It is a big number, but the earth is big. Everything emits light In the previous chapter we learned that everything with a temperature emits a spectrum of electromagneCc radiaon, or “light”. The wavelength of the peak of this distribuCon is given by L = 3000/T Where T is in Kelvin and L is in micro-meters. The surface temperature of the sun is 6000 K, so the peak wavelength emi"ed is 0.5 micro-meters, which is right in the middle of the visible spectrum. The average surface temperature of the earth is approximately 300 Kelvin, so the peak wavelength emi"ed is 10 micro-meters, which is in the infrared. visible light infrared light Equilibrium If the earth’s temperature is constant (equilibrium) then its temperature should be whatever it takes so the infrared light emi"ed by the earth has the same total power as what the earth absorbs from the sun. infrared light is emied some of the sun’s light is reflected visible light Power absorbed from sun equals power radiated in all direcCons. Oops If we set the power absorbed by the earth from the sun equal to the power that a solid body would radiate due to its temperature, we can calculate that temperature. This calculaon has been done and the answer one gets is 255 Kelvin (which is -18 CenCgrade or -1 Fahrenheit). But that is not correct. The average temperature of the earth is 289 Kelvin (16 C, or 61 F). What is wrong? Oops This calculaon has been done and the answer one gets is 255 Kelvin (which is -18 CenCgrade or -1 Fahrenheit). But that is not correct. The average temperature of the earth is 289 Kelvin (16 C, or 61 F). What is wrong? The earth has an atmosphere! With no atmosphere, these would be equal And the earth would be fairly cold. The main gasses in the atmosphere (nitrogen, oxygen, some argon) are transparent to visible light and also to infrared light. They go right through the atmosphere. However, other gases which are present in the atmosphere such as water vapor, carbon dioxide, methane, and others, strongly absorb infrared light. This absorpCon of visible and infrared light by these gasses is extremely well characterized by laboratory experiments. No doubts there. --Wikipedia Atmosphere absorbs infrared These two have to balance from the ground and emits it both into space and back to the ground And so the earth is much warmer. The actual situaon is more complicated, but can be modeled well. Some visible Some infrared light is is transmi"ed reflected by by the clouds. atmosphere. And so the earth is much warmer. The process is straighorward to understand quantavely hps://www.skepCcalscience.com This is pre"y basic science. ScienCsts would say they have an good theoreCcal understanding of the earth’s equilibrium temperature. It is clear that for current condiCons, increasing water vapor, carbon dioxide, or methane, will lead to a higher equilibrium temperature. It would be very stupid to argue about this. It would be like arguing about whether or not designing a car with a lower drag coefficient would reduce gasoline consumpCon. This is pre"y basic science. ScienCsts would say they have an good theoreCcal understanding of the earth’s equilibrium temperature. It is clear that for current condions, increasing water vapor, carbon dioxide, or methane, will lead to a higher equilibrium temperature. It would be very stupid to argue about this. It would be like arguing about whether or not designing a car with a lower drag coefficient would reduce gasoline consumpCon. So, what is the trend? Measurements. It is actually straigheorward to measure the amount of CO2 in the atmosphere with chemical sensors. The Naonal Oceanic and Atmospheric Administraon has maintained such a sensor near the summit of Mauna Loa in Hawaii since 1956. Measurements. The seasonal change is due to the seasonal change in the uptake of CO2 by the world’s land vegetaon, because most of the world’s land vegetaon is in the northern hemisphere. Further back The record extends back to 1956. Even Further back --From your book Much Further back The most direct method for measuring atmospheric carbon dioxide concentraons for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the AntarcCc or Greenland ice sheets. The most widely accepted of such studies come from a variety of AntarcCc cores and indicate that atmospheric CO2 concentraons were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. thousands of years ago --Wikipedia This is quite aenon geng From your book: Other measurements (not shown) tell us that the carbon dioxide level now is higher than it has been at any Cme in the last 20 million years. That fact is not disputed; it is astonishing but not surprising. We know how much carbon we are burning, and that is plenty to account for the increase. (Some of the CO2 dissolves in the oceans, making them more acid, and some is taken up by increased biomass.) This is quite aenon geng From your book: Other measurements (not shown) tell us that the carbon dioxide level now is higher than it has been at any Cme in the last 20 million years. That fact is not disputed; it is astonishing but not surprising. We know how much carbon we are burning, and that is plenty to account for the increase. (Some of the CO2 dissolves in the oceans, making them more acid, and some is taken up by increased biomass.) By the way, it is also accepted that 600 million years ago the level of CO2 in the atmosphere was 10-20 Cmes higher than it is now. And it is equally accepted that there was almost no frozen water on the planet. The poles were fully melted. So, what is the temperature doing? Physics teaches us that increasing carbon dioxide in the atmosphere must cause the earth’s temperature to rise. Measurements show that CO2 levels are increasing. So, what is temperature doing? So, what is the temperature doing? Physics teaches us that increasing carbon dioxide in the atmosphere must cause the earth’s temperature to rise. Measurements show that CO2 levels are increasing. So, what is temperature doing? First, let’s ask how we know. So, what is the temperature doing? First, let’s ask how we know. The simplest answer is that people have been keeping records of temperature all over the planet for a long Cme. “The instrumental temperature record provides the temperature of Earth's climate system from the historical network of in situ measurements of surface air temperatures and ocean surface temperatures. Data are collected at thousands of meteorological staons, buoys and ships around the globe. The longest-running temperature record is the Central England temperature data series, that starts in 1659. The longest-running quasi-global record starts in 1850. In recent decades more extensive sampling of ocean temperatures at various depths have begun allowing esCmates of ocean heat content but these do not form part of the global surface temperature datasets.” --Wikipedia So, what is the temperature doing? Instrumental record: Global mean surface temperature change from 1880 to 2016, relave to the 1951–1980 mean. The black line is the global annual mean and the red line is a smoothing. The blue uncertainty bars show a 95% confidence limit. Source: NASA GISS So, what is the temperature doing? Instrumental record: Pay aenCon to 1985 Global mean surface temperature change from 1880 to 2016, relave to the 1951–1980 mean. The black line is the global annual mean and the red line is a smoothing. The blue uncertainty bars show a 95% confidence limit. Source: NASA GISS So, what is the temperature doing? We also have satellites measuring the earth’s temperature. This sounds like it would be highly reliable and straigheorward, since a satellite can see the enCre earth. But it is actually challenging. Satellites do not measure temperature directly, but rather radiance at various wavelength from which temperature can be inferred. Also, the satellite data are not homogeneous, but are constructed from a series of satellites with similar but not idenCcal sensors.
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