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ATMS 321 Human-‐Induced Global Warming Chapter 12

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ATMS 321 Human-‐Induced Global Warming Chapter 12 ATMS 321 Human-Induced Global Warming Chapter 12 For more than 100 years scientists have been predicting that the release of carbon dioxide into the atmosphere might be causing the climate of Earth to warm. This has just become observable in the past few decades. Now it has become more of a public concern. Moana Loa Carbion Dioxide record since 1980. Carbon dioxide (CO2)is increased because of 1) Fossil fuel production, 2) cement manufacture and 3) deforestation. Methane, CH4, is also known as Natural Gas. It is produced during anerobic decay of organic material, as in organic sediments that produce petroleum and under rice paddies and in the guts of animals. The increase in sources likely come from the production of fossil fuels (Coal, petroleum and natural gas, itself), from increased agriculture, especially rice and cows. Nitrous Oxide (N2O) is formed during the oxidation of fixed nitrogen and is likely increased because of agriculture, especially the production of artificial fertilizers (use petroleum to fix nitrogen (NH3 – ammonia)). How do we know the increase in CO2 is caused by humans and not a natural cycle change? The Carbon Cycle The human production of carbon is small compared to the natural cycling of carbon http://en.wikipedia.org/wiki/Carbon_cycle Because more Anthropogenic CO2 production occurs in the NH, there is a N-S gradient in annual mean concentration. Isotopic evidence for Human-Caused CO2 increase. Circumstantial evidence It has increased in synchrony with human production of it by fossil fuel consumption. Isotopic Evidence Radioactive 14C has decreased as 14C-free fossil carbon has been added to the atmosphere. Carbon-13 has changed as you would expect if a lot of plant cycled carbon was added to the atmosphere. Plants have a strong preference for the lighter isotope, so the 13C/12C ratio in plant material is much less than average. Thus as fossil plant material in coal, oil and natural gas is added quickly to the atmosphere, the isotopic abundance of CO2 in air gets lighter. This is in fact observed. http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human- activities-updated/ Note the decline in 13C in a combined record from ice cores and instrumental records. This occurs because CO2 produced by combustion of fossil plant material has been added to the natural mixture of CO2 in the atmosphere over the past couple of centuries and increasingly rapidly in the last 50 years. Relative Contributions of Greenhouse gases, Circa 1990 The slice of the pie from carbon dioxide is increasing and is the major concern for the future. Nitrous Oxide over past 30 years, steady increase of about 0.75 ppb/yr IPCC 2007 Aerosol forcing Probably a cooling, can mask the greenhouse gas effect. But aerosols only last a few weeks in the atmosphere, whereas carbon dioxide anomalies are effectively permanent. Time scale is 200 years, but as you continue to add CO2 system changes to make time scale for recovery much longer. Modeled sulfate concentrations. Modeled Radiative effect of sulfate on TOA radiation balance, Wm-2 The first indirect effect of Aerosols – they can brighten clouds by distributing the water over more droplets. More ship tracks. Global Dimming and Brightening. If you observe downward solar radiation at the ground over a long period of time, you can measure changes with decreases “dimming” and increases “brightening”. These have been related to aerosol production by humans. An example is the plot below from Potsdam, Germany. We see a long period of decline from about 1950 to 1980, followed by a brightening from 1980 to 2006. This is thought to be related to increasing industrialization in Europe after WWII, especially fossil fuel combustion related effluents (coal plants and cars). Then in the 70’s and 80’s controls were put on both cars and industrial facilities to reduce effluents, especially sulfur. Then after the fall of the Berlin Wall in 1989, and subsequent reunification of Germany, the plants in Eastern Germany were modernized (or closed) and the air quality improved even more. Potsdam is in the central eastern part of Germany. Figure 2.5.3: Annual mean surface solar radiation (in Wm-2) as observed at Potsdam, Germany, from 1937 to 2006. Five year moving average in blue. Distinct phases of inclines (1930s-1940s, “early brightening”), declines (1950s-1980s, ”dimming”) and renewed inclines (since 1980s, “brightening”) can be seen. From Wild (2009). Climate Detection and Attribution: Two central questions must be answered. 1. Is the climate warming in the past 30-100 years? 2. Is it warming because of human activities? All kinds of evidence exists to suggest that the climate is currently warming. The first bit of evidence is the instrumental record, which suggests it has warmed about 0.8C over the past century and about 0.6C since 1980 or so. A great deal of research has been done to suggest that this reflects a good estimate of the true global temperature change over these periods. It shows up in both land and marine data, in various sectors, etc. Also lots of evidence exists from changes in plant and animal data, in mountain glaciers, etc. all pointing toward the fact that is has warmed over the past 30-100 years. Attribution Many different things change the global mean temperature, solar forcing, volcanic forcing, and natural unforced variability associated with the internal dynamics of the climate system, especially ocean transport. Total Solar irradiance and volcanic eruptions and aerosols have been measured pretty well over the past 30 years. The heat content of the ocean has also been measured to be increasing in step with the surface warming. The solar forcing and volcanic forcing seem too small to have produced the observed warming. In models we can put in the best estimates of the climate forcing from solar irradiance variations, volcanic aerosols, surface changes, etc., PLUS or MINUS the forcing from human-produced greenhouse gases and aerosols. The plot below summarizes the conclusions from the 2007 IPCC assessment. In these figures the black line shows the observed temperature change over the past century. The blue envelope shows the results of an ensemble of model runs with only the natural forcings (Sun, volcanoes), and the red shading shows the results from an ensemble of models in which natural and anthropogenic forcings are included. The conclusion is that the model envelope only includes the observed record, if one includes the human forcings, especially the greenhouse gas increases that warm theclimate most strongly in the past 30 years. The separation between the blue and red areas indicate the official beginning of the climate anthropocene, when humans took over the global temperature from naturally-occurring forcings and variability. Simple model of global mean temperature. It is useful to think about a very simple ordinary differential equation for global mean temperature. Change in energy content of climate system + extra cooling of climate associated with warming = extra heat input forcing dT ' C + !"1T ' = Q dt Here: C is the heat capacity of the system (mostly oceanic), units Joules per meter squared per degree C. T’ is a global mean temperature anomaly. ! is the climate sensitivity parameter , units ˚C per Wm-2. It is the equilibrium temperature response to a 1 Wm-2 climate forcing. It’s inverse is the rate at which the energy balance changes due to an increase in temperature. It would normally be negative, as the earth warms up it loses more energy (and vice versa), so you get a stable response to a climate forcing. If we wait for equilibrium, then T ' = !Q . Q is an applied forcing, like solar irradiance change, greenhouse gas forcing, volcanic eruption, etc. Divide by C to get, dT ' + ! "1T ' = c"1Q dt R Where ! R = c" is the inherent time scale, which is the product of the climate sensitivity and the heat capacity. t/! We can then solve this equation as we did before, by using an integrating factor, e R t/! d e R T ' ( ) Q t/! = e R dt c Assuming that the initial perturbation of temperature is zero (T’ = 0 at t=0), then t Q !t/" R t/" R T ' = e e dt ' #0 c If we suppose that the forcing is switched on instantaneously and kept constant, #! 0, for t < 0 Q = " then we get, Q , for t > 0 $# o "t/# R T ' = !RQo {1" e } Zonal mean temperature change caused by doubled Carbon Dioxide. Signatures of Greenhouse gas warming Predicted by global climate models. – zonal mean temperature anomalies 1. Stratosphere cools due to increased emission from CO2. Remember that stratospheric energy balance is between absorption of solar radiation by ozone and emission of terrestrial radiation by carbon dioxide. If we double carbon dioxide (and keep ozone fixed – not likely, but decent approximation) then the stratosphere must cool because emission has become more efficient. 2. Troposphere warms. It warms more in the tropical upper troposphere because the lapse rate there is thought to be constrained by the moist adiabat. As the climate warms, more water vapor is present in surface air. As the air rises in convection, more latent heat is released, so the lapse rate in equilibrium is decreased and more warming occurs at upper levels than at the surface, by about a factor of 2. 3. Polar amplification of warming. The poles warm more than the other parts of the planet at the surface because, especially in the northern hemisphere, the air has a strong inversion, which gets reduced as the climate warms and ice melts. Ice albedo feedback may also be a factor.
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