Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30
Lecture 3: The anthropogenic greenhouse effect: radiative forcings.
In the pre-industrial era (before major human interference with the atmosphere), the greenhouse gases in Earth's atmosphere were, in order of importance of their natural greenhouse effect:
The greenhouse effect from these naturally occurring gases is quite important, as your diagram in page 17 of Graedel and Crutzen shows. With the greenhouse gases, the surface of the planet emits 114% of the incoming energy in solar radiation, as infrared energy. But most of that is absorbed by the atmosphere, and the actual energy emitted to space is only 69%, of which only 12% is radiation that escapes directly from the surface and the other 57% is emitted from the atmosphere, from high layers in which the opacity to the infrared radiation is not so high. Now, imagine that we could somehow remove all the greenhouse gases from the atmosphere. Then all the infrared radiation from the surface would escape directly to space. The surface would have to cool, until its infrared emission would come down to 69% of the incoming energy in solar radiation (assuming that the fraction of solar light that is reflected directly out to space would stay constant at 31%), so that we could again have a balanced energy budget. This reduction in infrared emission implies that the average temperature would have to cool to -18 degree Celsius, or about 0 degree Fahrenheit, from the present 15 degree Celsius or 59 degree Fahrenheit.
We should really thankful for the natural greenhouse effect present in the Earth. Were it not for these greenhouse gases, the Earth would be an iceball!
Since the beginning of the industrial revolution, the concentration of greenhouse gases has been increasing.
This figure above shows how carbon dioxide used to be present at a level of 280 parts per million by volume (which is abbreviated as ppmv), and has by now increased to 370 ppmv. This means that at present, out of every one million molecules that are in the atmosphere, 370 are carbon dioxide. Methane has increased by a larger factor: it used to be 0.75 ppmv before the industrial revolution, and it is now at 1.7. Nitrous oxide has also increased a little bit, from 0.270 to 0.31 ppmv.
This increase in greenhouse gases results from several different human activities:
Why is water vapor, the most important greenhouse gas, not changed by human activities?
Contrary to all other greenhouse gases, water vapor has a very short lifetime in the atmosphere. It constant evaporates from the ocean and wet land, forms clouds, and rains. After evaporation, water typically spends only about two weeks in the atmosphere before it rains. Any additional water vapor that could be produced by humans would rain down in a short time. The amount of water vapor in the atmosphere is at an equilibrium point that depends on the global temperature of the Earth. If the planet warms up, there will be more water vapor. This is because the concentration of water vapor that you need to start condensation into water droplets increases with temperature. The abundance of water vapor is highly variable across the world, because it does not mix through the atmosphere in the short time it spends there. It takes about one year for the atmosphere of the Earth to get well mixed. The other greenhouse gases have lifetimes much longer than a year and are well mixed.
How have the concentrations of greenhouse gases been measured?
After 1958, we have records of the carbon dioxide concentrations measured directly from the atmosphere. Before this date, we can reconstruct the abundance of carbon dioxide and other gases in the atmosphere from analysis of ice extracted from glaciers. A new layer of snow falls onto glaciers every year, and the snow will be buried under other snow falling in the next years. The snow turns into ice under the weight of the layers above, and it retains a record of the atmosphere at the time the snow fell in the little bubbles of air and dust grains that are trapped within the snow. Cores of ice can be extracted from glaciers, the layers deposited over the years can be dated and the composition of air bubbles in the various layers can be measured.
What evidence do we have that the increase in greenhouse gases are due to human activities and not to some natural process?
There is a lot of evidence, in addition to the fact that their composition started to increase only with the industrial revolution. The carbon in the carbon dioxide has different isotopes, and the percentage of these isotopes has changed because fossil fuels contain different percentages than the carbon initially present in the atmosphere. The observed change in isotope composition is in agreement with what is expected for addition of fossil fuels. There has also been a decline of oxygen observed, consistent with the production of carbon dioxide by burning fossil fuels with oxygen from the air. The carbon dioxide is slightly more abundant in the Northern hemisphere because many more people live in the Northern hemisphere and much more carbon dioxide is emitted there. The difference in the concentration between North and South is small because it takes only about one year for the emitted carbon dioxide to be well mixed among both hemispheres.
Recall the energy budget diagram (page 17 in Graedel and Crutzen).
Up to now, we have always discussed that in terms of the percentage of energy of a certain transfer of energy compared to the total sunlight reaching the Earth. For example, we say that the energy that is directly absorbed by the surface is 49% of the total energy in sunlight reaching the top of the atmosphere of the Earth.
These fluxes of energy are also often referred to in physical units. To specify a flux of energy we need to measure a power per unit of area. The power per unit area from sunlight reaching the Earth at the top of the atmosphere is, on average, 342 Watts per square meter. Of this, 31% is reflected back to space and only 69% is absorbed (20% absorbed in the atmosphere and 49% on the surface), so this means 236 Watts per square meter are absorbed (obtained by multiplying 342 times 0.69). These units are not too difficult to understand. You are familiar with the unit of power called Watt, used to measure the power of any electrical appliance. If you take a lightbulb that you can buy in the store with a power of 100 Watts, and you were to spread all the light over an absorbing surface of, for example, 2 square meters, then that surface would be receiving 50 Watts per square meter. The Sun heats the Earth with the equivalent of, approximately, two and a half bulbs of 100 Watts shining over each square meter of the surface of the Earth.
Note that the 236 Watts per square meter received from the Sun is an average over all locations of the Earth. This already takes into account that during the night, there is no solar power being absorbed. And during the day, the amount of solar power depends on the angle of incidence of the light. For the point of the Earth in which the Sun is directly overhead, the power absorbed can be as much as 1000 Watts per square meter (that is when it feels really toasty if you lay under the Sun!). Similarly, the fraction of sunlight energy absorbed of 69% is only an average, it varies a lot over the Earth at any given time depending on cloud cover, angle of incidence of the sunlight, and the reflective properties of the surface.
The importance of each greenhouse gas for climate is measured by its contribution to the radiative forcing .
What is radiative forcing?
Radiative forcing is the amount by which a variation in the abundance of a greenhouse gas, compared to its abundance in pre-industrial times, changes the radiation energy budget of the Earth, if we change the abundance of the greenhouse gas but we do not alter the temperature and other properties of the surface and the troposphere.
For example, we know from our energy budget diagram in page 17 that the surface emits 114% of the incoming solar energy. This is 342x1.14 = 390 Watts per square meter. But because the atmosphere absorbs some infrared radiation, only 69%, or 342x0.69 = 237 Watts per square meter are actually emitted to space. If we now increase the abundance of carbon dioxide, the atmosphere becomes more opaque and even less infrared energy is emitted to space. For the case of the increase of carbon dioxide from 270 ppmv in pre-industrial times to 370 ppmv today, the outgoing infrared energy is reduced by 1.5 Watts per square. So the change in carbon dioxide has produced a radiative forcing of 1.5 Watts per square meter.
The radiative forcings have been calculated for all the greenhouse gases as a function of their abundance. These calculations are done by computing the way the radiation at each wavelength is absorbed and reradiated at different layers in the atmosphere, until it escapes to space. The present radiative forcings of each greenhouse gas (compared to their greenhouse effects in pre-industrial times) are:
In more popular physical units, the total radiative forcing is 10000 Watts for every acre of land or ocean in the Earth. This means that, if you take an acre of land or ocean anywhere in the Earth, the present anthropogenic greenhouse effect results in warming that is equivalent to what we would get if we had an electric heater of 10000 Watts turned on all the time, warming the surface and the air near the surface.
The calculations of the total radiative forcing from anthropogenic greenhouse gas has been determined to be accurate to 10%. Any errors in our knowledge of the absorption properties of the molecules or their distribution in the atmosphere could change the total radiative forcing only in the range between 2.2 and 2.7 Watts per square meter.